Answers to Review Questions

Answers to Review Questions

Table of Contents

Cover

Title Page

Copyright

Publisher's Note

Acknowledgments

About the Author

Introduction

Assessment Test

Answers to Assessment Test

Chapter 1: Internetworking

Internetworking Basics

Internetworking Models

The OSI Reference Model

Summary

Exam Essentials

Written Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 1.1

Answers to Written Lab 1.2

Answers to Written Lab 1.3

Chapter 2: Review of Ethernet Networking and Data Encapsulation

Ethernet Networks in Review

Ethernet Cabling

Data Encapsulation

The Cisco Three-Layer Hierarchical Model

Summary

Exam Essentials

Written Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 2.1

Answers to Written Lab 2.2

Answers to Written Lab 2.3

Answers to Written Lab 2.4

Chapter 3: Introduction to TCP/IP

Introducing TCP/IP

TCP/IP and the DoD Model

IP Addressing

IPv4 Address Types

Summary

Exam Essentials

Written Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 3.1

Answers to Written Lab 3.2

Chapter 4: Easy Subnetting

Subnetting Basics

Summary

Exam Essentials

Written Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 4.1

Answers to Written Lab 4.2

Answers to Written Lab 4.3

Chapter 5: Variable Length Subnet Masks (VLSMs), Summarization, and Troubleshooting

TCP/IP

Variable Length Subnet Masks (VLSMs)

Summarization

Troubleshooting IP Addressing

Summary

Exam Essentials

Written Lab 5

Review Questions

Answers to Review Questions

Answers to Written Lab 5

Chapter 6: Cisco’s Internetworking Operating System (IOS)

The IOS User Interface

Command-Line Interface (CLI)

Router and Switch Administrative Configurations

Router Interfaces

Viewing, Saving, and Erasing Configurations

Summary

Exam Essentials

Written Lab 6

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 6

Chapter 7: Managing a Cisco Internetwork

The Internal Components of a Cisco Router

The Router Boot Sequence

Managing Configuration Register

Backing Up and Restoring the Cisco IOS

Backing Up and Restoring the Cisco Configuration

Using Cisco Discovery Protocol (CDP)

Using Telnet

Resolving Hostnames

Checking Network Connectivity and Troubleshooting

Summary

Exam Essentials

Written Lab 7

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 7

Chapter 8: IP Routing

Routing Basics

The IP Routing Process

Configuring IP Routing in Our Network

Dynamic Routing

Distance-Vector Routing Protocols

Routing Information Protocol (RIP)

Verifying Your Configurations

Summary

Exam Essentials

Written Lab 8

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 8

Chapter 9: Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF)

EIGRP Features and Operation

Using EIGRP to Support Large Networks

Configuring EIGRP

Load Balancing with EIGRP

Verifying EIGRP

Open Shortest Path First (OSPF) Basics

Configuring OSPF

Verifying OSPF Configuration

OSPF DR and BDR Elections

OSPF and Loopback Interfaces

Troubleshooting OSPF

Configuring EIGRP and OSPF Summary Routes

Summary

Exam Essentials

Written Lab 9

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 9

Chapter 10: Layer 2 Switching and Spanning Tree Protocol (STP)

Before Layer 2 Switching

Switching Services

Spanning Tree Protocol (STP)

Configuring Catalyst Switches

Summary

Exam Essentials

Written Lab 10

Review Questions

Answers to Review Questions

Answers to Written Lab 10

Chapter 11: Virtual LANs (VLANs)

VLAN Basics

VLAN Memberships

Identifying VLANs

VLAN Trunking Protocol (VTP)

Routing between VLANs

Configuring VLANs

Configuring VTP

Telephony: Configuring Voice VLANs

Summary

Exam Essentials

Written Lab 11

Review Questions

Answers to Review Questions

Answers to Written Lab 11

Chapter 12: Security

Perimeter, Firewall, and Internal Routers

Introduction to Access Lists

Standard Access Lists

Extended Access Lists

Turning Off and Configuring Network Services

Monitoring Access Lists

Summary

Exam Essentials

Written Lab 12

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 12

Chapter 13: Network Address Translation (NAT)

When Do We Use NAT?

Types of Network Address Translation

NAT Names

How NAT Works

Testing and Troubleshooting NAT

Summary

Exam Essentials

Written Lab 13

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 13

Chapter 14: Cisco’s Wireless Technologies

Introduction to Wireless Technology

Basic Wireless Devices

Wireless Regulations

Wireless Topologies

Wireless Security

Summary

Exam Essentials

Written Lab 14

Review Questions

Answers to Review Questions

Answers to Written Lab 14

Chapter 15: Internet Protocol Version 6 (IPv6)

Why Do We Need IPv6?

The Benefits and Uses of IPv6

IPv6 Addressing and Expressions

How IPv6 Works in an Internetwork

IPv6 Routing Protocols

Migrating to IPv6

Summary

Exam Essentials

Written Lab 15

Review Questions

Answers to Review Questions

Answers to Written Lab 15

Chapter 16: Wide Area Networks

Introduction to Wide Area Networks

Cable and DSL

Cabling the Serial Wide Area Network

High-Level Data-Link Control (HDLC) Protocol

Point-to-Point Protocol (PPP)

Frame Relay

Virtual Private Networks

Summary

Exam Essentials

Written Lab 16

Hands-on Labs

Review Questions

Answers to Review Questions

Answers to Written Lab 16

Appendix: About the Companion CD

What You’ll Find on the CD

System Requirements

Using the CD

Troubleshooting

Index

End-User License Agreement

CD Information

Perf Card – Objectives Map

CCNA: Cisco Certified Network Associate Study Guide, Seventh Edition

Acquisitions Editor: Jeff Kellum

Development Editor: Kathi Duggan

Technical Editors: Dan Garfield and John Rouda

Production Editor: Christine O’Connor

Copy Editor: Judy Flynn

Editorial Manager: Pete Gaughan

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Vice President and Executive Group Publisher: Richard Swadley

Vice President and Publisher: Neil Edde

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Book Designers: Judy Fung and Bill Gibson

Compositor: Craig Woods, Happenstance Type-O-Rama

Proofreader: Jen Larsen, Word One

Indexer: Robert Swanson

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Cover Designer: Ryan Sneed

Copyright © 2011 by Wiley Publishing, Inc., Indianapolis, Indiana

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data

Lammle, Todd. CCNA : Cisco Certified Network Associate study guide / Todd

Lammle. — 7th ed. p. cm. ISBN 978-0-470-90107-6 (pbk.) 978-1-118-08804-3 (ebk.) 978-1-118-08805-0 (ebk.) 978-1-118-08806-7 (ebk.) 1.

Electronic data processing personnel—Certification. 2. Computer networks—Examinations—Study guides. I. Title. II. Title: Cisco certified network associate study guide. QA76.3.L348 2011 004.6—dc22 2011004111

TRADEMARKS: Wiley, the Wiley logo, and the Sybex logo are trademarks or registered trademarks of John Wiley & Sons, Inc. and/or its affiliates, in the United States and other countries, and may not be used without written permission. CISA and Certified Information Systems Auditor are registered trademarks of ISACA. All other trademarks are the property of their respective owners. Wiley Publishing, Inc., is not associated with any product or vendor mentioned in this book.

10 9 8 7 6 5 4 3 2 1

Dear Reader,

Thank you for choosing

CCNA: Cisco Certified Associate Study Guide, Seventh Edition.

This book is part of a family of premium-quality Sybex books, all of which are written by outstanding authors who combine practical experience with a gift for teaching.

Sybex was founded in 1976. More than 30 years later, we’re still committed to producing consistently exceptional books. With each of our titles, we’re working hard to set a new standard for the industry. From the paper we print on, to the authors we work with, our goal is to bring you the best books available.

I hope you see all that reflected in these pages. I’d be very interested to hear your comments and get your feedback on how we’re doing. Feel free to let me know what you think about this or any other Sybex book by sending me an email at [email protected]

. If you think you’ve found a technical error in this book, please visit http://sybex.custhelp.com

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Best regards,

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Vice President and Publisher

Sybex, an Imprint of Wiley

Acknowledgments

My development editor for this book was Kathi Duggan. She was very patient and kind, and easy to work with (as long as I was never late with my submissions!). Thank you, Kathi, for being fun to work with and for being a very hard worker (answering emails literally throughout the night) and making sure everything was done on time and with the very high-quality standard that my Sybex CCNA book has become known for. I’m very happy that you were my new DE for this project, Kathi—we created a great book together!

Next in line to thank was my new technical editor, Dan Garfield. His expertise in the Cisco technical field, and history of networking in general, is second to none. His detailed analysis of my work helped make this my best CCNA book in the last 13 years. Thank you, Dan, for working hard under pressure, with tight deadlines, and for staying the course of delivering high-quality work in a short time frame.

Jeff Kellum is instrumental to my success in the Cisco world and is my acquisitions editor. Jeff, thanks for your guidance and continued patience.

I look forward to our continued progress together in the Cisco certification world.

In addition, Christine O’Connor was an excellent production editor, and she worked really hard to get the book done as quickly as possible, without missing the small mistakes that are so easy to overlook. I am always very happy when Christine is on my list of editors for a book project!

Judy Flynn, my copy editor, was another return editor for my book who was patient and helpful, and I am happy she worked with me once again. I look forward to having Christine and Judy working with me on my next project.

Last, but in no way least, was Troy McMillian. Troy has become my defacto writer, technical editor, researcher, and he has always comes through on any unreasonable deadline. I always look forward to working with Troy.

Finally a big thanks to Craig Woods at Happenstance-Type-O-Rama and to the CD team.

About the Author

Todd Lammle

CCSI, CCNA/CCNA Wireless/CCNP/CCSP/CCVP, MCSE, CEH/CHFI, FCC RF Licensed, is the authority on Cisco certification and internetworking. He is a world-renowned author, speaker, trainer, and consultant. Todd has over 29 years of experience working with LANs,

WANs, and large licensed and unlicensed wireless networks and has published over 50 books, including the very popular Sybex

CCNA: Cisco

Certified Network Associate Study Guide

and the Sybex

CCNA Wireless Study Guide

. He runs an international training and consulting company based in Colorado and Texas. You can reach Todd through his forum and blog at www.lammle.com

.

Introduction

Welcome to the exciting world of Cisco certification! You have picked up this book because you want something better—namely, a better job with more satisfaction. Rest assured that you have made a good decision. Cisco certification can help you get your first networking job or more money and a promotion if you are already in the field.

Cisco certification can also improve your understanding of the internetworking of more than just Cisco products: You will develop a complete understanding of networking and how different network topologies work together to form a network. This is beneficial to every networking job and is the reason Cisco certification is in such high demand, even at companies with few Cisco devices.

Cisco is the king of routing, switching, and security, the Microsoft of the internetworking world. The Cisco certifications reach beyond the popular certifications, such as the CompTIA and Microsoft certifications, to provide you with an indispensable factor in understanding today’s network— insight into the Cisco world of internetworking. By deciding that you want to become Cisco certified, you are saying that you want to be the best— the best at routing and the best at switching. This book will lead you in that direction.

For up-to-the-minute updates covering additions or modifications to the CCNA certification exams, as well as additional study tools and review questions, be sure to visit the

Todd Lammle forum and website at www.lammle.com

.

Cisco’s Network Certifications

Initially, to secure the coveted Cisco CCIE certification, you took only one test and then you were faced with the (extremely difficult) hands-on lab, an all-or-nothing approach that made it tough to succeed.

In response to a high number of unsuccessful attempts, Cisco created a series of new certifications to help you get the coveted CCIE as well as aid prospective employers in measuring skill levels. With these new certifications, which make for a better approach to preparing for that almighty lab, Cisco opened doors that few were allowed through before.

This book covers everything CCNA routing and switching related. For up-to-date information on the CCENT and CCNA concentrations, as well as CCNP and CCIE certifications, please see www.lammle.com

and/ or www.globalnettc.com

.

Cisco Certified Network Associate (CCNA)

The CCNA certification was the first course and exam in the Cisco certification process, and the precursor to all current Cisco certifications. Now you can become a Cisco Certified Network Associate for the meager cost of this book and either one test (640-802) at $250 or two tests (640-816 and 640-822) at $125 each—although the CCNA exams are extremely hard and cover a lot of material, so you have to really know your stuff!

Taking a Cisco class or spending months with hands-on experience is not out of the norm.

And once you have your CCNA, you don’t have to stop there—you can choose to continue with your studies and achieve a higher certification, called the Cisco Certified Network Professional (CCNP). Someone with a CCNP has all the skills and knowledge they need to attempt the CCIE lab. But just becoming a CCNA can land you that job you’ve dreamed about.

Why Become a CCNA?

Cisco, not unlike Microsoft and other vendors that provide certification, has created the certification process to give administrators a set of skills and to equip prospective employers with a way to measure those skills or match certain criteria. Becoming a CCNA can be the initial step of a successful journey toward a new, highly rewarding, and sustainable career.

The CCNA program was created to provide a solid introduction not only to the Cisco Internetwork Operating System (IOS) and Cisco hardware, but also to internetworking in general, making it helpful to you in areas that are not exclusively Cisco’s. At this point in the certification process, it’s not unrealistic that network managers—even those without Cisco equipment—require Cisco certification for their job applicants.

If you make it through the CCNA and are still interested in Cisco and internetworking, you’re headed down a path to certain success.

What Skills Do You Need to Become a CCNA?

To meet the CCNA certification skill level, you must be able to understand or do the following:

Install, configure, and operate LAN, WAN, and wireless access services securely as well as troubleshoot and configure small to medium networks (500 nodes or fewer) for performance.

Use the protocols IP, IPv6, EIGRP, RIP, RIPv2, and OSPF as well as understand serial connections, Frame Relay, VPN, cable, DSL, PPPoE,

LAN switching, VLANs, VTP, STP, Ethernet, security, and access lists.

How Do You Become a CCNA?

The way to become a CCNA is to pass one little test (CCNA Composite exam 640-802). Then—poof!—you’re a CCNA. (Don’t you wish it were that easy?) True, it can be just one test, but you still have to possess enough knowledge to understand what the test writers are saying.

However, Cisco has a two-step process that you can take in order to become a CCNA that may be easier than taking one longer exam (this book is based on the one-step method, taking the 640-802 exam; however, the information it contains covers all three exams).

The two-test method involves passing the following:

Exam 640-822: Interconnecting Cisco Networking Devices 1(ICND1)

Exam 640-816: Introduction to Cisco Networking Devices 2 (ICND2)

I can’t stress this enough: it’s critical that you have some hands-on experience with Cisco routers. If you can get ahold of some basic routers or

Cisco’s Packet Tracer software, you’re set. But if you can’t, I’ve worked hard to provide hundreds of configuration examples throughout this book to help network administrators (or people who want to become network administrators) learn what they need to know to pass the CCNA exam.

Since the 640-802 exam is so hard, Cisco wants to reward you for taking the two-test approach. Or so it seems. If you take the ICND1 exam, you actually receive a certification called the CCENT (Cisco Certified Entry Networking Technician). This is one step toward your CCNA. To achieve your CCNA, you must still pass your ICND2 exam.

Again, this book was written for the CCNA 640-802 Composite exam—one exam and you get your certification.

For Cisco-authorized hands-on training with CCSI Todd Lammle, please see www.globalnetc.com

. Each student will get hands-on experience by configuring at least three routers and two switches—no sharing of equipment!

What Does This Book Cover?

This book covers everything you need to know to pass the CCNA 640-802 exam. However, taking the time to study and practice with routers or a router simulator is the real key to success.

You will learn the following information in this book:

Chapter 1 introduces you to internetworking. You will learn the basics of the Open Systems Interconnection (OSI) model the way Cisco wants you to learn it. There are written labs and plenty of review questions to help you. Do not skip the fundamental written labs in this chapter!

Chapter 2 will dive into Ethernet networking and standards. Data encapsulation is discussed in detail in this chapter as well. There are written labs and plenty of review questions in this chapter to help you.

Chapter 3 provides you with the background necessary for success on the exam as well as in the real world by discussing TCP/IP. This indepth chapter covers the very beginnings of the Internet Protocol stack and then goes all the way to IP addressing and understanding the difference between a network address and a broadcast address before finally ending with network troubleshooting.

Chapter 4 introduces you to easy subnetting. You will be able to subnet a network in your head after reading this chapter if you really want to.

Plenty of help is found in this chapter if you do not skip the written labs and review questions.

Chapter 5 will have you learn about Variable Length Subnet Masks (VLSMs) and how to design a network using VLSMs. This chapter will finish with summarization techniques and configurations. As with Chapter 4, plenty of help is found in this chapter if you do not skip the written lab and review questions.

Chapter 6 introduces you to the Cisco Internetworking Operating System (IOS) and command-line interface (CLI). In this chapter you will learn how to turn on a router and configure the basics of the IOS, including setting passwords, banners, and more. Hands-on labs will help you gain a firm grasp of the concepts taught in the chapter. Before you go through the hands-on labs, be sure to complete the written lab and review questions.

Chapter 7 provides you with the management skills needed to run a Cisco IOS network. Backing up and restoring the IOS, as well as router configuration, is covered, as are the troubleshooting tools necessary to keep a network up and running. Before performing the hands-on labs in this chapter, complete the written labs and review questions.

Chapter 8 teaches you about IP routing. This is a fun chapter because we will begin to build our network, add IP addresses, and route data between routers. You will also learn about static, default, and dynamic routing using RIP and RIPv2. Hands-on labs, a written lab, and the review questions will help you understand IP routing to the fullest.

Chapter 9 dives into the more complex dynamic routing with Enhanced IGRP and OSPF routing. The written lab, hands-on labs, and review questions will help you master these routing protocols.

Chapter 10 gives you background on layer 2 switching and how switches perform address learning and make forwarding and filtering decisions. Network loops and how to avoid them with the Spanning Tree Protocol (STP) will be discussed as well as the 802.1w RSTP version. Go through the written lab and review questions to make sure you really understand layer 2 switching.

Chapter 11 covers virtual LANs and how you can use them in your internetwork. This chapter covers the nitty-gritty of VLANs and the different concepts and protocols used with VLANs as well as troubleshooting. Voice VLANs and QoS are also discussed in this all-so-important chapter. The written lab and review questions will reinforce the VLAN material.

Chapter 12 covers security and access lists, which are created on routers to filter the network. IP standard, extended, and named access lists are covered in detail. Written and hands-on labs, along with review questions, will help you study for the security and access-list portion of the

CCNA Composite exam.

Chapter 13 covers Network Address Translation (NAT). This chapter has been on the Sybex website for a few years as an update to my last

CCNA book, but I updated it and added it to this edition. New information, commands, troubleshooting, and hands-on labs will help you nail the NAT CCNA objectives.

Chapter 14 covers wireless technologies. This is an introductory chapter regarding wireless technologies as Cisco views wireless. However,

I also added some advanced wireless topics that cover Cisco’s newest gear. At this time, advanced wireless gear is not covered within the

Cisco CCNA objectives, but that can change. Make sure you understand basic wireless technologies like access points and clients as well as the difference between 802.11a, b, and g.

Chapter 15 covers IPv6. This is a fun chapter and has some great information. IPv6 is not the big, bad scary monster that most people think it is. IPv6 is an objective on the latest exam, so study this chapter carefully. Keep an eye out at www.lammle.com

for late-breaking updates.

Chapter 16 concentrates on Cisco wide area network (WAN) protocols. This chapter covers HDLC, PPP, and Frame Relay in depth. VPNs and IPSec are also covered in this chapter. You must be proficient in all these protocols to be successful on the CCNA exam. Do not skip the written lab, review questions, or hands-on labs found in this chapter.

How to Use This Book

If you want a solid foundation for the serious effort of preparing for the Cisco Certified Network Associate (CCNA Composite) 640-802 exam, then look no further. I have spent hundreds of hours putting together this book with the sole intention of helping you to pass the CCNA exam and learn how to configure Cisco routers and switches.

This book is loaded with valuable information, and you will get the most out of your studying time if you understand how I put the book together.

To best benefit from this book, I recommend the following study method:

1.

Take the assessment test immediately following this introduction. (The answers are at the end of the test.) It’s okay if you don’t know any of the answers; that’s why you bought this book! Carefully read over the explanations for any question you get wrong and note the chapters in which the material is covered. This information should help you plan your study strategy.

2.

Study each chapter carefully, making sure you fully understand the information and the test objectives listed at the beginning of each one. Pay extra-close attention to any chapter that includes material covered in questions you missed.

3.

Complete the written labs at the end of each chapter. Do

not

skip these written exercises, because they directly relate to the CCNA exam and what you must glean from the chapters in which they appear. Do not just skim these labs! Make sure you understand completely the reason for each answer.

4.

Complete all hands-on labs in the chapter, referring to the text of the chapter so that you understand the reason for each step you take.

Try to get your hands on some real equipment, but if you don’t have Cisco equipment available, try to find Cisco’s Packet Tracer for a router simulator that you can use for all the hands-on labs needed for all your Cisco certification needs.

5.

Answer all of the review questions related to each chapter. (The answers appear at the end of the chapters.) Note the questions that confuse you and study the topics they cover again. Do not just skim these questions! Make sure you understand completely the reason for each answer. Remember that these will not be the exact questions you find on the exam; they are written to help you understand the chapter material.

6.

Try your hand at the practice exams that are included on the companion CD. The questions in these exams appear only on the CD.

Check out www.lammle.com

for more Cisco exam prep questions.

7.

Also on the companion CD is the first module from each of the first three CDs from my complete CCNA video series, which covers internetworking, TCP/IP, and subnetting. This is critical information for the CCNA exam. In addition, as an added bonus, I have included an audio section from my CCNA audio program. Do not skip the video and audio section!

Please understand that these are preview editions of the video and audios found at www.lammlepress.com

and not the full versions, but are still a great value, chock full of information.

8.

Test yourself using all the flashcards on the CD. These are brand-new and updated flashcard programs to help you prepare for the

CCNA exam. They are a great study tool!

To learn every bit of the material covered in this book, you’ll have to apply yourself regularly, and with discipline. Try to set aside the same time period every day to study, and select a comfortable and quiet place to do so. If you work hard, you will be surprised at how quickly you learn this material.

If you follow these steps and really study—

doing hands-on labs every single day

—in addition to using the review questions, the practice exams, the Todd Lammle video/audio sections, and the electronic flashcards, as well as all the written labs, it would be hard to fail the CCNA exam.

However, studying for the CCNA exam is like trying to get in shape—and if you do not go to the gym every day, you won’t get in shape.

What’s on the CD?

The folks at Sybex and I worked hard to provide some really great tools to help you with your certification process. All of the following tools should be loaded on your workstation when you’re studying for the test. As a fantastic bonus, I was able to add to the CD included with this book a preview section from both my CCNA video and audio series! Please understand that these are not the full versions, but they are still a great value for you included free with this book.

The Sybex Test Preparation Software

The test preparation software prepares you to pass the CCNA exam. In the test engine, you will find all the review and assessment questions from the book plus two practice exams with 140 questions that appear exclusively on the CD.

Electronic Flashcards

To prepare for the exam, you can read this book, study the review questions at the end of each chapter, and work through the practice exams included in the book and on the companion CD. But wait, there’s more! You can also test yourself with the 200 flashcards included on the CD. If you can get through these difficult questions and understand the answers, you’ll know you’re ready for the CCNA exam.

The CD includes 200 flashcards specifically written to hit you hard and make sure you are ready for the exam. With the review questions, practice exams, and flashcards on the CD, you’ll be more than prepared for the exam.

Bonus Material

The bonus material, found only on the CD, has a wealth of information that covers SDM and CC, recognizing and mitigating security threats, route authentication, layer-3 switching and switching types,and lastly, and probably the most valuable to you as a study tool, is the CCNA Simulation

Exam Practice Labs. Do not skip this bonus material when studying for the CCNA exam. Please see my web site and forum at www.lammle.com

for free up-to-the minute updates and new bonus material.

Todd Lammle Videos

I have created a full CCNA series of videos that can be purchased in either DVD or downloadable format from www.lammlepress.com

. However, as a bonus included with this book, the first module of this series is included on the CD as a “Preview.” Although this isn’t the full version, the video is over 1 hour of foundational CCNA information. This is a $149 value! Do not skip this video because it covers the internetworking objectives,

TCP/IP, and subnetting, which are very important to the CCNA exam.

Todd Lammle Audio

In addition to the videos included for free on the CD, I have included a “preview” section from my CCNA audio series. The CCNA audio series is a

$199 value! This is a great tool to add to your arsenal of study material to help you pass the CCNA exam.

To find more Todd Lammle videos and audios as well as other Cisco study material, please see www.lammlepress.com

.

Where Do You Take the Exams?

You may take the CCNA Composite exam at any of the Pearson VUE authorized testing centers ( www.vue.com

) or call 877-404-EXAM (3926).

To register for a Cisco Certified Network Associate exam, follow these steps:

1.

Determine the number of the exam you want to take. (The CCNA exam number is 640-802.)

2.

Register with the nearest Pearson VUE testing center. At this point, you will be asked to pay in advance for the exam. At the time of this writing, the exam is $250 and must be taken within one year of payment. You can schedule exams up to six weeks in advance or as late as the day you want to take it—but if you fail a Cisco exam, you must wait five days before you will be allowed to retake it. If something comes up and you need to cancel or reschedule your exam appointment, contact Pearson VUE at least 24 hours in advance.

3.

When you schedule the exam, you’ll get instructions regarding all appointment and cancellation procedures, the ID requirements, and information about the testing-center location.

Tips for Taking Your CCNA Exam

The CCNA Composite exam test contains about 55 to 60 questions and must be completed in 75 to 90 minutes or less. This information can change per exam. You must get a score of about 85 percent to pass this exam, but again, each exam can be different.

Many questions on the exam have answer choices that at first glance look identical—especially the syntax questions! Remember to read through the choices carefully because close doesn’t cut it. If you get commands in the wrong order or forget one measly character, you’ll get the question wrong. So, to practice, do the hands-on exercises at the end of this book’s chapters over and over again until they feel natural to you.

Also, never forget that the right answer is the Cisco answer. In many cases, more than one appropriate answer is presented, but the

correct

answer is the one that Cisco recommends. On the exam, it always tells you to pick one, two, or three, never “choose all that apply.” The CCNA

Composite exam may include the following test formats:

Multiple-choice single answer

Multiple-choice multiple answer

Drag-and-drop

Fill-in-the-blank

Router simulations

Cisco proctored exams will not show the steps to follow in completing a router interface configuration; however, they do allow partial command responses. For example, show config

or sho config

or sh conf

would be acceptable.

Router#show ip protocol

or router#show ip prot

would be acceptable.

Here are some general tips for exam success:

Arrive early at the exam center so you can relax and review your study materials.

Read the questions

carefully

. Don’t jump to conclusions. Make sure you’re clear about

exactly

what each question asks. Read twice, answer

once, is what I always tell my students.

When answering multiple-choice questions that you’re not sure about, use the process of elimination to get rid of the obviously incorrect answers first. Doing this greatly improves your odds if you need to make an educated guess.

You can no longer move forward and backward through the Cisco exams, so double-check your answer before clicking Next since you can’t change your mind.

After you complete an exam, you’ll get immediate, online notification of your pass or fail status, a printed examination score report that indicates your pass or fail status, and your exam results by section. (The test administrator will give you the printed score report.) Test scores are automatically forwarded to Cisco within five working days after you take the test, so you don’t need to send your score to them. If you pass the exam, you’ll receive confirmation from Cisco, typically within two to four weeks, sometimes longer.

How to Contact the Author

You can reach Todd Lammle through his forum at www.lammle.com

.

Assessment Test

1. What protocol does PPP use to identify the Network layer protocol?

A. NCP

B. ISDN

C. HDLC

D. LCP

2. Each field in an IPv6 address is how many bits long?

A. 4

B. 16

C. 32

D. 128

3. The RSTP provides which new port role?

A. Disabled

B. Enabled

C. Discarding

D. Forwarding

4. What does the command routerA(config)#

line cons 0

allow you to perform next?

A. Set the Telnet password.

B. Shut down the router.

C. Set your console password.

D. Disable console connections.

5. How long is an IPv6 address?

A. 32 bits

B. 128 bytes

C. 64 bits

D. 128 bits

6. What PPP protocol provides for dynamic addressing, authentication, and multilink?

A. NCP

B. HDLC

C. LCP

D. X.25

7. What command will display the line, protocol, DLCI, and LMI information of an interface?

A. sh pvc

B. show interface

C. show frame-relay pvc

D. sho runn

8. Which of the following is the valid host range for the subnet on which the IP address 192.168.168.188 255.255.255.192 resides?

A. 192.168.168.129–190

B. 192.168.168.129–191

C. 192.168.168.128–190

D. 192.168.168.128–192

9. What does the passive

command provide to the RIP dynamic routing protocol?

A. Stops an interface from sending or receiving periodic dynamic updates

B. Stops an interface from sending periodic dynamic updates but not from receiving updates

C. Stops the router from receiving any dynamic updates

D. Stops the router from sending any dynamic updates

10. Which protocol does Ping use?

A. TCP

B. ARP

C. ICMP

D. BootP

11. How many collision domains are created when you segment a network with a 12-port switch?

A. 1

B. 2

C. 5

D. 12

12. Which of the following commands will allow you to set your Telnet password on a Cisco router?

A. line telnet 0 4

B. line aux 0 4

C. line vty 0 4

D. line con 0

13. Which router command allows you to view the entire contents of all access lists?

A. show all access-lists

B. show access-lists

C. show ip interface

D. show interface

14. What does a VLAN do?

A. Acts as the fastest port to all servers

B. Provides multiple collision domains on one switch port

C. Breaks up broadcast domains in a layer 2 switch internetwork

D. Provides multiple broadcast domains within a single collision domain

15. If you wanted to delete the configuration stored in NVRAM, what would you type?

A. erase startup

B. erase nvram

C. delete nvram

D. erase running

16. Which protocol is used to send a destination network unknown message back to originating hosts?

A. TCP

B. ARP

C. ICMP

D. BootP

17. Which class of IP address has the most host addresses available by default?

A. A

B. B

C. C

D. A and B

18. How often are BPDUs sent from a layer 2 device?

A. Never

B. Every 2 seconds

C. Every 10 minutes

D. Every 30 seconds

19. Which one of the following is true regarding VLANs?

A. Two VLANs are configured by default on all Cisco switches.

B. VLANs only work if you have a complete Cisco switched internetwork. No off-brand switches are allowed.

C. You should not have more than 10 switches in the same VTP domain.

D. VTP is used to send VLAN information to switches in a configured VTP domain.

20. Which WLAN IEEE specification allows up to 54Mbps at 2.4GHz?

A. A

B. B

C. G

D. N

21. How many broadcast domains are created when you segment a network with a 12-port switch?

A. 1

B. 2

C. 5

D. 12

22. What flavor of Network Address Translation can be used to have one IP address allow many users to connect to the global Internet?

A. NAT

B. Static

C. Dynamic

D. PAT

23. What protocols are used to configure trunking on a switch? (Choose two.)

A. VLAN Trunking Protocol

B. VLAN

C. 802.1Q

D. ISL

24. What is a stub network?

A. A network with more than one exit point

B. A network with more than one exit and entry point

C. A network with only one entry and no exit point

D. A network that has only one entry and exit point

25. Where is a hub specified in the OSI model?

A. Session layer

B. Physical layer

C. Data Link layer

D. Application layer

26. What are the two main types of access control lists (ACLs)? (Choose two.)

A. Standard

B. IEEE

C. Extended

D. Specialized

27. To back up an IOS, what command will you use?

A. backup IOS disk

B. copy ios tftp

C. copy tftp flash

D. copy flash tftp

28. What command is used to create a backup configuration?

A. copy running backup

B. copy running-config startup-config

C. config mem

D. wr mem

29. What is the main reason the OSI model was created?

A. To create a layered model larger than the DoD model

B. So application developers can change only one layer’s protocols at a time

C. So different networks could communicate

D. So Cisco could use the model

30. Which protocol does DHCP use at the Transport layer?

A. IP

B. TCP

C. UDP

D. ARP

31. If your router is facilitating a CSU/DSU, which of the following commands do you need to use to provide the router with a 64000bps serial link?

A.

RouterA(config)#

bandwidth 64

B.

RouterA(config-if)#

bandwidth 64000

C.

RouterA(config)#

clockrate 64000

D.

RouterA(config-if)#

clock rate 64

E.

RouterA(config-if)

#clock rate 64000

32. Which command is used to determine if an IP access list is enabled on a particular interface?

A. show access-lists

B. show interface

C. show ip interface

D. show interface access-lists

33. Which command is used to upgrade an IOS on a Cisco router?

A. copy tftp run

B. copy tftp start

C. config net

D. copy tftp flash

34. The Protocol Data Unit Encapsulation (PDU) is completed in which order?

A. Bits, frames, packets, segments, data

B. Data, bits, segments, frames, packets

C. Data, segments, packets, frames, bits

D. Packets, frames, bits, segments, data

Answers to Assessment Test

1. A. Network Control Protocol is used to help identify the Network layer protocol used in the packet. See Chapter 16 for more information.

2. B. Each field in an IPv6 address is 16 bits long. An IPv6 address is a total of 128 bits. See Chapter 15 for more information.

3. C. The port roles used within RSTP include discarding, learning, and forwarding. The difference between 802.1d and RSTP is the discarding role. See Chapter 10 for more information.

4. C. The command line console 0

places you at a prompt where you can then set your console user-mode password. See Chapter 6 for more information.

5. D. An IPv6 address is 128 bits long, whereas an IPv4 address is only 32 bits long. See Chapter 15 for more information.

6. C. Link Control Protocol in the PPP stack provides negotiation of dynamic addressing, authentication, and multilink. See Chapter 16 for more information.

7. B. The show interface

command shows the line, protocol, DLCI, and LMI information of an interface. See Chapter 16 for more information.

8. A. 256 – 192 = 64, so 64 is our block size. Just count in increments of 64 to find our subnet: 64 + 64 = 128. 128 + 64 = 192. The subnet is 128, the broadcast address is 191, and the valid host range is the numbers in between, or 129–190. See Chapter 4 for more information.

9. B. The passive

command, short for passive-interface

, stops regular updates from being sent out an interface. However, the interface can still receive updates. See Chapter 8 for more information.

10. C. ICMP is the protocol at the Network layer that is used to send echo requests and replies. See Chapter 3 for more information.

11. D. Layer 2 switching creates individual collision domains per port. See Chapter 1 for more information.

12. C. The command line vty 0 4

places you in a prompt that will allow you to set or change your Telnet password. See Chapter 6 for more information.

13. B. To see the contents of all access lists, use the show access-lists

command. See Chapter 12 for more information.

14. C. VLANs break up broadcast domains at layer 2. See Chapter 11 for more information.

15. A. The command erase startup-config

deletes the configuration stored in NVRAM. See Chapter 6 for more information.

16. C. ICMP is the protocol at the Network layer that is used to send messages back to an originating router. See Chapter 3 for more information.

17. A. Class A addressing provides 24 bits for host addressing. See Chapter 3 for more information.

18. B. Every 2 seconds, BPDUs are sent out from all active bridge ports by default. See Chapter 10 for more information.

19. D. Switches do not propagate VLAN information by default; you must configure the VTP domain for this to occur. VLAN Trunking Protocol

(VTP) is used to propagate VLAN information across a trunk link. See Chapter 11 for more information.

20. C. IEEE 802.11bg is in the 2.4GHz range, with a top speed of 54Mbps. See Chapter 14 for more information.

21. A. By default, switches break up collision domains on a per-port basis but are one large broadcast domain. See Chapter 1 for more information.

22. D. Port Address Translation (PAT) allows a one-to-many approach to network address translation. See Chapter 13 for more information.

23. C, D. VTP is not right because it has nothing to do with trunking except that it sends VLAN information across a trunk link. 802.1Q and ISL encapsulations are used to configure trunking on a port. See Chapter 11 for more information.

24. D. Stub networks have only one connection to an internetwork. Default routes should be set on a stub network or network loops may occur; however, there are exceptions to this rule. See Chapter 9 for more information.

25. B. Hubs regenerate electrical signals, which are specified at the Physical layer. See Chapter 1 for more information.

26. A, C. Standard and extended access control lists (ACLs) are used to configure security on a router. See Chapter 12 for more information.

27. D. The command copy flash tftp

will prompt you to back up an existing file in flash to a TFTP host. See Chapter 7 for more information.

28. B. The command to back up the configuration on a router is copy running-config startup-config

. See Chapter 7 for more information.

29. C. The primary reason the OSI model was created was so that different networks could interoperate. See Chapter 1 for more information.

30. C. User Datagram Protocol is a connection network service at the Transport layer, and DHCP uses this connectionless service. See Chapter 3 for more information.

31. E. The clock rate

command is two words, and the speed of the line is in bps. See Chapter 6 for more information.

32. C. The show ip interface

command will show you if any interfaces have an outbound or inbound access list set. See Chapter 12 for more information.

33. D. The copy tftp flash

command places a new file in flash memory, which is the default location for the Cisco IOS in Cisco routers. See Chapter

7 for more information.

34. C. The PDU encapsulation method defines how data is encoded as it goes through each layer of the TCP/IP model. Data is segmented at the

Transport later, packets created at the Network layer, frames at the Data Link layer, and finally, the Physical layer encodes the 1s and 0s into a digital signal. See Chapter 2 for more information.

Chapter 1

Internetworking

The CCNA exam topics covered in this chapter include the following:

Describe how a network works>

Describe the purpose and functions of various network devices

Select the components required to meet a network specification

Use the OSI and TCP/IP models and their associated protocols to explain how data flows in a network

Describe common networked applications including web applications

Describe the purpose and basic operation of the protocols in the OSI and TCP models

Describe the impact of applications (Voice over IP and Video over IP) on a network

Interpret network diagrams

Describe the components required for network and Internet communications

Identify and correct common network problems at layers 1, 2, 3, and 7 using a layered model approach

Differentiate between LAN/WAN operation and features

Configure, verify, and troubleshoot a switch with VLANs and interswitch communications

Explain network segmentation and basic traffic management concepts

Implement an IP addressing scheme and IP Services to meet network requirements in a medium-size Enterprise branch office network

Explain the operation and benefits of using DHCP and DNS

Configure, verify, and troubleshoot basic router operation and routing on Cisco devices

Welcome to the exciting world of internetworking. This first chapter will really help you review your understanding of basic internetworking by focusing on how to connect networks together using Cisco routers and switches. This chapter was written with an assumption that you have already achieved your CompTIA Network+ certification or have the equivalent knowledge, and based on this, I will review internetworking only for the purpose of fully grasping the Cisco CCENT and/or CCNA objectives needed to help you achieve your certifications.

First, you need to know exactly what an internetwork is, right? You create an internetwork when you connect two or more networks via a router and configure a logical network addressing scheme with a protocol such as IP or IPv6.

I’ll be reviewing the following in this chapter:

Internetworking basics

Network segmentation

How bridges, switches, and routers are used to physically and logically segment a network

How routers are employed to create an internetwork

I’m also going to dissect the Open Systems Interconnection (OSI) model and describe each part to you in detail because you really need a good grasp of it for the solid foundation upon which you’ll build your Cisco networking knowledge. The OSI model has seven hierarchical layers that were developed to enable different networks to communicate reliably between disparate systems. Since this book is centering upon all things CCNA, it’s crucial for you to understand the OSI model as Cisco sees it, so that’s how I’ll be presenting the seven layers to you.

After you finish reading this chapter, you’ll encounter 20 review questions and three written labs. These are given to you to really lock the information from this chapter into your memory. So don’t skip them!

To find up-to-the-minute updates for this chapter, please see www.lammle.com

or www.sybex.com/go/ccna7e

.

Internetworking Basics

Before we explore internetworking models and the specifications of the OSI reference model, you’ve got to understand the big picture and learn the answer to the key question: Why is it so important to learn Cisco internetworking?

Networks and networking have grown exponentially over the last 20 years—understandably so. They’ve had to evolve at light speed just to keep up with huge increases in basic mission-critical user needs such as sharing data and printers as well as more advanced demands such as videoconferencing. Unless everyone who needs to share network resources is located in the same office area (an increasingly uncommon situation), the challenge is to connect the sometimes many relevant networks together so all users can share the networks’ wealth.

Starting with a look at

Figure 1-1 , you get a picture of a basic LAN network that’s connected together using a hub. This network is actually one collision domain and one broadcast domain—but no worries if you can’t remember what this means right now, because I’m going to talk so much about both collision and broadcast domains throughout this chapter and in Chapter 2 that you’ll probably even dream about them!

Okay, about Figure 1-1… How would you say the PC named Bob communicates with the PC named Sally? Well, they’re both on the same LAN connected with a multiport repeater (a hub). So does Bob just send out a data message, “Hey Sally, you there?” Or does Bob use Sally’s IP address and send a data message like this: “Hey 192.168.0.3, are you there?” Possibly you picked the IP address option, but even if you did, the news is still bad—both answers are wrong! Why? Because Bob is actually going to use Sally’s MAC address (known as a hardware address, which is burned right into the network card of Sally’s PC) to get ahold of her.

Figure 1-1:

The basic network

Great, but how does Bob get Sally’s MAC address if he knows only Sally’s name and doesn’t even have her IP address yet? Bob is going to start with name resolution (hostname to IP address resolution), something that’s usually accomplished using Domain Name Service (DNS). And of note, if these two are on the same LAN, Bob can just broadcast to Sally asking her for the information (no DNS needed)—welcome to Microsoft

Windows!

Here’s an output from a network analyzer depicting a simple initiation process from Bob to Sally:

Source Destination Protocol Info

192.168.0.2 192.168.0.255 NBNS Name query NB

SALLY<00>

As I already mentioned, since the two hosts are on a local LAN, Windows (Bob) will just broadcast to resolve the name

Sally

(the destination

192.168.0.255 is a broadcast address) and Sally will let Bob know her address is 192.168.0.3 (analyzer output not shown). Let’s take a look at the rest of the information:

EthernetII,Src:192.168.0.2(00:14:22:be:18:3b),Dst:Broadcast(ff:ff:-ff:ff:ff:ff)

What this output shows is that Bob knows his own MAC address and source IP address but not Sally’s IP address or MAC address, so Bob sends a broadcast address of all

f

s for the MAC address (a Data Link layer broadcast) and an IP LAN broadcast of 192.168.0.255. Again, don’t freak—you’re going to learn all about broadcasts in Chapter 3, “Introduction to TCP/IP.”

Now Bob has to broadcast on the LAN to get Sally’s MAC address so he can finally communicate to her PC and send data:

Source Destination Protocol Info

192.168.0.2 Broadcast ARP Who has 192.168.0.3? Tell 192.168.0.2

Next, check out Sally’s response:

Source Destination Protocol Info

192.168.0.3 192.168.0.2 ARP 192.168.0.3 is at 00:0b:db:99:d3:5e

192.168.0.3 192.168.0.2 NBNS Name query response NB

192.168.0.3

Okay, sweet— Bob now has both Sally’s IP address and her MAC address! These are both listed as the source address at this point because this information was sent from Sally back to Bob. So,

finally

, Bob has all the goods he needs to communicate with Sally. And just so you know, I’m going to tell you all about Address Resolution Protocol (ARP) and show you exactly how Sally’s IP address was resolved to a MAC address in

Chapter 8, “IP Routing.”

To complicate things further, it’s also likely that at some point you’ll have to break up one large network into a bunch of smaller ones because user response will have dwindled to a slow crawl as the network grew and grew. And with all that growth, your LAN’s traffic congestion has reached epic proportions. The answer to this is breaking up a really big network into a number of smaller ones—something called

network segmentation

.

You do this by using devices like

routers

,

switches

, and

bridges

. Figure 1-2 displays a network that’s been segmented with a switch so that each network segment connected to the switch is now a separate collision domain. But make note of the fact that this network is still one broadcast domain.

Figure 1-2:

A switch can replace the hub, breaking up collision domains.

Keep in mind that the hub used in Figure 1-2 just extended the one collision domain from the switch port. Here’s a list of some of the things that commonly cause LAN traffic congestion:

Too many hosts in a broadcast or collision domain

Broadcast storms

Too much multicast traffic

Low bandwidth

Adding hubs for connectivity to the network

Take another look at Figure 1-2—did you notice that I replaced the main hub from

Figure 1-1 with a switch? Whether you did or didn’t, the reason

I did that is because hubs don’t segment a network; they just connect network segments together. So basically, it’s an inexpensive way to connect a couple of PCs together, which is great for home use and troubleshooting, but that’s about it!

Now, routers are used to connect networks together and route packets of data from one network to another. Cisco became the de facto standard of routers because of its high-quality router products, great selection, and fantastic service. Routers, by default, break up a

broadcast domain

—the set of all devices on a network segment that hear all the broadcasts sent on that segment. Figure 1-3 shows a router in our little network that creates an internetwork and breaks up broadcast domains.

The network in

Figure 1-3 is a pretty cool network. Each host is connected to its own collision domain, and the router has created two broadcast domains. And don’t forget that the router provides connections to WAN services as well! The router uses something called a serial interface for

WAN connections, specifically, a V.35 physical interface on a Cisco router.

Figure 1-3:

Routers create an internetwork.

Breaking up a broadcast domain is important because when a host or server sends a network broadcast, every device on the network must read and process that broadcast—unless you’ve got a router. When the router’s interface receives this broadcast, it can respond by basically saying,

“Thanks, but no thanks,” and discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, it’s important to remember that they break up collision domains as well.

There are two advantages of using routers in your network:

They don’t forward broadcasts by default.

They can filter the network based on layer 3 (Network layer) information (e.g., IP address).

Four router functions in your network can be listed as follows:

Packet switching

Packet filtering

Internetwork communication

Path selection

Remember that routers are really switches; they’re actually what we call layer 3 switches (we’ll talk about layers later in this chapter). Unlike layer

2 switches, which forward or filter frames, routers (or layer 3 switches) use logical addressing and provide what is called packet switching. Routers can also provide packet filtering by using access lists, and when routers connect two or more networks together and use logical addressing (IP or

IPv6), this is called an internetwork. Lastly, routers use a routing table (map of the internetwork) to make path selections and to forward packets to remote networks.

Conversely, switches aren’t used to create internetworks (they do not break up broadcast domains by default); they’re employed to add functionality to a network LAN. The main purpose of a switch is to make a LAN work better—to optimize its performance—providing more bandwidth for the LAN’s users. And switches don’t forward packets to other networks as routers do. Instead, they only “switch” frames from one port to another within the switched network. Okay, you may be thinking, “Wait a minute, what are frames and packets?” I’ll tell you all about them later in this chapter, I promise!

By default, switches break up

collision domains

. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. If at the same time a different device tries to transmit, leading to a collision, both devices must retransmit, one at a time. Not very efficient! This situation is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. By contrast, each and every port on a switch represents its own collision domain.

Switches create separate collision domains but a single broadcast domain. Routers provide a separate broadcast domain for each interface.

The term

bridging

was introduced before routers and hubs were implemented, so it’s pretty common to hear people referring to bridges as switches and vice versa. That’s because bridges and switches basically do the same thing—break up collision domains on a LAN (in reality, you cannot buy a physical bridge these days, only LAN switches, but they use bridging technologies, so Cisco still refers to them as multiport bridges).

So what this means is that a switch is basically just a multiple-port bridge with more brainpower, right? Well, pretty much, but there are differences. Switches do provide this function, but they do so with greatly enhanced management ability and features. Plus, most of the time, bridges only had 2 or 4 ports. Yes, you could get your hands on a bridge with up to 16 ports, but that’s nothing compared to the hundreds available on some switches!

You would use a bridge in a network to reduce collisions within broadcast domains and to increase the number of collision domains in your network. Doing this provides more bandwidth for users. And keep in mind that using hubs in your network can contribute to congestion on your Ethernet network. As always, plan your network design carefully!

Figure 1-4 shows how a network would look with all these internetwork devices in place. Remember that the router will not only break up broadcast domains for every LAN interface, it will break up collision domains as well.

Figure 1-4:

Internetworking devices

When you looked at Figure 1-4 , did you notice that the router is found at center stage and that it connects each physical network together? We have to use this layout because of the older technologies involved—bridges and hubs.

On the top internetwork in

Figure 1-4 , you’ll notice that a bridge was used to connect the hubs to a router. The bridge breaks up collision domains, but all the hosts connected to both hubs are still crammed into the same broadcast domain. Also, the bridge only created two collision domains, so each device connected to a hub is in the same collision domain as every other device connected to that same hub. This is actually pretty lame, but it’s still better than having one collision domain for all hosts.

Notice something else: The three hubs at the bottom that are connected also connect to the router, creating one collision domain and one broadcast domain. This makes the bridged network look much better indeed!

Although bridges/switches are used to segment networks, they will not isolate broadcast or multicast packets.

The best network connected to the router is the LAN switch network on the left. Why? Because each port on that switch breaks up collision domains. But it’s not all good—all devices are still in the same broadcast domain. Do you remember why this can be a really bad thing? Because all devices must listen to all broadcasts transmitted, that’s why. And if your broadcast domains are too large, the users have less bandwidth and are required to process more broadcasts, and network response time will slow to a level that could cause office riots.

Once we have only switches in our network, things change a lot!

Figure 1-5 shows the network that is typically found today.

Figure 1-5:

Switched networks creating an internetwork

Okay, here I’ve placed the LAN switches at the center of the network world so the router is connecting only logical networks together. If I implemented this kind of setup, I’ve created virtual LANs (VLANs), something I’m going to tell you about in Chapter 11. So don’t stress. But it is really important to understand that even though you have a switched network, you still need a router (or layer 3 switch) to provide your inter-VLAN communication, or internetworking. Don’t forget that!

Obviously, the best network is one that’s correctly configured to meet the business requirements of the company it serves. LAN switches with routers, correctly placed in the network, are the best network design. This book will help you understand the basics of routers and switches so you

can make good, informed decisions on a case-by-case basis.

Let’s go back to

Figure 1-4 again. Looking at the figure, how many collision domains and broadcast domains are in this internetwork? Hopefully, you answered nine collision domains and three broadcast domains! The broadcast domains are definitely the easiest to see because only routers break up broadcast domains by default. And since there are three connections, that gives you three broadcast domains. But do you see the nine collision domains? Just in case that’s a no, I’ll explain. The all-hub network is one collision domain; the bridge network equals three collision domains. Add in the switch network of five collision domains—one for each switch port—and you’ve got a total of nine.

Now, in

Figure 1-5 , each port on the switch is a separate collision domain and each VLAN is a separate broadcast domain. But you still need a router for routing between VLANs. How many collision domains do you see here? I’m counting 10—remember that connections between the switches are considered a collision domain!

Should I Replace My Existing 10/100Mbps Switches?

You’re a network administrator at a large company in San Jose. The boss comes to you and says that he got your requisition to buy all new switches and is not sure about approving the expense; do you really need it?

Well, if you can, absolutely! The newest switches really add a lot of functionality to a network that older 10/100Mbps switches just don’t have (yes, five-year-old switches are considered just plain old today). But most of us don’t have an unlimited budget to buy all new gigabit switches. 10/100Mbps switches can still create a nice network—that is, of course, if you design and implement the network correctly—but you’ll still have to replace these switches eventually.

So do you need 1Gbps or better switch ports for all your users, servers, and other devices? Yes, you absolutely need new higher-end switches! With the new Windows networking stack and the IPv6 revolution shortly ahead of us, the server and hosts are no longer the bottlenecks of our internetworks. Our routers and switches are! We need at a minimum gigabit to the desktop and on every router interface—10Gbps would be better, or even higher if you can afford it.

So, go ahead! Put that requisition in to buy all new switches.

So now that you’ve gotten an introduction to internetworking and the various devices that live in an internetwork, it’s time to head into internetworking models.

Internetworking Models

When networks first came into being, computers could typically communicate only with computers from the same manufacturer. For example, companies ran either a complete DECnet solution or an IBM solution—not both together. In the late 1970s, the

Open Systems Interconnection

(OSI) reference model

was created by the International Organization for Standardization (ISO) to break this barrier.

The OSI model was meant to help vendors create interoperable network devices and software in the form of protocols so that different vendor networks could work with each other. Like world peace, it’ll probably never happen completely, but it’s still a great goal.

The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. The OSI reference model breaks this approach into layers.

In the following section, I am going to explain the layered approach and how we can use this approach to help us troubleshoot our internetworks.

The Layered Approach

A

reference model

is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called

layers

. When a communication system is designed in this manner, it’s known as

layered architecture

.

Think of it like this: You and some friends want to start a company. One of the first things you’ll do is sit down and think through what tasks must be done, who will do them, the order in which they will be done, and how they relate to each other. Ultimately, you might group these tasks into departments. Let’s say you decide to have an order-taking department, an inventory department, and a shipping department. Each of your departments has its own unique tasks, keeping its staff members busy and requiring them to focus on only their own duties.

In this scenario, I’m using departments as a metaphor for the layers in a communication system. For things to run smoothly, the staff of each department will have to trust and rely heavily upon the others to do their jobs and competently handle their unique responsibilities. In your planning sessions, you would probably take notes, recording the entire process to facilitate later discussions about standards of operation that will serve as your business blueprint, or reference model.

Once your business is launched, your department heads, each armed with the part of the blueprint relating to their own department, will need to develop practical methods to implement their assigned tasks. These practical methods, or protocols, will need to be compiled into a standard operating procedures manual and followed closely. Each of the various procedures in your manual will have been included for different reasons and have varying degrees of importance and implementation. If you form a partnership or acquire another company, it will be imperative that its business protocols—its business blueprint—match yours (or at least be compatible with it).

Similarly, software developers can use a reference model to understand computer communication processes and see what types of functions need to be accomplished on any one layer. If they are developing a protocol for a certain layer, all they need to concern themselves with is that specific layer’s functions, not those of any other layer. Another layer and protocol will handle the other functions. The technical term for this idea is

binding

. The communication processes that are related to each other are bound, or grouped together, at a particular layer.

Advantages of Reference Models

The OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. The primary purpose of all such models, especially the OSI model, is to allow different vendors’ networks to interoperate.

Advantages of using the OSI layered model include, but are not limited to, the following:

It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.

It allows multiple-vendor development through standardization of network components.

It encourages industry standardization by defining what functions occur at each layer of the model.

It allows various types of network hardware and software to communicate.

It prevents changes in one layer from affecting other layers, so it does not hamper development.

The OSI Reference Model

One of the greatest functions of the OSI specifications is to assist in data transfer between disparate hosts—meaning, for example, that they enable us to transfer data between a Unix host and a PC or a Mac.

The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end. Figure 1-6 shows the three upper layers and their functions, and Figure 1-7 shows the four lower layers and their functions.

When you study

Figure 1-6 , understand that the user interfaces with the computer at the Application layer and also that the upper layers are responsible for applications communicating between hosts. Remember that none of the upper layers knows anything about networking or network addresses. That’s the responsibility of the four bottom layers.

In

Figure 1-7 , you can see that it’s the four bottom layers that define how data is transferred through a physical wire or through switches and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host’s application.

The following network devices operate at all seven layers of the OSI model:

Network Management Stations (NMSs)

Web and application servers

Gateways (not default gateways)

Network hosts

Figure 1-6:

The upper layers

Figure 1-7:

The lower layers

Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as Ms. Post wrote the book setting the standards—or protocols—for human social interaction, the ISO developed the OSI reference model as the precedent and guide for an open network protocol set.

Defining the etiquette of communication models, it remains today the most popular means of comparison for protocol suites.

The OSI reference model has the following seven layers:

Application layer (layer 7)

Presentation layer (layer 6)

Session layer (layer 5)

Transport layer (layer 4)

Network layer (layer 3)

Data Link layer (layer 2)

Physical layer (layer 1)

Figure 1-8 shows a summary of the functions defined at each layer of the OSI model.

Figure 1-8:

Layer functions

With this in hand, you’re now ready to explore each layer’s function in detail.

The Application Layer

The

Application layer

of the OSI model marks the spot where users actually communicate to the computer. This layer comes into play only when it’s apparent that access to the network is going to be needed soon. Take the case of Internet Explorer (IE). You could uninstall every trace of networking components from a system, such as TCP/IP, NIC card, and so on, and you could still use IE to view a local HTML document—no problem. But things would definitely get messy if you tried to do something like view an HTML document that must be retrieved using HTTP or nab a file with FTP or TFTP. That’s because IE will respond to requests such as those by attempting to access the Application layer. And what’s happening is that the Application layer is acting as an interface between the actual application program—which isn’t at all a part of the layered structure—and the next layer down by providing ways for the application to send information down through the protocol stack. In other words, IE doesn’t truly reside within the Application layer—it interfaces with Application layer protocols when it needs to deal with remote resources.

The Application layer is also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist.

These tasks are important because computer applications sometimes require more than only desktop resources. Often, they’ll unite communicating components from more than one network application. Prime examples are file transfers and email as well as enabling remote access, network management activities, client/server processes, and information location. Many network applications provide services for communication over enterprise networks, but for present and future internetworking, the need is fast developing to reach beyond the limits of current

physical networking.

It’s important to remember that the Application layer is acting as an interface between the actual application programs. This means that Microsoft Word, for example, does not reside at the Application layer but instead interfaces with the Application layer protocols. Chapter 3 will present some programs that actually reside at the Application layer— for example, FTP and TFTP.

The Presentation Layer

The

Presentation layer

gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting.

This layer is essentially a translator and provides coding and conversion functions. A successful data-transfer technique is to adapt the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then convert the data back into its native format for actual reading (for example, EBCDIC to ASCII). By providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one.

The OSI has protocol standards that define how standard data should be formatted. Tasks like data compression, decompression, encryption, and decryption are associated with this layer. Some Presentation layer standards are involved in multimedia operations too.

The Session Layer

The

Session layer

is responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes:

simplex

,

half duplex

, and

full duplex

. To sum up, the Session layer basically keeps different applications’ data separate from other applications’ data.

The Transport Layer

The

Transport layer

segments and reassembles data into a data stream. Services located in the Transport layer segment and reassemble data from upper-layer applications and unite it into the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.

Some of you are probably familiar with TCP and UDP already. (But if you’re not, no worries—I’ll tell you all about them in Chapter 3.) If so, you know that both work at the Transport layer and that TCP is a reliable service and UDP is not. This means that application developers have more options because they have a choice between the two protocols when working with TCP/IP protocols.

The Transport layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuits. It also hides details of any network-dependent information from the higher layers by providing transparent data transfer.

The term reliable networking can be used at the Transport layer. It means that acknowledgments, sequencing, and flow control will be used.

The Transport layer can be connectionless or connection oriented. However, Cisco is mostly concerned with you understanding the connectionoriented portion of the Transport layer. The following sections will provide the skinny on the connection-oriented (reliable) protocol of the Transport layer.

Flow Control

Data integrity is ensured at the Transport layer by maintaining

flow control

and by allowing applications to request reliable data transport between systems. Flow control prevents a sending host on one side of the connection from overflowing the buffers in the receiving host—an event that can result in lost data. Reliable data transport employs a connection-oriented communications session between systems, and the protocols involved ensure that the following will be achieved:

The segments delivered are acknowledged back to the sender upon their reception.

Any segments not acknowledged are retransmitted.

Segments are sequenced back into their proper order upon arrival at their destination.

A manageable data flow is maintained in order to avoid congestion, overloading, and data loss.

The purpose of flow control is to provide a means for the receiver to govern the amount of data sent by the sender.

Connection-Oriented Communication

In reliable transport operation, a device that wants to transmit sets up a connection-oriented communication session with a remote device by creating a session. The transmitting device first establishes a connection-oriented session with its peer system, which is called a

call setup

or a

three-way handshake

. Data is then transferred; when the transfer is finished, a call termination takes place to tear down the virtual circuit.

Figure 1-9 depicts a typical reliable session taking place between sending and receiving systems. Looking at it, you can see that both hosts’ application programs begin by notifying their individual operating systems that a connection is about to be initiated. The two operating systems communicate by sending messages over the network confirming that the transfer is approved and that both sides are ready for it to take place.

After all of this required synchronization takes place, a connection is fully established and the data transfer begins (this virtual circuit setup is called overhead!).

Figure 1-9:

Establishing a connection-oriented session

While the information is being transferred between hosts, the two machines periodically check in with each other, communicating through their protocol software to ensure that all is going well and that the data is being received properly.

Here’s a summary of the steps in the connection-oriented session—the three-way handshake—pictured in

Figure 1-9 :

The first “connection agreement” segment is a request for synchronization.

The next segments acknowledge the request and establish connection parameters—the rules—between hosts. These segments request that the receiver’s sequencing is synchronized here as well so that a bidirectional connection is formed.

The final segment is also an acknowledgment. It notifies the destination host that the connection agreement has been accepted and that the actual connection has been established. Data transfer can now begin.

Sounds pretty simple, but things don’t always flow so smoothly. Sometimes during a transfer, congestion can occur because a high-speed computer is generating data traffic a lot faster than the network can handle transferring. A bunch of computers simultaneously sending datagrams through a single gateway or destination can also botch things up nicely. In the latter case, a gateway or destination can become congested even though no single source caused the problem. In either case, the problem is basically akin to a freeway bottleneck—too much traffic for too small a capacity. It’s not usually one car that’s the problem; there are simply too many cars on that freeway.

Okay, so what happens when a machine receives a flood of datagrams too quickly for it to process? It stores them in a memory section called a

buffer

. But this buffering action can solve the problem only if the datagrams are part of a small burst. If not, and the datagram deluge continues, a device’s memory will eventually be exhausted, its flood capacity will be exceeded, and it will react by discarding any additional datagrams that arrive.

No huge worries here, though. Because of the transport function, network flood control systems really work quite well. Instead of dumping data and allowing data to be lost, the transport can issue a “not ready” indicator to the sender, or source, of the flood (as shown in Figure 1-10 ). This mechanism works kind of like a stoplight, signaling the sending device to stop transmitting segment traffic to its overwhelmed peer. After the peer receiver processes the segments already in its memory reservoir—its buffer—it sends out a “ready” transport indicator. When the machine waiting to transmit the rest of its datagrams receives this “go” indicator, it resumes its transmission.

Figure 1-10:

Transmitting segments with flow control

In fundamental, reliable, connection-oriented data transfer, datagrams are delivered to the receiving host in exactly the same sequence they’re transmitted—and the transmission fails if this order is breached! If any data segments are lost, duplicated, or damaged along the way, a failure will occur. This problem is solved by having the receiving host acknowledge that it has received each and every data segment.

A service is considered connection oriented if it has the following characteristics:

A virtual circuit is set up (e.g., a three-way handshake).

It uses sequencing.

It uses acknowledgments.

It uses flow control.

The types of flow control are buffering, windowing, and congestion avoidance.

Windowing

Ideally, data throughput happens quickly and efficiently. And as you can imagine, it would be slow if the transmitting machine had to wait for an acknowledgment after sending each segment. But because there’s time available

after

the sender transmits the data segment and

before

it finishes processing acknowledgments from the receiving machine, the sender uses the break as an opportunity to transmit more data. The quantity of data segments (measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgment for them is called a

window

.

Windows are used to control the amount of outstanding, unacknowledged data segments.

So the size of the window controls how much information is transferred from one end to the other. While some protocols quantify information by observing the number of packets, TCP/IP measures it by counting the number of bytes.

As you can see in

Figure 1-11 , there are two window sizes—one set to 1 and one set to 3.

When you’ve configured a window size of 1, the sending machine waits for an acknowledgment for each data segment it transmits before transmitting another. If you’ve configured a window size of 3, it’s allowed to transmit three data segments before an acknowledgment is received.

In this simplified example, both the sending and receiving machines are workstations. In reality, this is not done in simple numbers but in the amount of bytes that can be sent.

If a receiving host fails to receive all the bytes that it should acknowledge, the host can improve the communication session by decreasing the window size.

Figure 1-11:

Windowing

Acknowledgments

Reliable data delivery ensures the integrity of a stream of data sent from one machine to the other through a fully functional data link. It guarantees that the data won’t be duplicated or lost. This is achieved through something called

positive acknowledgment with retransmission

—a technique that requires a receiving machine to communicate with the transmitting source by sending an acknowledgment message back to the sender when it receives data. The sender documents each segment measured in bytes; it then sends and waits for this acknowledgment before sending the next segment round of bytes. When it sends a segment, the transmitting machine starts a timer and retransmits if it expires before an acknowledgment is returned from the receiving end.

In

Figure 1-12 , the sending machine transmits segments 1, 2, and 3. The receiving node acknowledges it has received them by requesting segment 4. When it receives the acknowledgment, the sender then transmits segments 4, 5, and 6. If segment 5 doesn’t make it to the destination, the receiving node acknowledges that event with a request for the segment to be resent. The sending machine will then resend the lost segment and wait for an acknowledgment, which it must receive in order to move on to the transmission of segment 7.

The Network Layer

The

Network layer

(also called layer 3) manages device addressing, tracks the location of devices on the network, and determines the best way to move data, which means that the Network layer must transport traffic between devices that aren’t locally attached. Routers (layer 3 devices) are specified at the Network layer and provide the routing services within an internetwork.

Figure 1-12:

Transport layer reliable delivery

It happens like this: First, when a packet is received on a router interface, the destination IP address is checked. If the packet isn’t destined for that particular router, it will look up the destination network address in the routing table. Once the router chooses an exit interface, the packet will be sent to that interface to be framed and sent out on the local network. If the router can’t find an entry for the packet’s destination network in the routing table, the router drops the packet.

Two types of packets are used at the Network layer: data and route updates.

Data packets

Used to transport user data through the internetwork. Protocols used to support data traffic are called

routed protocols

; examples of routed protocols are IP and IPv6. You’ll learn about IP addressing in Chapters 3 and 4 and IPv6 in Chapter 15.

Route update packets

Used to update neighboring routers about the networks connected to all routers within the internetwork. Protocols that send route update packets are called

routing protocols

; examples of some common ones are RIP, RIPv2, EIGRP, and OSPF. Route update packets are used to help build and maintain routing tables on each router.

Figure 1-13 shows an example of a routing table.

The routing table used in a router includes the following information:

Network addresses

Protocol-specific network addresses. A router must maintain a routing table for individual routed protocol because each routed protocol keeps track of a network with a different addressing scheme (IP, IPv6, and IPX, for example). Think of it as a street sign in each of the different languages spoken by the residents that live on a particular street. So, if there were American, Spanish, and French folks on a street named Cat, the sign would read Cat/Gato/Chat.

Interface

The exit interface a packet will take when destined for a specific network.

Metric

The distance to the remote network. Different routing protocols use different ways of computing this distance. I’m going to cover routing protocols in Chapters 8 and 9, but for now, know that some routing protocols (namely RIP) use something called a

hop count

(the number of routers a packet passes through en route to a remote network), while others use bandwidth, delay of the line, or even tick count (1⁄18 of a second).

Figure 1-13:

Routing table used in a router

And as I mentioned earlier, routers break up broadcast domains, which means that by default, broadcasts aren’t forwarded through a router. Do you remember why this is a good thing? Routers also break up collision domains, but you can also do that using layer 2 (Data Link layer) switches.

Because each interface in a router represents a separate network, it must be assigned unique network identification numbers, and each host on the network connected to that router must use the same network number. Figure 1-14 shows how a router works in an internetwork.

Here are some points about routers that you should really commit to memory:

Routers, by default, will not forward any broadcast or multicast packets.

Routers use the logical address in a Network layer header to determine the next hop router to forward the packet to.

Routers can use access lists, created by an administrator, to control security on the types of packets that are allowed to enter or exit an interface.

Routers can provide layer 2 bridging functions if needed and can simultaneously route through the same interface.

Layer 3 devices (routers in this case) provide connections between virtual LANs (VLANs).

Routers can provide quality of service (QoS) for specific types of network traffic.

Switching and VLANs and are covered in Chapter 10, “Layer 2 Switching and Spanning Tree Protocol (STP),” and Chapter 11, “Virtual LANs (VLANs).”

Figure 1-14:

A router in an internetwork

The Data Link Layer

The

Data Link layer

provides the physical transmission of the data and handles error notification, network topology, and flow control. This means that the Data Link layer will ensure that messages are delivered to the proper device on a LAN using hardware addresses and will translate messages from the Network layer into bits for the Physical layer to transmit.

The Data Link layer formats the message into pieces, each called a

data frame

, and adds a customized header containing the hardware destination and source address. This added information forms a sort of capsule that surrounds the original message in much the same way that engines, navigational devices, and other tools were attached to the lunar modules of the Apollo project. These various pieces of equipment were useful only during certain stages of space flight and were stripped off the module and discarded when their designated stage was complete. Data traveling through networks is similar.

Figure 1-15 shows the Data Link layer with the Ethernet and IEEE specifications. When you check it out, notice that the IEEE 802.2 standard is used in conjunction with and adds functionality to the other IEEE standards.

It’s important for you to understand that routers, which work at the Network layer, don’t care at all about where a particular host is located. They’re only concerned about where networks are located and the best way to reach them—including remote ones. Routers are totally obsessive when it comes to networks. And for once, this is a good thing! It’s the Data Link layer that’s responsible for the actual unique identification of each device that resides on a local network.

Figure 1-15:

Data Link layer

For a host to send packets to individual hosts on a local network as well as transmit packets between routers, the Data Link layer uses hardware addressing. Each time a packet is sent between routers, it’s framed with control information at the Data Link layer, but that information is stripped off at the receiving router and only the original packet is left completely intact. This framing of the packet continues for each hop until the packet is finally delivered to the correct receiving host. It’s really important to understand that the packet itself is never altered along the route; it’s only encapsulated with the type of control information required for it to be properly passed on to the different media types.

The IEEE Ethernet Data Link layer has two sublayers:

Media Access Control (MAC) 802.3

Defines how packets are placed on the media. Contention media access is “first come/first served” access where everyone shares the same bandwidth—hence the name. Physical addressing is defined here as well as logical topologies.

What’s a logical topology? It’s the signal path through a physical topology. Line discipline, error notification (not correction), ordered delivery of frames, and optional flow control can also be used at this sublayer.

Logical Link Control (LLC) 802.2

Responsible for identifying Network layer protocols and then encapsulating them. An LLC header tells the

Data Link layer what to do with a packet once a frame is received. It works like this: A host will receive a frame and look in the LLC header to find out where the packet is destined—say, the IP protocol at the Network layer. The LLC can also provide flow control and sequencing of control bits.

The switches and bridges I talked about near the beginning of the chapter both work at the Data Link layer and filter the network using hardware

(MAC) addresses. We will look at these in the following section.

Switches and Bridges at the Data Link Layer

Layer 2 switching is considered hardware-based bridging because it uses specialized hardware called an

application-specific integrated circuit

(ASIC)

. ASICs can run up to gigabit speeds with very low latency rates.

Latency is the time measured from when a frame enters a port to when it exits a port.

Bridges and switches read each frame as it passes through the network. The layer 2 device then puts the source hardware address in a filter table and keeps track of which port the frame was received on. This information (logged in the bridge’s or switch’s filter table) is what helps the machine determine the location of the specific sending device. Figure 1-16 shows a switch in an internetwork.

Figure 1-16:

A switch in an internetwork

The real estate business is all about location, location, location, and it’s the same way for both layer 2 and layer 3 devices. Though both need to be able to negotiate the network, it’s crucial to remember that they’re concerned with very different parts of it. Primarily, layer 3 machines (such as routers) need to locate specific networks, whereas layer 2 machines (switches and bridges) need to eventually locate specific devices. So, networks are to routers as individual devices are to switches and bridges. And routing tables that “map” the internetwork are for routers as filter tables that “map” individual devices are for switches and bridges.

After a filter table is built on the layer 2 device, it will forward frames only to the segment where the destination hardware address is located. If the destination device is on the same segment as the frame, the layer 2 device will block the frame from going to any other segments. If the destination is on a different segment, the frame can be transmitted only to that segment. This is called

transparent bridging

.

When a switch interface receives a frame with a destination hardware address that isn’t found in the device’s filter table, it will forward the frame to all connected segments. If the unknown device that was sent the “mystery frame” replies to this forwarding action, the switch updates its filter table regarding that device’s location. But in the event the destination address of the transmitting frame is a broadcast address, the switch will forward all broadcasts to every connected segment by default.

All devices that the broadcast is forwarded to are considered to be in the same broadcast domain. This can be a problem; layer 2 devices propagate layer 2 broadcast storms that choke performance, and the only way to stop a broadcast storm from propagating through an internetwork is with a layer 3 device—a router.

The biggest benefit of using switches instead of hubs in your internetwork is that each switch port is actually its own collision domain.

(Conversely, a hub creates one large collision domain.) But even armed with a switch, you still don’t break up broadcast domains by default.

Neither switches nor bridges will do that. They’ll simply forward all broadcasts instead.

Another benefit of LAN switching over hub-centered implementations is that each device on every segment plugged into a switch can transmit simultaneously—at least, they can as long as there is only one host on each port and a hub isn’t plugged into a switch port. As you might have guessed, hubs allow only one device per network segment to communicate at a time.

The Physical Layer

Finally arriving at the bottom, we find that the

Physical layer

does two things: It sends bits and receives bits. Bits come only in values of 1 or 0—a

Morse code with numerical values. The Physical layer communicates directly with the various types of actual communication media. Different kinds of media represent these bit values in different ways. Some use audio tones, while others employ

state transitions

—changes in voltage from high to low and low to high. Specific protocols are needed for each type of media to describe the proper bit patterns to be used, how data is encoded into media signals, and the various qualities of the physical media’s attachment interface.

The Physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. This layer is also where you identify the interface between the

data terminal equipment (DTE)

and the

data communication equipment (DCE)

. (Some old phone-company employees still call DCE data circuit-terminating equipment.) The DCE is usually located at the service provider, while the DTE is the attached device. The services available to the DTE are most often accessed via a modem or

channel service unit/data service unit (CSU/DSU)

.

The Physical layer’s connectors and different physical topologies are defined by the OSI as standards, allowing disparate systems to communicate. The CCNA objectives are only interested in the IEEE Ethernet standards.

Hubs at the Physical Layer

A

hub

is really a multiple-port repeater. A repeater receives a digital signal and reamplifies or regenerates that signal and then forwards the digital signal out all active ports without looking at any data. An active hub does the same thing. Any digital signal received from a segment on a hub port is regenerated or reamplified and transmitted out all other ports on the hub. This means all devices plugged into a hub are in the same collision domain as well as in the same broadcast domain. Figure 1-17 shows a hub in a network.

Hubs, like repeaters, don’t examine any of the traffic as it enters and is then transmitted out to the other parts of the physical media. Every device connected to the hub, or hubs, must listen if a device transmits. A physical star network—where the hub is a central device and cables extend in all directions out from it—is the type of topology a hub creates. Visually, the design really does resemble a star, whereas Ethernet networks run a

logical bus topology, meaning that the signal has to run through the network from end to end.

Hubs and repeaters can be used to enlarge the area covered by a single LAN segment, although I do not recommend this. LAN switches are affordable for almost every situation.

Figure 1-17:

A hub in a network

Summary

Whew! I know this seemed like the chapter that wouldn’t end, but it did—and you made it through! You’re now armed with a ton of fundamental information; you’re ready to build upon it and are well on your way to certification.

I started by discussing simple, basic networking and the differences between collision and broadcast domains.

I then discussed the OSI model—the seven-layer model used to help application developers design applications that can run on any type of system or network. Each layer has its special jobs and select responsibilities within the model to ensure that solid, effective communications do, in fact, occur. I provided you with complete details of each layer and discussed how Cisco views the specifications of the OSI model.

In addition, each layer in the OSI model specifies different types of devices, and I described these different devices used at each layer.

Remember that hubs are Physical layer devices and repeat the digital signal to all segments except the one from which it was received.

Switches segment the network using hardware addresses and break up collision domains. Routers break up broadcast domains (and collision domains) and use logical addressing to send packets through an internetwork.

Exam Essentials

Identify the possible causes of LAN traffic congestion.

Too many hosts in a broadcast domain, broadcast storms, multicasting, and low bandwidth are all possible causes of LAN traffic congestion.

Describe the difference between a collision domain and a broadcast domain. Collision domain is an Ethernet term used to describe a network collection of devices in which one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it. On a broadcast domain, a set of all devices on a network segment hear all broadcasts sent on that segment.

Differentiate a MAC address and an IP address and describe how and when each address type is used in a network.

A MAC address is a hexadecimal number identifying the physical connection of a host. MAC addresses are said to operate on layer 2 of the OSI model. IP addresses, which can be expressed in binary or decimal format, are logical identifiers that are said to be on layer 3 of the OSI model. Hosts on the same physical segment locate one another with MAC addresses, while IP addresses are used when they reside on different LAN segments or subnets. Even when the hosts are in different subnets, a destination IP address will be converted to a MAC address when the packet reaches the destination network via routing.

Understand the difference between a hub, a bridge, a switch, and a router.

Hubs create one collision domain and one broadcast domain. Bridges break up collision domains but create one large broadcast domain. They use hardware addresses to filter the network.

Switches are really just multiple-port bridges with more intelligence. They break up collision domains but create one large broadcast domain by default. Switches use hardware addresses to filter the network. Routers break up broadcast domains (and collision domains) and use logical addressing to filter the network.

Identify the functions and advantages of routers.

Routers perform packet switching, filtering, and path selection, and they facilitate internetwork communication. One advantage of routers is that they reduce broadcast traffic.

Differentiate connection-oriented and connectionless network services and describe how each is handled during network communications

Connection-oriented services use acknowledgments and flow control to create a reliable session. More overhead is used than in a connectionless network service. Connectionless services are used to send data with no acknowledgments or flow control. This is considered unreliable.

Define the OSI layers, understand the function of each, and describe how devices and networking protocols can be mapped to each layer.

You must remember the seven layers of the OSI model and what function each layer provides. The Application, Presentation, and

Session layers are upper layers and are responsible for communicating from a user interface to an application. The Transport layer provides segmentation, sequencing, and virtual circuits. The Network layer provides logical network addressing and routing through an internetwork.

The Data Link layer provides framing and placing of data on the network medium. The Physical layer is responsible for taking 1s and 0s and encoding them into a digital signal for transmission on the network segment.

Written Labs

In this section, you’ll complete the following labs to make sure you’ve got the information and concepts contained within them fully dialed in:

Lab 1.1: OSI Questions

Lab 1.2: Defining the OSI Layers and Devices

Lab 1.3: Identifying Collision and Broadcast Domains

(The answers to the written labs can be found following the answers to the review questions for this chapter.)

Written Lab 1.1: OSI Questions

Answer the following questions about the OSI model:

1. Which layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery?

2. Which layer is responsible for converting data packets from the Data Link layer into electrical signals?

3. At which layer is routing implemented, enabling connections and path selection between two end systems?

4. Which layer defines how data is formatted, presented, encoded, and converted for use on the network?

5. Which layer is responsible for creating, managing, and terminating sessions between applications?

6. Which layer ensures the trustworthy transmission of data across a physical link and is primarily concerned with physical addressing, line discipline, network topology, error notification, ordered delivery of frames, and flow control?

7. Which layer is used for reliable communication between end nodes over the network and provides mechanisms for establishing, maintaining, and terminating virtual circuits; transport-fault detection and recovery; and controlling the flow of information?

8. Which layer provides logical addressing that routers will use for path determination?

9. Which layer specifies voltage, wire speed, and pinout cables and moves bits between devices?

10. Which layer combines bits into bytes and bytes into frames, uses MAC addressing, and provides error detection?

11. Which layer is responsible for keeping the data from different applications separate on the network?

12. Which layer is represented by frames?

13. Which layer is represented by segments?

14. Which layer is represented by packets?

15. Which layer is represented by bits?

16. Put the following in order of encapsulation:

Packets

Frames

Bits

Segments

17. Which layer segments and reassembles data into a data stream?

18. Which layer provides the physical transmission of the data and handles error notification, network topology, and flow control?

19. Which layer manages device addressing, tracks the location of devices on the network, and determines the best way to move data?

20. What is the bit length and expression form of a MAC address?

Written Lab 1.2: Defining the OSI Layers and Devices

Fill in the blanks with the appropriate layer of the OSI or hub, switch, or router device.

Description Device or OSI Layer

This device sends and receives information about the Network layer.

This layer creates a virtual circuit before transmitting between two end stations.

This device uses hardware addresses to filter a network.

Ethernet is defined at these layers.

This layer supports flow control, sequencing, and acknowledgments.

This device can measure the distance to a remote network.

Logical addressing is used at this layer.

Hardware addresses are defined at this layer.

This device creates one big collision domain and one large broadcast domain.

This device creates many smaller collision domains, but the network is still one large broadcast domain.

This device can never run full duplex.

This device breaks up collision domains and broadcast domains.

Written Lab 1.3: Identifying Collision and Broadcast Domains

1. In the following exhibit, identify the number of collision domains and broadcast domains in each specified device. Each device is represented by a letter:

A.

Hub

B.

Bridge

C.

Switch

D.

Router

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s introduction.

1. A receiving host has failed to receive all of the segments that it should acknowledge. What can the host do to improve the reliability of this communication session?

A. Send a different source port number.

B. Restart the virtual circuit.

C. Decrease the sequence number.

D. Decrease the window size.

2. When a station sends a transmission to the MAC address ff:ff:ff:ff:ff:ff

, what type of transmission is it?

A. Unicast

B. Multicast

C. Anycast

D. Broadcast

3. Which layer 1 devices can be used to enlarge the area covered by a single LAN segment? (Choose two.)

A. Switch

B. NIC

C. Hub

D. Repeater

E. RJ45 transceiver

4. Segmentation of a data stream happens at which layer of the OSI model?

A. Physical

B. Data Link

C. Network

D. Transport

5. Which of the following describe the main router functions? (Choose four.)

A. Packet switching

B. Collision prevention

C. Packet filtering

D. Broadcast domain enlargement

E. Internetwork communication

F. Broadcast forwarding

G. Path selection

6. Routers operate at layer ___. LAN switches operate at layer ___. Ethernet hubs operate at layer ___. Word processing operates at layer ___.

A. 3, 3, 1, 7

B. 3, 2, 1, none

C. 3, 2, 1, 7

D. 2, 3, 1, 7

E. 3, 3, 2, none

7. When data is encapsulated, which is the correct order?

A. Data, frame, packet, segment, bit

B. Segment, data, packet, frame, bit

C. Data, segment, packet, frame, bit

D. Data, segment, frame, packet, bit

8. Why does the data communication industry use the layered OSI reference model? (Choose two.)

A. It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.

B. It enables equipment from different vendors to use the same electronic components, thus saving research and development funds.

C. It supports the evolution of multiple competing standards and thus provides business opportunities for equipment manufacturers.

D. It encourages industry standardization by defining what functions occur at each layer of the model.

E. It provides a framework by which changes in functionality in one layer require changes in other layers.

9. What are two purposes for segmentation with a bridge?

A. To add more broadcast domains

B. To create more collision domains

C. To add more bandwidth for users

D. To allow more broadcasts for users

10. Which of the following is

not

a cause of LAN congestion?

A. Too many hosts in a broadcast domain

B. Adding switches for connectivity to the network

C. Broadcast storms

D. Low bandwidth

11. If a switch has three computers connected to it, with no VLANs present, how many broadcast and collision domains is the switch creating?

A. Three broadcast and one collision

B. Three broadcast and three collision

C. One broadcast and three collision

D. One broadcast and one collision

12. Acknowledgments, sequencing, and flow control are characteristics of which OSI layer?

A. Layer 2

B. Layer 3

C. Layer 4

D. Layer 7

13. Which of the following are types of flow control? (Choose all that apply.)

A. Buffering

B. Cut-through

C. Windowing

D. Congestion avoidance

E. VLANs

14. If a hub has three computers connected to it, how many broadcast and collision domains is the hub creating?

A. Three broadcast and one collision

B. Three broadcast and three collision

C. One broadcast and three collision

D. One broadcast and one collision

15. What is the purpose of flow control?

A. To ensure that data is retransmitted if an acknowledgment is not received

B. To reassemble segments in the correct order at the destination device

C. To provide a means for the receiver to govern the amount of data sent by the sender

D. To regulate the size of each segment

16. Which three statements are true about the operation of a full-duplex Ethernet network?

A. There are no collisions in full-duplex mode.

B. A dedicated switch port is required for each full-duplex node.

C. Ethernet hub ports are preconfigured for full-duplex mode.

D. In a full-duplex environment, the host network card must check for the availability of the network media before transmitting.

E. The host network card and the switch port must be capable of operating in full-duplex mode.

17. Which of the following is

not

a benefit of reference models such as the OSI model?

A. It allows changes on one layer to affect operations on all other layers as well.

B. It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.

C. It allows multiple-vendor development through standardization of network components.

D. It allows various types of network hardware and software to communicate.

18. Which of the following devices do

not

operate at all levels of the OSI model?

A. Network management stations (NMSs)

B. Routers

C. Web and application servers

D. Network hosts

19. When an HTTP document must be retrieved from a location other than the local machine, what layer of the OSI model must be accessed first?

A. Presentations

B. Transport

C. Application

D. Network

20. Which layer of the OSI model offers three different modes of communication:

simplex

,

half duplex

, and

full duplex

?

A. Presentation

B. Transport

C. Application

D. Session

Answers to Review Questions

1. D. A receiving host can control the transmitter by using flow control (TCP uses windowing by default). By decreasing the window size, the receiving host can slow down the transmitting host so the receiving host does not overflow its buffers.

2. D. A transmission to the MAC address ff:ff:ff:ff:ff:ff

is a broadcast transmission to all stations.

3. C, D. Not that you really want to enlarge a single collision domain, but a hub (multiport repeater) will provide this for you.

4. D. The Transport layer receives large data streams from the upper layers and breaks these up into smaller pieces called segments.

5. A, C, E, G. Routers provide packet switching, packet filtering, internetwork communication, and path selection. Although routers do create or terminate collision domains, this is not the main purpose of a router, so option B is not a correct answer to this question.

6. B. Routers operate at layer 3. LAN switches operate at layer 2. Ethernet hubs operate at layer 1. Word processing applications communicate to the Application layer interface, but do not operate at layer 7, so the answer would be none.

7. C. The encapsulation method is data, segment, packet, frame, bit.

8. A, D. The main advantage of a layered model is that it can allow application developers to change aspects of a program in just one layer of the layer model’s specifications. Advantages of using the OSI layered model include, but are not limited to, the following: It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting; it allows multiplevendor development through standardization of network components; it encourages industry standardization by defining what functions occur at each layer of the model; it allows various types of network hardware and software to communicate; and it prevents changes in one layer from affecting other layers, so it does not hamper development.

9. A, D. Unlike full duplex, half-duplex Ethernet operates in a shared collision domain, and it has a lower effective throughput than full duplex.

10. B. Adding switches for connectivity to the network would reduce LAN congestion rather than cause LAN congestion.

11. C. If a switch has three computers connected to it, with no VLANs present, one broadcast and three collision domains are created.

12. C. A reliable Transport layer connection uses acknowledgments to make sure all data is transmitted and received reliably. A reliable connection is defined by a virtual circuit that uses acknowledgments, sequencing, and flow control, which are characteristics of the Transport layer

(layer 4).

13. A, C, D. The common types of flow control are buffering, windowing, and congestion avoidance.

14. D. If a hub has three computers connected to it, one broadcast and one collision domain is created.

15. C. Flow control allows the receiving device to control the transmitter so the receiving device’s buffer does not overflow.

16. A, B, E. Full duplex means you are using both wire pairs simultaneously to send and receive data. You must have a dedicated switch port for each node, which means you will not have collisions. Both the host network card and the switch port must be capable and set to work in full-duplex mode.

17. A. Reference models prevent, rather than allow, changes on one layer to affect operations on other layers as well, so the model doesn’t hamper development.

18. B. Routers operate no higher than layer 3 of the OSI model.

19. C. When an HTTP document must be retrieved from a location other than the local machine, the Application layer must be accessed first.

20. D. The Session layer of the OSI model offers three different modes of communication:

simplex

,

half duplex

, and

full duplex.

Answers to Written Lab 1.1

1.

The Application layer is responsible for finding the network resources broadcast from a server and adding flow control and error control

(if the application developer chooses).

2.

The Physical layer takes frames from the Data Link layer and encodes the 1s and 0s into a digital signal for transmission on the network medium.

3.

The Network layer provides routing through an internetwork and logical addressing.

4.

The Presentation layer makes sure that data is in a readable format for the Application layer.

5.

The Session layer sets up, maintains, and terminates sessions between applications.

6.

PDUs at the Data Link layer are called frames and provide physical addressing, plus other options to place packets on the network medium.

7.

The Transport layer uses virtual circuits to create a reliable connection between two hosts.

8.

The Network layer provides logical addressing, typically IP addressing and routing.

9.

The Physical layer is responsible for the electrical and mechanical connections between devices.

10.

The Data Link layer is responsible for the framing of data packets.

11.

The Session layer creates sessions between different hosts’ applications.

12.

The Data Link layer frames packets received from the Network layer.

13.

The Transport layer segments user data.

14.

The Network layer creates packets out of segments handed down from the Transport layer.

15.

The Physical layer is responsible for transporting 1s and 0s (bits) in a digital signal.

16.

Segments, packets, frames, bits

17.

Transport

18.

Data Link

19.

Network

20.

48 bits (6 bytes) expressed as a hexadecimal number

Answers to Written Lab 1.2

Description

This device sends and receives information about the Network layer.

This layer creates a virtual circuit before transmitting between two end stations.

This device uses hardware addresses to filter a network.

Ethernet is defined at these layers.

This layer supports flow control, sequencing, and acknowledgments.

This device can measure the distance to a remote network.

Logical addressing is used at this layer.

Device or OSI Layer

Router

Transport

Bridge or switch

Data Link and Physical

Transport

Router

Network

Hardware addresses are defined at this layer.

This device creates one big collision domain and one large broadcast domain.

Data Link (MAC sublayer)

Hub

This device creates many smaller collision domains, but the network is still one large broadcast domain. Switch or bridge

This device can never run full duplex.

Hub

This device breaks up collision domains and broadcast domains.

Router

Answers to Written Lab 1.3

1.

Hub: One collision domain, one broadcast domain

2.

Bridge: Two collision domains, one broadcast domain

3.

Switch: Four collision domains, one broadcast domain

4.

Router: Three collision domains, three broadcast domains

Chapter 2

Review of Ethernet Networking and Data Encapsulation

The CCNA exam topics covered in this chapter include the following:

Describe How a Network Works

Configure, Verify, and Troubleshoot a Switch with VLANs and Interswitch Communications

Use the OSI and TCP/IP models and their associated protocols to explain how data flows in a network

Select the appropriate media, cables, ports, and connectors to connect switches to other network devices and hosts

Explain the technology and media access control method for Ethernet networks

Explain network segmentation and basic traffic management concepts

Before we move on and explore the TCP/IP and DoD models, IP addressing, subnetting, and routing in the upcoming chapters, you’ve got to understand the big picture of LANs and learn the answers to two key questions: How is Ethernet used in today’s networks? and, What are Media

Access Control (MAC) addresses and how are they used?

This chapter will answer those questions and more. I’ll not only discuss the basics of Ethernet and the way MAC addresses are used on an

Ethernet LAN, but I’ll cover the protocols used with Ethernet at the Data Link layer as well. You’ll also learn about the various Ethernet specifications.

As you learned in Chapter 1, there are a bunch of different types of devices specified at the different layers of the OSI model, and it’s very important to understand the many types of cables and connectors used for connecting all those devices to a network. This chapter will review the various cabling used with Cisco devices, describing how to connect to a router or switch and even how to connect a router or switch with a console connection.

Also in this chapter, I’ll provide an introduction to encapsulation. Encapsulation is the process of encoding data as it goes down the OSI stack.

The chapter concludes with a discussion of the three-layer hierarchical model that was developed by Cisco to help you design, implement, and troubleshoot internetworks.

After you finish reading this chapter, you’ll encounter 20 review questions and four written labs. These are given to you to really lock the information from this chapter into your memory. So don’t skip them!

To find up-to-the minute updates for this chapter, please see www.lammle.com

or www.sybex.com/go/ccna7e

.

Ethernet Networks in Review

Ethernet

is a contention-based media access method that allows all hosts on a network to share the same bandwidth of a link. Ethernet is popular because it’s readily scalable, meaning that it’s comparatively easy to integrate new technologies, such as upgrading from Fast Ethernet to Gigabit

Ethernet, into an existing network infrastructure. It’s also relatively simple to implement in the first place, and with it, troubleshooting is reasonably straightforward. Ethernet uses both Data Link and Physical layer specifications, and this chapter will give you both the Data Link layer and Physical layer information you need to effectively implement, troubleshoot, and maintain an Ethernet network.

Collision Domain

As mentioned in Chapter 1, the term

collision domain

is an Ethernet term that refers to a particular network scenario wherein one device sends a packet out on a network segment, thereby forcing every other device on that same physical network segment to pay attention to it. This can be bad because if two devices on one physical segment transmit at the same time, a collision event—a situation where each device’s digital signals interfere with another on the wire—occurs and forces the devices to retransmit later. Collisions can have a dramatically negative effect on network performance, so they’re definitely something you want to avoid!

The situation I just described is typically found in a hub environment where each host segment connects to a hub that represents only one collision domain and one broadcast domain. This begs the question that we discussed in Chapter 1: What’s a broadcast domain?

Broadcast Domain

Here’s the written definition:

Broadcast domain

refers to a group of devices on a network segment that hear all the broadcasts sent on that network segment.

Even though a broadcast domain is typically a boundary delimited by physical media like switches and routers, it can also reference a logical division of a network segment where all hosts can reach each other via a Data Link layer (hardware address) broadcast.

That’s the basic story, so now let’s take a look at a collision detection mechanism used in half-duplex Ethernet.

CSMA/CD

Ethernet networking uses

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

, a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes. And trust me—good collision management is crucial, because when a node transmits in a CSMA/CD network, all the other nodes on the network receive and examine that transmission. Only switches and routers can effectively prevent a transmission from propagating throughout the entire network!

So, how does the CSMA/CD protocol work? Let’s start by taking a look at

Figure 2-1 .

When a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear (no other host is transmitting), the host will then proceed with its transmission. But it doesn’t stop there. The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data (think busy signal). The nodes respond to that jam signal by waiting a while before attempting to transmit again.

Backoff algorithms determine when the colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit will then timeout. Pretty clean!

Figure 2-1:

CSMA/CD

When a collision occurs on an Ethernet LAN, the following happens:

A jam signal informs all devices that a collision occurred.

The collision invokes a random backoff algorithm.

Each device on the Ethernet segment stops transmitting for a short time until their backoff timers expire.

All hosts have equal priority to transmit after the timers have expired.

The following are the effects of having a CSMA/CD network sustaining heavy collisions:

Delay

Low throughput

Congestion

Backoff on an Ethernet network is the retransmission delay that’s enforced when a collision occurs. When a collision occurs, a host will resume transmission after the forced time delay has expired. After this backoff delay period has expired, all stations have equal priority to transmit data.

In the following sections, I am going to cover Ethernet in detail at both the Data Link layer (layer 2) and the Physical layer (layer 1).

Half- and Full-Duplex Ethernet

Half-duplex Ethernet is defined in the original IEEE 802.3 Ethernet specification; Cisco says it uses only one wire pair with a digital signal running in both directions on the wire. Certainly, the IEEE specifications discuss the process of half duplex somewhat differently, but what Cisco is talking about is a general sense of what is happening here with Ethernet.

It also uses the CSMA/CD protocol to help prevent collisions and to permit retransmitting if a collision does occur. If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions. Half-duplex Ethernet is only about 30 to 40 percent efficient because a large 100BaseT network will usually only give you 30 to 40Mbps, at most.

But full-duplex Ethernet uses two pairs of wires at the same time instead of one wire pair like half duplex. And full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device. This means that with full-duplex data transfer, you get a faster data transfer compared to half duplex. And because the transmitted data is sent on a different set of wires than the received data, no collisions will occur.

The reason you don’t need to worry about collisions is because now it’s like a freeway with multiple lanes instead of the single-lane road provided by half duplex. Full-duplex Ethernet is supposed to offer 100 percent efficiency in both directions—for example, you can get 20Mbps with a 10Mbps Ethernet running full duplex or 200Mbps for Fast Ethernet. But this rate is something known as an aggregate rate, which translates as

“you’re supposed to get” 100 percent efficiency. No guarantees, in networking as in life.

Full-duplex Ethernet can be used in the following five situations:

With a connection from a switch to a host

With a connection from a switch to a switch

With a connection from a host to a host using a crossover cable

With a connection from a switch to a router using a crossover cable

With a connection from a router to a router using a crossover cable

With a connection from a router to a host using a crossover cable

Full-duplex Ethernet requires a point-to-point connection when only two nodes are present. You can run full-duplex with just about any device except a hub.

Now, if it’s capable of all that speed, why wouldn’t it deliver? Well, when a full-duplex Ethernet port is powered on, it first connects to the remote end and then negotiates with the other end of the Fast Ethernet link. This is called an

auto-detect mechanism

. This mechanism first decides on the exchange capability, which means it checks to see if it can run at 10, 100, or even 1000Mbps. It then checks to see if it can run full duplex, and if it can’t, it will run half duplex.

Remember that half-duplex Ethernet shares a collision domain and provides a lower effective throughput than full-duplex Ethernet, which typically has a private per-port collision domain and a higher effective throughput.

Lastly, remember these important points:

There are no collisions in full-duplex mode.

A dedicated switch port is required for each full-duplex node.

The host network card and the switch port must be capable of operating in full-duplex mode.

Now let’s take a look at how Ethernet works at the Data Link layer.

Ethernet at the Data Link Layer

Ethernet at the Data Link layer is responsible for Ethernet addressing, commonly referred to as hardware addressing or MAC addressing. Ethernet is also responsible for framing packets received from the Network layer and preparing them for transmission on the local network through the

Ethernet contention-based media access method.

Ethernet Addressing

Here’s where we get into how Ethernet addressing works. It uses the

Media Access Control (MAC)

address burned into each and every Ethernet network interface card (NIC). The MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal format.

Figure 2-2 shows the 48-bit MAC addresses and how the bits are divided.

Figure 2-2:

Ethernet addressing using MAC addresses

The

organizationally unique identifier (OUI)

is assigned by the IEEE to an organization. It’s composed of 24 bits, or 3 bytes. The organization, in turn, assigns a globally administered address (24 bits, or 3 bytes) that is unique (supposedly, again—no guarantees) to each and every adapter it manufactures. Look closely at the figure. The high-order bit is the Individual/Group (I/G) bit. When it has a value of 0, we can assume that the address is the MAC address of a device and may well appear in the source portion of the MAC header. When it is a 1, we can assume that the address represents either a broadcast or multicast address in Ethernet or a broadcast or functional address in Token Ring and FDDI.

The next bit is the global/local bit, or just G/L bit (also known as U/L, where

U

means

universal

). When set to 0, this bit represents a globally administered address (as by the IEEE). When the bit is a 1, it represents a locally governed and administered address. The low-order 24 bits of an

Ethernet address represent a locally administered or manufacturer-assigned code. This portion commonly starts with 24 0s for the first card made and continues in order until there are 24 1s for the last (16,777,216th) card made. You’ll find that many manufacturers use these same six hex digits as the last six characters of their serial number on the same card.

Binary to Decimal and Hexadecimal Conversion

Before we get into working with the TCP/IP protocol and IP addressing (covered in Chapter 3), it’s really important for you to truly understand the differences between binary, decimal, and hexadecimal numbers and how to convert one format into the other.

So we’ll start with binary numbering. It’s pretty simple, really. The digits used are limited to either a 1 (one) or a 0 (zero), and each digit is called a

bit

(short for

bi

nary digi

t

). Typically, you count either 4 or 8 bits together, with these being referred to as a nibble and a byte, respectively.

What interests us in binary numbering is the value represented in a decimal format—the typical decimal format being the base-10 number scheme that we’ve all used since kindergarten. The binary numbers are placed in a value spot: starting at the right and moving left, with each spot having double the value of the previous spot.

Table 2-1 shows the decimal values of each bit location in a nibble and a byte. Remember, a nibble is 4 bits and a byte is 8 bits.

Table 2-1:

Binary values

Nibble Values Byte Values

8 4 2 1 128 64 32 16 8 4 2 1

What all this means is that if a one digit (1) is placed in a value spot, then the nibble or byte takes on that decimal value and adds it to any other value spots that have a 1. And if a zero (0) is placed in a bit spot, you don’t count that value.

Let me clarify things. If we have a 1 placed in each spot of our nibble, we would then add up 8 + 4 + 2 + 1 to give us a maximum value of 15.

Another example for our nibble values would be 1010; that means that the 8 bit and the 2 bit are turned on, which equals a decimal value of 10. If we have a nibble binary value of 0110, then our decimal value would be 6, because the 4 and 2 bits are turned on.

But the byte values can add up to a value that’s significantly higher than 15. This is how: If we counted every bit as a one (1), then the byte binary value would look like this (remember, 8 bits equal a byte):

11111111

We would then count up every bit spot because each is turned on. It would look like this, which demonstrates the maximum value of a byte:

128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

There are plenty of other decimal values that a binary number can equal. Let’s work through a few examples.

10010110

Which bits are on? The 128, 16, 4, and 2 bits are on, so we’ll just add them up: 128 + 16 + 4 + 2 = 150.

01101100

Which bits are on? The 64, 32, 8, and 4 bits are on, so we just need to add them up: 64 + 32 + 8 + 4 = 108.

11101000

Which bits are on? The 128, 64, 32, and 8 bits are on, so just add the values up: 128 + 64 + 32 + 8 = 232.

Table 2-2 is a table you should memorize before braving the IP sections in Chapters 3 and 4.

Table 2-2:

Binary to decimal memorization chart

Binary Value Decimal Value

10000000 128

11000000 192

11100000 224

11110000 240

11111000 248

11111100 252

11111110 254

11111111 255

Hexadecimal addressing is completely different than binary or decimal—it’s converted by reading nibbles, not bytes. By using a nibble, we can convert these bits to hex pretty simply. First, understand that the hexadecimal addressing scheme uses only the numbers 0 through 9. And since the

numbers 10, 11, 12, and so on can’t be used (because they are two-digit numbers), the letters

A

,

B

,

C

,

D

,

E

, and

F

are used to represent 10, 11,

12, 13, 14, and 15, respectively.

Hex is short for hexadecimal, which is a numbering system that uses the first six letters of the alphabet (A through F) to extend beyond the available 10 digits in the decimal system.

E

F

C

D

8

9

A

B

6

7

4

5

2

3

0

1

Table 2-3 shows both the binary value and the decimal value for each hexadecimal digit.

Table 2-3:

Hex to binary to decimal chart

Hexadecimal Value Binary Value Decimal Value

12

13

14

15

8

9

10

11

6

7

4

5

2

3

0

1

1000

1001

1010

1011

1100

1101

1110

1111

0000

0001

0010

0011

0100

0101

0110

0111

Did you notice that the first 10 hexadecimal digits (0–9) are the same value as the decimal values? If not, look again. This handy fact makes those values super easy to convert.

So suppose you have something like this: 0x6A. (Sometimes Cisco likes to put

0x

in front of characters so you know that they are a hex value. It doesn’t have any other special meaning.) What are the binary and decimal values? All you have to remember is that each hex character is one nibble and two hex characters together make a byte. To figure out the binary value, we need to put the hex characters into two nibbles and then put them together into a byte. 6 = 0110 and A (which is 10 in hex) = 1010, so the complete byte would be 01101010.

To convert from binary to hex, just take the byte and break it into nibbles. Here’s what I mean.

Say you have the binary number 01010101. First, break it into nibbles—0101 and 0101—with the value of each nibble being 5 since the 1 and 4 bits are on. This makes the hex answer 0x55. And in decimal format, the binary number is 01010101, which converts to 64 + 16 + 4 + 1 = 85.

Here’s another binary number:

11001100

Your answer would be 1100 = 12 and 1100 = 12 (therefore, it’s converted to CC in hex). The decimal conversion answer would be 128 + 64 + 8 + 4

= 204.

One more example, then we need to get working on the Physical layer. Suppose you had the following binary number:

10110101

The hex answer would be 0xB5, since 1011 converts to B and 0101 converts to 5 in hex value. The decimal equivalent is 128 + 32 + 16 + 4 + 1 =

181.

See Written Lab 2.1 for more practice with binary/hex/decimal conversion.

Ethernet Frames

The Data Link layer is responsible for combining bits into bytes and bytes into frames. Frames are used at the Data Link layer to encapsulate packets handed down from the Network layer for transmission on a type of media access.

The function of Ethernet stations is to pass data frames between each other using a group of bits known as a MAC frame format. This provides error detection from a

cyclic redundancy check

(

CRC

). But remember—this is error detection, not error correction. The 802.3 frames and Ethernet frame are shown in Figure 2-3 .

Encapsulating a frame within a different type of frame is called tunneling.

Figure 2-3:

802.3 and Ethernet frame formats

Following are the details of the different fields in the 802.3 and Ethernet frame types:

Preamble

An alternating 1,0 pattern provides a 5MHz clock at the start of each packet, which allows the receiving devices to lock the incoming bit stream.

Start Frame Delimiter (SFD)/Synch

The preamble is seven octets and the SFD is one octet (synch). The SFD is 10101011, where the last pair of 1s allows the receiver to come into the alternating 1,0 pattern somewhere in the middle and still sync up and detect the beginning of the data.

Destination Address (DA)

This transmits a 48-bit value using the least significant bit (LSB) first. The DA is used by receiving stations to determine whether an incoming packet is addressed to a particular node. The destination address can be an individual address or a broadcast or multicast MAC address. Remember that a broadcast is all 1s (or

F

s in hex) and is sent to all devices but a multicast is sent only to a similar subset of nodes on a network.

Source Address (SA)

The SA is a 48-bit MAC address used to identify the transmitting device, and it uses the LSB first. Broadcast and multicast address formats are illegal within the SA field.

Length or Type

802.3 uses a Length field, but the Ethernet_II frame uses a Type field to identify the Network layer protocol. 802.3 cannot identify the upper-layer protocol and must be used with a proprietary LAN—IPX, for example.

Data

This is a packet sent down to the Data Link layer from the Network layer. The size can vary from 46 to 1,500 bytes.

Frame Check Sequence (FCS)

FCS is a field at the end of the frame that’s used to store the cyclic redundancy check (CRC) answer. The

CRC is a mathematical algorithm that’s run when each frame is built. When a receiving host receives the frame and runs the CRC, the answer should be the same. If not, the frame is discarded, assuming errors have occurred.

Let’s pause here for a minute and take a look at some frames caught on our trusty network analyzer. You can see that the frame below has only three fields: Destination, Source, and Type (shown as Protocol Type on this analyzer):

Destination: 00:60:f5:00:1f:27

Source: 00:60:f5:00:1f:2c

Protocol Type: 08-00 IP

This is an Ethernet_II frame. Notice that the Type field is IP, or 08-00 (mostly just referred to as 0x800) in hexadecimal.

The next frame has the same fields, so it must be an Ethernet_II frame too:

Destination: ff:ff:ff:ff:ff:ff Ethernet Broadcast

Source: 02:07:01:22:de:a4

Protocol Type: 08-00 IP

Did you notice that this frame was a broadcast? You can tell because the destination hardware address is all 1s in binary, or all

F

s in hexadecimal.

Let’s take a look at one more Ethernet_II frame. I’ll talk about this next example again when we use IPv6 in Chapter 15, but you can see that the

Ethernet frame is the same Ethernet_II frame we use with the IPv4 routed protocol. The Type field has 0x86dd when the frame is carrying IPv6 data, and when we have IPv4 data, the frame uses 0x0800 in the protocol field:

Destination: IPv6-Neighbor-Discovery_00:01:00:03 (33:33:00:01:00:03)

Source: Aopen_3e:7f:dd (00:01:80:3e:7f:dd)

Type: IPv6 (0x86dd)

This is the beauty of the Ethernet_II frame. Because of the Type field, we can run any Network layer routed protocol and it will carry the data because it can identify the Network layer protocol.

Ethernet at the Physical Layer

Ethernet was first implemented by a group called DIX (Digital, Intel, and Xerox). They created and implemented the first Ethernet LAN specification, which the IEEE used to create the IEEE 802.3 committee. This was a 10Mbps network that ran on coax and then eventually twisted-pair and fiber physical media.

The IEEE extended the 802.3 committee to two new committees known as 802.3u (Fast Ethernet) and 802.3ab (Gigabit Ethernet on category 5) and then finally 802.3ae (10Gbps over fiber and coax).

Figure 2-4 shows the IEEE 802.3 and original Ethernet Physical layer specifications.

When designing your LAN, it’s really important to understand the different types of Ethernet media available to you. Sure, it would be great to run

Gigabit Ethernet to each desktop and 10Gbps between switches, and you need to figure out how to justify the cost of that network today. But if you

mix and match the different types of Ethernet media methods currently available, you can come up with a cost-effective network solution that works great.

Figure 2-4:

Ethernet Physical layer specifications

The EIA/TIA (which stands for the Electronic Industries Association and the newer Telecommunications Industry Alliance) is the standards body that creates the Physical layer specifications for Ethernet. The EIA/TIA specifies that Ethernet use a

registered jack (RJ) connector

on

unshielded twisted-pair (UTP)

cabling (RJ45). However, the industry is moving toward calling this just an 8-pin modular connector.

Each Ethernet cable type that is specified by the EIA/TIA has inherent attenuation, which is defined as the loss of signal strength as it travels the length of a cable and is measured in decibels (dB). The cabling used in corporate and home markets is measured in categories. A higher-quality cable will have a higher-rated category and lower attenuation. For example, category 5 is better than category 3 because category 5 cables have more wire twists per foot and therefore less crosstalk. Crosstalk is the unwanted signal interference from adjacent pairs in the cable.

Here are the original IEEE 802.3 standards:

10Base2

10Mbps, baseband technology, up to 185 meters in length. Known as

thinnet

and can support up to 30 workstations on a single segment. Uses a physical and logical bus with BNC connectors and thin coaxial cable. The 10 means 10Mbps,

Base

means baseband technology (which is a digital signaling method for communication on the network), and the 2 means almost 200 meters. 10Base2 Ethernet cards use BNC (British Naval Connector, Bayonet Neill Concelman, or Bayonet Nut Connector), T-connectors, and terminators to connect to a network.

10Base5

10Mbps, baseband technology, up to 500 meters in length using thick coaxial cable. Known as

thicknet

. Uses a physical and logical bus with AUI connectors. Up to 2,500 meters with repeaters and 1,024 users for all segments.

10BaseT

10Mbps using category 3 unshielded twisted pair (UTP) wiring for runs up to 100 meters. Unlike with the 10Base2 and 10Base5 networks, each device must connect into a hub or switch, and you can have only one host per segment or wire. Uses an RJ45 connector (8-pin modular connector) with a physical star topology and a logical bus.

Each of the 802.3 standards defines an AUI, which allows a one-bit-at-a-time transfer to the Physical layer from the Data Link media-access method. This allows the MAC address to remain constant but means the Physical layer can support both existing and new technologies. The thing is, the original AUI interface was a 15-pin connector, which allowed a transceiver (transmitter/receiver) that provided a 15-pin-to-twisted-pair conversion.

There’s an issue, though—the AUI interface can’t support 100Mbps Ethernet because of the high frequencies involved. So 100BaseT needed a new interface, and the 802.3u specifications created one called the Media Independent Interface (MII), which provides 100Mbps throughput. The MII uses a nibble, which you of course remember is defined as 4 bits. Gigabit Ethernet uses a Gigabit Media Independent Interface (GMII) and transmits 8 bits at a time. 802.3u (Fast Ethernet) is compatible with 802.3 Ethernet because they share the same physical characteristics. Fast

Ethernet and Ethernet use the same maximum transmission unit (MTU) and the same MAC mechanisms, and they both preserve the frame format that is used by 10BaseT Ethernet. Basically, Fast Ethernet is just based on an extension to the IEEE 802.3 specification, and because of that, it offers us a speed increase of 10 times that of 10BaseT.

Here are the expanded IEEE Ethernet 802.3 standards, starting with Fast Ethernet:

100Base-TX (IEEE 802.3u)

100Base-TX, most commonly known as Fast Ethernet, uses EIA/TIA category 5, 5E, or 6 UTP two-pair wiring.

One user per segment; up to 100 meters long. It uses an RJ45 connector with a physical star topology and a logical bus.

100Base-FX (IEEE 802.3u)

Uses fiber cabling 62.5/125-micron multimode fiber. Point-to-point topology; up to 412 meters long. It uses ST and SC connectors, which are media-interface connectors.

1000Base-CX (IEEE 802.3z)

Copper twisted-pair called twinax (a balanced coaxial pair) that can run only up to 25 meters and uses a special

9-pin connector known as the High Speed Serial Data Connector (HSSDC).

1000Base-T (IEEE 802.3ab)

Category 5, four-pair UTP wiring up to 100 meters long and up to 1Gbps.

1000Base-SX (IEEE 802.3z)

The implementation of 1 Gigabit Ethernet running over multimode fiber-optic cable (instead of copper twistedpair cable) and using short wavelength laser. Multimode fiber (MMF) using 62.5- and 50-micron core; uses an 850 nanometer (nm) laser and can go up to 220 meters with 62.5-micron, 550 meters with 50-micron.

1000Base-LX (IEEE 802.3z)

Single-mode fiber that uses a 9-micron core and 1300 nm laser and can go from 3 kilometers up to 10 kilometers.

1000BASE-ZX (Cisco standard)

1000BaseZX (or 1000Base-ZX) is a Cisco specified standard for gigabit Ethernet communication.

1000BaseZX operates on ordinary single-mode fiber-optic link with spans up to 43.5 miles (70 km).

10GBase-T

10GBase-T is a standard proposed by the IEEE 802.3an committee to provide 10Gbps connections over conventional UTP cables (category 5e, 6, or 7 cables). 10GBase-T allows the conventional RJ45 used for Ethernet LANs. It can support signal transmission at the full 100-meter distance specified for LAN wiring.

The following are all part of the IEEE 802.3ae standard.

10GBase-Short Range (SR)

An implementation of 10 Gigabit Ethernet that uses short-wavelength lasers at 850 nm over multimode fiber. It has a maximum transmission distance of between 2 and 300 meters, depending on the size and quality of the fiber.

10GBase-Long Range (LR)

An implementation of 10 Gigabit Ethernet that uses long-wavelength lasers at 1,310 nm over single-mode fiber.

It also has a maximum transmission distance between 2 meters and 10 km, depending on the size and quality of the fiber.

10GBase-Extended Range (ER)

An implementation of 10 Gigabit Ethernet running over single-mode fiber. It uses extra-long-wavelength lasers at 1,550 nm. It has the longest transmission distances possible of the 10-Gigabit technologies: anywhere from 2 meters up to 40 km, depending on the size and quality of the fiber used.

10GBase-Short Wavelength (SW)

10GBase-SW, as defined by IEEE 802.3ae, is a mode of 10GBase-S for MMF with an 850 nm laser transceiver with a bandwidth of 10Gbps. It can support up to 300 meters of cable length. This media type is designed to connect to SONET equipment.

10GBase-Long Wavelength (LW)

10GBase-LW is a mode of 10GBase-L supporting a link length of 10 km on standard single-mode fiber

(SMF) (G.652). This media type is designed to connect to SONET equipment.

10GBase-Extra Long Wavelength (EW)

10GBase-EW is a mode of 10GBase-E supporting a link length of up to 40 km on SMF based on

G.652 using optical-wavelength 1550 nm. This media type is designed to connect to SONET equipment.

If you want to implement a network medium that is not susceptible to electromagnetic interference (EMI), fiber-optic cable provides a more secure, long-distance cable that is not susceptible to EMI at high speeds.

Table 2-4 summarizes the cable types.

Table 2-4:

Common Ethernet cable types

Armed with the basics covered in this chapter, you’re equipped to go to the next level and put Ethernet to work using various Ethernet cabling.

Ethernet Cabling

A discussion about Ethernet cabling is an important one, especially if you are planning on taking the Cisco exams. You need to really understand the following three types of cables:

Straight-through cable

Crossover cable

Rolled cable

We will look at each in the following sections.

Straight-Through Cable

The

straight-through cable

is used to connect the following devices:

Host to switch or hub

Router to switch or hub

Four wires are used in straight-through cable to connect Ethernet devices. It is relatively simple to create this type;

Figure 2-5 shows the four wires used in a straight-through Ethernet cable.

Figure 2-5:

Straight-through Ethernet cable

Notice that only pins 1, 2, 3, and 6 are used. Just connect 1 to 1, 2 to 2, 3 to 3, and 6 to 6 and you’ll be up and networking in no time. However, remember that this would be an Ethernet-only cable and wouldn’t work with voice or other LAN or WAN technology.

Crossover Cable

The

crossover cable

can be used to connect the following devices:

Switch to switch

Hub to hub

Host to host

Hub to switch

Router direct to host

Router to Router via Fast Ethernet ports

The same four wires used in the straight-through cable are used in this cable; we just connect different pins together.

Figure 2-6 shows how the four wires are used in a crossover Ethernet cable.

Figure 2-6:

Crossover Ethernet cable

Notice that instead of connecting 1 to 1, 2 to 2, and so on, here we connect pins 1 to 3 and 2 to 6 on each side of the cable.

Rolled Cable

Although

rolled cable

isn’t used to connect any Ethernet connections together, you can use a rolled Ethernet cable to connect a host EIA-TIA 232 interface to a router console serial communication (COM) port.

If you have a Cisco router or switch, you would use this cable to connect your PC running HyperTerminal to the Cisco hardware. Eight wires are used in this cable to connect serial devices, although not all eight are used to send information, just as in Ethernet networking. Figure 2-7 shows the eight wires used in a rolled cable.

These are probably the easiest cables to make because you just cut the end off on one side of a straight-through cable, turn it over, and put it back on (with a new connector, of course).

Figure 2-7:

Rolled Ethernet cable

Once you have the correct cable connected from your PC to the Cisco router or switch console port, you can start HyperTerminal to create a console connection and configure the device. Set the configuration as follows:

1.

Open HyperTerminal and enter a name for the connection. It is irrelevant what you name it, but I always just use Cisco. Then click OK.

2.

Choose the communications port—either COM1 or COM2, whichever is open on your PC.

3.

Now set the port settings. The default values (2400bps and no flow control hardware) will not work; you must set the port settings as shown in Figure 2-8 .

Notice that the bit rate is now set to 9600 and the flow control is set to None. At this point, you can click OK and press the Enter key and you should be connected to your Cisco device console port.

We’ve taken a look at the various RJ45 unshielded twisted pair (UTP) cables. Keeping this in mind, what cable is used between the switches in

Figure 2-9 ?

In order for host A to ping host B, you need a crossover cable to connect the two switches together. But what types of cables are used in the network shown in Figure 2-10 ?

Figure 2-8:

Port settings for a rolled cable connection

Figure 2-9:

RJ45 UTP cable question #1

Figure 2-10:

RJ45 UTP cable question #2

In Figure 2-10 , there are a variety of cables in use. For the connection between the switches, we’d obviously use a crossover cable like we saw in Figure 2-6 . The trouble is, we have a console connection that uses a rolled cable. Plus, the connection from the router to the switch is a straightthrough cable, as is true for the hosts to the switches. Keep in mind that if we had a serial connection (which we don’t), it would be a V.35 that we’d use to connect us to a WAN.

Data Encapsulation

When a host transmits data across a network to another device, the data goes through

encapsulation

: It is wrapped with protocol information at each layer of the OSI model. Each layer communicates only with its peer layer on the receiving device.

To communicate and exchange information, each layer uses

Protocol Data Units (PDUs)

. These hold the control information attached to the data at each layer of the model. They are usually attached to the header in front of the data field but can also be at the trailer, or end, of it.

Each PDU attaches to the data by encapsulating it at each layer of the OSI model, and each has a specific name depending on the information provided in each header. This PDU information is read only by the peer layer on the receiving device. After it’s read, it’s stripped off and the data is

then handed to the next layer up.

Figure 2-11 shows the PDUs and how they attach control information to each layer. This figure demonstrates how the upper-layer user data is converted for transmission on the network. The data stream is then handed down to the Transport layer, which sets up a virtual circuit to the receiving device by sending over a synch packet. Next, the data stream is broken up into smaller pieces, and a Transport layer header is created and attached to the header of the data field; now the piece of data is called a

segment

(a

PDU

). Each segment can be sequenced so the data stream can be put back together on the receiving side exactly as it was transmitted.

Figure 2-11:

Data encapsulation

Each segment is then handed to the Network layer for network addressing and routing through the internetwork. Logical addressing (for example,

IP) is used to get each segment to the correct network. The Network layer protocol adds a control header to the segment handed down from the

Transport layer, and what we have now is called a

packet

or

datagram

. Remember that the Transport and Network layers work together to rebuild a data stream on a receiving host, but it’s not part of their work to place their PDUs on a local network segment—which is the only way to get the information to a router or host.

It’s the Data Link layer that’s responsible for taking packets from the Network layer and placing them on the network medium (cable or wireless).

The Data Link layer

encapsulates

each packet in a

frame

, and the frame’s header carries the hardware addresses of the source and destination hosts. If the destination device is on a remote network, then the frame is sent to a router to be routed through an internetwork. Once it gets to the destination network, a new frame is used to get the packet to the destination host.

To put this frame on the network, it must first be put into a digital signal. Since a frame is really a logical group of 1s and 0s, the Physical layer is responsible for encoding these digits into a digital signal, which is read by devices on the same local network. The receiving devices will synchronize on the digital signal and extract (decode) the 1s and 0s from the digital signal. At this point, the devices reconstruct the frames, run a

CRC, and then check their answer against the answer in the frame’s FCS field. If it matches, the packet is pulled from the frame and what’s left of the frame is discarded. This process is called

de-encapsulation

. The packet is handed to the Network layer, where the address is checked. If the address matches, the segment is pulled from the packet and what’s left of the packet is discarded. The segment is processed at the Transport layer, which rebuilds the data stream and acknowledges to the transmitting station that it received each piece. It then happily hands the data stream to the upper-layer application.

At a transmitting device, the data encapsulation method works like this:

1.

User information is converted to data for transmission on the network.

2.

Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts.

3.

Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork.

4.

Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment.

5.

Frames are converted to bits, and a digital encoding and clocking scheme is used.

6.

To explain this in more detail using the layer addressing, I’ll use Figure 2-12 .

Remember that a data stream is handed down from the upper layer to the Transport layer. As technicians, we really don’t care who the data stream comes from because that’s really a programmer’s problem. Our job is to rebuild the data stream reliably and hand it to the upper layers on the receiving device.

Before we go further in our discussion of

Figure 2-12 , let’s discuss port numbers and make sure we understand them. The Transport layer uses port numbers to define both the virtual circuit and the upper-layer processes, as you can see from Figure 2-13 .

Figure 2-12:

PDU and layer addressing

Figure 2-13:

Port numbers at the Transport layer

The Transport layer, when using a connection-oriented protocol (i.e., TCP), takes the data stream, makes segments out of it, and establishes a reliable session by creating a virtual circuit. It then sequences (numbers) each segment and uses acknowledgments and flow control. If you’re using

TCP, the virtual circuit is defined by the source and destination port number as well as source and destination IP address (this is called a socket).

Remember, the host just makes this up starting at port number 1024 (0 through 1023 are reserved for well-known port numbers). The destination port number defines the upper-layer process (application) that the data stream is handed to when the data stream is reliably rebuilt on the receiving host.

Now that you understand port numbers and how they are used at the Transport layer, let’s go back to

Figure 2-12 . Once the Transport layer header information is added to the piece of data, it becomes a segment and is handed down to the Network layer along with the destination IP address. (The destination IP address was handed down from the upper layers to the Transport layer with the data stream, and it was discovered through a name resolution method at the upper layers—probably DNS.)

The Network layer adds a header, and adds the logical addressing (IP addresses), to the front of each segment. Once the header is added to the segment, the PDU is called a packet. The packet has a protocol field that describes where the segment came from (either UDP or TCP) so it can hand the segment to the correct protocol at the Transport layer when it reaches the receiving host.

The Network layer is responsible for finding the destination hardware address that dictates where the packet should be sent on the local network.

It does this by using the Address Resolution Protocol (ARP)—something I’ll talk about more in Chapter 3. IP at the Network layer looks at the destination IP address and compares that address to its own source IP address and subnet mask. If it turns out to be a local network request, the hardware address of the local host is requested via an ARP request. If the packet is destined for a remote host, IP will look for the IP address of the default gateway (router) instead.

The packet, along with the destination hardware address of either the local host or default gateway, is then handed down to the Data Link layer.

The Data Link layer will add a header to the front of the packet and the piece of data then becomes a frame. (We call it a frame because both a header and a trailer are added to the packet, which makes it resemble bookends or a frame, if you will.) This is shown in Figure 2-12 . The frame uses an Ether-Type field to describe which protocol the packet came from at the Network layer. Now a cyclic redundancy check (CRC) is run on the frame, and the answer to the CRC is placed in the Frame Check Sequence field found in the trailer of the frame.

The frame is now ready to be handed down, one bit at a time, to the Physical layer, which will use bit timing rules to encode the data in a digital signal. Every device on the network segment will synchronize with the clock and extract the 1s and 0s from the digital signal and build a frame. After the frame is rebuilt, a CRC is run to make sure the frame is okay. If everything turns out to be all good, the hosts will check the destination MAC and

IP addresses to see if the frame is for them.

If all this is making your eyes cross and your brain freeze, don’t freak. I’ll be going over exactly how data is encapsulated and routed through an

internetwork in Chapter 8.

The Cisco Three-Layer Hierarchical Model

Most of us were exposed to hierarchy early in life. Anyone with older siblings learned what it was like to be at the bottom of the hierarchy.

Regardless of where you first discovered hierarchy, today most of us experience it in many aspects of our lives. It is

hierarchy

that helps us understand where things belong, how things fit together, and what functions go where. It brings order and understandability to otherwise complex models. If you want a pay raise, for instance, hierarchy dictates that you ask your boss, not your subordinate. That is the person whose role it is to grant (or deny) your request. So basically, understanding hierarchy helps us discern where we should go to get what we need.

Hierarchy has many of the same benefits in network design that it does in other areas of life. When used properly, it makes networks more predictable. It helps us define which areas should perform certain functions. Likewise, you can use tools such as access lists at certain levels in hierarchical networks and avoid them at others.

Let’s face it: Large networks can be extremely complicated, with multiple protocols, detailed configurations, and diverse technologies. Hierarchy helps us summarize a complex collection of details into an understandable model. Then, as specific configurations are needed, the model dictates the appropriate manner in which to apply them.

The Cisco hierarchical model can help you design, implement, and maintain a scalable, reliable, cost-effective hierarchical internetwork. Cisco defines three layers of hierarchy, as shown in Figure 2-14 , each with specific functions.

Figure 2-14:

The Cisco hierarchical model

The following are the three layers and their typical functions:

The core layer: backbone

The distribution layer: routing

The access layer: switching

Each layer has specific responsibilities. Remember, however, that the three layers are logical and are not necessarily physical devices.

Consider the OSI model, another logical hierarchy. The seven layers describe functions but not necessarily protocols, right? Sometimes a protocol maps to more than one layer of the OSI model, and sometimes multiple protocols communicate within a single layer. In the same way, when we build physical implementations of hierarchical networks, we may have many devices in a single layer, or we might have a single device performing functions at two layers. The definition of the layers is logical, not physical.

Now, let’s take a closer look at each of the layers.

The Core Layer

The

core layer

is literally the core of the network. At the top of the hierarchy, the core layer is responsible for transporting large amounts of traffic both reliably and quickly. The only purpose of the network’s core layer is to switch traffic as fast as possible. The traffic transported across the core is common to a majority of users. However, remember that user data is processed at the distribution layer, which forwards the requests to the core if needed.

If there is a failure in the core,

every single user

can be affected. Therefore, fault tolerance at this layer is an issue. The core is likely to see large volumes of traffic, so speed and latency are driving concerns here. Given the function of the core, we can now consider some design specifics.

Let’s start with some things we don’t want to do:

Don’t do anything to slow down traffic. This includes using access lists, routing between virtual local area networks (VLANs), and implementing packet filtering.

Don’t support workgroup access here.

Avoid expanding the core (i.e., adding routers) when the internetwork grows. If performance becomes an issue in the core, give preference to upgrades over expansion.

Now, there are a few things that we want to do as we design the core:

Design the core for high reliability. Consider data-link technologies that facilitate both speed and redundancy, such as Gigabit Ethernet (with redundant links), or even 10Gigabit Ethernet.

Design with speed in mind. The core should have very little latency.

Select routing protocols with lower convergence times. Fast and redundant data-link connectivity is no help if your routing tables are shot!

The Distribution Layer

The

distribution layer

is sometimes referred to as the

workgroup layer

and is the communication point between the access layer and the core. The primary functions of the distribution layer are to provide routing, filtering, and WAN access and to determine how packets can access the core, if needed. The distribution layer must determine the fastest way that network service requests are handled—for example, how a file request is forwarded to a server. After the distribution layer determines the best path, it forwards the request to the core layer if necessary. The core layer then quickly transports the request to the correct service.

The distribution layer is the place to implement policies for the network. Here you can exercise considerable flexibility in defining network operation. There are several actions that generally should be done at the distribution layer:

Routing

Implementing tools (such as access lists), packet filtering, and queuing

Implementing security and network policies, including address translation and firewalls

Redistributing between routing protocols, including static routing

Routing between VLANs and other workgroup support functions

Defining broadcast and multicast domains

Things to avoid at the distribution layer are limited to those functions that exclusively belong to one of the other layers.

The Access Layer

The

access layer

controls user and workgroup access to internetwork resources. The access layer is sometimes referred to as the

desktop layer

.

The network resources most users need will be available locally. The distribution layer handles any traffic for remote services. The following are some of the functions to be included at the access layer:

Continued (from distribution layer) use of access control and policies

Creation of separate collision domains (segmentation)

Workgroup connectivity into the distribution layer

Technologies such as Gigabit or Fast Ethernet switching are frequently seen in the access layer.

As already noted, three separate levels does not imply three separate devices. There could be fewer, or there could be more. Remember, this is a

layered

approach.

Summary

In this chapter, you learned the fundamentals of Ethernet networking, how hosts communicate on a network, and how CSMA/CD works in an

Ethernet half-duplex network.

I also talked about the differences between half- and full-duplex modes and discussed the collision detection mechanism CSMA/CD.

Also in this chapter was a description of the common Ethernet cable types used in today’s networks. And by the way, you’d be wise to study that section really well!

Important enough to not gloss over, this chapter provided an introduction to encapsulation. Encapsulation is the process of encoding data as it goes down the OSI stack.

Last, this chapter covered the Cisco three-layer hierarchical model. I described in detail the three layers and how each is used to help design and implement a Cisco internetwork. We are now going to move on to IP addressing in the next chapter.

Exam Essentials

Describe the operation of Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

CSMA/CD is a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. Although it does not eliminate collisions, it helps to greatly reduce them, which reduces retransmissions, resulting in a more efficient transmission of data for all devices.

Differentiate half-duplex and full-duplex communication and define the requirements to utilize each method.

Full-duplex Ethernet uses two pairs of wires instead of one wire pair like half duplex. Full duplex allows for sending and receiving at the same time, using different wires to eliminate collisions, while half duplex can send or receive but not at the same time and still can suffer collisions. To use full duplex, the devices at both ends of the cable must be capable of and configured to perform full duplex.

Describe the sections of a MAC address and the information contained in each section.

The MAC, or hardware, address is a 48-bit

(6-byte) address written in a hexadecimal format. The first 24 bits or 3 bytes are called the organizationally unique identifier (OUI), which is assigned by the IEEE to the manufacturer of the NIC. The balance of the number uniquely identifies the NIC.

Identify the binary and hexadecimal equivalent of a decimal number.

Any number expressed in one format can also be expressed in the other two. The ability to perform this conversion is critical to understanding IP addressing and subnetting. Be sure to go through the written labs covering binary to decimal to hexadecimal conversion.

Identify the fields in the Data Link portion of an Ethernet frame.

The fields in the Data Link portion of a frame include the Preamble, Start

Frame Delimiter, Destination MAC address, Source MAC address, Length or Type, Data, and Frame Check Sequence.

Identify the IEEE physical standards for Ethernet cabling.

These standards describe the capabilities and physical characteristics of various cable types and include but are not limited to 10Base2, 10Base5, and 10Base T.

Differentiate types of Ethernet cabling and identify their proper application.

The three types of cables that can be created from an

Ethernet cable are straight-through (to connect a PC’s or a router’s Ethernet interface to a hub or switch), crossover (to connect hub to hub, hub to switch, switch to switch, or PC to PC), and rolled (for a console connection from a PC to a router or switch).

Describe the data encapsulation process and the role it plays in packet creation.

Data encapsulation is a process whereby information is added to the frame from each layer of the OSI model. This is also called packet creation. Each layer communicates only with its peer layer on the receiving device.

Understand how to connect a console cable from a PC to a router and start HyperTerminal.

Take a rolled cable and connect it from the COM port of the host to the console port of a router. Start HyperTerminal and set the bits per second to 9600 and flow control to None.

Identify the layers in the Cisco three-layer model and describe the ideal function of each layer.

The three layers in the Cisco hierarchical model are the core (responsible for transporting large amounts of traffic both reliably and quickly), distribution (provides routing, filtering, and WAN access), and access (workgroup connectivity into the distribution layer).

Written Labs

In this section, you’ll complete the following labs to make sure you’ve got the information and concepts contained within them fully dialed in:

Lab 2.1: Binary/Decimal/Hexadecimal Conversion

Lab 2.2: CSMA/CD Operations

Lab 2.3: Cabling

Lab 2.4: Encapsulation

(The answers to the written labs can be found following the answers to the review questions for this chapter.)

Written Lab 2.1: Binary/Decimal/Hexadecimal Conversion

1.

Convert from decimal IP address to binary format.

Complete the following table to express 192.168.10.15 in binary format.

Complete the following table to express 172.16.20.55 in binary format.

Complete the following table to express 10.11.12.99 in binary format.

2.

Convert the following from binary format to decimal IP address.

Complete the following table to express 11001100.00110011.10101010.01010101 in decimal IP address format.

Complete the following table to express 11000110.11010011.00111001.11010001 in decimal IP address format.

Complete the following table to express 10000100.11010010.10111000.10100110 in decimal IP address format.

3.

Convert the following from binary format to hexadecimal.

Complete the following table to express 11011000.00011011.00111101.01110110 in hexadecimal.

Complete the following table to express 11001010.11110101.10000011.11101011 in hexadecimal.

Complete the following table to express 10000100.11010010.01000011.10110011 in hexadecimal.

Written Lab 2.2: CSMA/CD Operations

Carrier Sense Multiple Access with Collision Detection (CSMA/CD) helps to minimize collisions in the network, thereby increasing data transmission efficiency. Place the following steps of its operation in the order in which they occur.

All hosts have equal priority to transmit after the timers have expired.

Each device on the Ethernet segment stops transmitting for a short time until the timers expire.

The collision invokes a random backoff algorithm.

A jam signal informs all devices that a collision occurred.

Written Lab 2.3: Cabling

For each of the following situations determine whether a straight-through, crossover, or rolled cable would be used.

1.

Host to host

2.

Host to switch or hub

3.

Router direct to host

4.

Switch to switch

5.

Router to switch or hub

6.

Hub to hub

7.

Hub to switch

8.

Host to a router console serial communication (com) port

Written Lab 2.4: Encapsulation

Place the following steps of the encapsulation process in the proper order.

Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment.

Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork.

User information is converted to data for transmission on the network.

Frames are converted to bits, and a digital encoding and clocking scheme is used.

Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts.

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s introduction.

1. Which fields are contained within an IEEE Ethernet frame? (Choose two.)

A. Source and destination MAC address

B. Source and destination network address

C. Source and destination MAC address and source and destination network address

D. FCS field

2. Which of the following are unique characteristics of half-duplex Ethernet when compared to full-duplex Ethernet? (Choose two.)

A. Half-duplex Ethernet operates in a shared collision domain.

B. Half-duplex Ethernet operates in a private collision domain.

C. Half-duplex Ethernet has higher effective throughput.

D. Half-duplex Ethernet has lower effective throughput.

E. Half-duplex Ethernet operates in a private broadcast domain.

3. You want to implement a network medium that is not susceptible to EMI. Which type of cabling should you use?

A. Thicknet coax

B. Thinnet coax

C. Category 5 UTP cable

D. Fiber-optic cable

4. Which of the following types of connections can use full duplex? (Choose three.)

A. Hub to hub

B. Switch to switch

C. Host to host

D. Switch to hub

E. Switch to host

5. What type of RJ45 UTP cable is used between switches?

A. Straight-through

B. Crossover cable

C. Crossover with a CSU/DSU

D. Crossover with a router in between the two switches

6. How does a host on an Ethernet LAN know when to transmit after a collision has occurred? (Choose two.)

A. In a CSMA/CD collision domain, multiple stations can successfully transmit data simultaneously.

B. In a CSMA/CD collision domain, stations must wait until the media is not in use before transmitting.

C. You can improve the CSMA/CD network by adding more hubs.

D. After a collision, the station that detected the collision has first priority to resend the lost data.

E. After a collision, all stations run a random backoff algorithm. When the backoff delay period has expired, all stations have equal priority to transmit data.

F. After a collision, all stations involved run an identical backoff algorithm and then synchronize with each other prior to transmitting data.

7. What type of RJ45 UTP cable do you use to connect a PC’s COM port to a router or switch console port?

A. Straight-through

B. Crossover cable

C. Crossover with a CSU/DSU

D. Rolled

8. You have the following binary number: 10110111. What are the decimal and hexadecimal equivalents?

A. 69/0x2102

B. 183/B7

C. 173/A6

D. 83/0xC5

9. Which of the following contention mechanisms is used by Ethernet?

A. Token passing

B. CSMA/CD

C. CSMA/CA

D. Host polling

10. In the operation of CSMA/CD, which host(s) have priority after the expiration of the backoff algorithm?

A. All hosts have equal priority.

B. The two hosts that caused the collision will have equal priority.

C. The host that sent the jam signal after the collision.

D. The host with the highest MAC address.

11. Which of the following is correct?

A. Full-duplex Ethernet uses one pair of wires.

B. Full-duplex Ethernet uses two pairs of wires.

C. Half-duplex Ethernet uses two pairs of wires.

D. Full-duplex Ethernet uses three pairs of wires.

12. Which of the following statements is false with respect to full duplex?

A. There are no collisions in full-duplex mode.

B. A dedicated switch port is required for each full-duplex node.

C. There are few collisions in full-duplex mode.

D. The host network card and the switch port must be capable of operating in full-duplex mode.

13. Which statement is correct with regard to a MAC address?

A. A MAC, or logical, address is a 48-bit (6-byte) address written in a hexadecimal format.

B. A MAC, or hardware, address is a 64-bit (6-byte) address written in a hexadecimal format.

C. A MAC, or hardware, address is a 48-bit (6-byte) address written in a binary format.

D. A MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal format.

14. Which part of a MAC address is called the organizationally unique identifier (OUI)?

A. The first 24 bits, or 3 bytes

B. The first 12 bits, or 3 bytes

C. The first 24 bits, or 6 bytes

D. The first 32 bits, or 3 bytes

15. Which layer of the OSI model is responsible for combining bits into bytes and bytes into frames?

A. Presentation

B. Data Link

C. Application

D. Transport

16. What is the specific term for the unwanted signal interference from adjacent pairs in the cable?

A. EMI

B. RFI

C. Crosstalk

D. Attenuation

17. Which of the following is part of the IEEE 802.3u standard?

A. 100Base2

B. 10Base5

C. 100Base-TX

D. 1000Base-T

18. 10GBase-Long Wavelength is known as which IEEE standard?

A. 802.3F

B. 802.3z

C. 802.3ab

D. 802.3ae

19. 1000Base-T is which IEEE standard?

A. 802.3F

B. 802.3z

C. 802.3ab

D. 802.3ae

20. When making a HyperTerminal connection, what must the bit rate be set to?

A. 2400bps

B. 1200bps

C. 9600bps

D. 6400bps

Answers to Review Questions

1. A, D. An Ethernet frame has source and destination MAC addresses, an Ether-Type field to identify the Network layer protocol, the data, and the

FCS field that holds the answer to the CRC.

2. A, D. Half-duplex Ethernet works in a shared medium or collision domain. Half duplex provides a lower effective throughput than full duplex.

3. D. Fiber-optic cable provides a more secure, long-distance cable that is not susceptible to EMI interference at high speeds.

4. B, C, E. Hubs cannot run full-duplex Ethernet. Full duplex must be used on a point-to-point connection between two devices capable of running full duplex. Switches and hosts can run full duplex between each other, but a hub can never run full duplex.

5. B. To connect two switches together, you would use a RJ45 UTP crossover cable.

6. B, E. Once transmitting stations on an Ethernet segment hear a collision, they send an extended jam signal to ensure that all stations recognize the collision. After the jamming is complete, each sender waits a predetermined amount of time, plus a random time. After both timers expire, they are free to transmit, but they must make sure the media is clear before transmitting and that they all have equal priority.

7. D. To connect to a router or switch console port, you would use an RJ45 UTP rolled cable.

8. B. You must be able to take a binary number and convert it into both decimal and hexadecimal. To convert to decimal, just add up the 1s using their values. The values that are turned on with the binary number of 10110111 are 128 + 32 + 16 + 4 + 2 + 1 = 183. To get the hexadecimal equivalent, you need to break the eight binary digits into nibbles (4 bits), 1011 and 0111. By adding up these values, you get 11 and 7. In hexadecimal, 11 is B, so the answer is 0xB7.

9. B. Ethernet networking uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium.

10. A. After the expiration of the backoff algorithm, all hosts have equal priority.

11. B. Full-duplex Ethernet uses two pairs of wires.

12. C. There are no collisions in full-duplex mode.

13. D. A MAC, or hardware, address is a 48-bit (6-byte) address written in a hexadecimal format.

14. A. The first 24 bits, or 3 bytes, of a MAC address is called the organizationally unique identifier (OUI).

15. B. The Data Link layer of the OSI model is responsible for combining bits into bytes and bytes into frames.

16. C. The term for the unwanted signal interference from adjacent pairs in the cable is crosstalk.

17. C. IEEE 802.3.u is Fast Ethernet at 100Mbps and covers 100Base-TX, 100BaseT4, and 100Base-FX.

18. D. IEEE 802.3ae is the standard for 10Gbase-SR, -LR, -ER, -SW, -LW, and -E.

19. C. IEEE 802.3ab is the standard for 1Gbps on twisted-pair.

20. C. When making a HyperTerminal connection, the bit rate must be set to 9600bps.

Answers to Written Lab 2.1

1.

Convert from decimal IP address to binary format.

Complete the following table to express 192.168.10.15 in binary format.

Complete the following table to express 172.16.20.55 in binary format.

Complete the following table to express 10.11.12.99 in binary format.

2.

Convert the following from binary format to decimal IP address.

Complete the following table to express 11001100.00110011.10101010.01010101 in decimal IP address format.

Complete the following table to express 11000110.11010011.00111001.11010001 in decimal IP address format.

Complete the following table to express 10000100.11010010.10111000.10100110 in decimal IP address format.

3.

Convert the following from binary format to hexadecimal.

Complete the following table to express 11011000.00011011.00111101.01110110 in hexadecimal.

Complete the following table to express 11001010.11110101.10000011.11101011 in hexadecimal.

Complete the following table to express 10000100.11010010.01000011.10110011 in hexadecimal.

Answers to Written Lab 2.2

When a collision occurs on an Ethernet LAN, the following happens:

1.

A jam signal informs all devices that a collision occurred.

2.

The collision invokes a random backoff algorithm.

3.

Each device on the Ethernet segment stops transmitting for a short time until the timers expire.

4.

All hosts have equal priority to transmit after the timers have expired

Answers to Written Lab 2.3

1.

Crossover

2.

Straight-through

3.

Crossover

4.

Crossover

5.

Straight-through

6.

Crossover

7.

Crossover

8.

Rolled

Answers to Written Lab 2.4

At a transmitting device, the data encapsulation method works like this:

1.

User information is converted to data for transmission on the network.

2.

Data is converted to segments, and a reliable connection is set up between the transmitting and receiving hosts.

3.

Segments are converted to packets or datagrams, and a logical address is placed in the header so each packet can be routed through an internetwork.

4.

Packets or datagrams are converted to frames for transmission on the local network. Hardware (Ethernet) addresses are used to uniquely identify hosts on a local network segment.

5.

Frames are converted to bits, and a digital encoding and clocking scheme is used.

Chapter 3

Introduction to TCP/IP

The CCNA exam topics covered in this chapter include the following:

Describe how a network works

Describe the purpose and basic operation of the protocols in the OSI and TCP models.

Identify and correct common network problems at layers 1, 2, 3, and 7 using a layered model approach.

Implement an IP addressing scheme and IP Services to meet network requirements in a medium-size Enterprise branch office network.

Describe the operation and benefits of using private and public IP addressing.

The

Transmission Control Protocol/Internet Protocol (TCP/IP)

suite was created by the Department of Defense (DoD) to ensure and preserve data integrity as well as maintain communications in the event of catastrophic war. So it follows that if designed and implemented correctly, a

TCP/IP network can be a truly dependable and resilient one. In this chapter, I’ll cover the protocols of TCP/IP, and throughout this book, you’ll learn how to create a marvelous TCP/IP network—using Cisco routers, of course.

We’ll begin by taking a look at the DoD’s version of TCP/IP and then compare this version and its protocols with the OSI reference model discussed in Chapter 1, “Internetworking.”

Once you understand the protocols used at the various levels of the DoD model, I’ll cover IP addressing and the different classes of IP addresses used in networks today.

Subnetting will be covered in Chapter 4, “Easy Subnetting.”

Last, because IPv4 address types are so important to understanding IP addressing, as well as subnetting and Variable Length Subnet Masks

(VLSMs), an understanding of the various flavors of IPv4 addresses is critical. I’ll finish the chapter with various types of IPv4 addresses that you just must know.

Internet Protocol version 6 will not be discussed in this chapter; this chapter will focus solely on IPv4. IPv6 will be covered in Chapter 15, “Internet

Protocol Version 6 (IPv6).” Also, when I discuss Internet Protocol Version 4, you’ll see it written as just IP, not typically IPv4.

For up-to-the-minute updates for this chapter, please see www.lammle.com

and/or www.sybex.com/go/ccna7e

.

Introducing TCP/IP

Because TCP/IP is so central to working with the Internet and intranets, it’s essential for you to understand it in detail. I’ll begin by giving you some background on TCP/IP and how it came about and then move on to describing the important technical goals defined by the original designers. After that, you’ll find out how TCP/IP compares to a theoretical model—the Open Systems Interconnection (OSI) model.

A Brief History of TCP/IP

TCP/IP first came on the scene in 1973. Later, in 1978, it was divided into two distinct protocols: TCP and IP. Then, in 1983, TCP/IP replaced the

Network Control Protocol (NCP) and was authorized as the official means of data transport for anything connecting to ARPAnet, the Internet’s ancestor that was created by ARPA, the DoD’s Advanced Research Projects Agency, way back in 1957 in reaction to the Soviet’s launching of

Sputnik. ARPA was soon redubbed DARPA, and it was divided into ARPAnet and MILNET (also in 1983); both were finally dissolved in 1990.

But contrary to what you might think, most of the development work on TCP/IP happened at UC Berkeley in Northern California, where a group of scientists were simultaneously working on the Berkeley version of UNIX, which soon became known as the BSD, or Berkeley Software Distribution, series of UNIX versions. Of course, because TCP/IP worked so well, it was packaged into subsequent releases of BSD UNIX and offered to other universities and institutions if they bought the distribution tape. So basically, BSD Unix bundled with TCP/IP began as shareware in the world of academia and, as a result, became the basis of the huge success and exponential growth of today’s Internet as well as smaller, private and corporate intranets.

As usual, what may have started as a small group of TCP/IP aficionados evolved, and as it did, the U.S. government created a program to test any new published standards and make sure they passed certain criteria. This was to protect TCP/IP’s integrity and to ensure that no developer changed anything too dramatically or added any proprietary features. It’s this very quality—this open-systems approach to the TCP/IP family of protocols—that pretty much sealed its popularity because it guarantees a solid connection between myriad hardware and software platforms with no strings attached.

TCP/IP and the DoD Model

The DoD model is basically a condensed version of the OSI model—it’s composed of four, instead of seven, layers:

Process/Application layer

Host-to-Host layer

Internet layer

Network Access layer

Figure 3-1 shows a comparison of the DoD model and the OSI reference model. As you can see, the two are similar in concept, but each has a different number of layers with different names.

When the different protocols in the IP stack are discussed, the layers of the OSI and DoD models are interchangeable. In other words, the Internet layer and the Network layer describe the same thing, as do the Host-to-Host layer and the Transport layer.

Figure 3-1:

The DoD and OSI models

A vast array of protocols combine at the DoD model’s

Process/Application layer

to integrate the various activities and duties spanning the focus of the OSI’s corresponding top three layers (Application, Presentation, and Session). We’ll be looking closely at those protocols in the next part of this chapter. The Process/Application layer defines protocols for node-to-node application communication and also controls user-interface specifications.

The

Host-to-Host layer

parallels the functions of the OSI’s Transport layer, defining protocols for setting up the level of transmission service for applications. It tackles issues such as creating reliable end-to-end communication and ensuring the error-free delivery of data. It handles packet sequencing and maintains data integrity.

The

Internet layer

corresponds to the OSI’s Network layer, designating the protocols relating to the logical transmission of packets over the entire network. It takes care of the addressing of hosts by giving them an IP (Internet Protocol) address, and it handles the routing of packets among multiple networks.

At the bottom of the DoD model, the

Network Access layer

implements the data exchange between the host and the network. The equivalent of the Data Link and Physical layers of the OSI model, the Network Access layer oversees hardware addressing and defines protocols for the physical transmission of data.

The DoD and OSI models are alike in design and concept and have similar functions in similar layers.

Figure 3-2 shows the TCP/IP protocol suite and how its protocols relate to the DoD model layers.

In the following sections, we will look at the different protocols in more detail, starting with the Process/Application layer protocols.

The Process/Application Layer Protocols

In the following sections, I’ll describe the different applications and services typically used in IP networks. The following protocols and applications are covered:

Telnet

FTP

TFTP

NFS

SMTP

SSH

HTTP

HTTPS

NTP

NNTP

SCP

LDAP

IGMP

LPR

DNS

POP

IMAP4

TLS

SIP (VoIP)

RTP (VoIP)

LPD

X Window

SNMP

DHCP/BootP

Figure 3-2:

The TCP/IP protocol suite

Telnet

Telnet

is the chameleon of protocols—its specialty is terminal emulation. It allows a user on a remote client machine, called the Telnet client, to access the resources of another machine, the Telnet server. Telnet achieves this by pulling a fast one on the Telnet server and making the client machine appear as though it were a terminal directly attached to the local network. This projection is actually a software image—a virtual terminal that can interact with the chosen remote host.

These emulated terminals are of the text-mode type and can execute defined procedures such as displaying menus that give users the opportunity to choose options and access the applications on the duped server. Users begin a Telnet session by running the Telnet client software and then logging into the Telnet server.

File Transfer Protocol (FTP)

File Transfer Protocol (FTP)

is the protocol that actually lets us transfer files, and it can accomplish this between any two machines using it. But

FTP isn’t just a protocol; it’s also a program. Operating as a protocol, FTP is used by applications. As a program, it’s employed by users to perform file tasks by hand. FTP also allows for access to both directories and files and can accomplish certain types of directory operations, such as relocating into different ones.

Accessing a host through FTP is only the first step, though. Users must then be subjected to an authentication login that’s probably secured with passwords and usernames implemented by system administrators to restrict access. You can get around this somewhat by adopting the username

anonymous

—though what you’ll gain access to will be limited.

Even when employed by users manually as a program, FTP’s functions are limited to listing and manipulating directories, typing file contents, and copying files between hosts. It can’t execute remote files as programs.

Trivial File Transfer Protocol (TFTP)

Trivial File Transfer Protocol (TFTP)

is the stripped-down, stock version of FTP, but it’s the protocol of choice if you know exactly what you want and where to find it, plus it’s so easy to use and it’s fast too! It doesn’t give you the abundance of functions that FTP does, though. TFTP has no directory-browsing abilities; it can do nothing but send and receive files. This compact little protocol also skimps in the data department, sending much smaller blocks of data than FTP, and there’s no authentication as with FTP, so it’s even more insecure. Few sites support it because of the inherent security risks.

Network File System (NFS)

Network File System (NFS)

is a jewel of a protocol specializing in file sharing. It allows two different types of file systems to interoperate. It works like this: Suppose the NFS server software is running on a Windows server and the NFS client software is running on a Unix host. NFS allows for a portion of the RAM on the Windows server to transparently store Unix files, which can, in turn, be used by Unix users. Even though the Windows file system and Unix file system are unlike—they have different case sensitivity, filename lengths, security, and so on—both Unix users and Windows users can access that same file with their normal file systems, in their normal way.

When Should You Use FTP?

The folks at your San Francisco office need a 50 GB file emailed to them right away. What do you do? Most email servers would reject the email because they have size limits.

Even if there’s no size limit on the server, it still would take a while to send this big file to SF. FTP to the rescue!

If you need to give someone a large file or you need to get a large file from someone, FTP is a nice choice. Smaller files (less than 5 MB) can just be sent via email if you have the bandwidth of DSL or a cable modem. However, most ISPs don’t allow files larger than 5 or 10 MB to be emailed, so FTP is an option you should consider if you are in need of sending and receiving large files (and who isn’t these days?). To use FTP, you will need to set up an FTP server on the Internet so that the files can be shared.

Besides, FTP is faster than email, which is another reason to use FTP for sending or receiving large files. In addition, because it uses TCP and is connection-oriented, if the session dies, FTP can sometimes start up where it left off. Try that with your email client!

Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP)

, answering our ubiquitous call to email, uses a spooled, or queued, method of mail delivery. Once a message has been sent to a destination, the message is spooled to a device—usually a disk. The server software at the destination posts a vigil, regularly checking the queue for messages. When it detects them, it proceeds to deliver them to their destination. SMTP is used to send mail;

POP3 or IMAP is used to receive mail.

Post Office Protocol (POP)

Post Office Protocol (POP)

gives us a storage facility for incoming mail, and the latest version is called POP3 (sound familiar?). Basically, how this protocol works is when a client device connects to a POP3 server, messages addressed to that client are released for downloading. It doesn’t allow messages to be downloaded selectively, but once they are, the client/server interaction ends and you can delete and tweak your messages locally at will. Lately we’re seeing a newer standard, IMAP, being used more and more in place of POP3. Why?

Internet Message Access Protocol, Version 4 (IMAP4)

Because

Internet Message Access Protocol (IMAP

) makes it so you get control over how you download your mail, and with it, you also gain some much-needed security. It lets you peek at the message header or download just a part of a message—you can now just nibble at the bait instead of swallowing it whole and then choking on the hook hidden inside!

With it, you can choose to store messages on the email server hierarchically and link to documents and user groups too. IMAP even gives you search commands to use to hunt for messages based on their subject, header, or content. As you can imagine, it has some serious authentication features—it actually supports the Kerberos authentication scheme that MIT developed. And yes, IMAP4 is the current version.

Transport Layer Security (TLS)

Both

Transport Layer Security (TLS)

and its forerunner,

Secure Sockets Layer (SSL)

, are cryptographic protocols that come in really handy for enabling secure online data-transfer activities like browsing the Web, instant messaging, Internet faxing, and so on. They’re so similar it’s not within the scope of this book to detail the differences between them.

SIP (VoIP)

Session Initiation Protocol (SIP)

is a hugely popular signaling protocol used to construct and deconstruct multimedia communication sessions for many things like voice and video calls, video conferencing, streaming multimedia distribution, instant messaging, presence information, and online games over the Internet.

RTP (VoIP)

Real-time Transport Protocol (RTP

) describes a packet-formatting standard for delivering audio and video over the Internet. Although initially designed as a multicast protocol, it’s now used for unicast applications too. It’s commonly employed for streaming media, videoconferencing, and push-to-talk systems—all things that make it a de facto standard in Voice over IP (VoIP) industries.

Line Printer Daemon (LPD)

The

Line Printer Daemon (LPD)

protocol is designed for printer sharing. The LPD, along with the Line Printer (LPR) program, allows print jobs to be spooled and sent to the network’s printers using TCP/IP.

X Window

Designed for client/server operations,

X Window

defines a protocol for writing client/server applications based on a graphical user interface (GUI).

The idea is to allow a program, called a client, to run on one computer and have it display things through a window server on another computer.

Simple Network Management Protocol (SNMP)

Simple Network Management Protocol (SNMP)

collects and manipulates valuable network information. It gathers data by polling the devices on the network from a management station at fixed or random intervals, requiring them to disclose certain information. When all is well, SNMP receives something called a

baseline

—a report delimiting the operational traits of a healthy network. This protocol can also stand as a watchdog over the network, quickly notifying managers of any sudden turn of events. These network watchdogs are called

agents

, and when aberrations occur, agents send an alert called a

trap

to the management station.

SNMP Versions 1, 2, and 3

SNMP versions 1 and 2 are pretty much obsolete. This doesn’t mean you won’t see them in a network at some time, but v1 is super old and, well, obsolete. SNMPv2 provided improvements, especially in performance. But one of the best additions was what was called GETBULK, which allowed a host to retrieve a large amount of data at once.

However, v2 never really caught on in the networking world. SNMPv3 is now the standard and uses both TCP and UDP, unlike v1, which used only UDP. V3 added even more security and message integrity, authentication, and encryption.

Secure Shell (SSH)

Secure Shell (SSH)

protocol sets up a secure Telnet session over a standard TCP/IP connection and is employed for doing things like logging into systems, running programs on remote systems, and moving files from one system to another. And it does all of this while maintaining a nice, strong, encrypted connection. You can think of it as the new-generation protocol that’s now used in place of rsh

and rlogin

—even Telnet.

Hypertext Transfer Protocol (HTTP)

All those snappy websites comprising a mélange of graphics, text, links, and so on—the

Hypertext Transfer Protocol (HTTP)

is making it all possible. It’s used to manage communications between web browsers and web servers and opens the right resource when you click a link, wherever that resource may actually reside.

Hypertext Transfer Protocol Secure (HTTPS)

Hypertext Transfer Protocol Secure (HTTPS)

is also known as Secure Hypertext Transfer Protocol. It uses Secure Sockets Layer (SSL).

Sometimes you’ll see it referred to as SHTTP or S-HTTP (which is an extension of HTTP and doesn’t use SSL), but no matter—as indicated, it’s a secure version of HTTP that arms you with a whole bunch of security tools for keeping transactions between a web browser and a server secure.

It’s what your browser needs to fill out forms, sign in, authenticate, and encrypt an HTTP message when you make a reservation or buy something online.

Network Time Protocol (NTP)

Kudos to Professor David Mills of the University of Delaware for coming up with this handy protocol that’s used to synchronize the clocks on our computers to one standard time source (typically, an atomic clock).

Network Time Protocol (NTP)

works by synchronizing devices to ensure that all computers on a given network agree on the time. This may sound pretty simple, but it’s very important because so many of the transactions done today are time- and date-stamped. Think about your precious databases, for one. It can mess up a server pretty badly if it’s out of sync with the machines connected to it, even by mere seconds (think

crash!

). You can’t have a transaction entered by a machine at, say, 1:50 a.m. when the server records that transaction as having occurred at 1:45 a.m. So basically, NTP works to prevent “back to the future

sans

DeLorean” from bringing down the network—very important indeed!

Network News Transfer Protocol (NNTP)

Network News Transfer Protocol (NNTP)

is how you access the Usenet news servers that hold the legion of specific message boards called

newsgroups

. As you likely know, these groups represent pretty much any special interest humans have under the sun. For instance, if you happen to be a classic car buff or a WWII aircraft enthusiast, odds are good there’re lots of newsgroups available to join based upon those interests. NNTP is specified in RFC 977. And because it’s complicated to configure a news reader program, lots of websites—even search engines—are the entities we usually depend upon to access these many and varied resources.

Secure Copy Protocol (SCP)

FTP is great. It’s a super easy, user-friendly way to transfer files—if you don’t need to transfer those files securely. That’s because when you use

FTP for transferring data, usernames and passwords get sent right along with the file request in the clear for all to see with no encryption whatsoever! Kind of like Hail Mary passes, you basically just throw them out there and hope your information doesn’t fall into the wrong hands and get intercepted.

That’s where Secure Copy Protocol (SCP) comes to your rescue—its whole purpose is to protect your precious files through SSH. It first establishes and then sustains a secure, encrypted connection between the sending and receiving hosts until file transfer is complete. When armed

with SCP, your Hail Mary pass can be caught only by your intended receiver—snap! In today’s networks, however, the more robust SFTP is used more commonly than SCP.

Lightweight Directory Access Protocol (LDAP)

If you’re the system administrator of any decent-sized network, odds are you’ve got a type of directory in place that keeps track of all your network resources, such as devices and users. But how do you access those directories? Through the Lightweight Directory Access Protocol (LDAP), that’s how. This protocol standardizes how you access directories, and its first and second inceptions are described in RFCs 1487 and 1777, respectively. There were a few glitches in those two earlier versions, so a third version—the one most commonly used today—was created to address those issues, and is described in RFC 3377.

The Requests for Comments (RFCs) form a series of notes, started in 1969, about the Internet (originally the ARPAnet). The notes discuss many aspects of computer communication; they focus on networking protocols, procedures, programs, and concepts but also include meeting notes, opinion, and sometimes humor. You can find the

RFCs by visiting www.iana.org

.

Internet Group Management Protocol (IGMP)

Internet Group Management Protocol (IGMP) is the TCP/IP protocol used for managing IP multicast sessions. It accomplishes this by sending out unique IGMP messages over the network to reveal the multicast-group landscape and to find out which hosts belong to which multicast group. The host machines in an IP network also use IGMP messages to become members of a group and to quit the group, too. IGMP messages come in seriously handy for tracking group memberships as well as active multicast streams.

Line Printer Remote (LPR)

When printing in an unblended, genuine TCP/IP environment, a combination of Line Printer (LPR) and the Line Printer Daemon (LPD) is typically what’s used to get the job done. LPD, installed on all printing devices, handles both printers and print jobs. LPR acts on the client, or sending machine, and is used to send the data from a host machine to the network’s print resource so you end up with actual printed output.

Domain Name Service (DNS)

Domain Name Service (DNS)

resolves hostnames—specifically, Internet names, such as www.routersim.com

. You don’t have to use DNS; you can just type in the IP address of any device you want to communicate with. An IP address identifies hosts on a network and the Internet as well.

However, DNS was designed to make our lives easier. Think about this: What would happen if you wanted to move your web page to a different service provider? The IP address would change and no one would know what the new one was. DNS allows you to use a domain name to specify an IP address. You can change the IP address as often as you want and no one will know the difference.

DNS is used to resolve a

fully qualified domain name (FQDN)

—for example, www.lammle.com

or todd.lammle.com

. An FQDN is a hierarchy that can logically locate a system based on its domain identifier.

If you want to resolve the name

todd

, you either must type in the FQDN of todd.lammle.com

or have a device such as a PC or router add the suffix for you. For example, on a Cisco router, you can use the command ip domain-name lammle.com

to append each request with the lammle.com

domain. If you don’t do that, you’ll have to type in the FQDN to get DNS to resolve the name.

An important thing to remember about DNS is that if you can ping a device with an IP address but cannot use its FQDN, then you might have some type of DNS configuration failure.

Dynamic Host Configuration Protocol (DHCP)/Bootstrap Protocol (BootP)

Dynamic Host Configuration Protocol (DHCP)

assigns IP addresses to hosts. It allows easier administration and works well in small to even very large network environments. All types of hardware can be used as a DHCP server, including a Cisco router.

DHCP differs from BootP in that BootP assigns an IP address to a host but the host’s hardware address must be entered manually in a BootP table. You can think of DHCP as a dynamic BootP. But remember that BootP is also used to send an operating system that a host can boot from.

DHCP can’t do that.

But there is a lot of information a DHCP server can provide to a host when the host is requesting an IP address from the DHCP server. Here’s a list of the information a DHCP server can provide:

IP address

Subnet mask

Domain name

Default gateway (routers)

DNS server address

WINS server address

A DHCP server can give us even more information than this, but the items in the list are the most common.

A client that sends out a DHCP Discover message in order to receive an IP address sends out a broadcast at both layer 2 and layer 3.

The layer-2 broadcast is all

F

s in hex, which looks like this: FF:FF:FF:FF:FF:FF.

The layer-3 broadcast is 255.255.255.255, which means all networks and all hosts.

DHCP is connectionless, which means it uses User Datagram Protocol (UDP) at the Transport layer, also known as the Host-to-Host layer, which we’ll talk about next.

In case you don’t believe me, here’s an example of output from my trusty analyzer:

Ethernet II, Src: 0.0.0.0 (00:0b:db:99:d3:5e),Dst: Broadcast(ff:ff:ff:ff:ff:ff)

Internet Protocol, Src: 0.0.0.0 (0.0.0.0),Dst: 255.255.255.255(255.255.255.255)

The Data Link and Network layers are both sending out “all hands” broadcasts saying, “Help—I don’t know my IP address!”

Broadcast addresses will be discussed in more detail at the end of this chapter.

Figure 3-3 shows the process of a client/server relationship using a DHCP connection.

Figure 3-3:

DHCP client four-step process

The following is the four-step process a client takes to receive an IP address from a DHCP server:

1.

The DHCP client broadcasts a DHCP Discover message looking for a DHCP server (Port 67).

2.

The DHCP server that received the DHCP Discover message sends a unicast DHCP Offer message back to the host

3.

The client then broadcasts to the server a DHCP Request message asking for the offered IP address and possibly other information.

4.

The server finalizes the exchange with a unicast DHCP Acknowledgment message.

DHCP Conflicts

A DHCP address conflict occurs when two hosts use the same IP address. This sounds bad, doesn’t it? Well of course it is! We’ll never even have to discuss this problem in my IPv6 chapter!

During IP address assignment, a DHCP server checks for conflicts using the ping program to test the availability of the address before it is assigned from the pool. If no host replies, then the DHCP server assumes that the IP address is not already allocated. This helps the server know that it is providing a good address, but what about the host? To provide extra protection against the all-so-terrible IP conflict issue, the host can broadcast for its own address.

A host uses something called a gratuitous ARP to help avoid a possible duplicate address. The DHCP client sends an ARP broadcast out on

the local LAN or VLAN using its newly assigned address to solve conflicts before they occur.

So, if an IP address conflict is detected, the address is removed from the DHCP pool (scope), and it is all-so-important to remember that the address will not be assigned to a host until the administrator resolves the conflict by hand.

Please see Chapter 6 to see a DHCP configuration on a Cisco router and also to find out what happens when a DHCP client is on one side of a router and the DHCP server is on the other side (different networks)!

Automatic Private IP Addressing (APIPA)

Okay, so what happens if you have a few hosts connected together with a switch or hub and you don’t have a DHCP server? You can add IP information by hand (this is called

static IP addressing

), but Windows provides what is called Automatic Private IP Addressing (APIPA), a feature of later Windows operating systems. With APIPA, clients can automatically self-configure an IP address and subnet mask (basic IP information that hosts use to communicate) when a DHCP server isn’t available. The IP address range for APIPA is 169.254.0.1 through 169.254.255.254. The client also configures itself with a default class B subnet mask of 255.255.0.0.

However, when you’re in your corporate network working and you have a DHCP server running, and your host shows that it is using this IP address range, this means that either your DHCP client on the host is not working or the server is down or can’t be reached because of a network issue. I don’t know anyone who’s seen a host in this address range and has been happy about it!

Now, let’s take a look at the Transport layer, or what the DoD calls the Host-to-Host layer.

The Host-to-Host Layer Protocols

The main purpose of the Host-to-Host layer is to shield the upper-layer applications from the complexities of the network. This layer says to the upper layer, “Just give me your data stream, with any instructions, and I’ll begin the process of getting your information ready to send.”

The following sections describe the two protocols at this layer:

Transmission Control Protocol (TCP)

User Datagram Protocol (UDP)

In addition, we’ll look at some of the key host-to-host protocol concepts, as well as the port numbers.

Remember, this is still considered layer 4, and Cisco really likes the way layer 4 can use acknowledgments, sequencing, and flow control.

Transmission Control Protocol (TCP)

Transmission Control Protocol (TCP)

takes large blocks of information from an application and breaks them into segments. It numbers and sequences each segment so that the destination’s TCP stack can put the segments back into the order the application intended. After these segments are sent, TCP (on the transmitting host) waits for an acknowledgment of the receiving end’s TCP virtual circuit session, retransmitting those that aren’t acknowledged.

Before a transmitting host starts to send segments down the model, the sender’s TCP stack contacts the destination’s TCP stack to establish a connection. What is created is known as a

virtual circuit

. This type of communication is called

connection-oriented

. During this initial handshake, the two TCP layers also agree on the amount of information that’s going to be sent before the recipient’s TCP sends back an acknowledgment.

With everything agreed upon in advance, the path is paved for reliable communication to take place.

TCP is a full-duplex, connection-oriented, reliable, and accurate protocol, but establishing all these terms and conditions, in addition to error checking, is no small task. TCP is very complicated and, not surprisingly, costly in terms of network overhead. And since today’s networks are much more reliable than those of yore, this added reliability is often unnecessary. Most programmers use TCP because it removes a lot of programming work; however, real-time video and VoIP use UDP because they can’t afford the overhead.

TCP Segment Format

Since the upper layers just send a data stream to the protocols in the Transport layers, I’ll demonstrate how TCP segments a data stream and prepares it for the Internet layer. When the Internet layer receives the data stream, it routes the segments as packets through an internetwork. The segments are handed to the receiving host’s Host-to-Host layer protocol, which rebuilds the data stream to hand to the upper-layer applications or protocols.

Figure 3-4 shows the TCP segment format. The figure shows the different fields within the TCP header.

The TCP header is 20 bytes long, or up to 24 bytes with options. You need to understand what each field in the TCP segment is:

Source port

The port number of the application on the host sending the data. (Port numbers will be explained a little later in this section.)

Destination port

The port number of the application requested on the destination host.

Sequence number

A number used by TCP that puts the data back in the correct order or retransmits missing or damaged data, a process called

sequencing

.

Acknowledgment number

The TCP octet that is expected next.

Header length

The number of 32-bit words in the TCP header. This indicates where the data begins. The TCP header (even one including options) is an integral number of 32 bits in length.

Reserved

Always set to zero.

Code bits/flags

Control functions used to set up and terminate a session.

Window

The window size the sender is willing to accept, in octets.

Checksum

The cyclic redundancy check (CRC), because TCP doesn’t trust the lower layers and checks everything. The CRC checks the header and data fields.

Urgent

A valid field only if the Urgent pointer in the code bits is set. If so, this value indicates the offset from the current sequence number, in octets, where the segment of non-urgent data begins.

Options

May be 0 or a multiple of 32 bits, if any. What this means is that no options have to be present (option size of 0). However, if any options are used that do not cause the option field to total a multiple of 32 bits, padding of 0s must be used to make sure the data begins on a

32-bit boundary.

Data

Handed down to the TCP protocol at the Transport layer, which includes the upper-layer headers.

Figure 3-4:

TCP segment format

Let’s take a look at a TCP segment copied from a network analyzer:

TCP - Transport Control Protocol

Source Port: 5973

Destination Port: 23

Sequence Number: 1456389907

Ack Number: 1242056456

Offset: 5

Reserved: %000000

Code: %011000

Ack is valid

Push Request

Window: 61320

Checksum: 0x61a6

Urgent Pointer: 0

No TCP Options

TCP Data Area:

vL.5.+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35 11

2b 19 35 +. 11 2b 19

Frame Check Sequence: 0x0d00000f

Did you notice that everything I talked about earlier is in the segment? As you can see from the number of fields in the header, TCP creates a lot of overhead. Application developers may opt for efficiency over reliability to save overhead, so User Datagram Protocol was also defined at the

Transport layer as an alternative.

User Datagram Protocol (UDP)

If you were to compare

User Datagram Protocol (UDP)

with TCP, the former is basically the scaled-down economy model that’s sometimes referred to as a thin protocol. Like a thin person on a park bench, a thin protocol doesn’t take up a lot of room—or in this case, much bandwidth on a network.

UDP doesn’t offer all the bells and whistles of TCP either, but it does do a fabulous job of transporting information that doesn’t require reliable delivery—and it does so using far fewer network resources. (UDP is covered thoroughly in Request for Comments 768.)

There are some situations in which it would definitely be wise for developers to opt for UDP rather than TCP. One circumstance is when reliability is already handled at the Process/Application layer. Network File System (NFS) handles its own reliability issues, making the use of TCP both impractical and redundant. But ultimately, it’s up to the application developer to decide whether to use UDP or TCP, not the user who wants to transfer data faster.

UDP does

not

sequence the segments and does not care in which order the segments arrive at the destination. Rather, UDP sends the segments off and forgets about them. It doesn’t follow through, check up on them, or even allow for an acknowledgment of safe arrival—complete abandonment. Because of this, it’s referred to as an unreliable protocol. This does not mean that UDP is ineffective, only that it doesn’t handle

issues of reliability.

Further, UDP doesn’t create a virtual circuit, nor does it contact the destination before delivering information to it. Because of this, it’s also considered a

connectionless

protocol. Since UDP assumes that the application will use its own reliability method, it doesn’t use any. This gives an application developer a choice when running the Internet Protocol stack: TCP for reliability or UDP for faster transfers.

So, it is important to remember how this works because if the segments arrive out of order (very common in IP networks), they’ll just be passed up to the next OSI (DoD) layer in whatever order they’re received, possibly resulting in some seriously garbled data. On the other hand, TCP sequences the segments so they get put back together in exactly the right order—something UDP just can’t do.

UDP Segment Format

Figure 3-5 clearly illustrates UDP’s markedly low overhead as compared to TCP’s hungry usage. Look at the figure carefully—can you see that

UDP doesn’t use windowing or provide for acknowledgments in the UDP header?

Figure 3-5:

UDP segment

It’s important for you to understand what each field in the UDP segment is:

Source port

Port number of the application on the host sending the data

Destination port

Port number of the application requested on the destination host

Length

Length of UDP header and UDP data

Checksum

Checksum of both the UDP header and UDP data fields

Data

Upper-layer data

UDP, like TCP, doesn’t trust the lower layers and runs its own CRC. Remember that the Frame Check Sequence (FCS) is the field that houses the CRC, which is why you can see the FCS information.

The following shows a UDP segment caught on a network analyzer:

UDP - User Datagram Protocol

Source Port: 1085

Destination Port: 5136

Length: 41

Checksum: 0x7a3c

UDP Data Area:

..Z......00 01 5a 96 00 01 00 00 00 00 00 11 0000 00

...C..2._C._C 2e 03 00 43 02 1e 32 0a 00 0a 00 80 43 00 80

Frame Check Sequence: 0x00000000

Notice that low overhead! Try to find the sequence number, ack number, and window size in the UDP segment. You can’t because they just aren’t there!

Key Concepts of Host-to-Host Protocols

Since you’ve seen both a connection-oriented (TCP) and connectionless (UDP) protocol in action, it would be good to summarize the two here.

Table 3-1 highlights some of the key concepts that you should keep in mind regarding these two protocols. You should memorize this table.

Table 3-1:

Key features of TCP and UDP

TCP UDP

Sequenced

Reliable

Unsequenced

Unreliable

Connection-oriented Connectionless

Virtual circuit Low overhead

Acknowledgments No acknowledgment

Windowing flow control No windowing or flow control of any type

A telephone analogy could really help you understand how TCP works. Most of us know that before you speak to someone on a phone, you must first establish a connection with that other person—wherever they are. This is like a virtual circuit with the TCP protocol. If you were giving someone important information during your conversation, you might say, “You know?” or ask, “Did you get that?” Saying something like this is a lot like a TCP acknowledgment—it’s designed to get you verification. From time to time (especially on cell phones), people also ask, “Are you still there?” They end their conversations with a “Goodbye” of some kind, putting closure on the phone call. TCP also performs these types of functions.

Alternately, using UDP is like sending a postcard. To do that, you don’t need to contact the other party first. You simply write your message, address the postcard, and mail it. This is analogous to UDP’s connectionless orientation. Since the message on the postcard is probably not a matter of life or death, you don’t need an acknowledgment of its receipt. Similarly, UDP does not involve acknowledgments.

Let’s take a look at another figure, one that includes TCP, UDP, and the applications associated to each protocol:

Figure 3-6 (in the next

section).

Port Numbers

TCP and UDP must use

port numbers

to communicate with the upper layers because they’re what keep track of different conversations crossing the network simultaneously. Originating-source port numbers are dynamically assigned by the source host and will equal some number starting at

1024. 1023 and below are defined in RFC 3232 (or just see www.iana.org

), which discusses what are called well-known port numbers.

Virtual circuits that don’t use an application with a well-known port number are assigned port numbers randomly from a specific range instead.

These port numbers identify the source and destination application or process in the TCP segment.

Figure 3-6 illustrates how both TCP and UDP use port numbers.

Figure 3-6:

Port numbers for TCP and UDP

The different port numbers that can be used are explained next:

Numbers below 1024 are considered well-known port numbers and are defined in RFC 3232.

Numbers 1024 and above are used by the upper layers to set up sessions with other hosts and by TCP and UDP to use as source and destination addresses in the segment.

In the following sections, we’ll take a look at an analyzer output showing a TCP session.

TCP Session: Source Port

The following listing shows a TCP session captured with my analyzer software:

TCP - Transport Control Protocol

Source Port: 5973

Destination Port: 23

Sequence Number: 1456389907

Ack Number: 1242056456

Offset: 5

Reserved: %000000

Code: %011000

Ack is valid

Push Request

Window: 61320

Checksum: 0x61a6

Urgent Pointer: 0

No TCP Options

TCP Data Area:

vL.5.+.5.+.5.+.5 76 4c 19 35 11 2b 19 35 11 2b 19 35 11

2b 19 35 +. 11 2b 19

Frame Check Sequence: 0x0d00000f

Notice that the source host makes up the source port, which in this case is 5973. The destination port is 23, which is used to tell the receiving host the purpose of the intended connection (Telnet).

By looking at this session, you can see that the source host makes up the source port by using numbers from 1024 to 65535. But why does the source make up a port number? To differentiate between sessions with different hosts, my friend. How would a server know where information is coming from if it didn’t have a different number from a sending host? TCP and the upper layers don’t use hardware and logical addresses to understand the sending host’s address as the Data Link and Network layer protocols do. Instead, they use port numbers.

TCP Session: Destination Port

You’ll sometimes look at an analyzer and see that only the source port is above 1024 and the destination port is a well-known port, as shown in the following trace:

TCP - Transport Control Protocol

Source Port: 1144

Destination Port: 80 World Wide Web HTTP

Sequence Number: 9356570

Ack Number: 0

Offset: 7

Reserved: %000000

Code: %000010

Synch Sequence

Window: 8192

Checksum: 0x57E7

Urgent Pointer: 0

TCP Options:

Option Type: 2 Maximum Segment Size

Length: 4

MSS: 536

Option Type: 1 No Operation

Option Type: 1 No Operation

Option Type: 4

Length: 2

Opt Value:

No More HTTP Data

Frame Check Sequence: 0x43697363

And sure enough, the source port is over 1024, but the destination port is 80, or HTTP service. The server, or receiving host, will change the destination port if it needs to.

In the preceding trace, a “syn” packet is sent to the destination device. The syn sequence is what’s telling the remote destination device that it wants to create a session.

TCP Session: Syn Packet Acknowledgment

The next trace shows an acknowledgment to the syn packet:

TCP - Transport Control Protocol

Source Port: 80 World Wide Web HTTP

Destination Port: 1144

Sequence Number: 2873580788

Ack Number: 9356571

Offset: 6

Reserved: %000000

Code: %010010

Ack is valid

Synch Sequence

Window: 8576

Checksum: 0x5F85

Urgent Pointer: 0

TCP Options:

Option Type: 2 Maximum Segment Size

Length: 4

MSS: 1460

No More HTTP Data

Frame Check Sequence: 0x6E203132

Notice the

Ack is valid

, which means that the source port was accepted and the device agreed to create a virtual circuit with the originating host.

And here again, you can see that the response from the server shows that the source is 80 and the destination is the 1144 sent from the originating host—all’s well.

Table 3-2 gives you a list of the typical applications used in the TCP/IP suite, their well-known port numbers, and the Transport layer protocols used by each application or process. It’s important that you study and memorize this table.

Table 3-2:

Key protocols that use TCP and UDP

TCP UDP

Telnet 23 SNMP 161

SMTP 25 TFTP 69

HTTP 80 DNS 53

FTP 20, 21 BooTPS/DHCP 67

DNS 53

HTTPS 443

SSH 22

POP3 110

NTP 123

IMAP4 143

Notice that DNS uses both TCP and UDP. Whether it opts for one or the other depends on what it’s trying to do. Even though it’s not the only application that can use both protocols, it’s certainly one that you should remember in your studies.

What makes TCP reliable is sequencing, acknowledgments, and flow control (windowing). UDP does not have reliability.

The Internet Layer Protocols

In the DoD model, there are two main reasons for the Internet layer’s existence: routing and providing a single network interface to the upper layers.

None of the other upper- or lower-layer protocols have any functions relating to routing—that complex and important task belongs entirely to the

Internet layer. The Internet layer’s second duty is to provide a single network interface to the upper-layer protocols. Without this layer, application programmers would need to write “hooks” into every one of their applications for each different Network Access protocol. This would not only be a pain in the neck, but it would lead to different versions of each application—one for Ethernet, another one for wireless, and so on. To prevent this, IP provides one single network interface for the upper-layer protocols. That accomplished, it’s then the job of IP and the various Network Access protocols to get along and work together.

All network roads don’t lead to Rome—they lead to IP. And all the other protocols at this layer, as well as all those at the upper layers, use it.

Never forget that. All paths through the DoD model go through IP. The following sections describe the protocols at the Internet layer:

Internet Protocol (IP)

Internet Control Message Protocol (ICMP)

Address Resolution Protocol (ARP)

Reverse Address Resolution Protocol (RARP)

Proxy ARP

Gratuitous ARP

Internet Protocol (IP)

Internet Protocol (IP)

essentially is the Internet layer. The other protocols found here merely exist to support it. IP holds the big picture and could be said to “see all,” in that it’s aware of all the interconnected networks. It can do this because all the machines on the network have a software, or logical, address called an IP address, which I’ll cover more thoroughly later in this chapter.

IP looks at each packet’s address. Then, using a routing table, it decides where a packet is to be sent next, choosing the best path. The protocols of the Network Access layer at the bottom of the DoD model don’t possess IP’s enlightened scope of the entire network; they deal only with physical links (local networks).

Identifying devices on networks requires answering these two questions: Which network is it on? And what is its ID on that network? The first answer is the

software address

, or

logical address

(the correct street). The second answer is the hardware address (the correct mailbox). All hosts on a network have a logical ID called an IP address. This is the software, or logical, address and contains valuable encoded information, greatly simplifying the complex task of routing. (IP is discussed in RFC 791.)

IP receives segments from the Host-to-Host layer and fragments them into datagrams (packets) if necessary. IP then reassembles datagrams back into segments on the receiving side. Each datagram is assigned the IP address of the sender and of the recipient. Each router (layer-3 device) that receives a datagram makes routing decisions based on the packet’s destination IP address.

Figure 3-7 shows an IP header. This will give you an idea of what the IP protocol has to go through every time user data is sent from the upper layers and is to be sent to a remote network.

The following fields make up the IP header:

Version

IP version number.

Header length

Header length (HLEN) in 32-bit words.

Priority and Type of Service

Type of Service tells how the datagram should be handled. The first 3 bits are the priority bits which is now called the differentiated services bits.

Total length

Length of the packet including header and data.

Identification

Unique IP-packet value used to differentiate fragmented packets from different datagrams.

Flags

Specifies whether fragmentation should occur.

Fragment offset

Provides fragmentation and reassembly if the packet is too large to put in a frame. It also allows different maximum transmission units (MTUs) on the Internet.

Time To Live

The time to live is set into a packet when it is originally generated. If it doesn’t get to where it wants to go before the TTL expires, boom—it’s gone. This stops IP packets from continuously circling the network looking for a home.

Protocol

Port of upper-layer protocol (TCP is port 6 or UDP is port 17 ). Also supports Network layer protocols, like ARP and ICMP (this can be called Type field in some analyzers). We’ll talk about this field in more detail in a minute.

Header checksum

Cyclic redundancy check (CRC) on header only.

Source IP address

32-bit IP address of sending station.

Destination IP address

32-bit IP address of the station this packet is destined for.

Options

Used for network testing, debugging, security, and more.

Data

After the IP option field will be the upper-layer data.

Figure 3-7:

IP header

Here’s a snapshot of an IP packet caught on a network analyzer (notice that all the header information discussed previously appears here):

IP Header - Internet Protocol Datagram

Version: 4

Header Length: 5

Precedence: 0

Type of Service: %000

Unused: %00

Total Length: 187

Identifier: 22486

Fragmentation Flags: %010 Do Not Fragment

Fragment Offset: 0

Time To Live: 60

IP Type: 0x06 TCP

Header Checksum: 0xd031

Source IP Address: 10.7.1.30

Dest. IP Address: 10.7.1.10

No Internet Datagram Options

The Type field—it’s typically a Protocol field, but this analyzer sees it as an IP Type field—is important. If the header didn’t carry the protocol information for the next layer, IP wouldn’t know what to do with the data carried in the packet. The preceding example tells IP to hand the segment to

TCP.

Figure 3-8 demonstrates how the Network layer sees the protocols at the Transport layer when it needs to hand a packet to the upper-layer protocols.

Figure 3-8:

The Protocol field in an IP header

In this example, the Protocol field tells IP to send the data to either TCP port 6 or UDP port 17. But it will only be UDP or TCP if the data is part of a data stream headed for an upper-layer service or application. It could just as easily be destined for Internet Control Message Protocol (ICMP),

Address Resolution Protocol (ARP), or some other type of Network layer protocol.

Table 3-3 is a list of some other popular protocols that can be specified in the Protocol field.

Table 3-3:

Possible protocols found in the Protocol field of an IP header

Protocol Protocol Number

ICMP 1

IP in IP (tunneling) 4

TCP

IGRP

6

9

UDP

EIGRP

17

88

OSPF

IPv6

GRE

89

41

47

Layer 2 tunnel (L2TP) 115

You can find a complete list of Protocol field numbers at www.iana.org/assignments/protocol-numbers

.

Internet Control Message Protocol (ICMP)

Internet Control Message Protocol (ICMP)

works at the Network layer and is used by IP for many different services. ICMP is a management protocol and messaging service provider for IP. Its messages are carried as IP datagrams. RFC 1256 is an annex to ICMP, which affords hosts extended capability in discovering routes to gateways.

ICMP packets have the following characteristics:

They can provide hosts with information about network problems.

They are encapsulated within IP datagrams.

The following are some common events and messages that ICMP relates to:

Destination Unreachable

If a router can’t send an IP datagram any further, it uses ICMP to send a message back to the sender, advising it of the situation. For example, take a look at Figure 3-9 , which shows that interface E0 of the Lab_B router is down.

When Host A sends a packet destined for Host B, the Lab_B router will send an ICMP destination unreachable message back to the sending device (Host A in this example).

Buffer Full/Source Quence

If a router’s memory buffer for receiving incoming datagrams is full, it will use ICMP to send out this message until the congestion abates.

Hops/Time Exceeded

Each IP datagram is allotted a certain number of routers, called hops, to pass through. If it reaches its limit of hops before arriving at its destination, the last router to receive that datagram deletes it. The executioner router then uses ICMP to send an obituary message, informing the sending machine of the demise of its datagram.

Ping

Packet Internet Groper (Ping) uses ICMP echo request and reply messages to check the physical and logical connectivity of machines on an internetwork.

Traceroute

Using ICMP time-outs, Traceroute is used to discover the path a packet takes as it traverses an internetwork.

Figure 3-9:

ICMP error message is sent to the sending host from the remote router.

Both Ping and Traceroute (also just called Trace; Microsoft Windows uses tracert) allow you to verify address configurations in your internetwork.

The following data is from a network analyzer catching an ICMP echo request:

Flags: 0x00

Status: 0x00

Packet Length: 78

Timestamp: 14:04:25.967000 12/20/03

Ethernet Header

Destination: 00:a0:24:6e:0f:a8

Source: 00:80:c7:a8:f0:3d

Ether-Type: 08-00 IP

IP Header - Internet Protocol Datagram

Version: 4

Header Length: 5

Precedence: 0

Type of Service: %000

Unused: %00

Total Length: 60

Identifier: 56325

Fragmentation Flags: %000

Fragment Offset: 0

Time To Live: 32

IP Type: 0x01 ICMP

Header Checksum: 0x2df0

Source IP Address: 100.100.100.2

Dest. IP Address: 100.100.100.1

No Internet Datagram Options

ICMP - Internet Control Messages Protocol

ICMP Type: 8 Echo Request

Code: 0

Checksum: 0x395c

Identifier: 0x0300

Sequence Number: 4352

ICMP Data Area:

abcdefghijklmnop 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70

qrstuvwabcdefghi 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69

Frame Check Sequence: 0x00000000

Notice anything unusual? Did you catch the fact that even though ICMP works at the Internet (Network) layer, it still uses IP to do the Ping request? The Type field in the IP header is

0x01

, which specifies that the data we’re carrying is owned by the ICMP protocol. Remember, just as all roads lead to Rome, all segments or data

must

go through IP!

The Ping program uses the alphabet in the data portion of the packet as just a payload, typically around 100 bytes by default, unless, of course, you are pinging from a

Windows device, which thinks the alphabet stops at the letter W (and doesn’t include X, Y, or Z) and then starts at A again. Go figure!

If you remember reading about the Data Link layer and the different frame types in Chapter 2, you should be able to look at the preceding trace and tell what type of Ethernet frame this is. The only fields are destination hardware address, source hardware address, and Ether-Type. The only frame that uses an Ether-Type field exclusively is an Ethernet_II frame.

But before we get into the ARP protocol, let’s take another look at ICMP in action.

Figure 3-10 shows an internetwork (it has a router, so it’s an internetwork, right?).

Server1 (10.1.2.2) telnets to 10.1.1.5 from a DOS prompt. What do you think Server1 will receive as a response? Since Server1 will send the

Telnet data to the default gateway, which is the router, the router will drop the packet because there isn’t a network 10.1.1.0 in the routing table.

Because of this, Server1 will receive a destination unreachable back from ICMP.

Address Resolution Protocol (ARP)

Address Resolution Protocol (ARP)

finds the hardware address of a host from a known IP address. Here’s how it works: When IP has a datagram to send, it must inform a Network Access protocol, such as Ethernet or wireless, of the destination’s hardware address on the local network. (It has already been informed by upper-layer protocols of the destination’s IP address.) If IP doesn’t find the destination host’s hardware address in the

ARP cache, it uses ARP to find this information.

Figure 3-10:

ICMP in action

As IP’s detective, ARP interrogates the local network by sending out a broadcast asking the machine with the specified IP address to reply with its hardware address. So basically, ARP translates the software (IP) address into a hardware address—for example, the destination machine’s

Ethernet adapter address—and from it, deduces its whereabouts on the LAN by broadcasting for this address. Figure 3-11 shows how an ARP

looks to a local network.

ARP resolves IP addresses to Ethernet (MAC) addresses.

The following trace shows an ARP broadcast—notice that the destination hardware address is unknown and is all

F

s in hex (all 1s in binary)— and is a hardware address broadcast:

Flags: 0x00

Status: 0x00

Packet Length: 64

Timestamp: 09:17:29.574000 12/06/03

Ethernet Header

Destination: FF:FF:FF:FF:FF:FF Ethernet Broadcast

Source: 00:A0:24:48:60:A5

Protocol Type: 0x0806 IP ARP

ARP - Address Resolution Protocol

Hardware: 1 Ethernet (10Mb)

Protocol: 0x0800 IP

Hardware Address Length: 6

Protocol Address Length: 4

Operation: 1 ARP Request

Sender Hardware Address: 00:A0:24:48:60:A5

Sender Internet Address: 172.16.10.3

Target Hardware Address: 00:00:00:00:00:00 (ignored)

Target Internet Address: 172.16.10.10

Extra bytes (Padding):

................ 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A 0A

0A 0A 0A 0A 0A

Frame Check Sequence: 0x00000000

Figure 3-11:

Local ARP broadcast

Reverse Address Resolution Protocol (RARP)

When an IP machine happens to be a diskless machine, it has no way of initially knowing its IP address. But it does know its MAC address.

Reverse Address Resolution Protocol (RARP)

, as shown in Figure 3-12 , discovers the identity of the IP address for diskless machines by sending out a packet that includes its MAC address and a request for the IP address assigned to that MAC address. A designated machine, called a

RARP server

, responds with the answer and the identity crisis is over. RARP uses the information it does know about the machine’s MAC address to learn its IP address and complete the machine’s ID portrait.

Figure 3-12:

RARP broadcast example

RARP resolves Ethernet (MAC) addresses to IP addresses.

Proxy Address Resolution Protocol (Proxy ARP)

On a network, your hosts can’t have more than one default gateway configured. Think about this…What if the default gateway (router) happens to go down? The host won’t just start sending to another router automatically—you’ve got to reconfigure that host. But Proxy ARP can actually help machines on a subnet reach remote subnets without configuring routing or even a default gateway.

One advantage of using Proxy ARP is that it can be added to a single router on a network without disturbing the routing tables of all the other routers that live there too. But there’s a serious downside to using Proxy ARP. Using Proxy ARP will definitely increase the amount of traffic on your network segment, and hosts will have a larger ARP table than usual in order to handle all the IP-to-MAC-address mappings. And Proxy ARP is configured on all Cisco routers by default—you should disable it if you don’t think you’re going to use it.

One last thought on Proxy ARP: Proxy ARP isn’t really a separate protocol. It is a service run by routers on behalf of other devices (usually PCs) that are separated from their query to another device by a router, although they think they share the subnet with the remote device. This lets the router provide its own MAC address in response to ARP queries attempting to resolve a distant IP address to a functional MAC address.

If you can afford it, use Cisco’s Hot Standby Router Protocol (HSRP) instead. It means you have to buy two or more of your Cisco device(s), but it is well worth it. Check out the

Cisco website for more information on HSRP.

IP Addressing

One of the most important topics in any discussion of TCP/IP is IP addressing. An

IP address

is a numeric identifier assigned to each machine on an IP network. It designates the specific location of a device on the network.

An IP address is a software address, not a hardware address—the latter is hard-coded on a network interface card (NIC) and used for finding hosts on a local network. IP addressing was designed to allow hosts on one network to communicate with a host on a different network regardless of the type of LANs the hosts are participating in.

Before we get into the more complicated aspects of IP addressing, you need to understand some of the basics. First I’m going to explain some of the fundamentals of IP addressing and its terminology. Then you’ll learn about the hierarchical IP addressing scheme and private IP addresses.

IP Terminology

Throughout this chapter you’ll learn several important terms vital to your understanding of the Internet Protocol. Here are a few to get you started:

Bit

A

bit

is one digit, either a 1 or a 0.

Byte

A

byte

is 7 or 8 bits, depending on whether parity is used. For the rest of this chapter, always assume a byte is 8 bits.

Octet

An octet, made up of 8 bits, is just an ordinary 8-bit binary number. In this chapter, the terms

byte

and

octet

are completely interchangeable.

Network address

This is the designation used in routing to send packets to a remote network—for example, 10.0.0.0, 172.16.0.0, and

192.168.10.0.

Broadcast address

The address used by applications and hosts to send information to all nodes on a network is called the

broadcast address

. Examples include 255.255.255.255, which is any network, all nodes; 172.16.255.255, which is all subnets and hosts on network

172.16.0.0; and 10.255.255.255, which broadcasts to all subnets and hosts on network 10.0.0.0.

The Hierarchical IP Addressing Scheme

An IP address consists of 32 bits of information. These bits are divided into four sections, referred to as octets or bytes, each containing 1 byte (8 bits). You can depict an IP address using one of three methods:

Dotted-decimal, as in 172.16.30.56

Binary, as in 10101100.00010000.00011110.00111000

Hexadecimal, as in AC.10.1E.38

All these examples truly represent the same IP address. Hexadecimal isn’t used as often as dotted-decimal or binary when IP addressing is discussed, but you still might find an IP address stored in hexadecimal in some programs. The Windows Registry is a good example of a program that stores a machine’s IP address in hex.

The 32-bit IP address is a structured or hierarchical address, as opposed to a flat or nonhierarchical address. Although either type of addressing scheme could have been used,

hierarchical addressing

was chosen for a good reason. The advantage of this scheme is that it can handle a large number of addresses, namely 4.3 billion (a 32-bit address space with two possible values for each position—either 0 or 1—gives you 2

32

, or

4,294,967,296). The disadvantage of the flat addressing scheme, and the reason it’s not used for IP addressing, relates to routing. If every address were unique, all routers on the Internet would need to store the address of each and every machine on the Internet. This would make efficient routing

impossible, even if only a fraction of the possible addresses were used.

The solution to this problem is to use a two- or three-level hierarchical addressing scheme that is structured by network and host or by network, subnet, and host.

This two- or three-level scheme is comparable to a telephone number. The first section, the area code, designates a very large area. The second section, the prefix, narrows the scope to a local calling area. The final segment, the customer number, zooms in on the specific connection. IP addresses use the same type of layered structure. Rather than all 32 bits being treated as a unique identifier, as in flat addressing, a part of the address is designated as the network address and the other part is designated as either the subnet and host or just the node address.

In the following sections, I’m going to discuss IP network addressing and the different classes of address we can use to address our networks.

Network Addressing

The

network address

(which can also be called the network number) uniquely identifies each network. Every machine on the same network shares that network address as part of its IP address. In the IP address 172.16.30.56, for example, 172.16 is the network address.

The

node address

is assigned to, and uniquely identifies, each machine on a network. This part of the address must be unique because it identifies a particular machine—an individual—as opposed to a network, which is a group. This number can also be referred to as a

host address

.

In the sample IP address 172.16.30.56, the 30.56 is the node address.

The designers of the Internet decided to create classes of networks based on network size. For the small number of networks possessing a very large number of nodes, they created the rank

Class A network

. At the other extreme is the

Class C network

, which is reserved for the numerous networks with a small number of nodes. The class distinction for networks between very large and very small is predictably called the

Class B network

.

Subdividing an IP address into a network and node address is determined by the class designation of one’s network.

Figure 3-13 summarizes the three classes of networks—a subject I’ll explain in much greater detail throughout this chapter.

Figure 3-13:

Summary of the three classes of networks

To ensure efficient routing, Internet designers defined a mandate for the leading-bits section of the address for each different network class. For example, since a router knows that a Class A network address always starts with a 0, the router might be able to speed a packet on its way after reading only the first bit of its address. This is where the address schemes define the difference between a Class A, a Class B, and a Class C address. In the next sections, I’ll discuss the differences between these three classes, followed by a discussion of the Class D and Class E addresses (Classes A, B, and C are the only ranges that are used to address hosts in our networks).

Network Address Range: Class A

The designers of the IP address scheme said that the first bit of the first byte in a Class A network address must always be off, or 0. This means a

Class A address must be between 0 and 127 in the first byte, inclusive.

Consider the following network address:

0

xxxxxxx

If we turn the other 7 bits all off and then turn them all on, we’ll find the Class A range of network addresses:

0

0000000 = 0

0

1111111 = 127

So, a Class A network is defined in the first octet between 0 and 127, and it can’t be less or more. (Yes, I know 0 and 127 are not valid in a Class

A network. I’ll talk about reserved addresses in a minute.)

Network Address Range: Class B

In a Class B network, the RFCs state that the first bit of the first byte must always be turned on but the second bit must always be turned off. If you turn the other 6 bits all off and then all on, you will find the range for a Class B network:

10

000000 = 128

10

111111 = 191

As you can see, a Class B network is defined when the first byte is configured from 128 to 191.

Network Address Range: Class C

For Class C networks, the RFCs define the first 2 bits of the first octet as always turned on, but the third bit can never be on. Following the same process as the previous classes, convert from binary to decimal to find the range. Here’s the range for a Class C network:

110

00000 = 192

110

11111 = 223

So, if you see an IP address that starts at 192 and goes to 223, you’ll know it is a Class C IP address.

Network Address Ranges: Classes D and E

The addresses between 224 to 255 are reserved for Class D and E networks. Class D (224–239) is used for multicast addresses and Class E

(240–255) for scientific purposes, but I’m not going into these types of addresses in this book (and you don’t need to know them).

Network Addresses: Special Purpose

Some IP addresses are reserved for special purposes, so network administrators can’t ever assign these addresses to nodes.

Table 3-4 lists the members of this exclusive little club and the reasons why they’re included in it.

Table 3-4:

Reserved IP addresses

Address Function

Network address of all 0s

Network address of all 1s

Network 127.0.0.1

Node address of all 0s

Node address of all 1s

Interpreted to mean “this network or segment.”

Interpreted to mean “all networks.”

Reserved for loopback tests. Designates the local node and allows that node to send a test packet to itself without generating network traffic.

Interpreted to mean “network address” or any host on a specified network.

Interpreted to mean “all nodes” on the specified network; for example, 128.2.255.255 means “all nodes” on network 128.2 (Class

B address).

Used by Cisco routers to designate the default route. Could also mean “any network.” Entire IP address set to all 0s

Entire IP address set to all 1s (same as

255.255.255.255)

Broadcast to all nodes on the current network; sometimes called an “all 1s broadcast” or limited broadcast.

Class A Addresses

In a Class A network address, the first byte is assigned to the network address and the three remaining bytes are used for the node addresses.

The Class A format is as follows:

network.node.node.node

For example, in the IP address 49.22.102.70, the 49 is the network address and 22.102.70 is the node address. Every machine on this particular network would have the distinctive network address of 49.

Class A network addresses are 1 byte long, with the first bit of that byte reserved and the 7 remaining bits available for manipulation

(addressing). As a result, the maximum number of Class A networks that can be created is 128. Why? Because each of the 7 bit positions can be either a 0 or a 1, thus 2

7

, or 128.

To complicate matters further, the network address of all 0s (0000 0000) is reserved to designate the default route (see

Table 3-4 in the previous section). Additionally, the address 127, which is reserved for diagnostics, can’t be used either, which means that you can really only use the numbers 1 to 126 to designate Class A network addresses. This means the actual number of usable Class A network addresses is 128 minus 2, or 126.

The IP address 127.0.0.1 is used to test the IP stack on an individual node and cannot be used as a valid host address. However, the loopback address creates a shortcut method for TCP/IP applications and services that run on the same device to communicate with each other.

Each Class A address has 3 bytes (24-bit positions) for the node address of a machine. This means there are 2

24

—or 16,777,216—unique combinations and, therefore, precisely that many possible unique node addresses for each Class A network. Because node addresses with the two patterns of all 0s and all 1s are reserved, the actual maximum usable number of nodes for a Class A network is 2

24

minus 2, which equals

16,777,214. Either way, that’s a huge number of hosts on a network segment!

Class A Valid Host IDs

Here’s an example of how to figure out the valid host IDs in a Class A network address:

All host bits off is the network address: 10.0.0.0.

All host bits on is the broadcast address: 10.255.255.255.

The valid hosts are the numbers in between the network address and the broadcast address: 10.0.0.1 through 10.255.255.254. Notice that 0s and 255s can be valid host IDs. All you need to remember when trying to find valid host addresses is that the host bits can’t all be turned off or all be on at the same time.

Class B Addresses

In a Class B network address, the first 2 bytes are assigned to the network address and the remaining 2 bytes are used for node addresses. The format is as follows:

network.network.node.node

For example, in the IP address 172.16.30.56, the network address is 172.16 and the node address is 30.56.

With a network address being 2 bytes (8 bits each), there would be 2

16

unique combinations. But the Internet designers decided that all Class B network addresses should start with the binary digit 1, then 0. This leaves 14 bit positions to manipulate, therefore 16,384 (that is, 2

14

) unique

Class B network addresses.

A Class B address uses 2 bytes for node addresses. This is 2

16

minus the two reserved patterns (all 0s and all 1s), for a total of 65,534 possible node addresses for each Class B network.

Class B Valid Host IDs

Here’s an example of how to find the valid hosts in a Class B network:

All host bits turned off is the network address: 172.16.0.0.

All host bits turned on is the broadcast address: 172.16.255.255.

The valid hosts would be the numbers in between the network address and the broadcast address: 172.16.0.1 through 172.16.255.254.

Class C Addresses

The first 3 bytes of a Class C network address are dedicated to the network portion of the address, with only 1 measly byte remaining for the node address. Here’s the format:

network.network.network.node

Using the example IP address 192.168.100.102, the network address is 192.168.100 and the node address is 102.

In a Class C network address, the first three bit positions are always the binary 110. The calculation is as follows: 3 bytes, or 24 bits, minus 3 reserved positions leaves 21 positions. Hence, there are 2

21

, or 2,097,152, possible Class C networks.

Each unique Class C network has 1 byte to use for node addresses. This leads to 2

8

, or 256, minus the two reserved patterns of all 0s and all 1s, for a total of 254 node addresses for each Class C network.

Class C Valid Host IDs

Here’s an example of how to find a valid host ID in a Class C network:

All host bits turned off is the network ID: 192.168.100.0.

All host bits turned on is the broadcast address: 192.168.100.255.

The valid hosts would be the numbers in between the network address and the broadcast address: 192.168.100.1 through 192.168.100.254.

Private IP Addresses

The people who created the IP addressing scheme also created what we call private IP addresses. These addresses can be used on a private network, but they’re not routable through the Internet. This is designed for the purpose of creating a measure of well-needed security, but it also conveniently saves valuable IP address space.

If every host on every network had to have real routable IP addresses, we would have run out of IP addresses to hand out years ago. But by using private IP addresses, ISPs, corporations, and home users only need a relatively tiny group of bona fide IP addresses to connect their networks to the Internet. This is economical because they can use private IP addresses on their inside networks and get along just fine.

To accomplish this task, the ISP and the corporation—the end user, no matter who they are—need to use something called

Network Address

Translation (NAT)

, which basically takes a private IP address and converts it for use on the Internet. (NAT is covered in Chapter 13, “Network

Address Translation.”) Many people can use the same real IP address to transmit out onto the Internet. Doing things this way saves megatons of address space—good for us all!

The reserved private addresses are listed in

Table 3-5 .

Table 3-5:

Reserved IP address space

Address Class Reserved Address Space

Class A 10.0.0.0 through 10.255.255.255

Class B

Class C

172.16.0.0 through 172.31.255.255

192.168.0.0 through 192.168.255.255

You must know your private address space to become Cisco certified!

So, What Private IP Address Should I Use?

That’s a really great question: Should you use Class A, Class B, or even Class C private addressing when setting up your network? Let’s take Acme Corporation in SF as an example. This company is moving into a new building and needs a whole new network (what a treat this is!). It has 14 departments, with about 70 users in each. You could probably squeeze one or two Class C addresses to use, or maybe you could use a Class B, or even a Class A just for fun.

The rule of thumb in the consulting world is, when you’re setting up a corporate network—regardless of how small it is—you should use a Class A network address because it gives you the most flexibility and growth options. For example, if you used the 10.0.0.0 network address with a /24 mask, then you’d have 65,536 networks, each with 254 hosts. Lots of room for growth with that network!

But if you’re setting up a home network, you’d opt for a Class C address because it is the easiest for people to understand and configure. Using the default Class C mask gives you one network with 254 hosts—plenty for a home network.

With the Acme Corporation, a nice 10.1.x.0 with a /24 mask (the x is the subnet for each department) makes this easy to design, install, and troubleshoot.

IPv4 Address Types

Most people use the term

broadcast

as a generic term, and most of the time, we understand what they mean. But not always. For example, you might say, “The host broadcasted through a router to a DHCP server,” but, well, it’s pretty unlikely that this would ever really happen. What you probably mean—using the correct technical jargon—is, “The DHCP client broadcasted for an IP address; a router then forwarded this as a unicast packet to the DHCP server.” Oh, and remember that with IPv4, broadcasts are pretty important, but with IPv6, there aren’t any broadcasts sent at all

—now there’s something to get you excited about when you get to Chapter 15!

Okay, I’ve referred to broadcast addresses throughout Chapters 1 and 2, and even showed you some examples. But I really haven’t gone into the different terms and uses associated with them yet, and it’s about time I did. So here are the four IPv4 address types that I’d like to define for you:

Layer-2 broadcasts

These are sent to all nodes on a LAN.

Broadcasts (layer 3)

These are sent to all nodes on the network.

Unicast

This is an address for a single interface, and these are used to send packets to a single destination host.

Multicast

These are packets sent from a single source and transmitted to many devices on different networks. Referred to as “one-to-many.”

Layer-2 Broadcasts

First, understand that layer-2 broadcasts are also known as hardware broadcasts—they only go out on a LAN, and they don’t go past the LAN boundary (router).

The typical hardware address is 6 bytes (48 bits) and looks something like 45:AC:24:E3:60:A5. The broadcast would be all 1s in binary, which would be all

F

s in hexadecimal, as in FF.FF.FF.FF.FF.FF.

Layer-3 Broadcasts

Then there are the plain old broadcast addresses at layer 3. Broadcast messages are meant to reach all hosts on a broadcast domain. These are the network broadcasts that have all host bits on.

Here’s an example that you’re already familiar with: The network address of 172.16.0.0 255.255.0.0 would have a broadcast address of

172.16.255.255—all host bits on. Broadcasts can also be “any network and all hosts,” as indicated by 255.255.255.255.

A good example of a broadcast message is an Address Resolution Protocol (ARP) request. When a host has a packet, it knows the logical address (IP) of the destination. To get the packet to the destination, the host needs to forward the packet to a default gateway if the destination resides on a different IP network. If the destination is on the local network, the source will forward the packet directly to the destination. Because the source doesn’t have the MAC address to which it needs to forward the frame, it sends out a broadcast, something that every device in the local broadcast domain will listen to. This broadcast says, in essence, “If you are the owner of IP address 192.168.2.3, please forward your MAC address to me,” with the source giving the appropriate information.

Unicast Address

A unicast is a single IP address that is assigned to a network interface card and would be the destination IP address in a packet—in other words, directing packets to a specific host. A DHCP client request is a good example of how a unicast works.

Here’s an example: Your host on a LAN sends out an FF.FF.FF.FF.FF.FF layer-2 broadcast and destination 255.255.255.255 layer-3 broadcast, looking for a DHCP server on the LAN. The router will see that this is a broadcast meant for the DHCP server because it has a destination port number of 67 (BootP server) and will forward the request to the IP address of the DHCP server on another LAN. So, basically, if your DHCP server IP address is 172.16.10.1, your host just sends out a 255.255.255.255 DHCP client broadcast request, and the router changes that broadcast to the specific destination address of 172.16.10.1. (In order for the router to provide this service, you need to configure the interfaces with the ip helper-address

command—this is not a default service.)

Multicast Address

Multicast is a different beast entirely. At first glance, it appears to be a hybrid of unicast and broadcast communication, but that isn’t quite the case.

Multicast does allow point-to-multipoint communication, which is similar to broadcasts, but it happens in a different manner. The crux of

multicast

is that it enables multiple recipients to receive messages without flooding the messages to all hosts on a broadcast domain. However, this is not the default behavior—it’s what we

can

do with multicasting if it’s configured correctly!

Multicast works by sending messages or data to IP

multicast group

addresses. Routers then forward copies (unlike broadcasts, which are not forwarded) of the packet out to every interface that has hosts

subscribed

to that group address. This is where multicast differs from broadcast messages—with multicast communication, copies of packets, in theory, are sent only to subscribed hosts. When I say in theory, this means that the

hosts will receive, for example, a multicast packet destined for 224.0.0.9 (this is an EIGRP packet and only a router running the EIGRP protocol will read these). All hosts on the broadcast LAN (Ethernet is a broadcast multi-access LAN technology) will pick up the frame, read the destination address, and immediately discard the frame, unless they are in the multicast group. This saves PC processing, not LAN bandwidth. Multicasting can cause severe LAN congestion in some instances, if not implemented carefully.

There are several different groups that users or applications can subscribe to. The range of multicast addresses starts with 224.0.0.0 and goes through 239.255.255.255. As you can see, this range of addresses falls within IP Class D address space based on classful IP assignment.

Summary

If you made it this far and understood everything the first time through, you should be proud of yourself. We really covered a lot of ground in this chapter, but understand that the information in this chapter is key to being able to navigate through the rest of this book.

And even if you didn’t get a complete understanding the first time around, don’t stress. It really wouldn’t hurt you to read this chapter more than once. There is still a lot of ground to cover, so make sure you’ve got it all down, and get ready for more. What we’re doing is building a foundation, and you want a strong foundation, right?

After you learned about the DoD model, the layers, and associated protocols, you learned about the oh-so-important IP addressing. I discussed in detail the difference between each class of address and how to find a network address, broadcast address, and valid host range, which is critical information to understand before going on to Chapter 4.

Since you’ve already come this far, there’s no reason to stop now and waste all those brainwaves and new neurons. So don’t stop—go through the written lab and review questions at the end of this chapter and make sure you understand each answer’s explanation. The best is yet to come!

Exam Essentials

Differentiate the DoD and the OSI network models.

The DoD model is a condensed version of the OSI model, composed of four layers instead of seven, but is nonetheless like the OSI model in that it can be used to describe packet creation and devices and protocols can be mapped to its layers.

Identify Process/Application layer protocols.

Telnet is a terminal emulation program that allows you to log into a remote host and run programs. File Transfer Protocol (FTP) is a connection-oriented service that allows you to transfer files. Trivial FTP (TFTP) is a connectionless file transfer program. Simple Mail Transfer Protocol (SMTP) is a send-mail program.

Identify Host-to-Host layer protocols.

Transmission Control Protocol (TCP) is a connection-oriented protocol that provides reliable network service by using acknowledgments and flow control. User Datagram Protocol (UDP) is a connectionless protocol that provides low overhead and is considered unreliable.

Identify Internet layer protocols.

Internet Protocol (IP) is a connectionless protocol that provides network address and routing through an internetwork. Address Resolution Protocol (ARP) finds a hardware address from a known IP address. Reverse ARP (RARP) finds an IP address from a known hardware address. Internet Control Message Protocol (ICMP) provides diagnostics and destination unreachable messages.

Describe the functions of DNS and DHCP in the network.

Dynamic Host Configuration Protocol (DHCP) provides network configuration information (including IP addresses) to hosts, eliminating the need to perform the configurations manually. Domain Name Service (DNS) resolves hostnames—both Internet names such as www.routersim.com

and device names such as Workstation 2 to IP addresses, eliminating the need to know the IP address of a device for connection purposes.

Identify what is contained in the TCP header of a connection-oriented transmission.

The fields in the TCP header include the source port, destination port, sequence number, acknowledgment number, header length, a field reserved for future use, code bits, window size, checksum, urgent pointer, options field, and finally the data field.

Identify what is contained in the UDP header of a connectionless transmission.

The fields in the UDP header include only the source port, destination port, length, checksum, and data. The smaller number of fields as compared to the TCP header comes at the expense of providing none of the more advanced functions of the TCP frame.

Identify what is contained in the IP header.

The fields of an IP header include version, header length, priority or type of service, total length, identification, flags, fragment offset, time to live, protocol, header checksum, source IP address, destination IP address, options, and finally, data.

Compare and contrast UDP and TCP characteristics and features.

TCP is connection-oriented, acknowledged, and sequenced and has flow and error control, while UDP is connectionless, unacknowledged, and not sequenced and provides no error or flow control.

Understand the role of port numbers.

Port numbers are used to identify the protocol or service that is to be used in the transmission.

Identify the role of ICMP.

Internet Control Message Protocol (ICMP) works at the Network layer and is used by IP for many different services.

ICMP is a management protocol and messaging service provider for IP.

Define the Class A IP address range.

The IP range for a Class A network is 1–126. This provides 8 bits of network addressing and 24 bits of host addressing by default.

Define the Class B IP address range.

The IP range for a Class B network is 128–191. Class B addressing provides 16 bits of network addressing and 16 bits of host addressing by default.

Define the Class C IP address range.

The IP range for a Class C network is 192 through 223. Class C addressing provides 24 bits of network addressing and 8 bits of host addressing by default.

Identify the private IP ranges.

Class A private address range is 10.0.0.0 through 10.255.255.255.

Class B private address range is 172.16.0.0 through 172.31.255.255.

Class C private address range is 192.168.0.0 through 192.168.255.255.

Understand the difference between a broadcast, unicast, and multicast address.

A broadcast is all devices in a subnet, a unicast is to one device, and a multicast is to some but not all devices.

Written Labs

In this section, you’ll complete the following labs to make sure you’ve got the information and concepts contained within them fully dialed in:

Lab 3.1: TCP/IP

Lab 3.2: Mapping Applications to the DoD Model

(The answers to the written labs can be found following the answers to the review questions for this chapter.)

Written Lab 3.1: TCP/IP

Answer the following questions about TCP/IP:

1. What is the Class C address range in decimal and in binary?

2. What layer of the DoD model is equivalent to the Transport layer of the OSI model?

3. What is the valid range of a Class A network address?

4. What is the 127.0.0.1 address used for?

5. How do you find the network address from a listed IP address?

6. How do you find the broadcast address from a listed IP address?

7. What is the Class A private IP address space?

8. What is the Class B private IP address space?

9. What is the Class C private IP address space?

10. What are all the available characters that you can use in hexadecimal addressing?

Written Lab 3.2: Mapping Applications to the DoD Model

The four layers of the DoD model are Process/Application, Host-to-Host, Internet, and Network Access. Identify the layer of the DoD model each of these protocols operates.

1.

Internet Protocol (IP)

2.

Telnet

3.

FTP

4.

SNMP

5.

DNS

6.

Address Resolution Protocol (ARP)

7.

DHCP/BootP

8.

Transmission Control Protocol (TCP)

9.

X Window

10.

User Datagram Protocol (UDP)

11.

NFS

12.

Internet Control Message Protocol (ICMP)

13.

Reverse Address Resolution Protocol (RARP)

14.

Proxy ARP

15.

TFTP

16.

SMTP

17.

LPD

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s

Introduction.

1. What must happen if a DHCP IP conflict occurs?

A. Proxy ARP will fix the issue.

B. The client uses a gratuitous ARP to fix the issue.

C. The administrator must fix the conflict by hand at the DHCP server.

D. The dhcp server will reassign new IP addresses to both computers.

2. Which of the following allows a router to respond to an ARP request that is intended for a remote host?

A. Gateway DP

B. Reverse ARP (RARP)

C. Proxy ARP

D. Inverse ARP (IARP)

E. Address Resolution Protocol (ARP)

3. You want to implement a mechanism that automates the IP configuration, including IP address, subnet mask, default gateway, and DNS information. Which protocol will you use to accomplish this?

A. SMTP

B. SNMP

C. DHCP

D. ARP

4. What protocol is used to find the hardware address of a local device?

A. RARP

B. ARP

C. IP

D. ICMP

E. BootP

5. Which of the following are layers in the TCP/IP model? (Choose three.)

A. Application

B. Session

C. Transport

D. Internet

E. Data Link

F. Physical

6. Which class of IP address provides a maximum of only 254 host addresses per network ID?

A. Class A

B. Class B

C. Class C

D. Class D

E. Class E

7. Which of the following describe the DHCP Discover message? (Choose two.)

A. It uses FF:FF:FF:FF:FF:FF as a layer-2 broadcast.

B. It uses UDP as the Transport layer protocol.

C. It uses TCP as the Transport layer protocol.

D. It does not use a layer-2 destination address.

8. Which layer-4 protocol is used for a Telnet connection?

A. IP

B. TCP

C. TCP/IP

D. UDP

E. ICMP

9. How does a DHCP client ensure that no other computer has its assigned IP address?

A. Acknowledge receipt of a TCP segment.

B. Ping to its own address to see if a response is detected.

C. Broadcast a Proxy ARP

D. Broadcast a gratuitous ARP

E. Telnet to its own IP address

10. Which of the following services use TCP? (Choose three.)

A. DHCP

B. SMTP

C. SNMP

D. FTP

E. HTTP

F. TFTP

11. Which of the following services use UDP? (Choose three.)

A. DHCP

B. SMTP

C. SNMP

D. FTP

E. HTTP

F. TFTP

12. Which of the following are TCP/IP protocols used at the Application layer of the OSI model? (Choose three.)

A. IP

B. TCP

C. Telnet

D. FTP

E. TFTP

13. The following illustration shows a data structure header. What protocol is this header from?

A. IP

B. ICMP

C. TCP

D. UDP

E. ARP

F. RARP

14. If you use either Telnet or FTP, what layer are you using to generate the data?

A. Application

B. Presentation

C. Session

D. Transport

15. The DoD model (also called the TCP/IP stack) has four layers. Which layer of the DoD model is equivalent to the Network layer of the OSI model?

A. Application

B. Host-to-Host

C. Internet

D. Network Access

16. Which two of the following are private IP addresses?

A. 12.0.0.1

B. 168.172.19.39

C. 172.20.14.36

D. 172.33.194.30

E. 192.168.24.43

17. What layer in the TCP/IP stack is equivalent to the Transport layer of the OSI model?

A. Application

B. Host-to-Host

C. Internet

D. Network Access

18. Which statements are true regarding ICMP packets? (Choose two).

A. ICMP guarantees datagram delivery.

B. ICMP can provide hosts with information about network problems.

C. ICMP is encapsulated within IP datagrams.

D. ICMP is encapsulated within UDP datagrams.

19. What is the address range of a Class B network address in binary?

A. 01

xxxxxx

B. 0

xxxxxxx

C. 10

xxxxxx

D. 110

xxxxx

20. Which of the following protocols uses both TCP and UDP?

A. FTP

B. SMTP

C. Telnet

D. DNS

Answers to Review Questions

1. C. If a DHCP conflict is detected, either by the server sending a ping and getting a response or by a host using a gratuitous ARP (arp’ing for its own IP address and seeing if a host responds), then the server will hold that address and not use it again until it is fixed by an administrator.

2. C. Proxy ARP can help machines on a subnet reach remote subnets without configuring routing or a default gateway.

3. C. Dynamic Host Configuration Protocol (DHCP) is used to provide IP information to hosts on your network. DHCP can provide a lot of information, but the most common is IP address, subnet mask, default gateway, and DNS information.

4. B. Address Resolution Protocol (ARP) is used to find the hardware address from a known IP address.

5. A, C, D. This seems like a hard question at first because it doesn’t make sense. The listed answers are from the OSI model and the question

asked about the TCP/IP protocol stack (DoD model). However, let’s just look for what is wrong. First, the Session layer is not in the TCP/IP model; neither are the Data Link and Physical layers. This leaves us with the Transport layer (Host-to-Host in the DoD model), Internet layer (Network layer in the OSI), and Application layer (Application/Process in the DoD).

6. C. A Class C network address has only 8 bits for defining hosts: 2

8

– 2 = 254.

7. A, B. A client that sends out a DHCP Discover message in order to receive an IP address sends out a broadcast at both layer 2 and layer 3. The layer-2 broadcast is all Fs in hex, or FF:FF:FF:FF:FF:FF. The layer-3 broadcast is 255.255.255.255, which means any networks and all hosts.

DHCP is connectionless, which means it uses User Datagram Protocol (UDP) at the Transport layer, also called the Host-to-Host layer.

8. B. Although Telnet does use TCP and IP (TCP/IP), the question specifically asks about layer 4, and IP works at layer 3. Telnet uses TCP at layer

4.

9. D. To stop possible address conflicts, a DHCP client will use gratuitous ARP (broadcast an ARP request for its own IP address) to see if another host responds.

10. B, D, E. SMTP, FTP, and HTTP use TCP.

11. A, C, F. DHCP, SNMP, and TFTP use UDP. SMTP, FTP, and HTTP use TCP.

12. C, D, E. Telnet, File Transfer Protocol (FTP), and Trivial FTP (TFTP) are all Application layer protocols. IP is a Network layer protocol.

Transmission Control Protocol (TCP) is a Transport layer protocol.

13. C. First, you should know easily that only TCP and UDP work at the Transport layer, so now you have a 50/50 shot. However, since the header has sequencing, acknowledgment, and window numbers, the answer can only be TCP.

14. A. Both FTP and Telnet use TCP at the Transport layer; however, they both are Application layer protocols, so the Application layer is the best answer for this question.

15. C. The four layers of the DoD model are Application/Process, Host-to-Host, Internet, and Network Access. The Internet layer is equivalent to the

Network layer of the OSI model.

16. C, E. Class A private address range is 10.0.0.0 through 10.255.255.255. Class B private address range is 172.16.0.0 through

172.31.255.255, and Class C private address range is 192.168.0.0 through 192.168.255.255.

17. B. The four layers of the TCP/IP stack (also called the DoD model) are Application/Process, Host-to-Host, Internet, and Network Access. The

Host-to-Host layer is equivalent to the Transport layer of the OSI model.

18. B, C. ICMP is used for diagnostics and destination unreachable messages. ICMP is encapsulated within IP datagrams, and because it is used for diagnostics, it will provide hosts with information about network problems.

19. C. The range of a Class B network address is 128–191. This makes our binary range 10xxxxxx.

20. D. DNS uses TCP for zone exchanges between servers and UDP when a client is trying to resolve a hostname to an IP address.

Answers to Written Lab 3.1

1.

192 through 223, 110

xxxxx

2.

Host-to-Host

3.

1 through 126

4.

Loopback or diagnostics

5.

Turn all host bits off.

6.

Turn all host bits on.

7.

10.0.0.0 through 10.255.255.255

8.

172.16.0.0 through 172.31.255.255

9.

192.168.0.0 through 192.168.255.255

10.

0 through 9 and

A

,

B

,

C

,

D

,

E

, and

F

Answers to Written Lab 3.2

1.

Internet

2.

Process/Application

3.

Process/Application

4.

Process/Application

5.

Process/Application

6.

Internet

7.

Process/Application

8.

Host-to-Host

9.

Process/Application

10.

Host-to-Host

11.

Process/Application

12.

Internet

13.

Internet

14.

Internet

15.

Process/Application

16.

Process/Application

17.

Process/Application

Chapter 4

Easy Subnetting

The CCNA exam topics covered in this chapter include the following:

Describe how a network works.

Interpret network diagrams.

Implement an IP addressing scheme and IP Services to meet network requirements in a medium-size Enterprise branch office network.

Describe the operation and benefits of using private and public IP addressing.

Implement static and dynamic addressing services for hosts in a LAN environment.

This chapter will pick up right where we left off in the last chapter. We will continue our discussion of IP addressing.

We’ll start with subnetting an IP network. You’re going to have to really apply yourself because subnetting takes time and practice in order to nail it. So be patient. Do whatever it takes to get this stuff dialed in. This chapter truly is important—possibly the most important chapter in this book for you to understand.

I’ll thoroughly cover IP subnetting from the very beginning. I know this might sound weird to you, but I think you’ll be much better off if you can try to forget everything you’ve learned about subnetting before reading this chapter—especially if you’ve been to a Microsoft class!

So get psyched—you’re about to go for quite a ride! This chapter will truly help you understand IP addressing and networking, so don’t get discouraged or give up. If you stick with it, I promise that one day you’ll look back on this and you’ll be really glad you decided to hang on. It’s one of those things that after you understand it, you’ll wonder why you once thought it was so hard. Ready? Let’s go!

For up-to-the-minute updates for this chapter, please see www.lammle.com

or www.sybex.com/go/ccna7e

.

Subnetting Basics

In Chapter 3, you learned how to define and find the valid host ranges used in a Class A, Class B, and Class C network address by turning the host bits all off and then all on. This is very good, but here’s the catch: You were defining only one network. What happens if you wanted to take one network address and create six networks from it? You would have to do something called

subnetting

, because that’s what allows you to take one larger network and break it into a bunch of smaller networks.

There are loads of reasons in favor of subnetting, including the following benefits:

Reduced network traffic

We all appreciate less traffic of any kind. Networks are no different. Without trusty routers, packet traffic could grind the entire network down to a near standstill. With routers, most traffic will stay on the local network; only packets destined for other networks will pass through the router. Routers create broadcast domains. The more broadcast domains you create, the smaller the broadcast domains and the less network traffic on each network segment.

Optimized network performance

This is a result of reduced network traffic.

Simplified management

It’s easier to identify and isolate network problems in a group of smaller connected networks than within one gigantic network.

Facilitated spanning of large geographical distances

Because WAN links are considerably slower and more expensive than LAN links, a single large network that spans long distances can create problems in every area previously listed. Connecting multiple smaller networks makes the system more efficient.

In the following sections, I am going to move to subnetting a network address. This is the good part—ready?

IP Subnet-Zero

IP subnet-zero

is not a new command, but in the past, Cisco courseware and Cisco exam objectives, didn’t cover it—but it certainly does now! This command allows you to use the first and last subnet in your network design. For example, the Class C mask of 255.255.255.192 provides subnets

64 and 128 (discussed thoroughly later in this chapter), but with the ip subnet-zero

command, you now get to use subnets 0, 64, 128, and 192. That is two more subnets for every subnet mask we use.

Even though we don’t discuss the command line interface (CLI) until Chapter 6, “Cisco’s Internetworking Operating System (IOS),” it’s important for you to be familiar with this command:

P1R1#

sh running-config

Building configuration...

Current configuration : 827 bytes

!

hostname Pod1R1

!

ip subnet-zero

!

When studying for your Cisco exams, make sure you read very carefully and understand if Cisco is asking you not to use ip subnet-zero

. There are instances where this may happen.

How to Create Subnets

To create subnetworks, you take bits from the host portion of the IP address and reserve them to define the subnet address. This means fewer bits for hosts, so the more subnets, the fewer bits available for defining hosts.

Later in this chapter, you’ll learn how to create subnets, starting with Class C addresses. But before you actually implement subnetting, you need to determine your current requirements as well as plan for future conditions.

Before we move on to designing and creating a subnet mask, you need to understand that in this first section, we will be discussing classful routing, which means that all hosts (all nodes) in the network use the exact same subnet mask. When we move on to Variable Length Subnet Masks (VLSMs), I’ll discuss classless routing, which means that each network segment can use a different subnet mask.

To create a subnet, follow these steps:

1.

Determine the number of required network IDs:

One for each LAN subnet

One for each wide area network connection

2.

Determine the number of required host IDs per subnet:

One for each TCP/IP host

One for each router interface

3.

Based on the above requirements, create the following:

One subnet mask for your entire network

A unique subnet ID for each physical segment

A range of host IDs for each subnet

Subnet Masks

For the subnet address scheme to work, every machine on the network must know which part of the host address will be used as the subnet address. This is accomplished by assigning a

subnet mask

to each machine. A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address.

The network administrator creates a 32-bit subnet mask composed of 1s and 0s. The 1s in the subnet mask represent the positions that refer to the network or subnet addresses.

Not all networks need subnets, meaning they use the default subnet mask. This is basically the same as saying that a network doesn’t have a subnet address. Table 4-1 shows the default subnet masks for Classes A, B, and C. These default masks cannot change. In other words, you can’t make a Class B subnet mask read 255.0.0.0. If you try, the host will read that address as invalid and usually won’t even let you type it in. For a

Class A network, you can’t change the first byte in a subnet mask; it must read 255.0.0.0 at a minimum. Similarly, you cannot assign

255.255.255.255, as this is all 1s—a broadcast address. A Class B address must start with 255.255.0.0, and a Class C has to start with

255.255.255.0.

Understanding the Powers of 2

Powers of 2 are important to understand and memorize for use with IP subnetting. To review powers of 2, remember that when you see a number with another number to its upper right (called an exponent), this means you should multiply the number by itself as many times as the upper number specifies. For example, 23 is 2 ´ 2 ´ 2, which equals

8. Here’s a list of powers of 2 that you should commit to memory:

21 = 2

22 = 4

23 = 8

24 = 16

25 = 32

26 = 64

27 = 128

28 = 256

29 = 512

210 = 1,024

211 = 2,048

212 = 4,096

213 = 8,192

214 = 16,384

Before you get stressed out about knowing all these exponents, remember that it’s helpful to know them, but it’s not absolutely necessary. Here’s a little trick since you’re working with 2s: Each successive power of 2 is double the previous one.

For example, all you have to do to remember the value of 29 is to first know that 28 = 256. Why? Because when you double 2 to the eighth power (256), you get 29 (or 512). To determine the value of 210, simply start at 28 = 256, and then double it twice.

You can go the other way as well. If you needed to know what 26 is, for example, you just cut 256 in half two times: once to reach 27 and then one more time to reach 26.

A

B

C

Table 4-1:

Default subnet mask

Class Format Default Subnet Mask

network.node.node.node

255.0.0.0

network.network.node.node

255.255.0.0

network.network.network.node 255.255.255.0

Classless Inter-Domain Routing (CIDR)

Another term you need to familiarize yourself with is

Classless Inter-Domain Routing (CIDR)

. It’s basically the method that ISPs (Internet service providers) use to allocate a number of addresses to a company, a home—a customer. They provide addresses in a certain block size, something

I’ll be going into in greater detail later in this chapter.

When you receive a block of addresses from an ISP, what you get will look something like this: 192.168.10.32/28. This is telling you what your subnet mask is. The slash notation (/) means how many bits are turned on (1s). Obviously, the maximum could only be /32 because a byte is 8 bits and there are 4 bytes in an IP address: (4 × 8 = 32). But keep in mind that the largest subnet mask available (regardless of the class of address) can only be a /30 because you’ve got to keep at least 2 bits for host bits.

Take, for example, a Class A default subnet mask, which is 255.0.0.0. This means that the first byte of the subnet mask is all ones (1s), or

11111111. When referring to a slash notation, you need to count all the 1 bits to figure out your mask. The 255.0.0.0 is considered a /8 because it has 8 bits that are 1s—that is, 8 bits that are turned on.

A Class B default mask would be 255.255.0.0, which is a /16 because 16 bits are ones (1s): 11111111.11111111.00000000.00000000.

Table 4-2 has a listing of every available subnet mask and its equivalent CIDR slash notation.

Table 4-2:

CIDR values

Subnet Mask CIDR Value

255.0.0.0

255.128.0.0

255.192.0.0

255.224.0.0

255.240.0.0

255.248.0.0

/8

/9

/10

/11

/12

/13

255.252.0.0

255.254.0.0

/14

/15

255.255.0.0

/16

255.255.128.0

/17

255.255.192.0

/18

255.255.224.0

/19

255.255.240.0

/20

255.255.248.0

/21

255.255.252.0

/22

255.255.254.0

/23

255.255.255.0

/24

255.255.255.128 /25

255.255.255.192 /26

255.255.255.224 /27

255.255.255.240 /28

255.255.255.248 /29

255.255.255.252 /30

The /8 through /15 can only be used with Class A network addresses. /16 through /23 can be used by Class A and B network addresses. /24 through /30 can be used by Class A, B, and C network addresses. This is a big reason why most companies use Class A network addresses.

Since they can use all subnet masks, they get the maximum flexibility in network design.

No, you cannot configure a Cisco router using this slash format. But wouldn’t that be nice? Nevertheless, it’s really important for you to know subnet masks in the slash notation (CIDR).

Subnetting Class C Addresses

There are many different ways to subnet a network. The right way is the way that works best for you. In a Class C address, only 8 bits are available for defining the hosts. Remember that subnet bits start at the left and go to the right, without skipping bits. This means that the only Class C subnet masks can be the following:

Binary Decimal CIDR

---------------------------------------------------------

00000000 = 0 /24

10000000 = 128 /25

11000000 = 192 /26

11100000 = 224 /27

11110000 = 240 /28

11111000 = 248 /29

11111100 = 252 /30

We can’t use a /31 or /32 because we have to have at least 2 host bits for assigning IP addresses to hosts. In the past, I never discussed the /25 in a Class C network. Cisco always had been concerned with having at least 2 subnet bits, but now, because of Cisco recognizing the ip subnet-zero command in its curriculum and exam objectives, we can use just 1 subnet bit.

In the following sections, I’m going to teach you an alternate method of subnetting that makes it easier to subnet larger numbers in no time. Trust me, you need to be able to subnet fast!

Subnetting a Class C Address: The Fast Way!

When you’ve chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and broadcast addresses of a subnet that the mask provides, all you need to do is answer five simple questions:

How many subnets does the chosen subnet mask produce?

How many valid hosts per subnet are available?

What are the valid subnets?

What’s the broadcast address of each subnet?

What are the valid hosts in each subnet?

At this point, it’s important that you both understand and have memorized your powers of 2. Please refer to the sidebar “Understanding the

Powers of 2” earlier in this chapter if you need some help. Here’s how you get the answers to those five big questions:

How many subnets?

2 x

= number of subnets.

x

is the number of masked bits, or the 1s. For example, in 11000000, the number of 1s gives us

2

2

subnets. In this example, there are 4 subnets.

How many hosts per subnet?

2 y

– 2 = number of hosts per subnet.

y

is the number of unmasked bits, or the 0s. For example, in 11000000, the number of 0s gives us 2

6

– 2 hosts. In this example, there are 62 hosts per subnet. You need to subtract 2 for the subnet address and the broadcast address, which are not valid hosts.

What are the valid subnets?

256 – subnet mask = block size, or increment number. An example would be 256 – 192 = 64. The block size of a 192 mask is always 64. Start counting at zero in blocks of 64 until you reach the subnet mask value and these are your subnets. 0, 64, 128,

192. Easy, huh?

What’s the broadcast address for each subnet?

Now here’s the really easy part. Since we counted our subnets in the last section as 0, 64,

128, and 192, the broadcast address is always the number right before the next subnet. For example, the 0 subnet has a broadcast address of 63 because the next subnet is 64. The 64 subnet has a broadcast address of 127 because the next subnet is 128. And so on. And remember, the broadcast address of the last subnet is always 255.

What are the valid hosts?

Valid hosts are the numbers between the subnets, omitting the all-0s and all-1s. For example, if 64 is the subnet number and 127 is the broadcast address, then 65–126 is the valid host range—it’s

always

the numbers between the subnet address and the broadcast address.

I know this can truly seem confusing. But it really isn’t as hard as it seems to be at first—just hang in there! Why not try a few and see for yourself?

Subnetting Practice Examples: Class C Addresses

Here’s your opportunity to practice subnetting Class C addresses using the method I just described. Exciting, isn’t it! We’re going to start with the first Class C subnet mask and work through every subnet that we can using a Class C address. When we’re done, I’ll show you how easy this is with Class A and B networks too!

Practice Example #1C: 255.255.255.128 (/25)

Since 128 is 10000000 in binary, there is only 1 bit for subnetting and 7 bits for hosts. We’re going to subnet the Class C network address

192.168.10.0.

192.168.10.0 = Network address

255.255.255.128 = Subnet mask

Now, let’s answer the big five:

How many subnets?

Since 128 is 1 bit on (

1

0000000), the answer would be 2

1

= 2.

How many hosts per subnet?

We have 7 host bits off (1

0000000

), so the equation would be 2

7

– 2 = 126 hosts.

What are the valid subnets?

256 – 128 = 128. Remember, we’ll start at zero and count in our block size, so our subnets are 0, 128.

What’s the broadcast address for each subnet?

The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. For the zero subnet, the next subnet is 128, so the broadcast of the 0 subnet is 127.

What are the valid hosts?

These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the broadcast address. This way, the valid hosts are obvious. The following table shows the 0 and 128 subnets, the valid host ranges of each, and the broadcast address of both subnets:

Subnet

0 128

First host 1 129

Last host 126 254

Broadcast 127 255

Before moving on to the next example, take a look at Figure 4-1 . Okay, looking at a Class C /25, it’s pretty clear there are two subnets. But so what—why is this significant? Well actually, it’s not, but that’s not the right question. What you really want to know is what you would do with this information! You can see in Figure 4-1 that both subnets have been assigned to a router interface, which creates our broadcast domains and assigns our subnets. Use the command show ip route to see the routing table on a router (covered in detail throughout this book).

Figure 4-1:

Implementing a Class C /25 logical network

I know this isn’t exactly everyone’s favorite pastime, but it’s really important, so just hang in there; we’re going to talk about subnetting—period.

You need to know that the key to understanding subnetting is to understand the very reason you need to do it. And I’m going to demonstrate this by going through the process of building a physical network—and let’s add a router. (We now have an internetwork, as I truly hope you already know!)

All right, because we added that router, in order for the hosts on our internetwork to communicate, they must now have a logical network addressing scheme. We could use IPv6, but IPv4 is still the most popular, and it also just happens to be what we’re studying at the moment, so that’s what we’re going with. Okay—now take a look back to Figure 4-1 . There are two physical networks, so we’re going to implement a logical addressing scheme that allows for two logical networks. As always, it’s a really good idea to look ahead and consider likely growth scenarios—both short and long term, but for this example, a /25 will do the trick.

Practice Example #2C: 255.255.255.192 (/26)

In this second example, we’re going to subnet the network address 192.168.10.0 using the subnet mask 255.255.255.192.

192.168.10.0 = Network address

255.255.255.192 = Subnet mask

Now, let’s answer the big five:

How many subnets?

Since 192 is 2 bits on (

11

000000), the answer would be 2

2

= 4 subnets.

How many hosts per subnet?

We have 6 host bits off (11

000000

), so the equation would be 2

6

– 2 = 62 hosts.

What are the valid subnets?

256 – 192 = 64. Remember, we start at zero and count in our block size, so our subnets are 0, 64, 128, and

192.

What’s the broadcast address for each subnet?

The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. For the zero subnet, the next subnet is 64, so the broadcast address for the zero subnet is 63.

What are the valid hosts?

These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the broadcast address. This way, the valid hosts are obvious. The following table shows the 0, 64, 128, and 192 subnets, the valid host ranges of each, and the broadcast address of each subnet:

Okay, again, before getting into the next example, you can see that we can now subnet a /26. And what are you going to do with this fascinating information? Implement it! We’ll use Figure 4-2 to practice a /26 network implementation.

The /26 mask provides four subnetworks, and we need a subnet for each router interface. With this mask, in this example, we actually have room to add another router interface.

Practice Example #3C: 255.255.255.224 (/27)

This time, we’ll subnet the network address 192.168.10.0 and subnet mask 255.255.255.224.

192.168.10.0 = Network address

255.255.255.224 = Subnet mask

How many subnets?

224 is 11100000, so our equation would be 2

3

= 8.

How many hosts?

2

5

– 2 = 30.

What are the valid subnets?

256 – 224 = 32. We just start at zero and count to the subnet mask value in blocks (increments) of 32: 0,

32, 64, 96, 128, 160, 192, and 224.

What’s the broadcast address for each subnet (always the number right before the next subnet)?

What are the valid hosts (the numbers between the subnet number and the broadcast address)?

Figure 4-2:

Implementing a Class C /26 logical network

To answer the last two questions, first just write out the subnets, then write out the broadcast addresses—the number right before the next subnet.

Last, fill in the host addresses. The following table gives you all the subnets for the 255.255.255.224 Class C subnet mask:

Practice Example #4C: 255.255.255.240 (/28)

Let’s practice on another one:

192.168.10.0 = Network address

255.255.255.240 = Subnet mask

Subnets?

240 is 11110000 in binary. 2

4

= 16.

Hosts?

4 host bits, or 2

4

– 2 = 14.

Valid subnets?

256 – 240 = 16. Start at 0: 0 + 16 = 16. 16 + 16 = 32. 32 + 16 = 48. 48 + 16 = 64. 64 + 16 = 80. 80 + 16 = 96. 96 + 16 =

112. 112 + 16 = 128. 128 + 16 = 144. 144 + 16 = 160. 160 + 16 = 176. 176 + 16 = 192. 192 + 16 = 208. 208 + 16 = 224. 224 + 16 =

240.

Broadcast address for each subnet?

Valid hosts?

To answer the last two questions, check out the following table. It gives you the subnets, valid hosts, and broadcast addresses for each subnet.

First, find the address of each subnet using the block size (increment). Second, find the broadcast address of each subnet increment (it’s always the number right before the next valid subnet), then just fill in the host addresses. The following table shows the available subnets, hosts, and broadcast addresses provided from a Class C 255.255.255.240 mask:

Cisco has figured out that most people cannot count in 16s and therefore have a hard time finding valid subnets, hosts, and broadcast addresses with the Class C

255.255.255.240 mask. You’d be wise to study this mask.

Practice Example #5C: 255.255.255.248 (/29)

Let’s keep practicing:

192.168.10.0 = Network address

255.255.255.248 = Subnet mask

Subnets?

248 in binary = 11111000. 2

5

= 32.

Hosts?

2

3

– 2 = 6.

Valid subnets?

256 – 248 = 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88, 96, 104, 112, 120, 128, 136, 144, 152, 160, 168, 176, 184, 192,

200, 208, 216, 224, 232, 240, and 248.

Broadcast address for each subnet?

Valid hosts?

Take a look at the following table. It shows some of the subnets (first four and last four only), valid hosts, and broadcast addresses for the Class

C 255.255.255.248 mask:

If you try to configure a router interface with the address 192.168.10.6 255.255.255.248 (for example), and you receive the error

Bad mask /29 for address 192.168.10.6

This means that ip subnet-zero

is not enabled. You must be able to subnet to see that the address used in this example is in the zero subnet.

Practice Example #6C: 255.255.255.252 (/30)

Just one more:

192.168.10.0 = Network address

255.255.255.252 = Subnet mask

Subnets?

64.

Hosts?

2.

Valid subnets?

0, 4, 8, 12, etc., all the way to 252.

Broadcast address for each subnet (always the number right before the next subnet)?

Valid hosts (the numbers between the subnet number and the broadcast address)?

The following table shows you the subnet, valid host, and broadcast address of the first four and last four subnets in the 255.255.255.252 Class

C subnet:

Subnetting in Your Head: Class C Addresses

It really is possible to subnet in your head. Even if you don’t believe me, I’ll show you how. And it’s not all that hard either—take the following example:

192.168.10.33 = Node address

255.255.255.224 = Subnet mask

First, determine the subnet and broadcast address of the network in which the above IP address resides. You can do this by answering question

3 of the big five questions: 256 – 224 = 32. 0, 32, 64, and so on. The address of 33 falls between the two subnets of 32 and 64 and must be part of the 192.168.10.32 subnet. The next subnet is 64, so the broadcast address of the 32 subnet is 63. (Remember that the broadcast address of a subnet is always the number right before the next subnet.) The valid host range is 33–62 (the numbers between the subnet and broadcast address).

This is too easy!

Should We Really Use This Mask That Provides Only Two Hosts?

You are the network administrator for Acme Corporation in San Francisco, with dozens of WAN links connecting to your corporate office. Right now your network is a classful network, which means that the same subnet mask is on each host and router interface. You’ve read about classless routing where you can have different size masks, but don’t know what to use on your point-to-point WAN links. Is the 255.255.255.252 (/30) a helpful mask in this situation?

Yes, this is a very helpful mask in wide area networks.

If you use the 255.255.255.0 mask, then each network would have 254 hosts, but you only use 2 addresses with a WAN link! That is a waste of 252 hosts per subnet. If you use the 255.255.255.252 mask, then each subnet has only 2 hosts and you don’t waste precious addresses. This is a really important subject, one that we’ll address in a lot more detail in the section on VLSM network design in the next chapter.

Okay, let’s try another one. We’ll subnet another Class C address:

192.168.10.33 = Node address

255.255.255.240 = Subnet mask

What is the subnet and broadcast address of the network of which the above IP address is a member? 256 – 240 = 16. 0, 16, 32, 48, and so on.

Bingo—the host address is between the 32 and 48 subnets. The subnet is 192.168.10.32, and the broadcast address is 47 (the next subnet is 48).

The valid host range is 33–46 (the numbers between the subnet number and the broadcast address).

Okay, we need to do more, just to make sure you have this down.

You have a node address of 192.168.10.174 with a mask of 255.255.255.240. What is the valid host range?

The mask is 240, so we’d do a 256 – 240 = 16. This is our block size. Just keep adding 16 until we pass the host address of 174, starting at zero, of course: 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, and so on. The host address of 174 is between 160 and 176, so the subnet is

160. The broadcast address is 175; the valid host range is 161–174. That was a tough one.

One more—just for fun. This is the easiest one of all Class C subnetting:

192.168.10.17 = Node address

255.255.255.252 = Subnet mask

What is the subnet and broadcast address of the subnet in which the above IP address resides? 256 – 252 = 0 (always start at zero unless told otherwise), 4, 8, 12, 16, 20, etc. You’ve got it! The host address is between the 16 and 20 subnets. The subnet is 192.168.10.16, and the broadcast address is 19. The valid host range is 17–18.

Now that you’re all over Class C subnetting, let’s move on to Class B subnetting. But before we do, let’s have a quick review.

What Do We Know?

Okay—here’s where you can really apply what you’ve learned so far and begin committing it all to memory. This is a very cool section that I’ve been using in my classes for years. It will really help you nail down subnetting!

When you see a subnet mask or slash notation (CIDR), you should know the following:

/25

What do we know about a /25?

128 mask

1 bit on and 7 bits off (10000000)

Block size of 128

2 subnets, each with 126 hosts

/26

What do we know about a /26?

192 mask

2 bits on and 6 bits off (11000000)

Block size of 64

4 subnets, each with 62 hosts

/27

What do we know about a /27?

224 mask

3 bits on and 5 bits off (11100000)

Block size of 32

8 subnets, each with 30 hosts

/28

What do we know about a /28?

240 mask

4 bits on and 4 bits off

Block size of 16

16 subnets, each with 14 hosts

/29

What do we know about a /29?

248 mask

5 bits on and 3 bits off

Block size of 8

32 subnets, each with 6 hosts

/30

What do we know about a /30?

252 mask

6 bits on and 2 bits off

Block size of 4

64 subnets, each with 2 hosts

Regardless of whether you have a Class A, Class B, or Class C address, the /30 mask will provide you with only two hosts, ever. This mask is suited almost exclusively—as well as suggested by Cisco—for use on point-to-point links.

If you can memorize this “What Do We Know?” section, you’ll be much better off in your day-to-day job and in your studies. Try saying it out loud, which helps you memorize things—yes, your significant other and/or coworkers will think you’ve lost it, but they probably already do if you are in the networking field. And if you’re not yet in the networking field but are studying all this to break into it, you might as well have people start thinking you’re an odd bird now since they will eventually anyway.

It’s also helpful to write these on some type of flashcards and have people test your skill. You’d be amazed at how fast you can get subnetting down if you memorize block sizes as well as this “What Do We Know?” section.

Subnetting Class B Addresses

Before we dive into this, let’s look at all the possible Class B subnet masks first. Notice that we have a lot more possible subnet masks than we do with a Class C network address:

255.255.0.0 (/16)

255.255.128.0 (/17) 255.255.255.0 (/24)

255.255.192.0 (/18) 255.255.255.128 (/25)

255.255.224.0 (/19) 255.255.255.192 (/26)

255.255.240.0 (/20) 255.255.255.224 (/27)

255.255.248.0 (/21) 255.255.255.240 (/28)

255.255.252.0 (/22) 255.255.255.248 (/29)

255.255.254.0 (/23) 255.255.255.252 (/30)

We know the Class B network address has 16 bits available for host addressing. This means we can use up to 14 bits for subnetting (because we have to leave at least 2 bits for host addressing). Using a /16 means you are not subnetting with Class B, but it is a mask you can use.

By the way, do you notice anything interesting about that list of subnet values—a pattern, maybe? Ah ha! That’s exactly why I had you memorize the binary-to-decimal numbers earlier in the chapter. Since subnet mask bits start on the left and move to the right and bits can’t be skipped, the numbers are always the same regardless of the class of address. Memorize this pattern.

The process of subnetting a Class B network is pretty much the same as it is for a Class C, except that you just have more host bits and you start in the third octet.

Use the same subnet numbers for the third octet with Class B that you used for the fourth octet with Class C, but add a zero to the network portion and a 255 to the broadcast section in the fourth octet. The following table shows you an example host range of two subnets used in a Class B 240

(/20) subnet mask:

Subnet Address

16.0

32.0

Broadcast Address 31.255 47.255

Just add the valid hosts between the numbers, and you’re set!

The preceding example is true only until you get up to /24. After that, it’s numerically exactly like Class C.

Subnetting Practice Examples: Class B Addresses

The following sections will give you an opportunity to practice subnetting Class B addresses. Again, I have to mention that this is the same as subnetting with Class C, except we start in the third octet—with the exact same numbers!

Practice Example #1B: 255.255.128.0 (/17)

172.16.0.0 = Network address

255.255.128.0 = Subnet mask

Subnets?

2

1

= 2 (same as Class C).

Hosts?

2

15

– 2 = 32,766 (7 bits in the third octet, and 8 in the fourth).

Valid subnets?

256 – 128 = 128. 0, 128. Remember that subnetting is performed in the third octet, so the subnet numbers are really 0.0

and 128.0, as shown in the next table. These are the exact numbers we used with Class C; we use them in the third octet and add a 0 in the fourth octet for the network address.

Broadcast address for each subnet?

Valid hosts?

The following table shows the two subnets available, the valid host range, and the broadcast address of each:

Subnet

0.0

128.0

First host 0.1

128.1

Last host 127.254 255.254

Broadcast 127.255 255.255

Okay, notice that we just added the fourth octet’s lowest and highest values and came up with the answers. And again, it’s done exactly the same way as for a Class C subnet. We just use the same numbers in the third octet and added 0 and 255 in the fourth octet—pretty simple, huh? I really can’t say this enough: It’s just not hard. The numbers never change; we just use them in different octets!

Practice Example #2B: 255.255.192.0 (/18)

172.16.0.0 = Network address

255.255.192.0 = Subnet mask

Subnets?

2

2

= 4.

Hosts?

2

14

– 2 = 16,382 (6 bits in the third octet, and 8 in the fourth).

Valid subnets?

256 – 192 = 64. 0, 64, 128, 192. Remember that the subnetting is performed in the third octet, so the subnet numbers are really 0.0, 64.0, 128.0, and 192.0, as shown in the next table.

Broadcast address for each subnet?

Valid hosts?

The following table shows the four subnets available, the valid host range, and the broadcast address of each:

Again, it’s pretty much the same as it is for a Class C subnet—we just added 0 and 255 in the fourth octet for each subnet in the third octet.

Practice Example #3B: 255.255.240.0 (/20)

172.16.0.0 = Network address

255.255.240.0 = Subnet mask

Subnets?

2

4

= 16.

Hosts?

2

12

– 2 = 4094.

Valid subnets?

256 – 240 = 0, 16, 32, 48, etc., up to 240. Notice that these are the same numbers as a Class C 240 mask—we just put them in the third octet and add a 0 and 255 in the fourth octet.

Broadcast address for each subnet?

Valid hosts?

The following table shows the first four subnets, valid hosts, and broadcast addresses in a Class B 255.255.240.0 mask:

Practice Example #4B: 255.255.254.0 (/23)

172.16.0.0 = Network address

255.255.254.0 = Subnet mask

Subnets?

2

7

= 128.

Hosts?

2

9

– 2 = 510.

Valid subnets?

256 – 254 = 0, 2, 4, 6, 8, etc., up to 254.

Broadcast address for each subnet?

Valid hosts?

The following table shows the first five subnets, valid hosts, and broadcast addresses in a Class B 255.255.254.0 mask:

Practice Example #5B: 255.255.255.0 (/24)

Contrary to popular belief, 255.255.255.0 used with a Class B network address is not called a Class B network with a Class C subnet mask. It’s amazing how many people see this mask used in a Class B network and think it’s a Class C subnet mask. This is a Class B subnet mask with 8 bits of subnetting—it’s logically different from a Class C mask. Subnetting this address is fairly simple:

172.16.0.0 = Network address

255.255.255.0 = Subnet mask

Subnets?

2

8

= 256.

Hosts?

2

8

– 2 = 254.

Valid subnets?

256 – 255 = 1. 0, 1, 2, 3, etc., all the way to 255.

Broadcast address for each subnet?

Valid hosts?

The following table shows the first four and last two subnets, the valid hosts, and the broadcast addresses in a Class B 255.255.255.0 mask:

Practice Example #6B: 255.255.255.128 (/25)

This is one of the hardest subnet masks you can play with. And worse, it actually is a really good subnet to use in production because it creates over 500 subnets with 126 hosts for each subnet—a nice mixture. So, don’t skip over it!

172.16.0.0 = Network address

255.255.255.128 = Subnet mask

Subnets?

2

9

= 512.

Hosts?

2

7

– 2 = 126.

Valid subnets?

Okay, now for the tricky part. 256 – 255 = 1. 0, 1, 2, 3, etc. for the third octet. But you can’t forget the one subnet bit used in the fourth octet. Remember when I showed you how to figure one subnet bit with a Class C mask? You figure this the same way. (Now you know why I showed you the 1-bit subnet mask in the Class C section—to make this part easier.) You actually get two subnets for each third octet value, hence the 512 subnets. For example, if the third octet is showing subnet 3, the two subnets would actually be 3.0

and 3.128.

Broadcast address for each subnet?

Valid hosts?

The following table shows how you can create subnets, valid hosts, and broadcast addresses using the Class B 255.255.255.128 subnet mask

(the first eight subnets are shown, and then the last two subnets):

Practice Example #7B: 255.255.255.192 (/26)

Now, this is where Class B subnetting gets easy. Since the third octet has a 255 in the mask section, whatever number is listed in the third octet is a subnet number. However, now that we have a subnet number in the fourth octet, we can subnet this octet just as we did with Class C subnetting.

Let’s try it out:

172.16.0.0 = Network address

255.255.255.192 = Subnet mask

Subnets?

2

10

= 1024.

Hosts?

2

6

– 2 = 62.

Valid subnets?

256 – 192 = 64. The subnets are shown in the following table. Do these numbers look familiar?

Broadcast address for each subnet?

Valid hosts?

The following table shows the first eight subnet ranges, valid hosts, and broadcast addresses:

Notice that for each subnet value in the third octet, you get subnets 0, 64, 128, and 192 in the fourth octet.

Practice Example #8B: 255.255.255.224 (/27)

This is done the same way as the preceding subnet mask, except that we just have more subnets and fewer hosts per subnet available.

172.16.0.0 = Network address

255.255.255.224 = Subnet mask

Subnets?

2

11

= 2048.

Hosts?

2

5

– 2 = 30.

Valid subnets?

256 – 224 = 32. 0, 32, 64, 96, 128, 160, 192, 224.

Broadcast address for each subnet?

Valid hosts?

The following table shows the first eight subnets:

This next table shows the last eight subnets:

Subnetting in Your Head: Class B Addresses

Are you nuts? Subnet Class B addresses in our heads? It’s actually easier than writing it out—I’m not kidding! Let me show you how:

Question:

What is the subnet and broadcast address of the subnet in which 172.16.10.33 /27 resides?

Answer:

The interesting octet is the fourth octet. 256 – 224 = 32. 32 + 32 = 64. Bingo: 33 is between 32 and 64. However, remember that the third octet is considered part of the subnet, so the answer would be the 10.32 subnet. The broadcast is 10.63, since 10.64 is the next subnet. That was a pretty easy one.

Question:

What subnet and broadcast address is the IP address 172.16.66.10 255.255.192.0 (/18) a member of?

Answer:

The interesting octet is the third octet instead of the fourth octet. 256 – 192 = 64. 0, 64, 128. The subnet is 172.16.64.0. The broadcast must be 172.16.127.255 since 128.0 is the next subnet.

Question:

What subnet and broadcast address is the IP address 172.16.50.10 255.255.224.0 (/19) a member of?

Answer:

256 – 224 = 0, 32, 64 (remember, we always start counting at zero [0]). The subnet is 172.16.32.0, and the broadcast must be

172.16.63.255 since 64.0 is the next subnet.

Question:

What subnet and broadcast address is the IP address 172.16.46.255 255.255.240.0 (/20) a member of?

Answer:

256 – 240 = 16. The third octet is interesting to us. 0, 16, 32, 48. This subnet address must be in the 172.16.32.0 subnet, and the broadcast must be 172.16.47.255 since 48.0 is the next subnet. So, yes, 172.16.46.255 is a valid host.

Question:

What subnet and broadcast address is the IP address 172.16.45.14 255.255.255.252 (/30) a member of?

Answer:

Where is the interesting octet? 256 – 252 = 0, 4, 8, 12, 16 (in the fourth octet). The subnet is 172.16.45.12, with a broadcast of

172.16.45.15 because the next subnet is 172.16.45.16.

Question:

What is the subnet and broadcast address of the host 172.16.88.255/20?

Answer:

What is a /20? If you can’t answer this, you can’t answer this question, can you? A /20 is 255.255.240.0, which gives us a block size of 16 in the third octet, and since no subnet bits are on in the fourth octet, the answer is always 0 and 255 in the fourth octet. 0, 16, 32, 48,

64, 80, 96…bingo. 88 is between 80 and 96, so the subnet is 80.0 and the broadcast address is 95.255.

Question:

A router receives a packet on an interface with a destination address of 172.16.46.191/26. What will the router do with this packet?

Answer:

Discard it. Do you know why? 172.16.46.191/26 is a 255.255.255.192 mask, which gives us a block size of 64. Our subnets are then 0, 64, 128, 192. 191 is the broadcast address of the 128 subnet, so a router, by default, will discard any broadcast packets.

Subnetting Class A Addresses

Class A subnetting is not performed any differently than Classes B and C, but there are 24 bits to play with instead of the 16 in a Class B address and the 8 in a Class C address.

Let’s start by listing all the Class A masks:

255.0.0.0 (/8)

255.128.0.0 (/9) 255.255.240.0 (/20)

255.192.0.0 (/10) 255.255.248.0 (/21)

255.224.0.0 (/11) 255.255.252.0 (/22)

255.240.0.0 (/12) 255.255.254.0 (/23)

255.248.0.0 (/13) 255.255.255.0 (/24)

255.252.0.0 (/14) 255.255.255.128 (/25)

255.254.0.0 (/15) 255.255.255.192 (/26)

255.255.0.0 (/16) 255.255.255.224 (/27)

255.255.128.0 (/17) 255.255.255.240 (/28)

255.255.192.0 (/18) 255.255.255.248 (/29)

255.255.224.0 (/19) 255.255.255.252 (/30)

That’s it. You must leave at least 2 bits for defining hosts. And I hope you can see the pattern by now. Remember, we’re going to do this the same way as a Class B or C subnet. It’s just that, again, we simply have more host bits and we just use the same subnet numbers we used with Class B and C, but we start using these numbers in the second octet.

Subnetting Practice Examples: Class A Addresses

When you look at an IP address and a subnet mask, you must be able to distinguish the bits used for subnets from the bits used for determining hosts. This is imperative. If you’re still struggling with this concept, please reread the section “IP Addressing” in Chapter 3. It shows you how to determine the difference between the subnet and host bits and should help clear things up.

Practice Example #1A: 255.255.0.0 (/16)

Class A addresses use a default mask of 255.0.0.0, which leaves 22 bits for subnetting since you must leave 2 bits for host addressing. The

255.255.0.0 mask with a Class A address is using 8 subnet bits.

Subnets?

2

8

= 256.

Hosts?

2

16

– 2 = 65,534.

Valid subnets?

What is the interesting octet? 256 – 255 = 1. 0, 1, 2, 3, etc. (all in the second octet). The subnets would be 10.0.0.0, 10.1.0.0,

10.2.0.0, 10.3.0.0, etc., up to 10.255.0.0.

Broadcast address for each subnet?

Valid hosts?

The following table shows the first two and last two subnets, valid host range, and broadcast addresses for the private Class A 10.0.0.0 network:

Practice Example #2A: 255.255.240.0 (/20)

255.255.240.0 gives us 12 bits of subnetting and leaves us 12 bits for host addressing.

Subnets?

2

12

= 4096.

Hosts?

2

12

– 2 = 4094.

Valid subnets?

What is your interesting octet? 256 – 240 = 16. The subnets in the second octet are a block size of 1 and the subnets in the third octet are 0, 16, 32, etc.

Broadcast address for each subnet?

Valid hosts?

The following table shows some examples of the host ranges—the first three and the last subnets:

Practice Example #3A: 255.255.255.192 (/26)

Let’s do one more example using the second, third, and fourth octets for subnetting.

Subnets?

2

18

= 262,144.

Hosts?

2

6

– 2 = 62.

Valid subnets?

In the second and third octet, the block size is 1, and in the fourth octet, the block size is 64.

Broadcast address for each subnet?

Valid hosts?

The following table shows the first four subnets and their valid hosts and broadcast addresses in the Class A 255.255.255.192 mask:

The following table shows the last four subnets and their valid hosts and broadcast addresses:

Subnetting in Your Head: Class A Addresses

This sounds hard, but as with Class C and Class B, the numbers are the same; we just start in the second octet. What makes this easy? You only need to worry about the octet that has the largest block size (typically called the interesting octet; one that is something other than 0 or 255)—for example, 255.255.240.0 (/20) with a Class A network. The second octet has a block size of 1, so any number listed in that octet is a subnet. The third octet is a 240 mask, which means we have a block size of 16 in the third octet. If your host ID is 10.20.80.30, what is your subnet, broadcast address, and valid host range?

The subnet in the second octet is 20 with a block size of 1, but the third octet is in block sizes of 16, so we’ll just count them out: 0, 16, 32, 48, 64,

80, 96…voilà! (By the way, you can count by 16s by now, right?) This makes our subnet 10.20.80.0, with a broadcast of 10.20.95.255 because the next subnet is 10.20.96.0. The valid host range is 10.20.80.1 through 10.20.95.254. And yes, no lie! You really can do this in your head if you just get your block sizes nailed!

Okay, let’s practice on one more, just for fun!

Host IP: 10.1.3.65/23

First, you can’t answer this question if you don’t know what a /23 is. It’s 255.255.254.0. The interesting octet here is the third one: 256 – 254 = 2.

Our subnets in the third octet are 0, 2, 4, 6, etc. The host in this question is in subnet 2.0, and the next subnet is 4.0, so that makes the broadcast address 3.255. And any address between 10.1.2.1 and 10.1.3.254 is considered a valid host.

Summary

Did you read Chapters 3 and 4 and understand everything on the first pass? If so, that is fantastic—congratulations! The thing is, you probably got lost a couple of times—and as I told you, that’s what usually happens, so don’t stress. Don’t feel bad if you have to read each chapter more than once, or even 10 times, before you’re truly good to go.

This chapter provided you with an important understanding of IP subnetting. After reading this chapter, you should be able to subnet IP addresses in your head.

This chapter is extremely essential to your Cisco certification process, so if you just skimmed it, please go back and read it and do all the written labs.

Exam Essentials

Identify the advantages of subnetting.

Benefits of subnetting a physical network include reduced network traffic, optimized network performance, simplified management, and facilitated spanning of large geographical distances.

Describe the effect of the

ip subnet-zero

command.

This command allows you to use the first and last subnet in your network design.

Identify the steps to subnet a classful network.

Understand how IP addressing and subnetting work. First, determine your block size by using the 256-subnet mask math. Then count your subnets and determine the broadcast address of each subnet—it is always the number right before the next subnet. Your valid hosts are the numbers between the subnet address and the broadcast address.

Determine possible block sizes.

This is an important part of understanding IP addressing and subnetting. The valid block sizes are always

2, 4, 8, 16, 32, 64, 128, etc. You can determine your block size by using the 256-subnet mask math.

Describe the role of a subnet mask in IP addressing.

A subnet mask is a 32-bit value that allows the recipient of IP packets to distinguish

the network ID portion of the IP address from the host ID portion of the IP address.

Understand and apply the 2

n

– 2 formula.

Use this formula to determine the proper subnet mask for a particular size network given the application of that subnet mask to a particular classful network.

Explain the impact of Classless Inter-Domain Routing (CIDR).

CIDR allows the creation of networks of a size other than those allowed with the classful subnetting by allowing more than the three classful subnet masks.

Written Labs

In this section, you’ll complete the following labs to make sure you’ve got the information and concepts contained within them fully dialed in:

Lab 4.1: Written Subnet Practice #1

Lab 4.2: Written Subnet Practice #2

Lab 4.3: Written Subnet Practice #3

(The answers to the written labs can be found following the answers to the review questions for this chapter.)

Written Lab 4.1: Written Subnet Practice #1

Write the subnet, broadcast address, and valid host range for question 1 through question 6:

1.

192.168.100.25/30

2.

192.168.100.37/28

3.

192.168.100.66/27

4.

192.168.100.17/29

5.

192.168.100.99/26

6.

192.168.100.99/25

7.

You have a Class B network and need 29 subnets. What is your mask?

8.

What is the broadcast address of 192.168.192.10/29?

9.

How many hosts are available with a Class C /29 mask?

10.

What is the subnet for host ID 10.16.3.65/23?

/24

/25

/26

/27

/28

/29

/30

/20

/21

/22

/23

/16

/17

/18

/19

Written Lab 4.2: Written Subnet Practice #2

Given a Class B network and the net bits identified (CIDR), complete the following table to identify the subnet mask and the number of host addresses possible for each mask.

Classful Address Subnet Mask Number of Hosts per Subnet (2x – 2)

Written Lab 4.3: Written Subnet Practice #3

Complete the following based on the decimal IP address.

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s

Introduction.

1. What is the maximum number of IP addresses that can be assigned to hosts on a local subnet that uses the 255.255.255.224 subnet mask?

A. 14

B. 15

C. 16

D. 30

E. 31

F. 62

2. You have a network that needs 29 subnets while maximizing the number of host addresses available on each subnet. How many bits must you borrow from the host field to provide the correct subnet mask?

A. 2

B. 3

C. 4

D. 5

E. 6

F. 7

3. What is the subnetwork address for a host with the IP address 200.10.5.68/28?

A. 200.10.5.56

B. 200.10.5.32

C. 200.10.5.64

D. 200.10.5.0

4. The network address of 172.16.0.0/19 provides how many subnets and hosts?

A. 7 subnets, 30 hosts each

B. 7 subnets, 2,046 hosts each

C. 7 subnets, 8,190 hosts each

D. 8 subnets, 30 hosts each

E. 8 subnets, 2,046 hosts each

F. 8 subnets, 8,190 hosts each

5. Which two statements describe the IP address 10.16.3.65/23? (Choose two.)

A. The subnet address is 10.16.3.0 255.255.254.0.

B. The lowest host address in the subnet is 10.16.2.1 255.255.254.0.

C. The last valid host address in the subnet is 10.16.2.254 255.255.254.0.

D. The broadcast address of the subnet is 10.16.3.255 255.255.254.0.

E. The network is not subnetted.

6. If a host on a network has the address 172.16.45.14/30, what is the subnetwork this host belongs to?

A. 172.16.45.0

B. 172.16.45.4

C. 172.16.45.8

D. 172.16.45.12

E. 172.16.45.16

7. Which mask should you use on point-to-point WAN links in order to reduce the waste of IP addresses?

A. /27

B. /28

C. /29

D. /30

E. /31

8. What is the subnetwork number of a host with an IP address of 172.16.66.0/21?

A. 172.16.36.0

B. 172.16.48.0

C. 172.16.64.0

D. 172.16.0.0

9. You have an interface on a router with the IP address of 192.168.192.10/29. Including the router interface, how many hosts can have IP addresses on the LAN attached to the router interface?

A. 6

B. 8

C. 30

D. 62

E. 126

10. You need to configure a server that is on the subnet 192.168.19.24/29. The router has the first available host address. Which of the following should you assign to the server?

A. 192.168.19.0 255.255.255.0

B. 192.168.19.33 255.255.255.240

C. 192.168.19.26 255.255.255.248

D. 192.168.19.31 255.255.255.248

E. 192.168.19.34 255.255.255.240

11. You have an interface on a router with the IP address of 192.168.192.10/29. What is the broadcast address the hosts will use on this LAN?

A. 192.168.192.15

B. 192.168.192.31

C. 192.168.192.63

D. 192.168.192.127

E. 192.168.192.255

12. You need to subnet a network that has 5 subnets, each with at least 16 hosts. Which classful subnet mask would you use?

A. 255.255.255.192

B. 255.255.255.224

C. 255.255.255.240

D. 255.255.255.248

13. You configure a router interface with the IP address 192.168.10.62 255.255.255.192 and receive the following error:

Bad mask /26 for address 192.168.10.62

Why did you receive this error?

A. You typed this mask on a WAN link and that is not allowed.

B. This is not a valid host and subnet mask combination.

C. ip subnet-zero

is not enabled on the router.

D. The router does not support IP.

14. If an Ethernet port on a router were assigned an IP address of 172.16.112.1/25, what would be the valid subnet address of this interface?

A. 172.16.112.0

B. 172.16.0.0

C. 172.16.96.0

D. 172.16.255.0

E. 172.16.128.0

15. Using the following illustration, what would be the IP address of E0 if you were using the eighth subnet? The network ID is 192.168.10.0/28 and

you need to use the last available IP address in the range. The zero subnet should not be considered valid for this question.

A. 192.168.10.142

B. 192.168.10.66

C. 192.168.100.254

D. 192.168.10.143

E. 192.168.10.126

16. Using the illustration from the previous question, what would be the IP address of S0 if you were using the first subnet? The network ID is

192.168.10.0/28 and you need to use the last available IP address in the range. Again, the zero subnet should not be considered valid for this question.

A. 192.168.10.24

B. 192.168.10.62

C. 192.168.10.30

D. 192.168.10.127

17. Which configuration command must be in effect to allow the use of 8 subnets if the Class C subnet mask is 255.255.255.224?

A.

Router(config)#

ip classless

B.

Router(config)#

ip version 6

C.

Router(config)#

no ip classful

D.

Router(config)#

ip unnumbered

E.

Router(config)#

ip subnet-zero

F.

Router(config)#

ip all-nets

18. You have a network with a subnet of 172.16.17.0/22. Which is the valid host address?

A. 172.16.17.1 255.255.255.252

B. 172.16.0.1 255.255.240.0

C. 172.16.20.1 255.255.254.0

D. 172.16.16.1 255.255.255.240

E. 172.16.18.255 255.255.252.0

F. 172.16.0.1 255.255.255.0

19. Your router has the following IP address on Ethernet0: 172.16.2.1/23. Which of the following can be valid host IDs on the LAN interface attached to the router? (Choose two.)

A. 172.16.0.5

B. 172.16.1.100

C. 172.16.1.198

D. 172.16.2.255

E. 172.16.3.0

F. 172.16.3.255

20. To test the IP stack on your local host, which IP address would you ping?

A. 127.0.0.0

B. 1.0.0.127

C. 127.0.0.1

D. 127.0.0.255

E. 255.255.255.255

Answers to Review Questions

1. D. A /27 (255.255.255.224) is 3 bits on and 5 bits off. This provides 8 subnets, each with 30 hosts. Does it matter if this mask is used with a

Class A, B, or C network address? Not at all. The number of host bits would never change.

2. D. A 240 mask is 4 subnet bits and provides 16 subnets, each with 14 hosts. We need more subnets, so let’s add subnet bits. One more subnet bit would be a 248 mask. This provides 5 subnet bits (32 subnets) with 3 host bits (6 hosts per subnet). This is the best answer.

3. C. This is a pretty simple question. A /28 is 255.255.255.240, which means that our block size is 16 in the fourth octet. 0, 16, 32, 48, 64, 80, etc.

The host is in the 64 subnet.

4. F. A CIDR address of /19 is 255.255.224.0. This is a Class B address, so that is only 3 subnet bits, but it provides 13 host bits, or 8 subnets, each with 8,190 hosts.

5. B, D. The mask 255.255.254.0 (/23) used with a Class A address means that there are 15 subnet bits and 9 host bits. The block size in the third octet is 2 (256 – 254). So this makes the subnets in the interesting octet 0, 2, 4, 6, etc., all the way to 254. The host 10.16.3.65 is in the 2.0 subnet.

The next subnet is 4.0, so the broadcast address for the 2.0 subnet is 3.255. The valid host addresses are 2.1 through 3.254.

6. D. A /30, regardless of the class of address, has a 252 in the fourth octet. This means we have a block size of 4 and our subnets are 0, 4, 8, 12,

16, etc. Address 14 is obviously in the 12 subnet.

7. D. A point-to-point link uses only two hosts. A /30, or 255.255.255.252, mask provides two hosts per subnet.

8. C. A /21 is 255.255.248.0, which means we have a block size of 8 in the third octet, so we just count by 8 until we reach 66. The subnet in this question is 64.0. The next subnet is 72.0, so the broadcast address of the 64 subnet is 71.255.

9. A. A /29 (255.255.255.248), regardless of the class of address, has only 3 host bits. Six hosts is the maximum number of hosts on this LAN, including the router interface.

10. C. A /29 is 255.255.255.248, which is a block size of 8 in the fourth octet. The subnets are 0, 8, 16, 24, 32, 40, etc. 192.168.19.24 is the 24 subnet, and since 32 is the next subnet, the broadcast address for the 24 subnet is 31. 192.168.19.26 is the only correct answer.

11. A. A /29 (255.255.255.248) has a block size of 8 in the fourth octet. This means the subnets are 0, 8, 16, 24, etc. 10 is in the 8 subnet. The next subnet is 16, so 15 is the broadcast address.

12. B. You need 5 subnets, each with at least 16 hosts. The mask 255.255.255.240 provides 16 subnets with 14 hosts—this will not work. The mask 255.255.255.224 provides 8 subnets, each with 30 hosts. This is the best answer.

13. C. First, you cannot answer this question if you can’t subnet. The 192.168.10.62 with a mask of 255.255.255.192 is a block size of 64 in the fourth octet. The host 192.168.10.62 is in the zero subnet, and the error occurred because ip subnet-zero

is not enabled on the router.

14. A. A /25 mask is 255.255.255.128. Used with a Class B network, the third and fourth octets are used for subnetting with a total of 9 subnet bits,

8 bits in the third octet and 1 bit in the fourth octet. Since there is only 1 bit in the fourth octet, the bit is either off or on—which is a value of 0 or 128.

The host in the question is in the 0 subnet, which has a broadcast address of 127 since 112.128 is the next subnet.

15. A. A /28 is a 255.255.255.240 mask. Let’s count to the ninth subnet (we need to find the broadcast address of the eighth subnet, so we need to

count to the ninth subnet). Starting at 16 (remember, the question stated that we will not use subnet zero, so we start at 16, not 0), 16, 32, 48, 64,

80, 96, 112, 128, 144. The eighth subnet is 128 and the next subnet is 144, so our broadcast address of the 128 subnet is 143. This makes the host range 129–142. 142 is the last valid host.

16. C. A /28 is a 255.255.255.240 mask. The first subnet is 16 (remember that the question stated not to use subnet zero) and the next subnet is

32, so our broadcast address is 31. This makes our host range 17–30. 30 is the last valid host.

17. E. A Class C subnet mask of 255.255.255.224 is 3 bits on and 5 bits off (11100000) and provides 8 subnets, each with 30 hosts. However, if the command ip subnet-zero

is not used, then only 6 subnets would be available for use.

18. E. A Class B network ID with a /22 mask is 255.255.252.0, with a block size of 4 in the third octet. The network address in the question is in subnet 172.16.16.0 with a broadcast address of 172.16.19.255. Only option E has the correct subnet mask listed, and 172.16.18.255 is a valid host.

19. D, E. The router’s IP address on the E0 interface is 172.16.2.1/23, which is 255.255.254.0. This makes the third octet a block size of 2. The router’s interface is in the 2.0 subnet, and the broadcast address is 3.255 because the next subnet is 4.0. The valid host range is 2.1 through 3.254.

The router is using the first valid host address in the range.

20. C. To test the local stack on your host, ping the loopback interface of 127.0.0.1.

Answers to Written Lab 4.1

1.

192.168.100.25/30. A /30 is 255.255.255.252. The valid subnet is 192.168.100.24, broadcast is 192.168.100.27, and valid hosts are

192.168.100.25 and 26.

2.

192.168.100.37/28. A /28 is 255.255.255.240. The fourth octet is a block size of 16. Just count by 16s until you pass 37. 0, 16, 32, 48.

The host is in the 32 subnet, with a broadcast address of 47. Valid hosts 33–46.

3.

A /27 is 255.255.255.224. The fourth octet is a block size of 32. Count by 32s until you pass the host address of 66. 0, 32, 64, 96. The host is in the 64 subnet, and the broadcast address of 95. Valid host range of 65-94.

4.

192.168.100.17/29. A /29 is 255.255.255.248. The fourth octet is a block size of 8. 0, 8, 16, 24. The host is in the 16 subnet, broadcast of 23. Valid hosts 17–22.

5.

192.168.100.99/26. A /26 is 255.255.255.192. The fourth octet has a block size of 64. 0, 64, 128. The host is in the 64 subnet, broadcast of 127. Valid hosts 65–126.

6.

192.168.100.99/25. A /25 is 255.255.255.128. The fourth octet is a block size of 128. 0, 128. The host is in the 0 subnet, broadcast of

127. Valid hosts 1–126.

7.

A default Class B is 255.255.0.0. A Class B 255.255.255.0 mask is 256 subnets, each with 254 hosts. We need fewer subnets. If we used 255.255.240.0, this provides 16 subnets. Let’s add one more subnet bit. 255.255.248.0. This is 5 bits of subnetting, which provides

32 subnets. This is our best answer, a /21.

8.

A /29 is 255.255.255.248. This is a block size of 8 in the fourth octet. 0, 8, 16. The host is in the 8 subnet, broadcast is 15.

9.

A /29 is 255.255.255.248, which is 5 subnet bits and 3 host bits. This is only 6 hosts per subnet.

10.

A /23 is 255.255.254.0. The third octet is a block size of 2. 0, 2, 4. The subnet is in the 16.2.0 subnet; the broadcast address is

16.3.255.

Answers to Written Lab 4.2

Classful Address Subnet Mask Number of Hosts per Subnet (2n – 2)

/20

/21

/22

/23

/24

/25

/16

/17

/18

/19

/26

/27

/28

/29

/30

255.255.0.0

65,534

255.255.128.0

32,766

255.255.192.0

16,382

255.255.224.0

8,190

255.255.240.0

4,094

255.255.248.0

2,046

255.255.252.0

1,022

255.255.254.0

510

255.255.255.0

254

255.255.255.128 126

255.255.255.192 62

255.255.255.224 30

255.255.255.240 14

255.255.255.248 6

255.255.255.252 2

Answers to Written Lab 4.3

Chapter 5

Variable Length Subnet Masks (VLSMs), Summarization, and Troubleshooting

TCP/IP

The CCNA exam topics covered in this chapter include the following:

Implement an IP addressing scheme and IP Services to meet network requirements in a medium-size Enterprise branch office network.

Implement static and dynamic addressing services for hosts in a LAN environment.

Calculate and apply an addressing scheme including VLSM IP addressing design to a network.

Determine the appropriate classless addressing scheme using VLSM and summarization to satisfy addressing requirements in a

LAN/WAN environment.

Identify and correct common problems associated with IP addressing and host configurations.

After our discussion of IP subnetting in the last two chapters, I’m now going to tell you all about Variable Length Subnet Masks (VLSMs) as well as show you how to design and implement a network using VLSM networks.

Once you have mastered VLSM design and implementation, I’ll show you how to summarize classful boundaries. We’ll go into this further in

Chapter 9, “Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF),” where I’ll demonstrate summarizing using EIGRP and OSPF routing protocols.

I’ll wrap up the chapter by going over IP address troubleshooting and take you through the steps Cisco recommends when troubleshooting an IP network.

So get psyched—you’re about to go for quite a ride! This chapter will truly help you understand IP addressing and networking, so don’t get discouraged or give up. If you stick with it, I promise that one day you’ll look back on this and you’ll be really glad you decided to hang on. It’s one of those things that after you understand it, you’ll wonder why you once thought it was so hard. Ready? Let’s go!

For up-to-the-minute updates for this chapter, please see www.lammle.com

or www.sybex.com/go/ccna7e

.

Variable Length Subnet Masks (VLSMs)

In this chapter I’m going to show you a simple way to take one network and create many networks using subnet masks of different lengths on different types of network designs. This is called VLSM networking, and it does bring up another subject I mentioned in Chapter 4: classful and classless networking.

Neither RIPv1 nor IGRP routing protocols have a field for subnet information, so the subnet information gets dropped. What this means is that if a router running RIP has a subnet mask of a certain value, it assumes that

all

interfaces within the classful address space have the same subnet mask. This is called classful routing, and RIP and IGRP are both considered classful routing protocols. (I’ll be talking more about RIP and IGRP in

Chapter 8, “IP Routing.”) If you mix and match subnet mask lengths in a network running RIP or IGRP, that network just won’t work!

Classless routing protocols, however, do support the advertisement of subnet information. Therefore, you can use VLSM with routing protocols such as RIPv2, EIGRP, and OSPF. (EIGRP and OSPF will be discussed in Chapter 9.) The benefit of this type of network is that you save a bunch of IP address space with it.

As the name suggests, with VLSMs we can have different length subnet masks for different router interfaces. Look at

Figure 5-1 to see an example of why classful network designs are inefficient.

Figure 5-1:

Typical classful network

Looking at Figure 5-1 , you’ll notice that we have two routers, each with two LANs and connected together with a WAN serial link. In a typical classful network design (RIP or IGRP routing protocols), you could subnet a network like this:

192.168.10.0 = Network

255.255.255.240 (/28) = Mask

Our subnets would be (you know this part, right?) 0, 16, 32, 48, 64, 80, etc. This allows us to assign 16 subnets to our internetwork. But how many hosts would be available on each network? Well, as you probably know by now, each subnet provides only 14 hosts. This means that each LAN has

14 valid hosts available—one LAN doesn’t even have enough addresses needed for all the hosts! But the point-to-point WAN link also has 14 valid hosts. It’s too bad we can’t just nick some valid hosts from that WAN link and give them to our LANs!

All hosts and router interfaces have the same subnet mask—again, this is called classful routing. And if we want this network to be more efficient, we definitely need to add different masks to each router interface.

But there’s still another problem—the link between the two routers will never use more than two valid hosts! This wastes valuable IP address space, and it’s the big reason I’m going to talk to you about VLSM network design.

VLSM Design

Let’s take

Figure 5-1 and use a classless design…which will become the new network shown in Figure 5-2 . In the previous example, we wasted address space—one LAN didn’t have enough addresses because every router interface and host used the same subnet mask. Not so good. What would be good is to provide only the needed number of hosts on each router interface. To do this, we use what are referred to as Variable Length

Subnet Masks (VLSMs).

Figure 5-2:

Classless network design

Now remember that we can use different size masks on each router interface. And if we use a /30 on our WAN links and a /27, /28, and /29 on our LANs, we’ll get 2 hosts per WAN interface, and 30, 14, and 8 hosts per LAN interface—nice! This makes a huge difference—not only can we get just the right amount of hosts on each LAN, we still have room to add more WANs and LANs using this same network!

Remember, in order to implement a VLSM design on your network, you need to have a routing protocol that sends subnet mask information with the route updates. This would be RIPv2, EIGRP, and OSPF. RIPv1 and IGRP will not work in classless networks and are considered classful routing protocols.

Implementing VLSM Networks

To create VLSMs quickly and efficiently, you need to understand how block sizes and charts work together to create the VLSM masks.

Table 5-1 shows you the block sizes used when creating VLSMs with Class C networks. For example, if you need 25 hosts, then you’ll need a block size of

32. If you need 11 hosts, you’ll use a block size of 16. Need 40 hosts? Then you’ll need a block of 64. You cannot just make up block sizes—they’ve got to be the block sizes shown in Table 5-1 . So memorize the block sizes in this table—it’s easy. They’re the same numbers we used with

subnetting!

Why Bother with VLSM Design?

You have just been hired by a new company and need to add on to the existing network. There is no problem with starting over with a new IP address scheme. Should you use a VLSM classless network or a classful network?

Let’s just say you happen to have plenty of address space because you are using the Class A 10.0.0.0 private network address in your corporate environment and can’t even come close to imagining that you’d ever run out of IP addresses. Why would you want to bother with the VLSM design process?

Good question. There’s a good answer too!

Because by creating contiguous blocks of addresses to specific areas of your network, you can then easily summarize your network and keep route updates with a routing protocol to a minimum. Why would anyone want to advertise hundreds of networks between buildings when you can just send one summary route between buildings and achieve the same result?

If you’re confused about what summary routes are, let me explain. Summarization, also called supernetting, provides route updates in the most efficient way possible by advertising many routes in one advertisement instead of individually. This saves a ton of bandwidth and minimizes router processing. As always, you use blocks of addresses (remember that block sizes are used in all sorts of networks) to configure your summary routes and watch your network’s performance hum.

But know that summarization works only if you design your network carefully. If you carelessly hand out IP subnets to any location on the network, you’ll notice straight away that you no longer have any summary boundaries. And you won’t get very far with creating summary routes without those, so watch your step!

Table 5-1:

Block sizes

The next step is to create a VLSM table. Figure 5-3 shows you the table used in creating a VLSM network. The reason we use this table is so we don’t accidentally overlap networks.

Figure 5-3:

The VLSM table

You’ll find the sheet shown in Figure 5-3 very valuable because it lists every block size you can use for a network address. Notice that the block sizes are listed starting from a block size of 4 all the way to a block size of 128. If you have two networks with block sizes of 128, you’ll quickly see that you can have only two networks. With a block size of 64, you can have only four networks, and so on, all the way to having 64 networks if you use only block sizes of 4. Remember that this takes into account that you are using the command ip subnet-zero

in your network design.

Now, just fill in the chart in the lower-left corner, and then add the subnets to the worksheet and you’re good to go.

So let’s take what we’ve learned so far about our block sizes and VLSM table and create a VLSM network using a Class C network address

192.168.10.0 for the network in Figure 5-4 . Then fill out the VLSM table, as shown in Figure 5-5 .

Figure 5-4:

VLSM network example 1

In Figure 5-4 , we have four WAN links and four LANs connected together. We need to create a VLSM network that will allow us to save address space. Looks like we have two block sizes of 32, a block size of 16, and a block size of 8, and our WANs each have a block size of 4. Take a look and see how I filled out our VLSM chart in Figure 5-5 .

We still have plenty of room for growth with this VLSM network design.

We never could accomplish that with one subnet mask using classful routing. Let’s do another one.

Figure 5-6 shows a network with 11 networks, two block sizes of 64, one of 32, five of 16, and three of 4.

First, create your VLSM table and use your block size chart to fill in the table with the subnets you need.

Figure 5-7 shows a possible solution.

Figure 5-5:

VLSM table example 1

Notice that we filled in this entire chart and only have room for one more block size of 4! Only with a VLSM network can you provide this type of address space savings.

Keep in mind that it doesn’t matter where you start your block sizes as long as you always count from zero. For example, if you had a block size of 16, you must start at 0 and count from there—0, 16, 32, 48, etc. You can’t start a block size of 16 from, say, 40 or anything other than increments of 16.

Figure 5-6:

VLSM network example 2

Here’s another example. If you had block sizes of 32, you must start at zero like this: 0, 32, 64, 96, etc. Just remember that you don’t get to start wherever you want; you must always start counting from zero. In the example in Figure 5-7 , I started at 64 and 128, with my two block sizes of 64. I didn’t have much choice because my options are 0, 64, 128, and 192. However, I added the block size of 32, 16, 8, and 4 wherever I wanted just as long as they were in the correct increments of that block size.

Okay—you have three locations you need to address, and the IP network you have received is 192.168.55.0 to use as the addressing for the entire network. You’ll use ip subnet-zero

and RIPv2 as the routing protocol. (RIPv2 supports VLSM networks, RIPv1 does not—both of them will be discussed in Chapter 8.) Figure 5-8 shows the network diagram and the IP address of the RouterA S0/0 interface.

From the list of IP addresses on the right of the figure, which IP address will be placed in each router’s FastEthernet 0/0 interface and serial 0/1 of RouterB?

To answer this question, first look for clues in

Figure 5-8 . The first clue is that interface S0/0 on RouterA has IP address 192.168.55.2/30 assigned, which makes for an easy answer. A /30, as you know, is 255.255.255.252, which gives you a block size of 4. Your subnets are 0, 4, 8, etc. Since the known host has an IP address of 2, the only other valid host in the zero subnet is 1, so the third answer down is what you want for the

S0/1 interface of RouterB.

The next clues are the listed number of hosts for each of the LANs. RouterA needs 7 hosts, a block size of 16 (/28); RouterB needs 90 hosts, a block size of 128 (/25); and RouterC needs 23 hosts, a block size of 32 (/27).

Figure 5-9 shows the answers to this question.

Figure 5-7:

VLSM table example 2

Once you figured out the block size needed for each LAN, this was actually a pretty simple question—all you need to do is look for the right clues and, of course, know your block sizes.

One last example of VLSM design before we move on to summarization.

Figure 5-10 shows three routers, all running RIPv2. Which Class C addressing scheme would you use to satisfy the needs of this network yet save as much address space as possible?

Figure 5-8:

VLSM design example 1

Figure 5-9:

Solution to VLSM design example 1

Figure 5-10:

VLSM design example 2

This is a really sweet network, just waiting for you to fill out the chart. There are block sizes of 64, 32, and 16 and two block sizes of 4. This should be a slam dunk for you. Take a look at my answer in Figure 5-11 .

Figure 5-11:

Solution to VLSM design example 2

This is what I did: Starting at subnet 0, I used the block size of 64. (I didn’t have to—I could have started with a block size of 4, but I usually like to start with the largest block size and move to the smallest.) Okay, then I added the block sizes of 32 and 16 and the two block sizes of 4. There’s still a lot of room to add subnets to this network—very cool!

Summarization

Summarization, also called route aggregation, allows routing protocols to advertise many networks as one address. The purpose of this is to reduce the size of routing tables on routers to save memory, which also shortens the amount of time for IP to parse the routing table and find the

path to a remote network.

Figure 5-12 shows how a summary address would be used in an internetwork.

Figure 5-12:

Summary address used in an internetwork

Summarization is actually somewhat simple because all you really need to have down are the block sizes that we just used to learn subnetting and VLSM design. For example, if you wanted to summarize the following networks into one network advertisement, you just have to find the block size first; then you can easily find your answer:

192.168.16.0 through network 192.168.31.0

What’s the block size? There are exactly 16 Class C networks, so this neatly fits into a block size of 16.

Okay, now that you know the block size, you can find the network address and mask used to summarize these networks into one advertisement.

The network address used to advertise the summary address is always the first network address in the block—in this example, 192.168.16.0. To figure out a summary mask, in this same example, what mask is used to get a block size of 16? Yes, 240 is correct. This 240 would be placed in the third octet—the octet where we are summarizing. So, the mask would be 255.255.240.0.

You’ll learn how to apply these summary addresses to a router in Chapter 9.

Here’s another example:

Networks 172.16.32.0 through 172.16.50.0

This is not as clean as the previous example because there are two possible answers, and here’s why: Since you’re starting at network 32, your options for block sizes are 4, 8, 16, 32, 64, etc., and block sizes of 16 and 32 could work as this summary address.

Answer #1:

If you used a block size of 16, then the network address is 172.16.32.0 with a mask of 255.255.240.0 (240 provides a block of

16). However, this only summarizes from 32 to 47, which means that networks 48 through 50 would be advertised as single networks. This is probably the best answer, but that depends on your network design. Let’s look at the next answer.

Answer #2:

If you used a block size of 32, then your summary address would still be 172.16.32.0, but the mask would be 255.255.224.0 (224 provides a block of 32). The possible problem with this answer is that it will summarize networks 32 to 63 and we only have networks 32 to

50. No worries if you’re planning on adding networks 51 to 63 later into the same network, but you could have serious problems in your internetwork if somehow networks 51 to 63 were to show up and be advertised from somewhere else in your network! This is the reason answer number one is the safest answer.

Let’s take a look at another example, but let’s look at it from a host’s perspective.

Your summary address is 192.168.144.0/20—what’s the range of host addresses that would be forwarded according to this summary? The /20 provides a summary address of 192.168.144.0 and mask of 255.255.240.0.

The third octet has a block size of 16, and starting at summary address 144, the next block of 16 is 160, so our network summary range is 144 to

159 in the third octet (again, you

must

be able to count in 16s!).

A router that has this summary address in the routing table will forward any packet with destination IP addresses of 192.168.144.1 through

192.168.159.254.

Only two more summarization examples, then we’ll move on to troubleshooting.

In

Figure 5-13 , the Ethernet networks connected to router R1 are being summarized to R2 as 192.168.144.0/20. Which range of IP addresses will R2 forward to R1 according to this summary?

Figure 5-13:

Summarization example 1

No worries—this is really an easier question than it looks. The question actually has the summary address listed: 192.168.144.0/20. You already know that /20 is 255.255.240.0, which means you’ve got a block size of 16 in the third octet. Starting at 144 (this is also right there in the question), the next block size of 16 is 160, so you can’t go above 159 in the third octet. The IP addresses that will be forwarded are 192.168.144.1 through

192.168.159.254.

Okay, last one. In

Figure 5-14 , there are five networks connected to router R1. What’s the best summary address to R2?

Figure 5-14:

Summarization example 2

I’m going to be honest—this is a much harder question than the one in Figure 5-13 . You’re going to have to look pretty hard to see the answer.

The first thing to do with this is to write down all the networks and see if you can find anything in common with all six:

172.1.4.128/25

172.1.7.0/24

172.1.6.0/24

172.1.5.0/24

172.1.4.0/25

Do you see an octet that looks interesting to you? I do. It’s the third octet. 4, 5, 6, 7, and yes, it’s a block size of 4. So you can summarize

172.1.4.0 using a mask of 255.255.252.0, which means you will use a block size of 4 in the third octet. The IP addresses forwarded with this summary are 172.1.4.1 through 172.1.7.255.

Now to summarize this summarization section: Basically, if you’ve nailed down your block sizes, then finding and applying summary addresses and masks is actually fairly easy. But you’re going to get bogged down pretty quickly if you don’t know what a /20 is or if you can’t count by 16s!

Troubleshooting IP Addressing

Troubleshooting IP addressing is obviously an important skill because running into trouble somewhere along the way is pretty much a sure thing, and it’s going to happen to you. No—I’m not a pessimist; I’m just keeping it real. Because of this nasty fact, it will be great when you can save the day because you can both figure out (diagnose) the problem and fix it on an IP network whether you’re at work or at home!

So this is where I’m going to show you the “Cisco way” of troubleshooting IP addressing. Let’s use

Figure 5-15 as an example of your basic IP trouble—poor Sally can’t log in to the Windows server. Do you deal with this by calling the Microsoft team to tell them their server is a pile of junk and causing all your problems? Probably not such a great idea—let’s first double-check our network instead.

Okay let’s get started by going over the troubleshooting steps that Cisco follows. They’re pretty simple, but important nonetheless. Pretend you’re at a customer host and they’re complaining that they can’t communicate to a server that just happens to be on a remote network. Here are the four troubleshooting steps Cisco recommends:

1.

Open a Command window and ping 127.0.0.1. This is the diagnostic, or loopback, address, and if you get a successful ping, your IP stack is considered to be initialized. If it fails, then you have an IP stack failure and need to reinstall TCP/IP on the host.

C:\>

ping 127.0.0.1

Pinging 127.0.0.1 with 32 bytes of data:

Reply from 127.0.0.1: bytes=32 time<1ms TTL=128

Reply from 127.0.0.1: bytes=32 time<1ms TTL=128

Reply from 127.0.0.1: bytes=32 time<1ms TTL=128

Reply from 127.0.0.1: bytes=32 time<1ms TTL=128

Ping statistics for 127.0.0.1:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

Figure 5-15:

Basic IP troubleshooting

2.

From the Command window, ping the IP address of the local host. If that’s successful, your network interface card (NIC) is functioning. If it fails, there is a problem with the NIC. Success here doesn’t just mean that a cable is plugged into the NIC, only that the IP protocol stack on the host can communicate to the NIC (via the LAN driver).

C:\>

ping 172.16.10.2

Pinging 172.16.10.2 with 32 bytes of data:

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Reply from 172.16.10.2: bytes=32 time<1ms TTL=128

Ping statistics for 172.16.10.2:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

3.

From the DOS window, ping the default gateway (router). If the ping works, it means that the NIC is plugged into the network and can communicate on the local network. If it fails, you have a local physical network problem that could be anywhere from the NIC to the router.

C:\>

ping 172.16.10.1

Pinging 172.16.10.1 with 32 bytes of data:

Reply from 172.16.10.1: bytes=32 time<1ms TTL=128

Reply from 172.16.10.1: bytes=32 time<1ms TTL=128

Reply from 172.16.10.1: bytes=32 time<1ms TTL=128

Reply from 172.16.10.1: bytes=32 time<1ms TTL=128

Ping statistics for 172.16.10.1:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

4.

If steps 1 through 3 were successful, try to ping the remote server. If that works, then you know that you have IP communication between the local host and the remote server. You also know that the remote physical network is working.

C:\>

ping 172.16.20.2

Pinging 172.16.20.2 with 32 bytes of data:

Reply from 172.16.20.2: bytes=32 time<1ms TTL=128

Reply from 172.16.20.2: bytes=32 time<1ms TTL=128

Reply from 172.16.20.2: bytes=32 time<1ms TTL=128

Reply from 172.16.20.2: bytes=32 time<1ms TTL=128

Ping statistics for 172.16.20.2:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 0ms, Maximum = 0ms, Average = 0ms

If the user still can’t communicate with the server after steps 1 through 4 are successful, you probably have some type of name resolution problem and need to check your Domain Name System (DNS) settings. But if the ping to the remote server fails, then you know you have some type of remote physical network problem and need to go to the server and work through steps 1 through 3 until you find the snag.

Before we move on to determining IP address problems and how to fix them, I just want to mention some basic commands that you can use to help troubleshoot your network from both a PC and a Cisco router (the commands might do the same thing, but they are implemented differently).

Packet InterNet Groper (

ping

)

Uses ICMP echo request and replies to test if a node IP stack is initialized and alive on the network.

traceroute

Displays the list of routers on a path to a network destination by using TTL time-outs and ICMP error messages. This command will not work from a Command prompt.

tracert

Same function as traceroute

, but it’s a Microsoft Windows command and will not work on a Cisco router.

arp -a

Displays IP-to-MAC-address mappings on a Windows PC.

show ip arp

Same function as arp -a

, but displays the ARP table on a Cisco router. Like the commands traceroute and tracert, arp -a

and show ip arp are not interchangeable through DOS and Cisco.

ipconfig /all

Used only from a Command prompt, shows you the PC network configuration.

Once you’ve gone through all these steps and used the appropriate DOS commands, if necessary, what do you do if you find a problem? How do you go about fixing an IP address configuration error? Let’s move on and discuss how to determine the IP address problems and how to fix them.

Determining IP Address Problems

It’s common for a host, router, or other network device to be configured with the wrong IP address, subnet mask, or default gateway. Because this happens way too often, I’m going to teach you how to both determine and fix IP address configuration errors.

Once you’ve worked through the four basic steps of troubleshooting and determined there’s a problem, you obviously then need to find and fix it.

It really helps to draw out the network and IP addressing scheme. If it’s already done, consider yourself lucky and go buy a lottery ticket because although it should be done, it rarely is. And if it is, it’s usually outdated or inaccurate anyway. Typically it is not done, and you’ll probably just have to bite the bullet and start from scratch.

I’ll show you how to draw out your network using the Cisco Discovery Protocol (CDP) in Chapter 7, “Managing a Cisco Internetwork.”

Once you have your network accurately drawn out, including the IP addressing scheme, you need to verify each host’s IP address, mask, and default gateway address to determine the problem. (I’m assuming that you don’t have a physical layer problem or that if you did, you’ve already fixed it.)

Let’s check out the example illustrated in

Figure 5-16 . A user in the sales department calls and tells you that she can’t get to ServerA in the marketing department. You ask her if she can get to ServerB in the marketing department, but she doesn’t know because she doesn’t have rights to log on to that server. What do you do?

You ask the client to go through the four troubleshooting steps that you learned about in the preceding section. Steps 1 through 3 work, but step 4 fails. By looking at the figure, can you determine the problem? Look for clues in the network drawing. First, the WAN link between the Lab_A router and the Lab_B router shows the mask as a /27. You should already know that this mask is 255.255.255.224 and then determine that all networks are using this mask. The network address is 192.168.1.0. What are our valid subnets and hosts? 256 – 224 = 32, so this makes our subnets 32,

64, 96, 128, etc. So, by looking at the figure, you can see that subnet 32 is being used by the sales department, the WAN link is using subnet 96, and the marketing department is using subnet 64.

Figure 5-16:

IP address problem 1

Now you’ve got to determine what the valid host ranges are for each subnet. From what you learned at the beginning of this chapter, you should now be able to easily determine the subnet address, broadcast addresses, and valid host ranges. The valid hosts for the Sales LAN are 33 through

62—the broadcast address is 63 because the next subnet is 64, right? For the Marketing LAN, the valid hosts are 65 through 94 (broadcast 95), and for the WAN link, 97 through 126 (broadcast 127). By looking at the figure, you can determine that the default gateway on the Lab_B router is incorrect. That address is the broadcast address of the 64 subnet, so there’s no way it could be a valid host.

Did you get all that? Maybe we should try another one, just to make sure.

Figure 5-17 shows a network problem. A user in the Sales LAN can’t get to ServerB. You have the user run through the four basic troubleshooting steps and find that the host can communicate to the local network but not to the remote network. Find and define the IP addressing problem.

If you use the same steps used to solve the last problem, you can see first that the WAN link again provides the subnet mask to use— /29, or

255.255.255.248. Assuming classful addressing, you need to determine what the valid subnets, broadcast addresses, and valid host ranges are to solve this problem.

The 248 mask is a block size of 8 (256 – 248 = 8, as discussed in Chapter 4), so the subnets both start and increment in multiples of 8. By looking at the figure, you see that the Sales LAN is in the 24 subnet, the WAN is in the 40 subnet, and the Marketing LAN is in the 80 subnet. Can you see the problem yet? The valid host range for the Sales LAN is 25–30, and the configuration appears correct. The valid host range for the WAN link is 41–46, and this also appears correct. The valid host range for the 80 subnet is 81–86, with a broadcast address of 87 because the next subnet is 88. ServerB has been configured with the broadcast address of the subnet.

Figure 5-17:

IP address problem 2

Okay, now that you can figure out misconfigured IP addresses on hosts, what do you do if a host doesn’t have an IP address and you need to assign one? What you need to do is look at other hosts on the LAN and figure out the network, mask, and default gateway. Let’s take a look at a couple of examples of how to find and apply valid IP addresses to hosts.

You need to assign a server and router IP addresses on a LAN. The subnet assigned on that segment is 192.168.20.24/29, and the router needs to be assigned the first usable address and the server the last valid host ID. What are the IP address, mask, and default gateway assigned to the server?

To answer this, you must know that a /29 is a 255.255.255.248 mask, which provides a block size of 8. The subnet is known as 24, the next subnet in a block of 8 is 32, so the broadcast address of the 24 subnet is 31, which makes the valid host range 25–30.

Server IP address: 192.168.20.30

Server mask: 255.255.255.248

Default gateway: 192.168.20.25 (router’s IP address)

As another example, let’s take a look at

Figure 5-18 and solve this problem.

Look at the router’s IP address on Ethernet0. What IP address, subnet mask, and valid host range could be assigned to the host?

Figure 5-18:

Find the valid host #1.

The IP address of the router’s Ethernet0 is 192.168.10.33/27. As you already know, a /27 is a 224 mask with a block size of 32. The router’s interface is in the 32 subnet. The next subnet is 64, so that makes the broadcast address of the 32 subnet 63 and the valid host range 33–62.

Host IP address: 192.168.10.34–62 (any address in the range except for 33, which is assigned to the router)

Mask: 255.255.255.224

Default gateway: 192.168.10.33

Figure 5-19 shows two routers with Ethernet configurations already assigned. What are the host addresses and subnet masks of hosts A and B?

Figure 5-19:

Find the valid host #2.

RouterA has an IP address of 192.168.10.65/26 and RouterB has an IP address of 192.168.10.33/28. What are the host configurations?

RouterA Ethernet0 is in the 192.168.10.64 subnet and RouterB Ethernet0 is in the 192.168.10.32 network.

Host A IP address: 192.168.10.66–126

Host A mask: 255.255.255.192

Host A default gateway: 192.168.10.65

Host B IP address: 192.168.10.34–46

Host B mask: 255.255.255.240

Host B default gateway: 192.168.10.33

Just a couple more examples and then this chapter is history. Hang in there!

Figure 5-20 shows two routers; you need to configure the S0/0 interface on RouterA. The network assigned to the serial link is 172.16.17.0/22.

What IP address can be assigned?

Figure 5-20:

Find the valid host address #3.

First, you must know that a /22 CIDR is 255.255.252.0, which makes a block size of 4 in the third octet. Since 17 is listed, the available range is

16.1 through 19.254; so, for example, the IP address S0/0 could be 172.16.18.255 since that’s within the range.

Okay, last one! You have one Class C network ID and you need to provide one usable subnet per city while allowing enough usable host addresses for each city specified in Figure 5-21 . What is your mask?

Figure 5-21:

Find the valid subnet mask.

Actually, this is probably the easiest thing you’ve done all day! I count 5 subnets needed and the Wyoming office needs 16 users (always look for the network that needs the most hosts). What block size is needed for the Wyoming office? 32. (Remember, you cannot use a block size of 16 because you always have to subtract 2!) What mask provides you with a block size of 32? 224. Bingo! This provides 8 subnets, each with 30 hosts.

You’re done, the diva has sung, the chicken has crossed the road…whew! Okay, take a good break (but skip the shot and the beer for now), then come back and go through the written lab and review questions.

Summary

Did you read Chapters 3, 4, and 5 and understand everything on the first pass? If so, that is fantastic—congratulations! The thing is, you probably got lost a couple of times—and as I already mentioned, that’s what usually happens, so don’t stress. Don’t feel bad if you have to read each of these chapters more than once, or even 10 times, before you’re truly good to go.

This chapter provided you with an important understanding of Variable Length Subnet Masks. You should also know how to design and implement simple VLSM networks and summarization.

You should also understand the Cisco troubleshooting methods. You must remember the four steps that Cisco recommends you take when trying to narrow down exactly where a network/IP addressing problem is and then know how to proceed systematically in order to fix it. In addition, you should be able to find valid IP addresses and subnet masks by looking at a network diagram.

Exam Essentials

Describe the benefits of Variable Length Subnet Masks (VLSMs).

VLSMs enable the creation of subnets of specific sizes and allows the division of a classless network into smaller networks that do not need to be equal in size. This makes use of the address space more efficient as many times IP addresses are wasted with classful subnetting.

Understand the relationship between the subnet mask value and the resulting block size and the allowable IP addresses in each resulting subnet.

The relationship between the classful network being subdivided and the subnet mask used determines the number of possible hosts or the block size. It also determines where each subnet begins and ends and which IP addresses cannot be assigned to a host within each subnet.

Describe the process of summarization or route aggregation and its relationship to subnetting.

Summarization is the combining of subnets derived from a classful network for the purpose of advertising a single route to neighboring routers instead of multiple routes, reducing the size of routing tables and speeding the route process.

Calculate the summary mask that will advertise a single network representing all subnets.

The network address used to advertise the summary address is always the first network address in the block of subnets. The mask is the subnet mask value that yields the same block size.

Remember the four diagnostic steps.

The four simple steps that Cisco recommends for troubleshooting are ping the loopback address, ping the NIC, ping the default gateway, and ping the remote device.

Identify and mitigate an IP addressing problem.

Once you go through the four troubleshooting steps that Cisco recommends, you must be able to determine the IP addressing problem by drawing out the network and finding the valid and invalid hosts addressed in your network.

Understand the troubleshooting tools that you can use from your host and a Cisco router.

The ping 127.0.0.1

command tests your local IP stack, and tracert

is a Windows DOS command to track the path a packet takes through an internetwork to a destination. Cisco routers use the command traceroute

, or just trace

for short. Don’t confuse the Windows and Cisco commands. Although they produce the same output, they don’t work from the same prompts. The command ipconfig /all

will display your PC network configuration from a DOS prompt, and arp a

(again from a DOS prompt) will display IP-to-MAC-address mapping on a Windows PC.

Written Lab 5

For each of the following sets of networks, determine the summary address and the mask to be used that will summarize the subnets.

1. 192.168.1.0/24 through 192.168.12.0/24

2. 172.148.0.0/13 through 172.156.0.0/13

3. 192.168.32.0 through 192.168.63.0

4. 203.168.6.0/24 and 203.168.60.0/24

5. 66.66.0.0 through 66.66.15.0

6. 192.168.1.0 through 192.168.120.0

7. 172.16.1.0 through 172.16.7.0

8. 192.168.128.0 through 192.168.190.0

9. 53.60.96.0 through 53.60.127.0

10. 172.16.10.0 through 172.16.63.0

(The answers to Written Lab 5 can be found following the answers to the review questions for this chapter.)

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s

Introduction.

1. On a VLSM network, which mask should you use on point-to-point WAN links in order to reduce the waste of IP addresses?

A. /27

B. /28

C. /29

D. /30

E. /31

2. To test the IP stack on your local host, which IP address would you ping?

A. 127.0.0.0

B. 1.0.0.127

C. 127.0.0.1

D. 127.0.0.255

E. 255.255.255.255

3. What is the only connection type that supports the use of the /30 mask?

A. Point-to-multipoint

B. Point-to-point

C. Multipoint-to-multipoint

D. Host to switch

4. To use VLSM, what capability must the routing protocols in use possess?

A. Support for multicast

B. Multi-protocol support

C. Transmission of subnet mask information

D. Support for unequal load balancing

5. What is another term for

route aggregation

?

A. VLSM

B. Load balancing

C. Subnetting

D. Summarization

6. Which of the following is a result of route aggregation?

A. Smaller routing tables

B. More complete routing tables

C. Increased memory usage

D. Increased CPU usage

7. The network address used to advertise a summary address is always which of the following?

A. The last network address in the block

B. The next to last network in the block

C. The second network in the block

D. The first network in the block

8. When a ping to the loopback address fails, what can you assume?

A. The IP address of the local host is incorrect.

B. The IP address of the remote host is incorrect.

C. The NIC is not functional.

D. The IP stack has failed to initialize.

9. When a ping to the local host IP address fails, what can you assume?

A. The IP address of the local host is incorrect.

B. The IP address of the remote host is incorrect.

C. The NIC is not functional.

D. The IP stack has failed to initialize.

10. When a ping to the local host IP address succeeds but a ping to the default gateway IP address fails, what can you rule out? (Choose all that apply.)

A. The IP address of the local host is incorrect.

B. The IP address of the gateway is incorrect.

C. The NIC is not functional.

D. The IP stack has failed to initialize.

11. If a remote host can be pinged, what problems can you rule out?

A. The IP address of the local host is incorrect.

B. The IP address of the gateway is incorrect.

C. The NIC is not functional.

D. The IP stack has failed to initialize.

E. All of the above.

12. What network service is the most likely problem if you can ping a computer by IP address but not by name?

A. DNS

B. DHCP

C. ARP

D. ICMP

13. When you issue the ping

command, what protocol are you using?

A. DNS

B. DHCP

C. ARP

D. ICMP

14. Which of the following commands displays the networks traversed on a path to a network destination?

A. ping

B. traceroute

C. pingroute

D. pathroute

15. Which of the following commands uses ICMP echo requests and replies?

A. ping

B. traceroute

C. arp

D. tracert

16. What command is the Windows version of the Cisco command that displays the networks traversed on a path to a network destination?

A. ping

B. traceroute

C. arp

D. tracert

17. Which command displays IP-to-MAC-address mappings on a Windows PC?

A. ping

B. traceroute

C. arp -a

D. tracert

18. What command displays the ARP table on a Cisco router?

A. show ip arp

B. traceroute

C. arp -a

D. tracert

19. What switch must be added to the ipconfig

command on a PC to verify DNS configuration?

A.

/dns

B.

-dns

C.

/all

D.

-all

20. Which of the following is the best summarization of the following networks: 192.168.128.0 through 192.168.159.0

A. 192.168.0.0/24

B. 192.168.128.0/16

C. 192.168.128.0/19

D. 192.168.128.0/20

Answers to Review Questions

1. D. A point-to-point link uses only two hosts. A /30, or 255.255.255.252, mask provides two hosts per subnet.

2. C. To test the local stack on your host, ping the loopback interface of 127.0.0.1.

3. B. The only connection type that supports the use of the /30 mask is point-to-point.

4. C. To use VLSM, the routing protocols in use possess the capability to transmit subnet mask information.

5. D. Another term for route aggregation is summarization.

6. A. Route aggregation results in smaller routing tables.

7. D. The network address used to advertise a summary address is always the first network in the block.

8. D. When a ping to the loopback address fails, you can assume the IP stack has failed to initialize.

9. C. When a ping to the local host IP address fails, you can assume the NIC is not functional.

10. C, D. If a ping to the local host succeeds, you can rule out IP stack or NIC failure.

11. E. If you can ping a remote host, everything is working locally.

12. A. The most likely problem if you can ping a computer by IP address but not by name is a failure of DNS.

13. D. When you issue the ping

command, you are using the ICMP protocol.

14. B. The traceroute

command displays the networks traversed on a path to a network destination.

15. A. The ping

command uses ICMP echo requests and replies.

16. D. tracert

is the Windows version of the Cisco command that displays the networks traversed on a path to a network destination.

17. C. The arp -a

command displays IP-to-MAC-address mappings on a Windows PC.

18. A. The command that displays the ARP table on a Cisco router is show ip arp

.

19. C. The

/all

switch must be added to the ipconfig

command on a PC to verify DNS configuration.

20. C. If you start at 192.168.128.0 and go through 192.168.159.0, you can see this is a block of 32 in the third octet. Since the network address is always the first one in the range, the summary address is 192.168.128.0. What mask provides a block of 32 in the third octet?

The answer is 255.255.224.0, or /19.

1.

192.168.0.0/20.

2.

172.144.0.0/16

3.

192.168.32.0 255.255.224.0

4.

192.168.96.0 255.255.240.0

5.

66.66.0.0/16.

6.

192.168.0.0/25

7.

172.16.1.0 255.255.248.0

8.

192.168.128.0 255.255.192.0

9.

53.60.96.0 255.255.224.0

10.

172.16.0.0 255.255.192.0

Answers to Written Lab 5

Chapter 6

Cisco’s Internetworking Operating System (IOS)

The CCNA exam topics covered in this chapter include the following:

Implement an IP addressing scheme and IP Services to meet network requirements in a medium-size Enterprise branch office network

Configure, verify, and troubleshoot DHCP and DNS operation on a router (including CLI/SDM).

Configure, verify, and troubleshoot basic router operation and routing on Cisco devices.

Describe the operation of Cisco routers (including: router bootup process, POST, router components).

Access and utilize the router to set basic parameters (including CLI/SDM).

Connect, configure, and verify operation status of a device interface.

Verify device configuration and network connectivity using ping, traceroute, telnet, SSH, or other utilities.

Verify network connectivity (including: using ping, traceroute, and telnet or SSH).

Troubleshoot routing issues.

Verify router hardware and software operation using SHOW and DEBUG commands.

The time has come to introduce you to the Cisco Internetwork Operating System (IOS). The IOS is what runs Cisco routers as well as Cisco’s switches, and it’s what allows you to configure the devices as well.

So that’s what you’re going to learn about in this chapter. I’m going to show you how to configure a Cisco IOS router using the Cisco IOS command-line interface (CLI). When you become proficient with this interface, you’ll be able to configure hostnames, banners, passwords, and more as well as troubleshoot using the Cisco IOS.

I’m also going to get you up to speed on the vital basics of router configurations and command verifications. Here’s a list of the subjects we’ll be covering in this chapter:

Understanding and configuring the Cisco Internetwork Operating System (IOS)

Connecting to a router

Bringing up a router

Logging into a router

Understanding the router prompts

Understanding the CLI prompts

Performing editing and help features

Gathering basic routing information

Setting administrative functions

Setting hostnames

Setting banners

Setting passwords

Setting interface descriptions

Performing interface configurations

Viewing, saving, and erasing configurations

Verifying routing configurations

And just as it was with preceding chapters, the fundamentals that you’ll learn in this chapter are foundational building blocks that really need to be in place before you go on to the next chapters in the book.

For up-to-the-minute updates for this chapter, please see www.lammle.com

or www.sybex.com/go/ccna7e

.

The IOS User Interface

The

Cisco Internetwork Operating System (IOS)

is the kernel of Cisco routers and most switches. In case you didn’t know, a kernel is the basic, indispensable part of an operating system that allocates resources and manages things such as low-level hardware interfaces and security.

In the following sections, I’ll show you the Cisco IOS and how to configure a Cisco router using the command-line interface (CLI).

I’m going to save Cisco switch configurations for Chapter 10, “Layer 2 Switching and Spanning Tree Protocol (STP).”

Cisco Router IOS

The Cisco IOS is a proprietary kernel that provides routing, switching, internetworking, and telecommunications features. The first IOS was written by William Yeager in 1986, and it enabled networked applications. It runs on most Cisco routers as well as an ever-increasing number of Cisco

Catalyst switches, like the Catalyst 2960 and 3560 series switches.

These are some important things that the Cisco router IOS software is responsible for:

Carrying network protocols and functions

Connecting high-speed traffic between devices

Adding security to control access and stop unauthorized network use

Providing scalability for ease of network growth and redundancy

Supplying network reliability for connecting to network resources

You can access the Cisco IOS through the console port of a router, from a modem into the auxiliary (or Aux) port, or even through Telnet. Access to the IOS command line is called an

EXEC session

.

Connecting to a Cisco Router

You can connect to a Cisco router to configure it, verify its configuration, and check statistics. There are different ways to do this, but most often, the first place you would connect to is the console port. The

console port

is usually an RJ-45 (8-pin modular) connection located at the back of the router—by default, there may or may not be a password set. The new ISR routers use

cisco

as the username and

cisco

as the password by default.

See Chapter 2, “Review of Ethernet Networking and Data Encapsulation,” for an explanation of how to configure a PC to connect to a router console port.

You can also connect to a Cisco router through an

auxiliary port

—which is really the same thing as a console port, so it follows that you can use it as one. But an auxiliary port also allows you to configure modem commands so that a modem can be connected to the router. This is a cool feature

—it lets you dial up a remote router and attach to the auxiliary port if the router is down and you need to configure it

out-of-band

(meaning from outside of the network).

The third way to connect to a Cisco router is in-band, through the program

Telnet

. (

In-band

means configuring the router through the network, the opposite of

out-of-band

.)Telnet is a terminal emulation program that acts as though it’s a dumb terminal. You can use Telnet to connect to any active interface on a router, such as an Ethernet or serial port. I’ll discuss something called Secure Shell (SSH) later in this chapter, which is a more secure way to connect in-band through the network.

Figure 6-1 shows an illustration of a Cisco 2600 series modular router, which is a cut above routers populating the 2500 series because it has a faster processor and can handle many more interfaces. Both the 2500 and 2600 series routers are end of life (EOL), and you can only buy them used. However, many 2600 series routers are still found in production, so it’s important to understand them. Pay close attention to all the different kinds of interfaces and connections.

Figure 6-1:

A Cisco 2600 router

The 2600 series router can have multiple serial interfaces, which can be used for connecting a T1 using a serial V.35 WAN connection. Multiple

Ethernet or FastEthernet ports can be used on the router, depending on the model. This router also has one console and one auxiliary connection via RJ-45 connectors.

Another router I want to talk about is the 2800 series (shown in

Figure 6-2 ). This router has replaced the 2600 series router and is referred to as an Integrated Services Router (ISR) but yet again has been updated to the 2900 since my previous edition of this book. The ISR series gets its name because many of the services, like security, are built into it. It’s a modular device like the 2600, but it’s much faster and a lot more sleek—it’s elegantly designed to support a broad new range of interface options.

Figure 6-2:

A Cisco 2800 router

You need to keep in mind that for the most part, you get some serious bang for your buck with the 2800/2900—unless you start adding a lot of interfaces to it. You’ve got to pony up for each one of those little beauties, and things can really start to add up—fast!

There are a couple of other series of routers that are less expensive than the 2800 series: the 1800/1900 and 800/900 series. You may want to look into these routers if you’re looking for a less-expensive alternative to the 2800/2900 but still want to run the same IOS.

Figure 6-3 shows an 1841 router that holds most of the same interfaces as the 2800, but it’s smaller and less expensive. The real reason you would opt for a 2800/2900 instead of an 1800/1900 series router comes down to the more advanced interfaces you can run on the 2800/2900— things like the wireless controller and switching modules.

Figure 6-3:

A Cisco 1841 router

As a heads up, I’m going to be using mostly 2800, 1800, and 800 series routers throughout this book to demonstrate examples of router configurations. But understand that you can use the 2600 and even the older 2500 routers to practice routing principles.

You can find more information about all Cisco routers at www.cisco.com/en/US/products/hw/routers/index.html

.

Bringing Up a Router

When you first bring up a Cisco router, it will run a power-on self-test (POST). If it passes, it will then look for and load the Cisco IOS from flash memory—if an IOS file is present and expands it into RAM. (Just in case you don’t know, flash memory is electronically erasable programmable read-only memory—an EEPROM.) After that, the IOS loads and looks for a valid configuration—the startup-config—that’s stored in nonvolatile

RAM, or NVRAM.

The following messages appear when you first boot or reload a router (I am using my 2811 router):

System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 2006 by cisco Systems, Inc.

Initializing memory for ECC c2811 platform with 262144 Kbytes of main memory

Main memory is configured to 64 bit mode with ECC enabled

Upgrade ROMMON initialized program load complete, entry point: 0x8000f000, size: 0xcb80 program load complete, entry point: 0x8000f000, size: 0xcb80

This is the first part of the router boot process output. It’s information about the bootstrap program that first runs the POST. It then tells the router how to load, which by default is to find the IOS in flash memory. It also lists the amount of RAM in the router.

The next part shows us that the IOS is being decompressed into RAM: program load complete, entry point: 0x8000f000, size: 0x14b45f8

Self decompressing the image :

####################################################################

############################################ [OK]

The pound signs are telling us that the IOS is being decompressed into RAM. After it is decompressed into RAM, the IOS is loaded and starts running the router, as shown below. Notice that the IOS version is stated as advanced security version 12.4(12):

[some output cut]

Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version

12.4(12), RELEASE SOFTWARE (fc1)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by Cisco Systems, Inc.

Compiled Fri 17-Nov-06 12:02 by prod_rel_team

Image text-base: 0x40093160, data-base: 0x41AA0000

A sweet new feature of the new ISR routers is that the IOS name is no longer cryptic. The filename actually tells you what the IOS can do, as in

Advanced Security. Once the IOS is loaded, the information learned from the POST will be displayed next, as you can see here:

[some output cut]

Cisco 2811 (revision 49.46) with 249856K/12288K bytes of memory.

Processor board ID FTX1049A1AB

2 FastEthernet interfaces

4 Serial(sync/async) interfaces

1 Virtual Private Network (VPN) Module

DRAM configuration is 64 bits wide with parity enabled.

239K bytes of non-volatile configuration memory.

62720K bytes of ATA CompactFlash (Read/Write)

There are two FastEthernet interfaces, four serial interfaces, plus a VPN module. The amount of RAM, NVRAM, and flash are also displayed.

The above router output shows us that there’s 256MB of RAM, 239K of NVRAM, and 64MB of flash.

When the IOS is loaded and up and running, a preconfiguration (called startup-config) will be copied from NVRAM into RAM. The copy of this file will be placed in RAM and is called running-config.

My 1841 and 871W routers boot exactly the same as the 2811 router. The 1841 and 871W do show less memory and different interfaces, but other than that, they have the

same bootup procedure and the same preconfigured startup-config file.

Bringing Up a Non-ISR Router (a 2600 For Example)

As you’re about to see, the boot cycle is about the same for non-ISR routers as for the ISR routers. The following messages appear when you first boot or reload a 2600 router:

System Bootstrap, Version 11.3(2)XA4, RELEASE SOFTWARE (fc1)

Copyright (c) 1999 by cisco Systems, Inc.

TAC:Home:SW:IOS:Specials for info

C2600 platform with 65536 Kbytes of main memory

The next part shows us that the IOS is being decompressed into RAM: program load complete, entry point:0x80008000, size:0x43b7fc

Self decompressing the image :

#######################################################

#######################################################

#######################################################

#######################################################

#######################################################

#######################################################

####################################################################### [OK]

So far, everything is pretty much the same. Notice below that the IOS version is stated as version 12.3(20):

Cisco Internetwork Operating System Software

IOS (tm) C2600 Software (C2600-IK9O3S3-M), Version 12.3(20), RELEASE

SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by cisco Systems, Inc.

Compiled Tue 08-Aug-06 20:50 by kesnyder

Image text-base: 0x80008098, data-base: 0x81A0E7A8

Just as with the 2800 series, once the IOS is loaded, the information learned from the POST will be displayed: cisco 2610 (MPC860) processor (revision 0x202) with 61440K/4096K bytes

of memory.

Processor board ID JAD03348593 (1529298102)

M860 processor: part number 0, mask 49

Bridging software.

X.25 software, Version 3.0.0.

1 Ethernet/IEEE 802.3 interface(s)

1 Serial network interface(s)

2 Serial(sync/async) network interface(s)

32K bytes of non-volatile configuration memory.

16384K bytes of processor board System flash (Read/Write)

Okay—finally what we see here is one Ethernet interface and three serial interfaces. The amount of RAM and flash is also displayed, and the above router output shows there are 64 MB of RAM and 16 MB of flash.

And as I mentioned, when the IOS is loaded and up and running, a valid configuration called the startup-config will be loaded from NVRAM. But here’s where it differs from the default bootup of the ISR routers—if there isn’t a configuration in NVRAM, the router will broadcast looking for a valid one on a TFTP host. (This can only happen if the router senses carrier detect, or CD, on any interface.) If the broadcast fails, it will then go into what is called

setup mode

—a step-by-step process to help you configure the router. So you need to remember that if you plug any interface of your router into your network and then boot your router, you may have to wait a couple minutes while the router searches for the configuration.

You can also enter setup mode at any time from the command line by typing the command

setup

from something called privileged mode, which I’ll get to in a minute. Setup mode covers only some commands and is generally just unhelpful. Here is an example:

Would you like to enter the initial configuration dialog? [yes/no]:

y

At any point you may enter a question mark '?' for help.

Use ctrl-c to abort configuration dialog at any prompt.

Default settings are in square brackets '[]'.

Basic management setup configures only enough connectivity for management of the system, extended setup will ask you to configure each interface on the system

Would you like to enter basic management setup? [yes/no]:

y

Configuring global parameters:

Enter host name [Router]:

Ctrl+C

Configuration aborted, no changes made.

You can exit setup mode at any time by pressing Ctrl+C.

I highly recommend going through setup mode once, then never again. You should always use the CLI.

Command-Line Interface (CLI)

I sometimes refer to the CLI as “Cash Line Interface” because if you can create advanced configurations on Cisco routers and switches using the

CLI, then you’ll get the cash!

Entering the CLI

After the interface status messages appear and you press Enter, the

Router>

prompt will appear. This is called

user exec mode

(user mode), and it’s mostly used to view statistics, but it’s also a stepping stone to logging in to privileged mode.

You can only view and change the configuration of a Cisco router in

privileged exec mode

(privileged mode), which you can enter with the enable command.

Here’s how:

Router>

enable

Router#

You now end up with a

Router#

prompt, which indicates that you’re in

privileged mode

, where you can both view and change the router’s configuration. You can go back from privileged mode into user mode by using the disable

command, as seen here:

Router#

disable

Router>

At this point, you can type

logout

from either mode to exit the console:

Router>

logout

Router con0 is now available

Press RETURN to get started.

In the following sections, I am going to show you how to perform some basic administrative configurations.

Overview of Router Modes

To configure from a CLI, you can make global changes to the router by typing configure terminal

(or config t

for short), which puts you in global configuration mode and changes what’s known as the running-config. A global command (a command run from global config) is set only once and affects the entire router.

You can type config

from the privileged-mode prompt and then just press Enter to take the default of terminal

, as seen here:

Router#

config

Configuring from terminal, memory, or network [terminal]? [

press enter

]

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

At this point, you make changes that affect the router as a whole (globally), hence the term

global configuration mode

. To change the runningconfig—the current configuration running in dynamic RAM (DRAM)—you use the configure terminal

command, as I just demonstrated.

Here are some of the other options under the configure

command:

Router(config)#

exit

or press

cntl-z

Router#config ?

memory Configure from NV memory

network Configure from a TFTP network host

overwrite-network Overwrite NV memory from TFTP network host

terminal Configure from the terminal

<cr>

We’ll go through these commands in Chapter 7.

CLI Prompts

It’s really important that you understand the different prompts you can find when configuring a router. Knowing these well will help you navigate and recognize where you are at any time within configuration mode. In the following sections, I’m going to demonstrate the prompts that are used on a

Cisco router and discuss the various terms used. (Always check your prompts before making any changes to a router’s configuration!)

I’m not going into every different command prompt offered because doing that would be reaching beyond the scope of this book. Instead, I’m going to describe all the different prompts you’ll see throughout this chapter and the rest of the book. These command prompts really are the ones you’ll use most in real life anyway; plus, they’re the ones you’ll need to know for the exam.

Don’t freak! It’s not important that you understand what each of these command prompts accomplishes yet because I’m going to completely fill you in on all of them really soon. So right now, just relax and focus on becoming familiar with the different prompts available and all will be well!

Interfaces

To make changes to an interface, you use the interface

command from global configuration mode:

Router(config)#

interface ?

Async Async interface

BVI Bridge-Group Virtual Interface

CDMA-Ix CDMA Ix interface

CTunnel CTunnel interface

Dialer Dialer interface

FastEthernet FastEthernet IEEE 802.3

Group-Async Async Group interface

Lex Lex interface

Loopback Loopback interface

MFR Multilink Frame Relay bundle interface

Multilink Multilink-group interface

Null Null interface

Port-channel Ethernet Channel of interfaces

Serial Serial

Tunnel Tunnel interface

Vif PGM Multicast Host interface

Virtual-PPP Virtual PPP interface

Virtual-Template Virtual Template interface

Virtual-TokenRing Virtual TokenRing

range interface range command

Router(config)#

interface fastEthernet 0/0

Router(config-if)#

Did you notice that the prompt changed to

Router(config-if)#

? This tells you that you’re in

interface configuration mode

. And wouldn’t it be nice if the prompt also gave you an indication of what interface you were configuring? Well, at least for now we’ll have to live without the prompt information, because it doesn’t. One thing is for sure: You really have to pay attention when configuring a router!

Subinterfaces

Subinterfaces allow you to create logical interfaces within the router. The prompt then changes to

Router(config-subif)#

:

Router(config-if)#

interface f0/0.1

Router(config-subif)#

You can read more about subinterfaces in Chapter 11, “Virtual LANs (VLANS)” and Chapter 16, “Wide Area Networks,” but don’t skip ahead just yet!

Line Commands

To configure user-mode passwords, use the line

command. The prompt then becomes

Router(config-line)#

:

Router#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

line ?

<0-337> First Line number

aux Auxiliary line

console Primary terminal line

tty Terminal controller

vty Virtual terminal

The line console 0

command is known as a major command (also called a

global command

), and any command typed from the

(config-line) prompt is known as a subcommand.

Routing Protocol Configurations

To configure routing protocols such as RIP and EIGRP, you’ll need to get to the prompt

Router(config-router#)

:

Router#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

router rip

Router(config-router)#

version 2

Router(config-router)#

Defining Router Terms

Table 6-1 defines some of the terms we’ve used so far.

Table 6-1:

Router terms

Mode Definition

User EXEC mode

Privileged EXEC mode

Limited to basic monitoring commands

Provides access to all other router commands

Global configuration mode Commands that affect the entire system

Specific configuration modes Commands that affect interfaces/processes only

Setup mode Interactive configuration dialog

Editing and Help Features

You can use the Cisco advanced editing features to help you configure your router. If you type in a question mark (

?

) at any prompt, you’ll be given a list of all the commands available from that prompt:

Router#

?

Exec commands:

access-enable Create a temporary Access-List entry

access-profile Apply user-profile to interface

access-template Create a temporary Access-List entry

archive manage archive files

auto Exec level Automation

bfe For manual emergency modes setting

calendar Manage the hardware calendar

cd Change current directory

clear Reset functions

clock Manage the system clock

cns CNS agents

configure Enter configuration mode

connect Open a terminal connection

copy Copy from one file to another

crypto Encryption related commands.

ct-isdn Run an ISDN component test command

debug Debugging functions (see also 'undebug')

delete Delete a file

dir List files on a filesystem

disable Turn off privileged commands

disconnect Disconnect an existing network connection

--More--

Plus, at this point you can press the spacebar to get another page of information, or you can press Enter to go one command at a time. You can also press Q (or any other key, for that matter) to quit and return to the prompt.

Here’s a shortcut: To find commands that start with a certain letter, use the letter and the question mark with no space between them:

Router#

c?

calendar cd clear clock cns configure connect copy crypto ct-isdn

Router#c

By typing

c?

, we received a response listing all the commands that start with

c

. Also notice that the

Router#c

prompt reappears after the list of commands is displayed. This can be helpful when you have long commands and need the next possible command. It would be pretty lame if you had to retype the entire command every time you used a question mark!

To find the next command in a string, type the first command and then a question mark:

Router#

clock ?

read-calendar Read the hardware calendar into the clock

set Set the time and date

update-calendar Update the hardware calendar from the clock

Router#

clock set ?

hh:mm:ss Current Time

Router#

clock set 11:15:11 ?

<1-31> Day of the month

MONTH Month of the year

Router#

clock set 11:15:11 25 april ?

<1993-2035> Year

Router#

clock set 11:15:11 25 april 2011 ?

<cr>

Router#

clock set 11:15:11 25 april 2011

*April 25 11:15:11.000: %SYS-6-CLOCKUPDATE: System clock has been updated from 18:52:53 UTC Wed Feb 28 2011 to 11:15:11 UTC Sat April 25 2011, configured from console by cisco on console.

By typing the

clock ?

command, you’ll get a list of the next possible parameters and what they do. Notice that you should just keep typing a command, a space, and then a question mark until

<cr>

(carriage return) is your only option.

If you’re typing commands and receive

Router#

clock set 11:15:11

% Incomplete command.

you’ll know that the command string isn’t done yet. Just press the up arrow key to redisplay the last command entered, and then continue with the command by using your question mark.

And if you receive the error

Router(config)#

access-list 110 permit host 1.1.1.1

^

% Invalid input detected at '^' marker.

you’ve entered a command incorrectly. See that little caret—the

^

? It’s a very helpful tool that marks the exact point where you blew it and entered the command incorrectly. Here’s another example of when you’ll see the caret:

Router#

sh serial 0/0/0

^

% Invalid input detected at '^' marker.

This command looks right, but be careful! The problem is that the full command is show interface serial 0/0/0

.

Now if you receive the error

Router#

sh ru

% Ambiguous command: "sh ru" it means there are multiple commands that begin with the string you entered and it’s not unique. Use the question mark to find the command you need:

Router#

sh ru?

rudpv1 running-config

As you can see, there are two commands that start with show ru

.

Table 6-2 lists the enhanced editing commands available on a Cisco router.

Table 6-2:

Enhanced editing commands

Command Meaning

Ctrl+A

Ctrl+E

Esc+B

Ctrl+B

Moves your cursor to the beginning of the line

Moves your cursor to the end of the line

Moves back one word

Moves back one character

Ctrl+F

Esc+F

Moves forward one character

Moves forward one word

Ctrl+D Deletes a single character

Backspace Deletes a single character

Ctrl+R

Ctrl+U

Ctrl+W

Ctrl+Z

Tab

Redisplays a line

Erases a line

Erases a word

Ends configuration mode and returns to EXEC

Finishes typing a command for you

Another cool editing feature I want to show you is the automatic scrolling of long lines. In the following example, the command typed had reached the right margin and automatically moved 11 spaces to the left (the dollar sign [

$

] indicates that the line has been scrolled to the left):

Router#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

$110 permit host 171.10.10.10 0.0.0.0 eq 23

You can review the router-command history with the commands shown in Table 6-3 .

Table 6-3:

Router-command history

Command Meaning

Ctrl+P or up arrow Shows last command entered

Ctrl+N or down arrow Shows previous commands entered show history show terminal

Shows last 20 commands entered by default

Shows terminal configurations and history buffer size terminal history size

Changes buffer size (max 256)

The following example demonstrates the show history

command and how to change the history size as well as how to verify it with the show terminal command. First, use the show history

command to see the last 20 commands that were entered on the router, although my router only has 10 commands shown here:

Router#

show history

en

sh history

show terminal

sh cdp neig

sh ver

sh flash

sh int fa0

sh history

sh int s0/0

sh int s0/1

Now use the show terminal

command to verify the terminal history size:

Router#

show terminal

Line 0, Location: "", Type: ""

[output cut]

Modem type is unknown.

Session limit is not set.

Time since activation: 00:21:41

Editing is enabled.

History is enabled, history size is 20.

DNS resolution in show commands is enabled

Full user help is disabled

Allowed input transports are none.

Allowed output transports are pad telnet rlogin lapb-ta mop v120 ssh.

Preferred transport is telnet.

No output characters are padded

No special data dispatching characters

The terminal history size

command, used from privileged mode, can change the size of the history buffer:

Router#

terminal history size ?

<0-256> Size of history buffer

Router#

terminal history size 25

You verify the change with the show terminal

command:

Router#

show terminal

Line 0, Location: "", Type: ""

[output cut]

Editing is enabled.

History is enabled, history size is 25.

Full user help is disabled

Allowed transports are lat pad v120 telnet mop rlogin

nasi. Preferred is lat.

No output characters are padded

No special data dispatching characters

Group codes: 0

When Do You Use the Cisco Editing Features?

A couple of editing features are used quite often and some not so much, if at all. Understand that Cisco didn’t make these up; these are just old Unix commands. However,

Ctrl+A is really helpful to negate a command.

For example, if you were to put in a long command and then decide you didn’t want to use that command in your configuration after all, or if it didn’t work, then you could just press your up arrow key to show the last command entered, press Ctrl+A, type no and then a space, press Enter—and poof! The command is negated. This doesn’t work on every command, but it works on a lot of them.

Gathering Basic Routing Information

The show version

command will provide basic configuration for the system hardware as well as the software version and the boot images. Here’s an example:

Router#

show version

Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version

12.4(12), RELEASE SOFTWARE (fc1)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by Cisco Systems, Inc.

Compiled Fri 17-Nov-06 12:02 by prod_rel_team

The preceding section of output describes the Cisco IOS running on the router. The following section describes the read-only memory (ROM) used, which is used to boot the router and holds the POST:

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

The next section shows how long the router has been running, how it was restarted (if you see a system restarted by bus

error, that is a very bad thing), the location from which the Cisco IOS was loaded, and the IOS name. Flash is the default:

Router uptime is 2 hours, 30 minutes

System returned to ROM by power-on

System restarted at 09:04:07 UTC Sat Aug 25 2007

System image file is "flash:c2800nm-advsecurityk9-mz.124-12.bin"

This next section displays the processor, the amount of DRAM and flash memory, and the interfaces the POST found on the router:

[some output cut]

Cisco 2811 (revision 53.50) with 249856K/12288K bytes of memory.

Processor board ID FTX1049A1AB

2 FastEthernet interfaces

4 Serial(sync/async) interfaces

1 Virtual Private Network (VPN) Module

DRAM configuration is 64 bits wide with parity enabled.

239K bytes of non-volatile configuration memory.

62720K bytes of ATA CompactFlash (Read/Write)

Configuration register is 0x2102

The configuration register value is listed last—it’s something I’ll cover in Chapter 7.

In addition, the show interfaces

and show ip interface brief

commands are very useful in verifying and troubleshooting a router as well as network issues. These commands are covered later in this chapter. Don’t miss it!

Router and Switch Administrative Configurations

Even though the following sections aren’t critical to making a router or switch

work

on a network, they’re still really important; in them, I’m going to

lead you through configuring commands that will help you administer your network.

The administrative functions that you can configure on a router and switch are as follows:

Hostnames

Banners

Passwords

Interface descriptions

Remember, none of these will make your routers or switches work better or faster, but trust me, your life will be a whole lot better if you just take the time to set these configurations on each of your network devices. That’s because doing this makes troubleshooting and maintaining your network sooooo much easier—seriously! In this next section, I’ll be demonstrating commands on a Cisco router, but these commands are exactly the same on a Cisco switch.

Hostnames

You can set the identity of the router with the hostname

command. This is only locally significant, which means that it has no bearing on how the router performs name lookups or how the router works on the internetwork. However, I’ll use the hostname in Chapter 16 for authentication purposes when

I discuss the WAN protocol PPP.

Here’s an example:

Router#

config t

Enter configuration commands, one per line. End with

CNTL/Z.

Router(config)#

hostname Todd

Todd(config)#

hostname Atlanta

Atlanta(config)#

hostname Todd

Todd(config)#

Even though it’s pretty tempting to configure the hostname after your own name, it’s definitely a better idea to name the router something pertinent to the location. This is because giving it a hostname that’s somehow relevant to where the device actually lives will make finding it a whole lot easier. And it also helps you confirm that you are, indeed, configuring the right device. For this chapter, we’ll leave it at

Todd

for now because it’s fun.

Banners

A

banner

is more than just a little cool—one very good reason for having a banner is to give any and all who dare attempt to telnet or dial into your internetwork a little security notice. And you can create a banner to give anyone who shows up on the router exactly the information you want them to have.

Make sure you’re familiar with these four available banner types: exec process creation banner, incoming terminal line banner, login banner, and message of the day banner (all illustrated in the following code):

Todd(config)#

banner ?

LINE c banner-text c, where 'c' is a delimiting character

exec Set EXEC process creation banner

incoming Set incoming terminal line banner

login Set login banner

motd Set Message of the Day banner

prompt-timeout Set Message for login authentication timeout

slip-ppp Set Message for SLIP/PPP

Message of the day (MOTD) is the most extensively used banner. It gives a message to every person dialing into or connecting to the router via

Telnet or an auxiliary port, or even through a console port as seen here:

Todd(config)#

banner motd ?

LINE c banner-text c, where 'c' is a delimiting character

Todd(config)#

banner motd #

Enter TEXT message. End with the character '#'.

$

Acme.com network, then you must disconnect immediately.

#

Todd(config)#

^Z

Todd#

00:25:12: %SYS-5-CONFIG_I: Configured from console by

console

Todd#

exit

Router con0 is now available

Press RETURN to get started.

If you are not authorized to be in Acme.com network, then you must disconnect immediately.

Todd#

The preceding MOTD banner essentially tells anyone connecting to the router to get lost if they’re not on the guest list! The part to understand is the delimiting character—the thing that’s used to tell the router when the message is done. You can use any character you want for it, but (I hope this

is obvious) you can’t use the delimiting character in the message itself. Also, once the message is complete, press Enter, then the delimiting character, and then Enter again. It’ll still work if you don’t do that, but if you have more than one banner, they’ll be combined as one message and put on a single line.

For example, you can set a banner on one line as shown:

Todd(config)#

banner motd x Unauthorized access prohibited! x

This example will work just fine, but if you add another MOTD banner message, they would end up on a single line.

Here are some details of the other banners I mentioned:

Exec banner

You can configure a line-activation (exec) banner to be displayed when an EXEC process (such as a line activation or incoming connection to a VTY line) is created. By simply starting a user exec session through a console port, you’ll activate the exec banner.

Incoming banner

You can configure a banner to be displayed on terminals connected to reverse Telnet lines. This banner is useful for providing instructions to users who use reverse Telnet.

Login banner

You can configure a login banner to be displayed on all connected terminals. This banner is displayed after the MOTD banner but before the login prompts. The login banner can’t be disabled on a per-line basis, so to globally disable it, you’ve got to delete it with the no banner login

command.

Here is an example of a login banner:

!

banner login ^C

-----------------------------------------------------------------

Cisco Router and Security Device Manager (SDM) is installed on this device.

This feature requires the one-time use of the username "cisco" with the password "cisco". The default username and password have a privilege level of 15.

Please change these publicly known initial credentials using

SDM or the IOS CLI.

Here are the Cisco IOS commands.

username <myuser> privilege 15 secret 0 <mypassword> no username cisco

Replace <myuser> and <mypassword> with the username and password you want to use.

For more information about SDM please follow the instructions in the QUICK START GUIDE for your router or go to http://www.cisco.com/go/sdm

-----------------------------------------------------------------

^C

!

The above login banner could look pretty familiar to anyone who’s ever logged into an ISR router—it’s the banner that Cisco has in its default configuration for its ISR routers.

The login banner is displayed before the login prompts but after the MOTD banner.

Setting Passwords

Five passwords are used to secure your Cisco routers: console, auxiliary, telnet (VTY), enable password, and enable secret. The enable secret and enable password are used to set the password that’s used to secure privileged mode. This will prompt a user for a password when the enable command is used. The other three are used to configure a password when user mode is accessed through the console port, through the auxiliary port, or via Telnet.

Let’s take a look at each of these now.

Enable Passwords

You set the enable passwords from global configuration mode like this:

Todd(config)#

enable ?

last-resort Define enable action if no TACACS servers

respond

password Assign the privileged level password

secret Assign the privileged level secret

use-tacacs Use TACACS to check enable passwords

The following points describe the enable password parameters: last-resort

Allows you to still enter the router if you set up authentication through a TACACS server and it’s not available. But it isn’t used if the

TACACS server is working.

password

Sets the enable password on older, pre-10.3 systems, and isn’t ever used if an enable secret is set.

secret

This is the newer, encrypted password that overrides the enable password if it’s set.

use-tacacs

This tells the router to authenticate through a TACACS server. It’s convenient if you have anywhere from a dozen to multitudes of routers because, well, would you like to face the fun task of changing the password on all those routers? If you’re sane, no, you wouldn’t. So instead, just go through the TACACS server and you only have to change the password once—yeah!

Here’s an example of setting the enable passwords:

Todd(config)#

enable secret todd

Todd(config)#

enable password todd

The enable password you have chosen is the same as your

enable secret. This is not recommended. Re-enter the

enable password.

If you try to set the enable secret and enable passwords the same, the router will give you a nice, polite warning to change the second password.

If you don’t have older legacy routers, don’t even bother to use the enable password.

User-mode passwords are assigned by using the line

command:

Todd(config)#

line ?

<0-337> First Line number

aux Auxiliary line

console Primary terminal line

tty Terminal controller

vty Virtual terminal

Here are the lines to be concerned with for the exam objectives: aux

Sets the user-mode password for the auxiliary port. It’s usually used for attaching a modem to the router, but it can be used as a console as well.

console

Sets a console user-mode password.

vty

Sets a Telnet password on the router. If this password isn’t set, then Telnet can’t be used by default.

To configure the user-mode passwords, you configure the line you want and use either the login

or no login

command to tell the router to prompt for authentication. The next sections will provide a line-by-line example of the configuration of each line.

Auxiliary Password

To configure the auxiliary password, go into global configuration mode and type

line aux ?

. You can see here that you only get a choice of 0–0 (that’s because there’s only one port):

Todd#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Todd(config)#

line aux ?

<0-0> First Line number

Todd(config)#

line aux 0

Todd(config-line)#

login

% Login disabled on line 1, until 'password' is set

Todd(config-line)#

password aux

Todd(config-line)#

login

It’s important to remember to apply the login

command or the auxiliary port won’t prompt for authentication.

Cisco has begun this process of not letting you set the login

command before a password is set on a line because if you set the login

command under a line and then don’t set a password, the line won’t be usable. And it will prompt for a password that doesn’t exist. So this is a good thing—a feature, not a hassle!

Definitely remember that although Cisco has this “password feature” on its routers starting in its newer IOS (12.2 and above), it’s not in all its IOSs.

Console Password

To set the console password, use the line console 0

command. But look at what happened when I tried to type

line console ?

from the

(configline)# prompt—I received an error. You can still type

line console 0

and it will accept it, but the help screens just don’t work from that prompt. Type

exit

to get back one level and you’ll find that your help screens now work. This is a “feature.” Really.

Here’s the example:

Todd(config-line)#

line console ?

% Unrecognized command

Todd(config-line)#

exit

Todd(config)#

line console ?

<0-0> First Line number

Todd(config-line)#

password console

Todd(config-line)#

login

Since there’s only one console port, I can only choose line console 0. You can set all your line passwords to the same password, but for security reasons, I’d recommend that you make them different.

There are a few other important commands to know for the console port.

For one, the exec-timeout 0 0

command sets the time-out for the console EXEC session to zero, which means to never time out. The default timeout is 10 minutes. (If you’re feeling mischievous, try this on people at work: Set it to 0 1. That will make the console time out in 1 second! And to fix it, you have to continually press the down arrow key while changing the time-out time with your free hand!) logging synchronous

is a very cool command, and it should be a default command, but it’s not. It stops annoying console messages from popping up and disrupting the input you’re trying to type. The messages still pop up, but you are returned to your router prompt without your input interrupted.

This makes your input messages oh-so-much easier to read.

Here’s an example of how to configure both commands:

Todd(config-line)#

line con 0

Todd(config-line)#

exec-timeout ?

<0-35791> Timeout in minutes

Todd(config-line)#

exec-timeout 0 ?

<0-2147483> Timeout in seconds

<cr>

Todd(config-line)#

exec-timeout 0 0

Todd(config-line)#

logging synchronous

You can set the console to go from never timing out (0 0) to timing out in 35,791 minutes and 2,147,483 seconds. The default is 10 minutes.

Telnet Password

To set the user-mode password for Telnet access into the router, use the line vty

command. Routers that aren’t running the Enterprise edition of the

Cisco IOS default to five VTY lines, 0 through 4. But if you have the Enterprise edition, you’ll have significantly more. The best way to find out how many lines you have is to use that question mark:

Todd(config-line)#

line vty 0 ?

% Unrecognized command

Todd(config-line)#

exit

Todd(config)#

line vty 0 ?

<1-1180> Last Line number

<cr>

Todd(config)#

line vty 0 1180

Todd(config-line)#

password telnet

Todd(config-line)#

login

Remember, you cannot get help from your

(config-line)#

prompt. You must go back to global config mode in order to use the question mark (

?

).

So what will happen if you try to telnet into a router that doesn’t have a VTY password set? You’ll receive an error stating that the connection is refused because, well, the password isn’t set. So, if you telnet into a router and receive the message

Todd#

telnet SFRouter

Trying SFRouter (10.0.0.1)…Open

Password required, but none set

[Connection to SFRouter closed by foreign host]

Todd# then the remote router (SFRouter in this example) does not have the VTY (Telnet) password set. But you can get around this and tell the router to allow Telnet connections without a password by using the no login

command:

SFRouter(config-line)#

line vty 0 4

SFRouter(config-line)#

no login

I do not recommend using the no login

command to allow Telnet connections without a password unless you are in a testing or classroom environment! In a production network, you should always set your VTY password.

After your routers are configured with an IP address, you can use the Telnet program to configure and check your routers instead of having to use a console cable. You can use the Telnet program by typing

telnet

from any command prompt (DOS or Cisco). Anything Telnet is covered more thoroughly in Chapter 7.

Setting Up Secure Shell (SSH)

Instead of Telnet, you can use Secure Shell, which creates a more secure session than the Telnet application that uses an unencrypted data stream. Secure Shell (SSH) uses encryption keys to send data so that your username and password are not sent in the clear.

Here are the steps to setting up SSH:

1.

Set your hostname:

Router(config)#

hostname Todd

2.

Set the domain name (both the hostname and domain name are required for the encryption keys to be generated):

Todd(config)#

ip domain-name Lammle.com

3.

Set the username to allow SSH client access

Todd(config)#

username Todd password Lammle

4.

Generate the encryption keys for securing the session:

Todd(config)#

crypto key generate rsa general-keys modulus ?

<360-2048> size of the key modulus [360-2048]

Todd(config)#

crypto key generate rsa general-keys modulus 1024

The name for the keys will be: Todd.Lammle.com

% The key modulus size is 1024 bits

% Generating 1024 bit RSA keys, keys will be non-exportable...[OK]

*June 24 19:25:30.035: %SSH-5-ENABLED: SSH 1.99 has been enabled

5.

Enable SSH version 2 on the router; although this isn’t mandatory it is highly suggested:

Todd(config)#

ssh version 2

6.

Connect to the VTY lines of the router:

Todd(config)#

line vty 0 1180

7.

Last, configure SSH and then Telnet as access protocols:

Todd(config-line)#

transport input ssh telnet

If you do not use the keyword telnet

at the end of the command string, then only SSH will work on the router. I am not suggesting you use either way, but just understand that SSH is more secure than Telnet.

Encrypting Your Passwords

Because only the enable secret password is encrypted by default, you’ll need to manually configure the user-mode and enable passwords for encryption.

Notice that you can see all the passwords except the enable secret when performing a show running-config

on a router:

Todd#

sh running-config

Building configuration...

[output cut]

!

enable secret 5 $1$2R.r$DcRaVo0yBnUJBf7dbG9XE0 enable password todd

!

[output cut]

!

line con 0

exec-timeout 0 0

password console

logging synchronous

login line aux 0

password aux

login line vty 0 4

password telnet

login

transport input telnet ssh line vty 5 15 password telnet

login

transport input telnet ssh line vty 16 1180

password telnet

login

!

end

To manually encrypt your passwords, use the service password-encryption

command. Here’s an example of how to do it:

Todd#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Todd(config)#

service password-encryption

Todd(config)#

exit

Todd#

sh run

Building configuration...

[output cut]

!

enable secret 5 $1$2R.r$DcRaVo0yBnUJBf7dbG9XE0 enable password 7 131118160F

!

[output cut]

!

line con 0

exec-timeout 0 0

password 7 0605002F5F41051C

logging synchronous

login line aux 0

password 7 03054E13

login line vty 0 4

access-class 23 in password 7 01070308550E12

login

transport input telnet ssh line vty 5 15 password 7 01070308550E12

login

transport input telnet ssh line vty 16 1180

password 7 120D001B1C0E18

login

!

end

Todd#

config t

Todd(config)#

no service password-encryption

Todd(config)#

^Z

Todd#

There you have it! The passwords will now be encrypted. You just encrypt the passwords, perform a show run

, and then turn off the command. You can see that the enable password and the line passwords are all encrypted.

But before I get into showing you all about setting descriptions on your routers, let’s talk about encrypting passwords a bit more. As I said, if you set your passwords and then turn on the service password-encryption

command, you have to perform a show running-config

before you turn off the encryption service or your passwords won’t be encrypted. You don’t have to turn off the encryption service at all; you’d only do that if your router is running low on processes. And if you turn on the service before you set your passwords, then you don’t even have to view them to get them encrypted.

Descriptions

Setting descriptions on an interface is helpful to the administrator and, as with the hostname, only locally significant. The description

command is a helpful one because you can, for instance, use it to keep track of circuit numbers.

Here’s an example:

Todd#

config t

Todd(config)#

int s0/0/0

Todd(config-if)#

description Wan to SF circuit number 6fdda12345678

Todd(config-if)#

int fa0/0

Todd(config-if)#

description Sales VLAN

Todd(config-if)#

^Z

Todd#

You can view the description of an interface with either the show running-config

command or the show interface

command:

Todd#

sh run

[output cut]

!

interface FastEthernet0/0

description Sales VLAN

ip address 10.10.10.1 255.255.255.248

duplex auto

speed auto

!

interface Serial0/0/0

description Wan to SF circuit number 6fdda 12345678

no ip address

shutdown

!

[output cut]

Todd#

sh int f0/0

FastEthernet0/0 is up, line protocol is down

Hardware is MV96340 Ethernet, address is 001a.2f55.c9e8 (bia 001a.2f55.c9e8)

Description: Sales VLAN

[output cut]

Todd#

sh int s0/0/0

Serial0/0/0 is administratively down, line protocol is down

Hardware is GT96K Serial

Description: Wan to SF circuit number 6fdda12345678

description: A Helpful Command

Bob, a senior network administrator at Acme Corporation in San Francisco, has over 50 WAN links to various branches throughout the U.S. and Canada. Whenever an interface goes down, Bob spends a lot of time trying to figure out the circuit number as well as the phone number of the provider of the WAN link.

The interface description

command would be very helpful to Bob because he can use this command on his LAN links to discern exactly where every router interface is connected. And Bob would benefit tremendously by adding circuit numbers to each and every WAN interface, along with the phone number of the responsible provider.

So by spending the few hours it would take to add this information to each and every router interface, Bob can save a huge amount of precious time when his WAN links go down—and you know they will!

Doing the do Command

Beginning with IOS version 12.3, Cisco has finally added a command to the IOS that allows you to view the configuration and statistics from within configuration mode. (In the examples I gave you in the previous section, all show

commands were run from privileged mode.)

In fact, with any IOS, you’d get the following error if you tried to view the configuration from global config:

Router(config)#

sh run

^

% Invalid input detected at '^' marker.

Compare that to the output I get from entering that same command on my router that’s running the 12.4 IOS and using the “do” syntax:

Enter configuration commands, one per line. End with CNTL/Z.

Todd(config)#

do show run

Building configuration...

Current configuration : 3276 bytes

!

[output cut]

Todd(config)#

do sh int f0/0

FastEthernet0/0 is up, line protocol is down

Hardware is MV96340 Ethernet, address is 001a.2f55.c9e8 (bia

001a.2f55.c9e8)

Description: Sales VLAN

[output cut]

So basically, you can pretty much run any command from any configuration prompt now—cool, huh? Going back to the example of encrypting our passwords, the do

command would definitely have gotten the party started sooner—so, my friends, this is a very, very good thing indeed!

Router Interfaces

Interface configuration is one of the most important router configurations because without interfaces, a router is pretty much a completely useless object. Plus, interface configurations must be totally precise to enable communication with other devices. Network layer addresses, media type, bandwidth, and other administrator commands are all used to configure an interface.

Different routers use different methods to choose the interfaces used on them. For instance, the following command shows a Cisco router with

10 serial interfaces, labeled 0 through 9:

Router(config)#

int serial ?

<0-9> Serial interface number

Now it’s time to choose the interface you want to configure. Once you do that, you will be in interface configuration for that specific interface. The following command would be used to choose serial port 5, for example:

Router(config)#

int serial 5

Router(config)-if)#

The old 2522 router I am using in this example has one Ethernet 10BaseT port, and typing

interface ethernet 0

can configure that interface, as seen here:

Router(config)#

int ethernet ?

<0-0> Ethernet interface number

Router(config)#

int ethernet 0

Router(config-if)#

As I showed you earlier, the 2500 router is a fixed-configuration router. This means that when you bought that model, you were stuck with that

physical configuration—a huge reason why I don’t use them much. I certainly never would use them in a production setting anymore, but for studying for your exam they can be used quite effectively at a very low cost.

To configure an interface, we always used the interface

type number

sequence, but with the 2600 and 2800 series routers (actually, any ISR router for that matter), there’s a physical slot in the router, and there’s a port number on the module plugged into that slot. So on a modular router, the configuration would be interface type slot/port

, as seen here:

Router(config)#

int fastethernet ?

<0-1> FastEthernet interface number

Router(config)#

int fastethernet 0

% Incomplete command.

Router(config)#

int fastethernet 0?

/

Router(config)#

int fastethernet 0/?

<0-1> FastEthernet interface number

Make note of the fact that you can’t just type

int fastethernet 0

. You must type the full command:

type slot/port

or

int fastethernet 0/0

(or

int fa 0/0

).

For the ISR series, it’s basically the same, only you get even more options. For example, the built-in FastEthernet interfaces work with the same configuration we used with the 2600 series:

Todd(config)#

int fastEthernet 0/?

<0-1> FastEthernet interface number

Todd(config)#

int fastEthernet 0/0

Todd(config-if)#

But the rest of the modules are different—they use three numbers instead of two. The first 0 is the router itself, and then you choose the slot, and then the port. Here’s an example of a serial interface on my 2811:

Todd(config)#

interface serial ?

<0-2> Serial interface number

Todd(config)#

interface serial 0/0/?

<0-1> Serial interface number

Todd(config)#

interface serial 0/0/0

Todd(config-if)#

This can look a little dicey, I know, but I promise it’s really not that hard! It helps to remember that you should always view a running-config output first so you know what interfaces you have to deal with. Here’s my 2801 output:

Todd(config-if)#

do show run

Building configuration...

[output cut]

!

interface FastEthernet0/0

no ip address

shutdown

duplex auto

speed auto

!

interface FastEthernet0/1

no ip address

shutdown

duplex auto

speed auto

!

interface Serial0/0/0

no ip address

shutdown

no fair-queue

!

interface Serial0/0/1

no ip address

shutdown

!

interface Serial0/1/0

no ip address

shutdown

!

interface Serial0/2/0

no ip address

shutdown

clock rate 2000000

!

[output cut]

For the sake of brevity, I didn’t include my complete running-config, but I’ve displayed all you need. You can see the two built-in FastEthernet interfaces, the two serial interfaces in slot 0 (0/0/0 and 0/0/1), the serial interface in slot 1 (0/1/0), and the serial interface in slot 2 (0/2/0). Once you see the interfaces like this, it makes it a lot easier for you to understand how the modules are inserted into the router.

Just understand that if you type

interface e0

on a 2500,

interface fastethernet 0/0

on a 2600, or

interface serial 0/1/0

on a 2800, all you’re doing is choosing an interface to configure, and basically, they’re all configured the same way after that.

I’m going to continue with our router interface discussion in the next sections, and I’ll include how to bring up the interface and set an IP address on it.

Bringing Up an Interface

You can disable an interface with the interface command shutdown

and enable it with the no shutdown

command.

If an interface is shut down, it’ll display administratively down when you use the show interfaces command (sh int for short):

Todd#

sh int f0/1

FastEthernet0/1 is administratively down, line protocol is down

[output cut]

Another way to check an interface’s status is via the show running-config

command. All interfaces are shut down by default. You can bring up the interface with the no shutdown

command ( no shut

for short):

Todd#

config t

Todd(config)#

int f0/1

Todd(config-if)#

no shutdown

Todd(config-if)#

*Feb 28 22:45:08.455: %LINK-3-UPDOWN: Interface FastEthernet0/1,

changed state to up

Todd(config-if)#

do show int f0/1

FastEthernet0/1 is up, line protocol is up

[output cut]

Configuring an IP Address on an Interface

Even though you don’t have to use IP on your routers, it’s most often what people actually do use. To configure IP addresses on an interface, use the ip address

command from interface configuration mode:

Todd(config)#

int f0/1

Todd(config-if)#

ip address 172.16.10.2 255.255.255.0

Don’t forget to enable the interface with the no shutdown

command. Remember to look at the command show interface int

to see if the interface is administratively shut down or not. show running-config

will also give you this information.

The ip address address mask command starts the IP processing on the interface.

If you want to add a second subnet address to an interface, you have to use the secondary

parameter. If you type another IP address and press

Enter, it will replace the existing primary IP address and mask. This is definitely a most excellent feature of the Cisco IOS.

So let’s try it. To add a secondary IP address, just use the secondary

parameter:

Todd(config-if)#

ip address 172.16.20.2 255.255.255.0 ?

secondary Make this IP address a secondary address

<cr>

Todd(config-if)#

ip address 172.16.20.2 255.255.255.0 secondary

Todd(config-if)#

^Z

Todd(config-if)#

do sh run

Building configuration...

[output cut] interface FastEthernet0/1

ip address 172.16.20.2 255.255.255.0 secondary

ip address 172.16.10.2 255.255.255.0

duplex auto

speed auto

!

I really wouldn’t recommend having multiple IP addresses on an interface because it’s ugly and inefficient, but I showed you anyway just in case you someday find yourself dealing with an MIS manager who’s in love with really bad network design and makes you administer it! And who knows? Maybe someone will ask you about it someday and you’ll get to seem really smart because you know this.

Using the Pipe

No, not that pipe. I mean the output modifier. (Although with some of the router configurations I’ve seen in my career, sometimes I wonder!) This pipe ( | ) allows us to wade through all the configurations or other long outputs and get straight to our goods fast. Here’s an example:

Todd#

sh run | ?

append Append redirected output to URL (URLs supporting append operation

only)

begin Begin with the line that matches

exclude Exclude lines that match

include Include lines that match

redirect Redirect output to URL

section Filter a section of output

tee Copy output to URL

Todd#

sh run | begin interface

interface FastEthernet0/0

description Sales VLAN

ip address 10.10.10.1 255.255.255.248

duplex auto

speed auto

!

interface FastEthernet0/1

ip address 172.16.20.2 255.255.255.0 secondary

ip address 172.16.10.2 255.255.255.0

duplex auto

speed auto

!

interface Serial0/0/0

description Wan to SF circuit number 6fdda 12345678

no ip address

!

So basically, the pipe symbol (output modifier) is what you need to help you get where you want to go light years faster than mucking around in a router’s entire configuration. I use it a lot when I am looking at a large routing table to find out whether a certain route is in the routing table. Here’s an example:

Todd#

sh ip route | include 192.168.3.32

R 192.168.3.32 [120/2] via 10.10.10.8, 00:00:25, FastEthernet0/0

Todd#

First, you need to know that this routing table had over 100 entries, so without my trusty pipe, I’d probably still be looking through that output! It’s a powerfully efficient tool that saves you major time and effort by quickly finding a line in a configuration—or as the preceding example shows, a single route in a huge routing table.

Give yourself a little time to play around with the pipe command; get the hang of it, and you’ll be seriously high on your newfound ability to quickly parse through router output.

Serial Interface Commands

Wait! Before you just jump in and configure a serial interface, you need some key information—like knowing that the interface will usually be attached to a CSU/DSU type of device that provides clocking for the line to the router, as I’ve shown in Figure 6-4 .

Figure 6-4:

A typical WAN connection

Here you can see that the serial interface is used to connect to a DCE network via a CSU/DSU that provides the clocking to the router interface.

But if you have a back-to-back configuration (for example, one that’s used in a lab environment like I’ve shown you in Figure 6-5 ), one end—the data communication equipment (DCE) end of the cable—must provide clocking!

By default, Cisco router serial interfaces are all data terminal equipment (DTE) devices, which means that you must configure an interface to provide clocking if you need it to act like a DCE device. Again, you would not provide clocking on a production T1 connection, for example, because you would have a CSU/DSU connected to your serial interface, as Figure 6-4 shows.

Figure 6-5:

Providing clocking on a nonproduction network

You configure a DCE serial interface with the clock rate

command:

Todd#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Todd(config)#

int s0/0/0

Todd(config-if)#

clock rate ?

Speed (bits per second)

1200

2400

4800

9600

14400

19200

28800

32000

38400

48000

56000

57600

64000

72000

115200

125000

128000

148000

192000

250000

256000

384000

500000

512000

768000

800000

1000000

2000000

4000000

5300000

8000000

<300-8000000> Choose clockrate from list above

Todd(config-if)#

clock rate 1000000

The clock rate

command is set in bits per second. Besides looking at the cable end to check for a label of DCE or DTE, you can see if a router’s serial interface has a DCE cable connected with the show controllers int

command:

Todd#

sh controllers s0/0/0

Interface Serial0/0/0

Hardware is GT96K

DTE V.35idb at 0x4342FCB0, driver data structure at 0x434373D4

Here is an example of an output that shows a DCE connection:

Todd#

sh controllers s0/2/0

Interface Serial0/2/0

Hardware is GT96K

DCE V.35, clock rate 1000000

The next command you need to get acquainted with is the bandwidth

command. Every Cisco router ships with a default serial link bandwidth of T1

(1.544Mbps). But this has nothing to do with how data is transferred over a link. The bandwidth of a serial link is used by routing protocols such as

EIGRP and OSPF to calculate the best cost (path) to a remote network. So if you’re using RIP routing, the bandwidth setting of a serial link is irrelevant since RIP uses only hop count to determine that. If you’re rereading this part thinking, “Huh—what? Routing protocols? Metrics?”—don’t freak! I’m going over all that soon in Chapter 8, “IP Routing.”

Here’s an example of using the bandwidth

command:

Todd#

config t

Todd(config)#

int s0/0/0

Todd(config-if)#

bandwidth ?

<1-10000000> Bandwidth in kilobits

inherit Specify that bandwidth is inherited

receive Specify receive-side bandwidth

Todd(config-if)#

bandwidth 1000

Did you notice that, unlike the clock rate

command, the bandwidth

command is configured in kilobits per second?

After going through all these configuration examples regarding the clock rate command, understand that the new ISR routers automatically detect DCE connections and set the clock rate

to 2000000. However, you still need to understand the clock rate

command for the Cisco objectives, even though the new routers set it for you automatically!

Viewing, Saving, and Erasing Configurations

If you run through setup mode, you’ll be asked if you want to use the configuration you just created. If you say yes, then it will copy the configuration

running in DRAM (known as the running-config) into NVRAM and name the file

startup-config

. Hopefully, you will always use the CLI and not setup mode.

You can manually save the file from DRAM (usually just called RAM) to NVRAM by using the copy running-config startup-config

command (you can use the shortcut copy run start

also):

Todd#

copy running-config startup-config

Destination filename [startup-config]? [

press enter

]

Building configuration...

[OK]

Todd#

Building configuration...

When you see a question with an answer in

[]

, it means that if you just press Enter, you’re choosing the default answer.

Also, when the command asked for the destination filename, the default answer was startup-config. The reason it asks is because you can copy the configuration pretty much anywhere you want. Take a look:

Todd#

copy running-config ?

archive: Copy to archive: file system

flash: Copy to flash: file system

ftp: Copy to ftp: file system

http: Copy to http: file system

https: Copy to https: file system

ips-sdf Update (merge with) IPS signature configuration

null: Copy to null: file system

nvram: Copy to nvram: file system

rcp: Copy to rcp: file system

running-config Update (merge with) current system configuration

scp: Copy to scp: file system

startup-config Copy to startup configuration

syslog: Copy to syslog: file system

system: Copy to system: file system

tftp: Copy to tftp: file system

xmodem: Copy to xmodem: file system

ymodem: Copy to ymodem: file system

We’ll take a closer look at how and where to copy files in Chapter 7.

You can view the files by typing

show running-config

or

show startup-config

from privileged mode. The sh run

command, which is a shortcut for show running-config

, tells us that we are viewing the current configuration:

Todd#

show running-config

Building configuration...

Current configuration : 3343 bytes

!

version 12.4

[output cut]

The sh start

command—one of the shortcuts for the show startup-config

command—shows us the configuration that will be used the next time the router is reloaded. It also tells us how much NVRAM is being used to store the startup-config file. Here’s an example:

Todd#

show startup-config

Using 1978 out of 245752 bytes

!

version 12.4

[output cut]

Deleting the Configuration and Reloading the Router

You can delete the startup-config file by using the erase startup-config

command:

Todd#

erase startup-config

Erasing the nvram filesystem will remove all configuration files!

Continue? [confirm][

enter

]

[OK]

Erase of nvram: complete

Todd#

*Feb 28 23:51:21.179: %SYS-7-NV_BLOCK_INIT: Initialized the geometry of nvram

Todd#

sh startup-config

startup-config is not present

Todd#

reload

Proceed with reload? [confirm]System configuration has been modified.

Save? [yes/no]:

n

If you reload or power down and up the router after using the erase startup-config

command, you’ll be offered setup mode because there’s no configuration saved in NVRAM. You can press Ctrl+C to exit setup mode at any time (the reload

command can only be used from privileged mode).

At this point, you shouldn’t use setup mode to configure your router. So just say

no

to setup mode, because it’s there to help people who don’t know how to use the Cash Line Interface (CLI), and this no longer applies to you. Be strong—you can do it!

Verifying Your Configuration

Obviously, show running-config

would be the best way to verify your configuration and show startup-config

would be the best way to verify the configuration that’ll be used the next time the router is reloaded—right?

Well, once you take a look at the running-config, if all appears well, you can verify your configuration with utilities such as Ping and Telnet. Ping is a program that uses ICMP echo requests and replies. (ICMP is discussed in Chapter 3, “Introduction to TCP/IP.”) Ping sends a packet to a remote host, and if that host responds, you know that it’s alive. But you don’t know if it’s alive and also

well

—just because you can ping a Microsoft server does not mean you can log in! Even so, Ping is an awesome starting point for troubleshooting an internetwork.

Did you know that you can ping with different protocols? You can, and you can test this by typing

ping ?

at either the router user-mode or privileged-mode prompt:

Router#

ping ?

WORD Ping destination address or hostname

appletalk Appletalk echo

clns CLNS echo

decnet DECnet echo

ip IP echo

ipv6 IPv6 echo

ipx Novell/IPX echo

srb srb echo

tag Tag encapsulated IP echo

<cr>

If you want to find a neighbor’s Network layer address, either you need to go to the router or switch itself or you can type

show cdp entry * protocol

to get the Network layer addresses you need for pinging.

You can also use an extended ping to change the default variables, as shown here:

Router#

ping

Protocol [ip]: [enter]

Target IP address:

1.1.1.1

Repeat count [5]:

100

Datagram size [100]:

1500

Timeout in seconds [2]:

Extended commands [n]:

y

Source address or interface:

Fastethernet0/0

Type of service [0]:

Set DF bit in IP header? [no]:

Validate reply data? [no]:

Data pattern [0xABCD]:

Loose, Strict, Record, Timestamp, Verbose[none]:

verbose

Loose, Strict, Record, Timestamp, Verbose[V]:

Sweep range of sizes [n]:

Type escape sequence to abort.

Sending 100, 1500-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

Packet sent with a source address of 10.10.10.1

Notice that extended ping allows you to set the repeat count higher than the default of 5 and the datagram size larger, which raises the MTU and allows a better testing of throughput, and one last important piece I’ll pull out of the output, the source interface. You can choose which interface the ping is sourced from. This is helpful in some diagnostic situations.

Cisco Discovery Protocol (CDP) is covered in Chapter 7.

Traceroute uses ICMP with IP time to live (TTL) time-outs to track the path a packet takes through an internetwork, in contrast to Ping, which just finds the host and responds. And Traceroute can also be used with multiple protocols.

Router#

traceroute ?

WORD Trace route to destination address or hostname

appletalk AppleTalk Trace

clns ISO CLNS Trace

ip IP Trace

ipv6 IPv6 Trace

ipx IPX Trace

<cr>

Telnet, FTP, and HTTP are really the best tools because they use IP at the Network layer and TCP at the Transport layer to create a session with a remote host. If you can telnet, ftp, or http into a device, your IP connectivity just has to be good.

Router#

telnet ?

WORD IP address or hostname of a remote system

<cr>

From the router prompt, you just type a hostname or IP address and it will assume you want to telnet—you don’t need to type the actual command, telnet

.

In the following sections, I am going to show you how to verify the interface statistics.

Verifying with the show interface Command

Another way to verify your configuration is by typing show interface

commands, the first of which is show interface ?

. That will reveal all the available

interfaces to verify and configure.

The show interfaces

command displays the configurable parameters and statistics of all interfaces on a router.

This command is very useful for verifying and troubleshooting router and network issues.

The following output is from my freshly erased and rebooted 2811 router:

Router#

sh int ?

Async Async interface

BVI Bridge-Group Virtual Interface

CDMA-Ix CDMA Ix interface

CTunnel CTunnel interface

Dialer Dialer interface

FastEthernet FastEthernet IEEE 802.3

Loopback Loopback interface

MFR Multilink Frame Relay bundle interface

Multilink Multilink-group interface

Null Null interface

Port-channel Ethernet Channel of interfaces

Serial Serial

Tunnel Tunnel interface

Vif PGM Multicast Host interface

Virtual-PPP Virtual PPP interface

Virtual-Template Virtual Template interface

Virtual-TokenRing Virtual TokenRing

accounting Show interface accounting

counters Show interface counters

crb Show interface routing/bridging info

dampening Show interface dampening info

description Show interface description

etherchannel Show interface etherchannel information

irb Show interface routing/bridging info

mac-accounting Show interface MAC accounting info

mpls-exp Show interface MPLS experimental accounting info

precedence Show interface precedence accounting info

pruning Show interface trunk VTP pruning information

rate-limit Show interface rate-limit info

stats Show interface packets & octets, in & out, by switching

path

status Show interface line status

summary Show interface summary

switching Show interface switching

switchport Show interface switchport information

trunk Show interface trunk information

| Output modifiers

<cr>

The only “real” physical interfaces are FastEthernet, Serial, and Async; the rest are all logical interfaces or commands you can use to verify with.

The next command is show interface fastethernet 0/0

. It reveals to us the hardware address, logical address, and encapsulation method as well as statistics on collisions, as seen here:

Router#

sh int f0/0

FastEthernet0/0 is up, line protocol is up

Hardware is MV96340 Ethernet, address is 001a.2f55.c9e8 (bia 001a.2f55.c9e8)

Internet address is 192.168.1.33/27

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Auto-duplex, Auto Speed, 100BaseTX/FX

ARP type: ARPA, ARP Timeout 04:00:00

Last input never, output 00:02:07, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

0 packets input, 0 bytes

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog

0 input packets with dribble condition detected

16 packets output, 960 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

Router#

As you probably guessed, we’re going to discuss the important statistics from this output, but first, for fun (this is all fun, right?), I’ve got to ask you, what subnet is the FastEthernet 0/0 a member of and what’s the broadcast address and valid host range?

And, my friend, you really have to be able to nail these things NASCAR fast! Just in case you didn’t, the address is 192.168.1.33/27. And I’ve gotta be honest—if you don’t know what a /27 is at this point, you’ll need a miracle to pass the exam. (A /27 is 255.255.255.224.) The fourth octet is a block size of 32. The subnets are 0, 32, 64, etc.; the FastEthernet interface is in the 32 subnet; the broadcast address is 63; and the valid hosts are 33–62.

If you struggled with any of this, please save yourself from certain doom and get yourself back into Chapter 4, “Easy Subnetting,” now! Read and reread it until you’ve got it dialed in!

The preceding interface is working and looks to be in good shape. The show interfaces

command will show you if you are receiving errors on the interface, and it will show you the maximum transmission unit (MTU), which is the maximum packet size allowed that can transmit on that interface; bandwidth (BW) for use with routing protocols; reliability (255/255 means perfect!); and load (1/255 means no load).

Continuing to use the previous output, what is the bandwidth of the interface? Well, other than the easy giveaway of the interface being called a

“FastEthernet” interface, we can see that the bandwidth is 100000 Kbit, which is 100,000,000 (Kbit means to add three zeros), which is 100Mbits per second, or FastEthernet. Gigabit would be 1000000Kbits per second.

The most important statistic of the show interface

command is the output of the line and Data Link protocol status. If the output reveals that

FastEthernet 0/0 is up and the line protocol is up, then the interface is up and running:

Router#

sh int fa0/0

FastEthernet0/0 is up, line protocol is up

The first parameter refers to the Physical layer, and it’s up when it receives carrier detect. The second parameter refers to the Data Link layer, and it looks for keepalives from the connecting end. (Keepalives are used between devices to make sure connectivity has not dropped.)

Here’s an example of where the problem usually is found—on serial interfaces:

Router#

sh int s0/0/0

Serial0/0 is up, line protocol is down

If you see that the line is up but the protocol is down, as shown here, you’re experiencing a clocking (keepalive) or framing problem—possibly an encapsulation mismatch. Check the keepalives on both ends to make sure that they match; that the clock rate is set, if needed; and that the encapsulation type is the same on both ends. The preceding output would be considered a Data Link layer problem.

If you discover that both the line interface and the protocol are down, it’s a cable or interface problem. The following output would be considered a

Physical layer problem:

Router#

sh int s0/0/0

Serial0/0 is down, line protocol is down

If one end is administratively shut down (as shown next), the remote end would present as down and down:

Router#

sh int s0/0/0

Serial0/0 is administratively down, line protocol is down

To enable the interface, use the command no shutdown

from interface configuration mode.

The next show interface serial 0/0/0

command demonstrates the serial line and the maximum transmission unit (MTU)—1,500 bytes by default. It also shows the default bandwidth (BW) on all Cisco serial links: 1.544 Kbps. This is used to determine the bandwidth of the line for routing protocols such as EIGRP and OSPF. Another important configuration to notice is the keepalive, which is 10 seconds by default. Each router sends a keepalive message to its neighbor every 10 seconds, and if both routers aren’t configured for the same keepalive time, it won’t work.

Router#

sh int s0/0/0

Serial0/0 is up, line protocol is up

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation HDLC, loopback not set, keepalive set

(10 sec)

Last input never, output never, output hang never

Last clearing of "show interface" counters never

Queueing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

0 packets input, 0 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

0 abort

0 packets output, 0 bytes, 0 underruns

0 output errors, 0 collisions, 16 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=down DSR=down DTR=down RTS=down CTS=down

You can clear the counters on the interface by typing the command

clear counters

:

Router#

clear counters ?

Async Async interface

BVI Bridge-Group Virtual Interface

CTunnel CTunnel interface

Dialer Dialer interface

FastEthernet FastEthernet IEEE 802.3

Group-Async Async Group interface

Line Terminal line

Loopback Loopback interface

MFR Multilink Frame Relay bundle interface

Multilink Multilink-group interface

Null Null interface

Serial Serial

Tunnel Tunnel interface

Vif PGM Multicast Host interface

Virtual-Template Virtual Template interface

Virtual-TokenRing Virtual TokenRing

<cr>

Router#

clear counters s0/0/0

Clear "show interface" counters on this interface

[confirm]

[enter]

Router#

00:17:35: %CLEAR-5-COUNTERS: Clear counter on interface

Serial0/0/0 by console

Router#

Verifying with the show ip interface Command

The show ip interface

command will provide you with information regarding the layer 3 configurations of a router’s interfaces:

Router#

sh ip interface

FastEthernet0/0 is up, line protocol is up

Internet address is 1.1.1.1/24

Broadcast address is 255.255.255.255

Address determined by setup command

MTU is 1500 bytes

Helper address is not set

Directed broadcast forwarding is disabled

Outgoing access list is not set

Inbound access list is not set

Proxy ARP is enabled

Security level is default

Split horizon is enabled

[output cut]

The status of the interface, the IP address and mask, information on whether an access list is set on the interface, and basic IP information are included in this output.

Using the show ip interface brief Command

The show ip interface brief

command is probably one of the most helpful commands that you can ever use on a Cisco router. This command provides a quick overview of the router’s interfaces, including the logical address and status:

Router#

sh ip int brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 unassigned YES unset up up

FastEthernet0/1 unassigned YES unset up up

Serial0/0/0 unassigned YES unset up down

Serial0/0/1 unassigned YES unset administratively down down

Serial0/1/0 unassigned YES unset administratively down down

Serial0/2/0 unassigned YES unset administratively down down

Remember, administratively down means that you need to type no shutdown

under the interface. Notice that Serial0/0/0 is up/down, which means that the physical layer is good and carrier detect is sensed but no keepalives are being received from the remote end. In a nonproduction network, like the one I am working with, the clock rate isn’t set.

Verifying with the show protocols Command

The show protocols

command is a really helpful command you’d use in order to quickly see the status of layers 1 and 2 of each interface as well as the

IP addresses used.

Here’s a look at one of my production routers:

Router#

sh protocols

Global values:

Internet Protocol routing is enabled

Ethernet0/0 is administratively down, line protocol is down

Serial0/0 is up, line protocol is up

Internet address is 100.30.31.5/24

Serial0/1 is administratively down, line protocol is down

Serial0/2 is up, line protocol is up

Internet address is 100.50.31.2/24

Loopback0 is up, line protocol is up

Internet address is 100.20.31.1/24

Using the show controllers Command

The show controllers

command displays information about the physical interface itself. It’ll also give you the type of serial cable plugged into a serial port. Usually, this will only be a DTE cable that plugs into a type of data service unit (DSU).

Router#

sh controllers serial 0/0

HD unit 0, idb = 0x1229E4, driver structure at 0x127E70 buffer size 1524 HD unit 0,

V.35 DTE cable

Router#

sh controllers serial 0/1

HD unit 1, idb = 0x12C174, driver structure at 0x131600 buffer size 1524 HD unit 1,

V.35 DCE cable

Notice that serial 0/0 has a DTE cable, whereas the serial 0/1 connection has a DCE cable. Serial 0/1 would have to provide clocking with the clock rate

command. Serial 0/0 would get its clocking from the DSU.

Let’s look at this command again. In

Figure 6-6 , see the DTE/DCE cable between the two routers? Know that you will not see this in production networks!

Figure 6-6:

The show controllers

command

Router R1 has a DTE connection—typically the default for all Cisco routers. Routers R1 and R2 can’t communicate. Check out the output of the show controllers s0/0

command here:

R1#

sh controllers serial 0/0

HD unit 0, idb = 0x1229E4, driver structure at 0x127E70 buffer size 1524 HD unit 0,

V.35 DCE cable

The show controllers s0/0

command shows that the interface is a V.35 DCE cable. This means that R1 needs to provide clocking of the line to router R2. Basically, the interface has the wrong label on the cable on the R1 router’s serial interface. But if you add clocking on the R1 router’s serial interface, the network should come right up.

Let’s check out another issue, shown in

Figure 6-7 , that you can solve by using the show controllers

command. Again, routers R1 and R2 can’t communicate.

Figure 6-7:

The show controllers

command used with the show ip interface

command

Here’s the output of R1’s show controllers s0/0

command and show ip interface s0/0:

R1#

sh controllers s0/0

HD unit 0, idb = 0x1229E4, driver structure at 0x127E70 buffer size 1524 HD unit 0,

DTE V.35 clocks stopped

cpb = 0xE2, eda = 0x4140, cda = 0x4000

R1#

sh ip interface s0/0

Serial0/0 is up, line protocol is down

Internet address is 192.168.10.2/24

Broadcast address is 255.255.255.255

If you use the show controllers

command and the show ip interface

command, you’ll see that router R1 isn’t receiving clocking of the line. This network is a nonproduction network, so no CSU/DSU is connected to provide clocking of the line. This means the DCE end of the cable will be providing the clock rate—in this case, the R2 router. The show ip interface

indicates that the interface is up but the protocol is down, which means that no keepalives are being received from the far end. In this example, the likely culprit is the result of bad cable, or no clocking.

Summary

This was a fun chapter! I showed you a lot about the Cisco IOS and I really hope you gained a lot of insight into the Cisco router world. This chapter started off by explaining the Cisco Internetwork Operating System (IOS) and how you can use the IOS to run and configure Cisco routers. You learned how to bring a router up and what setup mode does. Oh, and by the way, since you can now basically configure Cisco routers, you should never use setup mode, right?

After I discussed how to connect to a router with a console and LAN connection, I covered the Cisco help features and how to use the CLI to find commands and command parameters. In addition, I discussed some basic show

commands to help you verify your configurations.

Administrative functions on a router help you administer your network and verify that you are configuring the correct device. Setting router

passwords is one of the most important configurations you can perform on your routers. I showed you the five passwords to set. In addition, I used the hostname, interface description, and banners to help you administer your router.

Well, that concludes your introduction to the Cisco IOS. And, as usual, it’s super-important for you to have the basics that we went over in this chapter before you move on to the following chapters.

Exam Essentials

Describe the responsibilities of the IOS.

The Cisco router IOS software is responsible for network protocols and providing supporting functions, connecting high-speed traffic between devices, adding security to control access and prevent unauthorized network use, providing scalability for ease of network growth and redundancy, and supplying network reliability for connecting to network resources.

List the options available to connect to a Cisco device for management purposes.

The three options available are the console port and auxiliary port and through Telnet. A Telnet connection is not possible until an IP address has been configured and a Telnet username and password have been configured.

Understand the boot sequence of a router.

When you first bring up a Cisco router, it will run a power-on self-test (POST), and if that passes, it will look for and load the Cisco IOS from flash memory, if a file is present. The IOS then proceeds to load and looks for a valid configuration in NVRAM called the startup-config. If no file is present in NVRAM, the router will go into setup mode.

Describe the use of setup mode.

Setup mode is automatically started if a router boots and no startup-config is in NVRAM. You can also bring up setup mode by typing

setup

from privileged mode. Setup provides a minimum amount of configuration in an easy format for someone who does not understand how to configure a Cisco router from the command line.

Differentiate user, privileged, and global configuration modes, both visually and from a command capabilities perspective.

User mode, indicated by the routername>

prompt, provides a command-line interface with very few available commands by default. User mode does not allow the configuration to be viewed or changed. Privileged mode, indicated by the routername#

prompt, allows a user to both view and change the configuration of a router. You can enter privileged mode by typing the command

enable

and entering the enable password or enable secret password, if set. Global configuration mode, indicated by the routername(config)#

prompt, allows configuration changes to be made that apply to the entire router (as opposed to a configuration change that might affect only one interface, for example).

Recognize additional prompts available in other modes and describe their use.

Additional modes are reached via the global configuration prompt, routername(config)#

, and their prompts include interface, router(config-if)#

, for making interface settings; subinterface, router(config-subif)#

, used when a physical interface must be logically subdivided; line configuration mode, router(config-line)#

, used to set passwords and make other settings to various connection methods; and routing protocol modes for various routing protocols, router(configrouter)#

, used to enable and configure routing protocols.

Access and utilize editing and help features.

Make use of typing a question mark at the end of commands for help in using the commands.

Additionally, understand how to filter command help with the same question mark and letters. Use the command history to retrieve commands previously utilized without retyping. Understand the meaning of the caret when an incorrect command is rejected. Finally, identify useful hot key combinations.

Identify the information provided by the

show version

command.

The show version

command will provide basic configuration for the system hardware as well as the software version, the names and sources of configuration files, the configuration register setting, and the boot images.

Set the hostname of a router.

The command sequence to set the hostname of a router is as follows: enable config t hostname Todd

Differentiate the enable password and enable secret password.

Both of these passwords are used to gain access into privileged mode.

However, the enable secret password is newer and is always encrypted by default. Also, if you set the enable password and then set the enable secret, only the enable secret will be used.

Describe the configuration and use of banners.

Banners provide information to users accessing the device and can be displayed at various login prompts. They are configured with the banner

command and a keyword describing the specific type of banner.

Set the enable secret on a router.

To set the enable secret, you use the global config command enable secret

. Do not use enable secret password

password

or you will set your password to

password password

. Here is an example: enable config t enable secret todd

Set the console password on a router.

To set the console password, use the following sequence: enable config t line console 0 password todd login

Set the Telnet password on a router.

To set the Telnet password, the sequence is as follows: enable config t line vty 0 4 password todd login

Describe the advantages of using Secure Shell and list its requirements.

Secure Shell (SSH) uses encrypted keys to send data so that usernames and passwords are not sent in the clear. It requires that a hostname and domain name be configured and that encryption keys be generated.

Describe the process of preparing an interface for use.

To use an interface, you must configure it with an IP address and subnet mask in the same subnet of the hosts that will be connecting to the switch that is connected to that interface. It also must be enabled with the no shutdown command. A serial interface that is connected back-to-back with another router serial interface must also be configured with a clock rate on the DCE end of the serial cable.

Understand how to troubleshoot a serial link problem.

If you type

show interface serial 0

and see down, line protocol is down

, this will be considered a Physical layer problem. If you see it as up, line protocol is down

, then you have a Data Link layer problem.

Understand how to verify your router with the

show interfaces

command.

If you type

show interfaces

, you can view the statistics for the interfaces on the router, verify whether the interfaces are shut down, and see the IP address of each interface.

Describe how to view, edit, delete, and save a configuration.

The show running-config

command is used to view the current configuration being used by the router. The show startup-config

command displays the last configuration that was saved and is the one that will be used at next startup. The copy running-config startup-config

command is used to save changes made to the running configuration in NVRAM. The erase startupconfig

command deletes the saved configuration and will result in the invocation of the setup menu when the router is rebooted because there will be no configuration present.

Written Lab 6

Write out the command or commands for the following questions:

1.

What command is used to set a serial interface to provide clocking to another router at 64 Kb?

2.

If you telnet into a router and get the response connection refused, password not set

, what commands would you execute on the destination router to stop receiving this message and not be prompted for a password?

3.

If you type

show inter ethernet 0

and notice the port is administratively down, what commands would you execute to enable the interface?

4.

If you wanted to delete the configuration stored in NVRAM, what command(s) would you type?

5.

If you wanted to set the user-mode password to

todd

for the console port, what command(s) would you type?

6.

If you wanted to set the enable secret password to

cisco

, what command(s) would you type?

7.

If you wanted to determine if serial interface 0/2 should provide clocking, what command would you use?

8.

What command would you use to see the terminal history size?

9.

You want to reinitialize the router and totally replace the running-config with the current startup-config. What command will you use?

10.

How would you set the name of a router to

Chicago

?

(The answers to Written Lab 6 can be found following the answers to the review questions for this chapter.)

Hands-on Labs

In this section, you will perform commands on a Cisco router that will help you understand what you learned in this chapter.

You’ll need at least one Cisco router—two would be better, three would be outstanding. The hands-on labs in this section are included for use with real Cisco routers. All of these labs work with the Cisco Packet Tracer router simulator. Lastly, for the CCNA it doesn’t matter what series type router you use with these labs (i.e., 2500, 2600, 800, 1800, or 2800).

It is assumed that the router you’re going to use has no current configuration present. If necessary, erase any existing configuration with Hands-on

Lab 6.1; otherwise, proceed to Hands-on lab 6.2:

Lab 6.1: Erasing an Existing Configuration

Lab 6.2: Exploring User, Privileged, and Configuration Modes

Lab 6.3: Using the Help and Editing Features

Lab 6.4: Saving a Router Configuration

Lab 6.5: Setting Passwords

Lab 6.6: Setting the Hostname, Descriptions, IP Address, and Clock Rate

Hands-on Lab 6.1: Erasing an Existing Configuration

The following lab may require the knowledge of a username and password to enter privileged mode. If the router has a configuration with an unknown username and password for privileged mode, this procedure will not be possible. It is possible to erase a configuration without a

privileged mode password, but the exact steps depend on the router model and will not be covered until Chapter 7.

1.

Start the router up and when prompted, press Enter.

2.

At the

Routername>

prompt, type

enable

.

3.

If prompted, enter the username and press Enter. Then enter the correct password and press Enter.

4.

At the privileged mode prompt, type

erase startup-config

.

5.

At the privileged mode prompt, type

reload

, and when prompted to save the configuration, type

n

for no.

Hands-on Lab 6.2: Exploring User, Privileged, and Configuration Modes

1.

Turn the router on. If you just erased the configuration as in Hands-on Lab 6.1, when prompted to continue with the configuration dialog, enter

n

for no and press Enter. When prompted, press Enter to connect to your router. This will put you into user mode.

2.

At the

Router>

prompt, type a question mark (

?

).

3.

Notice the

–more–

at the bottom of the screen.

4.

Press the Enter key to view the commands line by line. Press the spacebar to view the commands a full screen at a time. You can type

q

at any time to quit.

5.

Type

enable

or

en

and press Enter. This will put you into privileged mode where you can change and view the router configuration.

6.

At the

Router#

prompt, type a question mark (

?

). Notice how many options are available to you in privileged mode.

7.

Type

q

to quit.

8.

Type

config

and press Enter.

9.

When prompted for a method, press Enter to configure your router using your terminal (which is the default).

10.

At the

Router(config)#

prompt, type a question mark (

?

), then

q

to quit, or press the spacebar to view the commands.

11.

Type

interface e0

or

int e0

(or even

int fa0/0

) and press Enter. This will allow you to configure interface Ethernet 0.

12.

At the

Router(config-if)#

prompt, type a question mark (

?

).

13.

Type

int s0 (int s0/0)

or

interface s0

(same as the interface serial 0

command) and press Enter. This will allow you to configure interface serial 0. Notice that you can go from interface to interface easily.

14.

Type

encapsulation ?

.

15.

Type

exit

. Notice how this brings you back one level.

16.

Press Ctrl+Z. Notice how this brings you out of configuration mode and places you back into privileged mode.

17.

Type

disable

. This will put you into user mode.

18.

Type

exit

, which will log you out of the router.

Hands-on Lab 6.3: Using the Help and Editing Features

1.

Log into the router and go to privileged mode by typing

en

or

enable

.

2.

Type a question mark (

?

).

3.

Type

cl?

and then press Enter. Notice that you can see all the commands that start with

cl

.

4.

Type

clock ?

and press Enter.

Notice the difference between steps 3 and 4. Step 3 has you type letters with no space and a question mark, which will give you all the commands that start with cl. Step

4 has you type a command, space, and question mark. By doing this, you will see the next available parameter.

5.

Set the router’s clock by typing

clock ?

and, following the help screens, setting the router’s time and date. The following steps walk you through setting the date and time:

6.

Type

clock ?

.

7.

Type

clock set ?

.

8.

Type

clock set 10:30:30 ?

.

9.

Type

clock set 10:30:30 14 May ?

.

10.

Type

clock set 10:30:30 14 March 2011

.

11.

Press Enter.

12.

Type

show clock

to see the time and date.

13.

From privileged mode, type

show access-list 10

. Don’t press Enter.

14.

Press Ctrl+A. This takes you to the beginning of the line.

15.

Press Ctrl+E. This should take you back to the end of the line.

16.

The Ctrl-A takes you back to the beginning of the line, and then the Ctrl+F moves you forward one character.

17.

Press Ctrl+B, which will move you back one character.

18.

Press Enter, then press Ctrl+P. This will repeat the last command.

19.

Press the up arrow key on your keyboard. This will also repeat the last command.

20.

Type

sh history

. This shows you the last 10 commands entered.

21.

Type

terminal history size ?

. This changes the history entry size. The

?

is the number of allowed lines.

22.

Type

show terminal

to gather terminal statistics and history size.

23.

Type

terminal no editing

. This turns off advanced editing. Repeat steps 14 through 18 to see that the shortcut editing keys have no effect until you type

terminal editing

.

24.

Type

terminal editing

and press Enter to re-enable advanced editing.

25.

Type

sh run

, then press your Tab key. This will finish typing the command for you.

26.

Type

sh start

, then press your Tab key. This will finish typing the command for you.

Hands-on Lab 6.4: Saving a Router Configuration

1.

Log into the router and go into privileged mode by typing

en

or

enable

, then press Enter.

2.

To see the configuration stored in NVRAM, type

sh start

and press Tab and Enter, or type

show startup-config

and press Enter. However, if no configuration has been saved, you will get an error message.

3.

To save a configuration to NVRAM, which is known as startup-config, you can do one of the following:

Type

copy run start

and press Enter.

Type

copy running

, press Tab, type

start

, press Tab, and press Enter.

Type

copy running-config startup-config

and press Enter.

4.

Type

sh start

, press Tab, then press Enter.

5.

Type

sh run

, press Tab, then press Enter.

6.

Type

erase start

, press Tab, then press Enter.

7.

Type

sh start

, press Tab, then press Enter. The router will either tell you that NVRAM is not present, or some other type of message, depending on the IOS and hardware.

8.

Type

reload

, then press Enter. Acknowledge the reload by pressing Enter. Wait for the router to reload.

9.

Say no to entering setup mode, or just press Ctrl+C.

Hands-on Lab 6.5: Setting Passwords

1.

Log into the router and go into privileged mode by typing

en

or

enable

.

2.

Type

config t

and press Enter.

3.

Type

enable ?

.

4.

Set your enable secret password by typing

enable secret password

(the third word should be your own personalized password) and pressing Enter. Do not add the parameter password

after the parameter secret

(this would make your password the word

password

). An example would be enable secret todd

.

5.

Now let’s see what happens when you log all the way out of the router and then log in. Log out by pressing Ctrl+Z, and then type

exit

and press Enter. Go to privileged mode. Before you are allowed to enter privileged mode, you will be asked for a password. If you successfully enter the secret password, you can proceed.

6.

Remove the secret password. Go to privileged mode, type

config t

, and press Enter. Type

no enable secret

and press Enter. Log out and then log back in again; now you should not be asked for a password.

7.

One more password used to enter privileged mode is called the enable password. It is an older, less secure password and is not used if an enable secret password is set. Here is an example of how to set it: config t enable password todd1

8.

Notice that the enable secret and enable passwords are different. They cannot be the same.

9.

Type

config t

to be at the right level to set your console and auxiliary passwords, then type

line ?

.

10.

Notice that the parameters for the line commands are auxiliary

, vty

, and console

. You will set all three.

11.

To set the Telnet or VTY password, type

line vty 0 4

and then press Enter. The

0 4

is the range of the five available virtual lines used to connect with Telnet. If you have an enterprise IOS, the number of lines may vary. Use the question mark to determine the last line number available on your router.

12.

The next command is used to set the authentication on or off. Type

login

and press Enter to prompt for a user-mode password when telnetting into the router. You will not be able to telnet into a router if the password is not set.

You can use the no login

command to disable the user-mode password prompt when using Telnet.

13.

One more command you need to set for your VTY password is password

. Type

password password

to set the password. (

password

is your password.)

14.

Here is an example of how to set the VTY password: config t line vty 0 4 login

password todd

15.

Set your auxiliary password by first typing

line auxiliary 0

or

line aux 0

.

16.

Type

login

.

17.

Type

password

password

.

18.

Set your console password by first typing

line console 0

or

line con 0

.

19.

Type

login

.

20.

Type

password

password

. Here is an example of the last two command sequences: config t line con 0 login password todd1 line aux 0 login password todd

21.

You can add the

Exec-timeout 0 0

command to the console 0

line. This will stop the console from timing out and logging you out. The command sequence will now look like this: config t line con 0 login password todd2 exec-timeout 0 0

22.

Set the console prompt to not overwrite the command you’re typing with console messages by using the command logging synchronous

.

config t line con 0 logging synchronous

Hands-on Lab 6.6: Setting the Hostname, Descriptions, IP Address, and Clock Rate

1.

Log into the router and go into privileged mode by typing

en

or

enable

. If required, enter a username and password.

2.

Set your hostname on your router by using the hostname command. Notice that it is one word. Here is an example of setting your hostname:

Router#config t

Router(config)#hostname RouterA

RouterA(config)#

Notice that the hostname of the router changed in the prompt as soon as you pressed Enter.

3.

Set a banner that the network administrators will see by using the banner

command, as shown in the following steps.

4.

Type

config t

, then

banner ?

.

5.

Notice that you can set at least four different banners. For this lab we are only interested in the login and message of the day (MOTD) banners.

6.

Set your MOTD banner, which will be displayed when a console, auxiliary, or Telnet connection is made to the router, by typing this: config t banner motd #

This is an motd banner

#

7.

The preceding example used a

#

sign as a delimiting character. This tells the router when the message is done. You cannot use the delimiting character in the message itself.

8.

You can remove the MOTD banner by typing the following command: config t no banner motd

9.

Set the login banner by typing this:

config t banner login #

This is a login banner

#

10.

The login banner will display immediately after the MOTD but before the user-mode password prompt. Remember that you set your user-mode passwords by setting the console, auxiliary, and VTY line passwords.

11.

You can remove the login banner by typing this: config t no banner login

12.

You can add an IP address to an interface with the ip address

command. You need to get into interface configuration mode first; here is an example of how you do that: config t int e0 (you can use int Ethernet 0 too) ip address 1.1.1.1 255.255.0.0

no shutdown

Notice that the IP address (

1.1.1.1

) and subnet mask (

255.255.0.0

) are configured on one line. The no shutdown

(or no shut

for short) command is used to enable the interface. All interfaces are shut down by default.

13.

You can add identification to an interface by using the description

command. This is useful for adding information about the connection.

Here is an example: config t int s0 ip address 2.2.2.1 255.255.0.0

no shut description Wan link to Miami

14.

You can add the bandwidth of a serial link as well as the clock rate when simulating a DCE WAN link. Here is an example: config t int s0 bandwidth 64 clock rate 64000

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s

Introduction.

1. You type

show running-config

and get this output:

[output cut] line console 0

Exec-timeout 1 44

Password 7 098C0BQR

Login

[output cut]

What do the two numbers following the exec-timeout

command mean?

A. If no command has been typed in 44 seconds, the console connection will be closed.

B. If no router activity has been detected in 1 hour and 44 minutes, the console will be locked out.

C. If no commands have been typed in 1 minute and 44 seconds, the console connection will be closed.

D. If you’re connected to the router by a Telnet connection, input must be detected within 1 minute and 44 seconds or the connection will be closed.

2. Which of the following connection methods available to connect to a router is considered

out-of-band

?

A. Serial port

B. VTY port

C. HTTP port

D. Aux port

3. Which two of the following commands are required when configuring SSH on your router?

A. enable secret password

B. exec-timeout 0 0

C. ip domain-name name

D. username name password password

E.

ip ssh version 2

4. Which command will show you whether a DTE or a DCE cable is plugged into serial 0?

A. sh int s0

B. sh int serial 0

C. show controllers s 0

D. show serial 0 controllers

5. Which of the following is a correct combination of file type and default location in a Cisco router?

A. IOS/NVRAM

B. Startup configuration/flash memory

C. IOS/flash memory

D. Running configuration/NVRAM

6. You set the console password, but when you display the configuration, the password doesn’t show up; it looks like this:

[output cut]

Line console 0

Exec-timeout 1 44

Password 7 098C0BQR

Login

[output cut]

What command would configure the password to be stored this way?

A. encrypt password

B. service password-encryption

C. service-password-encryption

D. exec-timeout 1 44

7. Which of the following commands will configure all the default VTY ports on a router?

A.

Router#

line vty 0 4

B.

Router(config)#

line vty 0 4

C.

Router(config-if)#

line console 0

D.

Router(config)#

line vty all

8. Which of the following commands sets the secret password to Cisco?

A. enable secret password Cisco

B. enable secret cisco

C. enable secret Cisco

D. enable password Cisco

9. If you wanted administrators to see a message when logging into the router, which command would you use?

A. message banner motd

B. banner message motd

C. banner motd

D. message motd

10. Which of the following prompts indicate that the router is currently in privileged mode?

A. router(config)#

B. router>

C. router#

D. router(config-if) router(config)# -- global configuration mode router> -- user mode router# -- privileged mode router(config-if)# -- interface configuration mode

11. What command do you type to save the configuration stored in RAM to NVRAM?

A.

Router(config)#

copy current to starting

B.

Router#

copy starting to running

C.

Router(config)#

copy running-config startup-config

D.

Router#

copy run start

12. You try to telnet into SFRouter from router Corp and receive this message:

Corp#

telnet SFRouter

Trying SFRouter (10.0.0.1)…Open

Password required, but none set

[Connection to SFRouter closed by foreign host]

Corp#

Which of the following sequences will address this problem correctly?

A.

Corp(config)#line console 0

Corp (config-line)#password password

Corp (config-line)#login

B.

SFRemote(config)#line console 0

Corp (config-line)#enable secret password

Corp (config-line)#login

C.

Corp(config)#line vty 0 4

Corp (config-line)#password password

Corp (config-line)#login

D.

SFRemote(config)#line vty 0 4

Corp (config-line)#password password

Corp (config-line)#login

13. Which command will delete the contents of NVRAM on a router?

A. delete NVRAM

B. delete startup-config

C. erase NVRAM

D. erase start

14. What is the problem with an interface if you type

show interface serial 0

and receive the following message?

Serial0 is administratively down, line protocol is down

A. The keepalives are different times.

B. The administrator has the interface shut down.

C. The administrator is pinging from the interface.

D. No cable is attached.

15. Which of the following commands displays the configurable parameters and statistics of all interfaces on a router?

A. show running-config

B. show startup-config

C. show interfaces

D. show versions

16. If you delete the contents of NVRAM and reboot the router, what mode will you be in?

A. Privileged mode

B. Global mode

C. Setup mode

D. NVRAM loaded mode

17. You type the following command into the router and receive the following output:

Router#show serial 0/0

^

% Invalid input detected at '^' marker.

Why was this error message displayed?

A. You need to be in privileged mode.

B. You cannot have a space between serial and 0/0.

C. The router does not have a serial0/0 interface.

D. Part of the command is missing.

18. You type

Router#sh ru

and receive a

% ambiguous command

error. Why did you receive this message?

A. The command requires additional options or parameters.

B. There is more than one show

command that starts with the letters

ru

.

C. There is no show

command that starts with

ru

.

D. The command is being executed from the wrong router mode.

19. Which of the following commands will display the current IP addressing and the layer 1 and 2 status of an interface? (Choose two.)

A. show version

B. show interfaces

C. show controllers

D. show ip interface

E. show running-config

20. At which layer of the OSI model would you assume the problem is if you type

show interface serial 1

and receive the following message?

Serial1 is down, line protocol is down

A. Physical layer

B. Data Link layer

C. Network layer

D. None; it is a router problem.

Answers to Review Questions

1. C. The exec-timeout

command is set in minutes and seconds.

2. D. The auxiliary port can be configured with modem commands so that a modem can be connected to the router. It lets you dial up a remote router and attach to the auxiliary port if the router is down and you need to configure it out-of-band (meaning out of the network).

3. C, D. To configure SSH on your router, you need to set the username

command, the ip domain-name

, login local

, and the transport input ssh

under the VTY

lines, and the crypto key

command. However, SSH version 2 is not required, but suggested.

4. C. The show controllers serial 0

command will show you whether either a DTE or DCE cable is connected to the interface. If it is a DCE connection, you need to add clocking with the clock rate

command.

5. C. The default locations of the files are IOS in flash memory, startup configuration in NVRAM, and running configuration in RAM.

6. B. The command service password-encryption

, from global configuration mode, will encrypt the passwords.

7. B. From global configuration mode, use the line vty 0 4

command to set all five default VTY lines.

8. C. The enable secret password is case sensitive, so the second option is wrong. To set the enable secret password, use the enable secret

password

command from global configuration mode.

9. C. The typical banner is a message of the day (MOTD) and is set by using the global configuration mode command banner motd

.

10. C. The prompts offered as options indicate the following modes: router(config)# -- global configuration mode router> -- user mode router# -- privileged mode router(config-if)# -- interface configuration mode

11. D. To copy the running-config to NVRAM so that it will be used if the router is restarted, use the privileged mode ( copy run start

for short).

copy running-config startup-config

command in

12. D. To allow a VTY (Telnet) session into your router, you must set the VTY password. Option C is wrong because it is setting the password on the wrong router. Notice that the answers you have to set the password before you set the login command. Remember, Cisco may have you set the password before the login command.

13. D. The erase startup-config command erases the contents of NVRAM and will put you in setup mode if the router is restarted.

14. B. If an interface is shut down, the show interface

command will show the interface as administratively down. (It is possible that no cable is attached, but you can’t tell that from this message.)

15. C. With the show interfaces

command, you can view the configurable parameters, get statistics for the interfaces on the router, verify if the interfaces are shut down, and see the IP address of each interface.

16. C. If you delete the startup-config and reload the router, the router will automatically enter setup mode. You can also type

setup

from privileged mode at any time.

17. D. You can view the interface statistics from user mode, but the command is show interface serial 0/0

.

18. B. The

% ambiguous command

error means that there is more than one possible show

command that starts with ru. Use a question mark to find the correct command.

19. B, D. The commands show interfaces

and show ip interface

will show you the layer 1 and 2 status and the IP addresses of your router’s interfaces.

20. A. If you see that a serial interface and the protocol are both down, then you have a Physical layer problem. If you see serial1 is up, line protocol is down

, then you are not receiving (Data Link) keepalives from the remote end.

Answers to Written Lab 6

1.

router(config)#clock rate 64000

2.

router#config t router(config)# line vty 0 4 router(config-line)# no login

3.

router#config t router(config)# int e0 router(config-if)# no shut

4.

router#erase startup-config

5.

router#config t router(config)# line console 0 router(config)# login router(config)# password todd

6.

router#config t router(config)# enable secret cisco

7.

router#show controllers serial 0/1

8.

router#show terminal

9.

router#reload

10.

router#config t router(config)#hostname Chicago

Chapter 7

Managing a Cisco Internetwork

The CCNA exam topics covered in this chapter include the following:

Configure, verify, and troubleshoot basic router operation and routing on Cisco devices.

Manage IOS configuration files (including: save, edit, upgrade, restore).

Manage Cisco IOS.

Verify network connectivity (including: using ping, traceroute, and telnet or SSH).

Here in Chapter 7, I’m going to show you how to manage Cisco routers on an internetwork. The Internetwork Operating System (IOS) and configuration files reside in different locations in a Cisco device, so it’s really important to understand both where these files are located and how they work.

You’ll be learning about the main components of a router, the router boot sequence, and the configuration register, including how to use the configuration register for password recovery. After that, you’ll find out how to manage routers by using the copy

command with a TFTP host when using the Cisco IOS File System (IFS).

We’ll wrap up the chapter with an exploration of the Cisco Discovery Protocol (CDP), and you’ll learn how to resolve hostnames and some important Cisco IOS troubleshooting techniques.

For up-to-the-minute updates for this chapter, please see www.lammle.com

and/or www.sybex.com/go/ccna7e

.

The Internal Components of a Cisco Router

To configure and troubleshoot a Cisco internetwork, you need to know the major components of Cisco routers and understand what each one does.

Table 7-1 describes the major Cisco router components.

Table 7-1:

Cisco router components

Component Description

Bootstrap

POST (power-on self-test)

ROM monitor

Stored in the microcode of the ROM, the bootstrap is used to bring a router up during initialization. It will boot the router and then load the IOS.

Stored in the microcode of the ROM, the POST is used to check the basic functionality of the router hardware and determines which interfaces are present.

Mini-IOS

RAM (random access memory)

ROM (read-only memory)

Stored in the microcode of the ROM, the ROM monitor is used for manufacturing, testing, and troubleshooting.

Called the RXBOOT or bootloader by Cisco, the mini-IOS is a small IOS in ROM that can be used to bring up an interface and load a Cisco IOS into flash memory.

The mini-IOS can also perform a few other maintenance operations.

Used to hold packet buffers, ARP cache, routing tables, and also the software and data structures that allow the router to function. Running-config is stored in RAM, and most routers expand the IOS from flash into RAM upon boot.

Used to start and maintain the router. Holds the POST and the bootstrap program as well as the mini-IOS.

Flash memory

Stores the Cisco IOS by default. Flash memory is not erased when the router is reloaded. It is EEPROM (electronically erasable programmable read-only memory) created by Intel.

NVRAM (nonvolatile

RAM)

Used to hold the router and switch configuration. NVRAM is not erased when the router or switch is reloaded. Does not store an IOS. The configuration register is stored in NVRAM.

Configuration register

Used to control how the router boots up. This value can be found as the last line of the show version

command output and by default is set to 0x2102, which tells the router to load the IOS from flash memory as well as to load the configuration from NVRAM.

The Router Boot Sequence

When a router boots up, it performs a series of steps, called the

boot sequence

, to test the hardware and load the necessary software. The boot sequence consists of the following steps:

1.

The router performs a POST. The POST tests the hardware to verify that all components of the device are operational and present. For example, the POST checks for the different interfaces on the router. The POST is stored in and run from

ROM (read-only memory)

.

2.

The bootstrap then looks for and loads the Cisco IOS software. The bootstrap is a program in ROM that is used to execute programs.

The bootstrap program is responsible for finding where each IOS program is located and then loading the file. By default, the IOS software is loaded from flash memory in all Cisco routers.

The default order of an IOS loading from a router is flash, TFTP server, then ROM.

3.

The IOS software looks for a valid configuration file stored in NVRAM. This file is called startup-config and is only there if an administrator copies the running-config file into NVRAM. (As you already know, the new ISR routers have a small startup-config file preloaded.)

4.

If a startup-config file is in NVRAM, the router will copy this file and place it in RAM and call the file running-config. The router will use this file to run the router. The router should now be operational. If a startup-config file is not in NVRAM, the router will broadcast out any interface that detects carrier detect (CD) for a TFTP host looking for a configuration, and when that fails (typically it will fail—most people won’t even realize the router has attempted this process), it will start the setup mode configuration process.

Managing Configuration Register

All Cisco routers have a 16-bit software register that’s written into NVRAM. By default, the

configuration register

is set to load the Cisco IOS from

flash memory

and to look for and load the startup-config file from NVRAM. In the following sections, I am going to discuss the configuration register settings and how to use these settings to provide password recovery on your routers.

Understanding the Configuration Register Bits

The 16 bits (2 bytes) of the configuration register are read from 15 to 0, from left to right. The default configuration setting on Cisco routers is

0x2102. This means that bits 13, 8, and 1 are on, as shown in Table 7-2 . Notice that each set of 4 bits (called a nibble) is read in binary with a value of 8, 4, 2, 1.

Table 7-2:

The configuration register bit numbers

Add the prefix 0x to the configuration register address. The 0x means that the digits that follow are in hexadecimal.

Table 7-3 lists the software configuration bit meanings. Notice that bit 6 can be used to ignore the NVRAM contents. This bit is used for password recovery—something I’ll go over with you soon in the section “Recovering Passwords” later in this chapter.

Remember that in hex, the scheme is 0–9 and A–F (A = 10, B = 11, C = 12, D = 13, E = 14, and F = 15). This means that a 210F setting for the configuration register is actually

210(15), or 1111 in binary.

Table 7-3:

Software configuration meanings

Bit Hex Description

7

8

0–3

6

0x0000–0x000F Boot field (see Table 7-4 ).

0x0040 Ignore NVRAM contents.

0x0080

0x101

OEM bit enabled.

Break disabled.

10 0x0400 IP broadcast with all zeros.

5, 11–12 0x0800–0x1000 Console line speed.

13

14

15

0x2000

0x4000

0x8000

Boot default ROM software if network boot fails.

IP broadcasts do not have net numbers.

Enable diagnostic messages and ignore NVRAM contents.

The boot field, which consists of bits 0–3 in the configuration register, controls the router boot sequence. Table 7-4 describes the boot field bits.

Table 7-4:

The boot field (configuration register bits 00–03)

Boot

Field

Meaning Use

00

01

ROM monitor mode

To boot to ROM monitor mode, set the configuration register to 2100. You must manually boot the router with the b

command. The router will show the rommon>

prompt.

To boot the mini-IOS image stored in ROM, set the configuration register to 2101. The router will show the

Router(boot)>

prompt.

02–F

Boot image from ROM

Specifies a default boot filename

Any value from 2102 through 210F tells the router to use the boot commands specified in NVRAM.

Checking the Current Configuration Register Value

You can see the current value of the configuration register by using the show version

command ( sh version

or show ver

for short), as demonstrated here:

Router>

sh version

Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version

12.4(12), RELEASE SOFTWARE (fc1)

[output cut]

Configuration register is 0x2102

The last information given from this command is the value of the configuration register. In this example, the value is 0x2102—the default setting.

The configuration register setting of 0x2102 tells the router to look in NVRAM for the boot sequence.

Notice that the show version

command also provides the IOS version, and in the preceding example, it shows the IOS version as 12.4(12).

The show version

command will display system hardware configuration information, software version, and the names of the boot images on a router.

Changing the Configuration Register

You can change the configuration register value to modify how the router boots and runs. These are the main reasons you would want to change the configuration register:

To force the system into the ROM monitor mode

To select a boot source

To enable or disable the

Break

function

To control broadcast addresses

To set the console terminal baud rate

To load operating software from ROM

To enable booting from a Trivial File Transfer Protocol (TFTP) server

Before you change the configuration register, make sure you know the current configuration register value. Use the show version

command to get this information.

You can change the configuration register by using the config-register

command. Here’s an example. The following commands tell the router to boot a small IOS from ROM and then show the current configuration register value:

Router(config)#

config-register 0x2101

Router(config)#

^Z

Router#

sh ver

[output cut]

Configuration register is 0x2102 (will be 0x2101 at next

reload)

Notice that the show version

command displays the current configuration register value and also what that value will be when the router reboots. Any change to the configuration register won’t take effect until the router is reloaded. The 0x2101 will load the IOS from ROM the next time the router is rebooted. You may see it listed as 0x101—that’s basically the same thing, and it can be written either way.

Here is our router after setting the configuration register to 0x2101 and reloading:

Router(boot)#

sh ver

Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version

12.4(12), RELEASE SOFTWARE (fc1)

[output cut]

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

Router uptime is 3 minutes

System returned to ROM by power-on

System image file is "flash:c2800nm-advsecurityk9-mz.124-12.bin"

[output cut]

Configuration register is 0x2101

At this point, if you typed

show flash

, you’d still see the IOS in flash memory ready to go. But we told our router to load from ROM, which is why the hostname shows up with (boot).

Router(boot)#

sh flash

-#- --length-- -----date/time------ path

1 21710744 Jan 2 2007 22:41:14 +00:00 c2800nm-advsecurityk9-mz.124-12.bin

2 1823 Dec 5 2006 14:46:26 +00:00 sdmconfig-2811.cfg

3 4734464 Dec 5 2006 14:47:12 +00:00 sdm.tar

4 833024 Dec 5 2006 14:47:38 +00:00 es.tar

5 1052160 Dec 5 2006 14:48:10 +00:00 common.tar

6 1038 Dec 5 2006 14:48:32 +00:00 home.shtml

7 102400 Dec 5 2006 14:48:54 +00:00 home.tar

8 491213 Dec 5 2006 14:49:22 +00:00 128MB.sdf

9 1684577 Dec 5 2006 14:50:04 +00:00 securedesktop-ios-3.1.1.27-k9.pkg

10 398305 Dec 5 2006 14:50:34 +00:00 sslclient-win-1.1.0.154.pkg

32989184 bytes available (31027200 bytes used)

So even though we have our full IOS in flash, we changed the default loading of the router’s software by changing the configuration register. If you want to set the configuration register back to the default, just type this:

Router(boot)#

config t

Router(boot)(config)#

config-register 0x2102

Router(boot)(config)#

^Z

Router(boot)#

reload

In the next section, I’ll show you how to load the router into ROM monitor mode so you can perform password recovery.

Recovering Passwords

If you’re locked out of a router because you forgot the password, you can change the configuration register to help you get back on your feet. As I said earlier, bit 6 in the configuration register is used to tell the router whether to use the contents of NVRAM to load a router configuration.

The default configuration register value is 0x2102, meaning that bit 6 is off. With the default setting, the router will look for and load a router configuration stored in NVRAM (startup-config). To recover a password, you need to turn on bit 6. Doing this will tell the router to ignore the NVRAM contents. The configuration register value to turn on bit 6 is 0x2142.

Here are the main steps to password recovery:

1.

Boot the router and interrupt the boot sequence by performing a break, which will take the router into ROM monitor mode.

2.

Change the configuration register to turn on bit 6 (with the value 0x2142).

3.

Reload the router.

4.

Enter privileged mode.

5.

Copy the startup-config file to running-config.

6.

Change the password.

7.

Reset the configuration register to the default value.

8.

Save the router configuration.

9.

Reload the router (optional).

I’m going to cover these steps in more detail in the following sections. I’ll also show you the commands to restore access to ISR, 2600, and even

2500 series routers. (You can still use 2500s for labs and you never know when you might need this information!)

As I said, you can enter ROM monitor mode by pressing Ctrl+Break during router bootup. But if the IOS is corrupt or missing, if there’s no network connectivity available to find a TFTP host, or if the mini-IOS from ROM doesn’t load (meaning the default router fallback failed), the router will enter ROM monitor mode by default.

Interrupting the Router Boot Sequence

Your first step is to boot the router and perform a break. This is usually done by pressing the Ctrl+Break key combination when using

HyperTerminal (personally, I use SecureCRT or Putty) while the router first reboots.

After you’ve performed a break, you should see something like this for a 2600 series router (it is pretty much the same output for the ISR series):

System Bootstrap, Version 11.3(2)XA4, RELEASE SOFTWARE (fc1)

Copyright (c) 1999 by cisco Systems, Inc.

TAC:Home:SW:IOS:Specials for info

PC = 0xfff0a530, Vector = 0x500, SP = 0x680127b0

C2600 platform with 32768 Kbytes of main memory

PC = 0xfff0a530, Vector = 0x500, SP = 0x80004374 monitor: command "boot" aborted due to user interrupt rommon 1 >

Notice the line monitor: command "boot" aborted due to user interrupt

. At this point, you will be at the rommon 1>

prompt, which is called the ROM monitor mode.

Changing the Configuration Register

As I explained earlier, you can change the configuration register from within the IOS by using the config-register

command. To turn on bit 6, use the configuration register value 0x2142.

Remember that if you change the configuration register to 0x2142, the startup-config will be bypassed and the router will load into setup mode.

Cisco ISR/2600 Series Commands

To change the bit value on a Cisco ISR/2600 series router, you just enter the command at the rommon 1>

prompt: rommon 1 >

confreg 0x2142

You must reset or power cycle for new config to take effect rommon 2 >

reset

Cisco 2500 Series Commands

To change the configuration register on a 2500 series router, type

o

after creating a break sequence on the router. This brings up a menu of configuration register option settings. To change the configuration register, enter the command

o/r

, followed by the new register value. Here’s an example of turning on bit 6 on a 2501 router:

System Bootstrap, Version 11.0(10c), SOFTWARE

Copyright (c) 1986-1996 by cisco Systems

2500 processor with 14336 Kbytes of main memory

Abort at 0x1098FEC (PC)

>

o

Configuration register = 0x2102 at last boot

Bit# Configuration register option settings:

15 Diagnostic mode disabled

14 IP broadcasts do not have network numbers

13 Boot default ROM software if network boot fails

12-11 Console speed is 9600 baud

10 IP broadcasts with ones

08 Break disabled

07 OEM disabled

06 Ignore configuration disabled

03-00 Boot file is cisco2-2500 (or 'boot system' command)

>

o/r 0x2142

Notice that the last entry in the router output is 03-00. This tells the router what the IOS boot file is. By default, the router will use the first file found in the flash memory, so if you want to boot a different filename, you can use the boot system flash:ios_name

command. (I’ll show you the boot system command in a minute.)

Reloading the Router and Entering Privileged Mode

At this point, you need to reset the router like this:

From the ISR/2600 series router, type

I

(for initialize) or

reset

.

From the 2500 series router, type

I

.

The router will reload and ask if you want to use setup mode (because no startup-config is used). Answer no to entering setup mode, press Enter to go into user mode, and then type

enable

to go into privileged mode.

Viewing and Changing the Configuration

Now you’re past the point where you would need to enter the user-mode and privileged-mode passwords in a router. Copy the startup-config file to the running-config file:

copy startup-config running-config

Or use the shortcut:

copy start run

The configuration is now running in

random access memory (RAM)

, and you’re in privileged mode, meaning that you can now view and change the configuration. But you can’t view the enable secret setting for the password since it is encrypted. To change the password, do this:

config t enable secret todd

Resetting the Configuration Register and Reloading the Router

After you’re finished changing passwords, set the configuration register back to the default value with the config-register

command:

config t config-register 0x2102

Finally, save the new configuration with a copy running-config startup-config

and reload

the router.

If you save your configuration and reload the router and it comes up in setup mode, the configuration register setting is probably incorrect.

Boot System Commands

Did you know that you can configure your router to boot another IOS if the flash is corrupted? Well, you can. In fact, you just might want all your routers to boot from a TFTP host each time anyway because that way, you’ll never have to upgrade each router individually. This may be a smooth way to go because it allows you to just change one file on a TFTP host to perform an upgrade.

There are some boot

commands you can play with that will help you manage the way your router boots the Cisco IOS—but remember, we’re talking about the router’s IOS here,

not

the router’s configuration!

Router>

en

Router#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

boot ?

bootstrap Bootstrap image file

config Configuration file

host Router-specific config file

network Network-wide config file

system System image file

The boot

command truly gives you a wealth of options, but first, I’ll show you the typical settings that Cisco recommends. So let’s get started—the boot system

command will allow you to tell the router which file to boot from flash memory. Remember that the router, by default, boots the first file found in flash. You can change that with the following commands:

Router(config)#

boot system ?

WORD TFTP filename or URL

flash Boot from flash memory

ftp Boot from a server via ftp

mop Boot from a Decnet MOP server

rcp Boot from a server via rcp

rom Boot from rom

tftp Boot from a tftp server

Router(config)#

boot system flash c2800nm-advsecurityk9-mz.124-12.bin

The preceding command configures the router to boot the IOS listed in it. This is a helpful command for when you load a new IOS into flash and want to test it, or even when you want to totally change which IOS is loading by default.

The next command is considered a fall-back routine, but as I said, you can make it a permanent way to have your routers boot from a TFTP host.

Personally, I wouldn’t necessarily recommend doing this (single point of failure); I’m just showing you that it’s possible:

Router(config)#

boot system tftp ?

WORD System image filename

Router(config)#

boot system tftp c2800nm-advsecurityk9-mz.124-12.bin ?

Hostname or A.B.C.D Address from which to download the file

<cr>

Router(config)#

boot system tftp c2800nm-advsecurityk9-mz.124-12.bin 1.1.1.2

Router(config)#

As your last recommended fall-back option—the one to go to if the IOS in flash doesn’t load and the TFTP host does not produce the IOS—load the mini-IOS from ROM like this:

Router(config)#

boot system rom

Router(config)#

do show run | include boot system

boot system flash c2800nm-advsecurityk9-mz.124-12.bin

boot system tftp c2800nm-advsecurityk9-mz.124-12.bin 1.1.1.2

boot system rom

Router(config)#

To sum this up, we now have Cisco’s suggested IOS backup routine configured on our router: flash, TFTP host, ROM.

Backing Up and Restoring the Cisco IOS

Before you upgrade or restore a Cisco IOS, you really should copy the existing file to a

TFTP host

as a backup just in case the new image crashes and burns.

And you can use any TFTP host to accomplish this. By default, the flash memory in a router is used to store the Cisco IOS. In the following sections, I’ll describe how to check the amount of flash memory, how to copy the Cisco IOS from flash memory to a TFTP host, and how to copy the

IOS from a TFTP host to flash memory.

You’ll learn how to use the Cisco IFS to manage your IOS files after first learning how to manage them with a TFTP host.

But before you back up an IOS image to a network server on your intranet, you’ve got to do these three things:

Make sure you can access the network server.

Ensure that the network server has adequate space for the code image.

Verify the file naming and path requirement.

And if you have a laptop or workstation’s Ethernet port directly connected to a router’s Ethernet interface, as shown in

Figure 7-1 , you need to verify the following before attempting to copy the image to or from the router:

TFTP server software must be running on the administrator’s workstation.

The Ethernet connection between the router and the workstation must be made with a crossover cable.

The workstation must be on the same subnet as the router’s Ethernet interface.

The copy flash tftp

command must be supplied the IP address of the workstation if you are copying from the router flash.

And if you’re copying “into” flash, you need to verify that there’s enough room in flash memory to accommodate the file to be copied.

Figure 7-1:

Copying an IOS from a workstation to a router

Verifying Flash Memory

Before you attempt to upgrade the Cisco IOS on your router with a new IOS file, it’s a good idea to verify that your flash memory has enough room to hold the new image. You verify the amount of flash memory and the file or files being stored in flash memory by using the show flash

command ( sh flash

for short):

Router#

sh flash

-#- --length-- -----date/time------ path

1 21710744 Jan 2 2007 22:41:14 +00:00 c2800nm-advsecurityk9-mz.124-12.bin

[output cut]

32989184 bytes available (31027200 bytes used)

The router above has 64MB of RAM, and roughly half of the memory is in use.

The show flash

command will display the amount of memory consumed by the current IOS image as well as tell you if there’s enough room available to hold both current and new images. You should know that if there’s not enough room for both the old and new image you want to load, the old image will be erased!

The amount of flash is actually easier to tally using the show version

command on routers:

Router#

show version

[output cut]

Cisco 2811 (revision 49.46) with 249856K/12288K bytes of memory.

Processor board ID FTX1049A1AB

2 FastEthernet interfaces

4 Serial(sync/async) interfaces

1 Virtual Private Network (VPN) Module

DRAM configuration is 64 bits wide with parity enabled.

239K bytes of non-volatile configuration memory.

62720K bytes of ATA CompactFlash (Read/Write)

You can see that the amount of flash shows up on the last line. By averaging up, we get the amount of flash to 64MB.

Notice that the filename in this example is c2800nm-advsecurityk9-mz.124-12.bin

. The main difference in the output of the show flash

and

show version commands is that the show flash

command displays all files in flash and the show version

command shows the actual name of the file that the router is using to run the router.

Backing Up the Cisco IOS

To back up the Cisco IOS to a TFTP server, you use the filename and the IP address of the TFTP server.

copy flash tftp

command. It’s a straightforward command that requires only the source

The key to success in this backup routine is to make sure you’ve got good, solid connectivity to the TFTP server. Check this by pinging the TFTP device from the router console prompt like this:

Router#

ping 1.1.1.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout

is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max

= 4/4/8 ms

The Packet Internet Groper (Ping) utility is used to test network connectivity, and I use it in some of the examples in this chapter. I’ll be talking about it in more detail in the section “Checking Network Connectivity and Troubleshooting” later in the chapter.

After you ping the TFTP server to make sure that IP is working, you can use the copy flash tftp

command to copy the IOS to the TFTP server as shown next:

Router#

copy flash tftp

Source filename []?

c2800nm-advsecurityk9-mz.124-12.bin

Address or name of remote host []?

1.1.1.2

Destination filename [c2800nm-advsecurityk9-mz.124-12.bin]?

[enter]

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

21710744 bytes copied in 60.724 secs (357532 bytes/sec)

Router#

Just copy the IOS filename from either the show flash

or show version

command and then paste it when prompted for the source filename.

In the preceding example, the contents of flash memory were copied successfully to the TFTP server. The address of the remote host is the IP address of the TFTP host, and the source filename is the file in flash memory.

The copy flash tftp

command won’t prompt you for the location of any file or ask you where to put the file. TFTP is just a “grab it and place it” program in this situation. This means that the TFTP server must have a default directory specified or it won’t work!

Restoring or Upgrading the Cisco Router IOS

What happens if you need to restore the Cisco IOS to flash memory to replace an original file that has been damaged or if you want to upgrade the

IOS? You can download the file from a TFTP server to flash memory by using the copy tftp flash

command. This command requires the IP address of the TFTP host and the name of the file you want to download.

But before you begin, make sure the file you want to place in flash memory is in the default TFTP directory on your host. When you issue the command, TFTP won’t ask you where the file is, so if the file you want to use isn’t in the default directory of the TFTP host, this just won’t work.

Router#

copy tftp flash

Address or name of remote host []?

1.1.1.2

Source filename []?

c2800nm-advsecurityk9-mz.124-12.bin

Destination filename [c2800nm-advsecurityk9-mz.124-12.bin]?

[enter]

%Warning: There is a file already existing with this name

Do you want to over write? [confirm]

[enter]

Accessing tftp://1.1.1.2/c2800nm-advsecurityk9-mz.124-12.bin...

Loading c2800nm-advsecurityk9-mz.124-12.bin from 1.1.1.2 (via

FastEthernet0/0): !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[OK - 21710744 bytes]

21710744 bytes copied in 82.880 secs (261954 bytes/sec)

Router#

In the preceding example, I copied the same file into flash memory, so it asked me if I wanted to overwrite it. Remember that we are “playing” with files in flash memory. If I had just corrupted my file by overwriting it, I won’t know until I reboot the router. Be careful with this command! If the file is corrupted, you’ll need to do an IOS restore from ROM monitor mode.

If you are loading a new file and you don’t have enough room in flash memory to store both the new and existing copies, the router will ask to erase the contents of flash memory before writing the new file into flash memory.

A Cisco router can become a TFTP server host for a router system image that’s run in flash memory. The global configuration command is tftp-server flash:ios_name

.

Using the Cisco IOS File System (Cisco IFS)

Cisco has created a file system called Cisco IFS that allows you to work with files and directories just as you would from a Windows DOS prompt.

The commands you use are dir

, copy

, more

, delete

, erase

or format

, cd

and pwd

, and mkdir

and rmdir

.

Working with IFS gives you the ability to view all files—even those on remote servers. And you definitely want to find out if an image on one of your remote servers is valid before you copy it, right? You also need to know how big it is—size matters here! It’s also a really good idea to take a look at the remote server’s configuration and make sure it’s all good before loading that file on your router.

It’s very cool that IFS makes the file system user interface universal—it’s not platform specific anymore. You now get to use the same syntax for all your commands on all of your routers, no matter the platform!

Sound too good to be true? Well, it kind of is because you’ll find out that support for all commands on each file system and platform just isn’t there. But it’s really no big deal since various file systems differ in the actions they perform; the commands that aren’t relevant to a particular file system are the very ones that aren’t supported. Be assured that any file system or platform will fully support all the commands you need to manage it.

Another cool IFS feature is that it cuts down on all those obligatory prompts for a lot of the commands. If you want to enter a command, all you have to do is type all the necessary info straight into the command line—no more jumping through hoops of prompts! So, if you want to copy a file to an FTP server, all you’d do is first indicate where the desired source file is on your router, pinpoint where the destination file is to be on the FTP server, determine the username and password you’re going to use when you want to connect to that server, and type it all in on one line—sleek!

And for those of you resistant to change, you can still have the router prompt you for all the information it needs and enjoy entering a more elegantly minimized version of the command than you did before.

But even in spite of all this, your router might still prompt you—even if you did everything right in your command line. It comes down to how you’ve got the file prompt

command configured and which command you’re trying to use. But no worries—if that happens, the default value will be entered right there in the command, and all you have to do is hit Enter to verify the correct values.

IFS also lets you explore various directories and inventory files in any directory you want. Plus, you can make subdirectories in flash memory or on a card, but you only get to do that if you’re working on one of the more recent platforms.

And get this—the new file system interface uses URLs to determine the whereabouts of a file. So just as they pinpoint places on the Web, URLs

now indicate where files are on your Cisco router, or even on a remote file server! You just type URLs right into your commands to identify where the file or directory is. It’s really that easy—to copy a file from one place to another, you simply enter the copy source-url destination-url

command—sweet!

IFS URLs are a tad different than what you’re used to though, and there’s an array of formats to use that vary depending on where, exactly, the file is that you’re after.

We’re going to use Cisco IFS commands pretty much the same way that we used the copy

command in the IOS section earlier:

For backing up the IOS

For upgrading the IOS

For viewing text files

Okay—with all that down, let’s take a look at the common IFS commands available to us for managing the IOS. I’ll get into configuration files soon, but for now I’m going to get you started with going over the basics used to manage the new Cisco IOS.

dir

Same as with Windows, this command lets you view files in a directory. Type

dir

, hit Enter, and by default you get the contents of the flash:/ directory output.

copy

This is one popular command, often used to upgrade, restore, or back up an IOS. But as I said, when you use it, it’s really important to focus on the details—what you’re copying, where it’s coming from, and where it’s going to land.

more

Same as with Unix, this will take a text file and let you look at it on a card. You can use it to check out your configuration file or your backup configuration file. I’ll go over it more when we get into actual configuration.

show file

This command will give you the skinny on a specified file or file system, but it’s kind of obscure because people don’t use it a lot.

delete

Three guesses—yep, it deletes stuff. But with some types of routers, not as well as you’d think. That’s because even though it whacks the file, it doesn’t always free up the space it was using. To actually get the space back, you have to use something called the squeeze

command too.

erase/format

Use these with care—make sure that when you’re copying files, you say no to the dialog that asks you if you want to erase the file system! The type of memory you’re using determines if you can nix the flash drive or not.

cd/pwd

Same as with Unix and DOS, cd

is the command you use to change directories. Use the pwd

command to print (show) the working directory.

mkdir/rmdir

Use these commands on certain routers and switches to create and delete directories—the mkdir

command for creation and the rmdir command for deletion. Use the cd

and pwd

commands to change into these directories.

Using the Cisco IFS to Upgrade an IOS

Let’s take a look at some of these Cisco IFS commands on my ISR router (1841 series) with a hostname of R1.

We’ll start with the pwd

command to verify our default directory and then use the dir

command to verify the contents of the default directory ( flash:/

):

R1#

pwd

flash:

R1#

dir

Directory of flash:/

1 -rw- 13937472 Dec 20 2006 19:58:18 +00:00 c1841-ipbase-

mz.124-1c.bin

2 -rw- 1821 Dec 20 2006 20:11:24 +00:00 sdmconfig-18xx.cfg

3 -rw- 4734464 Dec 20 2006 20:12:00 +00:00 sdm.tar

4 -rw- 833024 Dec 20 2006 20:12:24 +00:00 es.tar

5 -rw- 1052160 Dec 20 2006 20:12:50 +00:00 common.tar

6 -rw- 1038 Dec 20 2006 20:13:10 +00:00 home.shtml

7 -rw- 102400 Dec 20 2006 20:13:30 +00:00 home.tar

8 -rw- 491213 Dec 20 2006 20:13:56 +00:00 128MB.sdf

9 -rw- 1684577 Dec 20 2006 20:14:34 +00:00 securedesktop-

ios-3.1.1.27-k9.pkg

10 -rw- 398305 Dec 20 2006 20:15:04 +00:00 sslclient-win-

1.1.0.154.pkg

32071680 bytes total (8818688 bytes free)

What we can see here is that we have the basic IP IOS ( c1841-ipbase-mz.124-1c.bin

). Looks like we need to upgrade our 1841. You’ve just got to love how Cisco puts the IOS type in the filename now! First, let’s check the size of the file that’s in flash with the show file

command ( show flash

would also work):

R1#

show file info flash:c1841-ipbase-mz.124-1c.bin

flash:c1841-ipbase-mz.124-1c.bin:

type is image (elf) []

file size is 13937472 bytes, run size is 14103140 bytes

Runnable image, entry point 0x8000F000, run from ram

With a file that size, the existing IOS will have to be erased before we can add our new IOS file ( c1841-advipservicesk9-mz.124-12.bin

), which is over

21MB. We’ll use the delete

command, but remember, we can play with any file in flash memory and nothing serious will happen until we reboot—that is, if we made a mistake. So obviously, and as I pointed out earlier, we need to be majorly careful here!

R1#

delete flash:c1841-ipbase-mz.124-1c.bin

Delete filename [c1841-ipbase-mz.124-1c.bin]?

[enter]

Delete flash:c1841-ipbase-mz.124-1c.bin? [confirm]

[enter]

R1#

sh flash

-#- --length-- -----date/time------ path

1 1821 Dec 20 2006 20:11:24 +00:00 sdmconfig-18xx.cfg

2 4734464 Dec 20 2006 20:12:00 +00:00 sdm.tar

3 833024 Dec 20 2006 20:12:24 +00:00 es.tar

4 1052160 Dec 20 2006 20:12:50 +00:00 common.tar

5 1038 Dec 20 2006 20:13:10 +00:00 home.shtml

6 102400 Dec 20 2006 20:13:30 +00:00 home.tar

7 491213 Dec 20 2006 20:13:56 +00:00 128MB.sdf

8 1684577 Dec 20 2006 20:14:34 +00:00 securedesktop-ios-3.1.1.27-k9.pkg

9 398305 Dec 20 2006 20:15:04 +00:00 sslclient-win-1.1.0.154.pkg

22757376 bytes available (9314304 bytes used)

R1#

sh file info flash:c1841-ipbase-mz.124-1c.bin

%Error opening flash:c1841-ipbase-mz.124-1c.bin (File not found)

R1#

So with the preceding commands, I deleted the existing file and then verified the deletion by using both the show flash

and

show file

commands.

Let’s add the new file with the copy

command, but again, I’m going to make sure I’m careful because this doesn’t make it safer than the first method I showed you earlier:

R1#

copy tftp://1.1.1.2//c1841-advipservicesk9-mz.124-12.bin/ flash:/

c1841-advipservicesk9-mz.124-12.bin

Source filename [/c1841-advipservicesk9-mz.124-12.bin/]?

[enter]

Destination filename [c1841-advipservicesk9-mz.124-12.bin]?

[enter]

Loading /c1841-advipservicesk9-mz.124-12.bin/ from 1.1.1.2 (via

FastEthernet0/0): !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[output cut]

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[OK - 22103052 bytes]

22103052 bytes copied in 72.008 secs (306953 bytes/sec)

R1#

sh flash

-#- --length-- -----date/time------ path

1 1821 Dec 20 2006 20:11:24 +00:00 sdmconfig-18xx.cfg

2 4734464 Dec 20 2006 20:12:00 +00:00 sdm.tar

3 833024 Dec 20 2006 20:12:24 +00:00 es.tar

4 1052160 Dec 20 2006 20:12:50 +00:00 common.tar

5 1038 Dec 20 2006 20:13:10 +00:00 home.shtml

6 102400 Dec 20 2006 20:13:30 +00:00 home.tar

7 491213 Dec 20 2006 20:13:56 +00:00 128MB.sdf

8 1684577 Dec 20 2006 20:14:34 +00:00 securedesktop-ios-3.1.1.27-k9.pkg

9 398305 Dec 20 2006 20:15:04 +00:00 sslclient-win-1.1.0.154.pkg

10 22103052 Mar 10 2007 19:40:50 +00:00 c1841-advipservicesk9-mz.124-12.bin

651264 bytes available (31420416 bytes used)

R1#

We can check the file information as well with the show file

command:

R1#

sh file information flash:c1841-advipservicesk9-mz.124-12.bin

flash:c1841-advipservicesk9-mz.124-12.bin:

type is image (elf) []

file size is 22103052 bytes, run size is 22268736 bytes

Runnable image, entry point 0x8000F000, run from ram

Remember that the IOS is expanded into RAM when the router boots, so the new IOS will not run until you reload the router.

I really recommend that you play with the Cisco IFS commands on a router just to get a good feel for them because, as I’ve said, they can definitely give you some grief at first!

I mention “safer methods” a lot in this chapter. Clearly, I’ve caused myself some serious pain not being careful enough when working in flash memory! I cannot tell you enough—pay attention when messing around with flash memory!

One of the brilliant features of the ISR routers is that they use the physical flash cards that are accessible from the front or back of any router. You can pull these flash cards out, put them in an appropriate slot in your PC, and the card will show up as a drive. You can then add, change, and delete files. Just put the flash card back in your router and power up—instant upgrade. Nice!

Backing Up and Restoring the Cisco Configuration

Any changes that you make to the router configuration are stored in the running-config file. And if you don’t enter a copy run start

command after you make a change to running-config, that change will go poof if the router reboots or gets powered down. So you probably want to make another backup of the configuration information just in case the router or switch completely dies on you. Even if your machine is healthy and happy, it’s good to have for reference and documentation reasons.

In the following sections, I’ll describe how to copy the configuration of a router to a TFTP server and how to restore that configuration.

Backing Up the Cisco Router Configuration

To copy the router’s configuration from a router to a TFTP server, you can use either the copy running-config tftp

or the copy startup-config tftp command. Either one will back up the router configuration that’s currently running in DRAM or that’s stored in NVRAM.

Verifying the Current Configuration

To verify the configuration in DRAM, use the show running-config

command ( sh run

for short) like this:

Router#

show running-config

Building configuration...

Current configuration : 776 bytes

!

version 12.4

The current configuration information indicates that the router is running version 12.4 of the IOS.

Verifying the Stored Configuration

Next, you should check the configuration stored in NVRAM. To see this, use the show startup-config

command ( sh start

for short) like this:

Router#

show startup-config

Using 776 out of 245752 bytes

!

version 12.4

The first line shows you how much room your backup configuration is using. Here, we can see that NVRAM is 245KB (again, memory is easier to see with the show version

command when you’re using an ISR router) and that only 776 bytes of it are used.

If you’re not sure that the files are the same and the running-config file is what you want to use, then use the copy running-config startup-config command

.

This will help you ensure that both files are in fact the same. I’ll go through this with you in the next section.

Copying the Current Configuration to NVRAM

By copying running-config to NVRAM as a backup, as shown in the following output, you’re assured that your running-config will always be reloaded if the router gets rebooted. In the new IOS version 12.0, you’re prompted for the filename you want to use:

Router#

copy running-config startup-config

Destination filename [startup-config]?

[enter]

Building configuration...

[OK]

Router#

The reason the filename prompt appears is that there are now so many options you can use when using the copy

command:

Router#

copy running-config ?

archive: Copy to archive: file system

flash: Copy to flash: file system

ftp: Copy to ftp: file system

http: Copy to http: file system

https: Copy to https: file system

ips-sdf Update (merge with) IPS signature configuration

null: Copy to null: file system

nvram: Copy to nvram: file system

rcp: Copy to rcp: file system

running-config Update (merge with) current system configuration

scp: Copy to scp: file system

startup-config Copy to startup configuration

syslog: Copy to syslog: file system

system: Copy to system: file system

tftp: Copy to tftp: file system

xmodem: Copy to xmodem: file system

ymodem: Copy to ymodem: file system

We’ll go over the copy

command again in a minute.

Copying the Configuration to a TFTP Server

Once the file is copied to NVRAM, you can make a second backup to a TFTP server by using the copy running-config tftp

command ( copy run tftp

for short), like this:

Router#

copy running-config tftp

Address or name of remote host []?

1.1.1.2

Destination filename [router-confg]?

todd-confg

!!

776 bytes copied in 0.800 secs (970 bytes/sec)

Router#

In the preceding example, I named the file todd-confg

because I had not set a hostname for the router. If you have a hostname already configured, the command will automatically use the hostname plus the extension

-confg as the name of the file.

Restoring the Cisco Router Configuration

If you’ve changed your router’s running-config file and want to restore the configuration to the version in the startup-config file, the easiest way to do

this is to use the copy startup-config running-config

command ( copy start run

for short). You can also use the older Cisco command config mem

to restore a configuration. Of course, this will work only if you copied running-config into NVRAM before making any changes!

If you did copy the router’s configuration to a TFTP server as a second backup, you can restore the configuration using the copy tftp running-config command ( copy tftp run

for short) or the copy tftp startup-config

command ( copy tftp start

for short), as shown here (the old command that provides this function is config net

):

Router#

copy tftp running-config

Address or name of remote host []?

1.1.1.2

Source filename []?

todd-confg

Destination filename[running-config]?

[enter]

Accessing tftp://1.1.1.2/todd-confg...

Loading todd-confg from 1.1.1.2 (via FastEthernet0/0): !

[OK - 776 bytes]

776 bytes copied in 9.212 secs (84 bytes/sec)

Router#

*Mar 7 17:53:34.071: %SYS-5-CONFIG_I: Configured from

tftp://1.1.1.2/todd-confg by console

Router#

The configuration file is an ASCII text file, meaning that before you copy the configuration stored on a TFTP server back to a router, you can make changes to the file with any text editor. Last, notice that the command was changed to a URL of tftp://1.1.1.2/todd-config

. This is the Cisco IOS

File System (IFS)—as discussed earlier—and we’ll use that to back up and restore our configuration in a minute.

It is important to remember that when you copy or merge a configuration from a TFTP server to a freshly erased and rebooted router’s RAM, the interfaces are shut down by default and you must manually go and enable each interface with the no shutdown

command.

Erasing the Configuration

To delete the startup-config file on a Cisco router, use the command erase startup-config

, like this:

Router#

erase startup-config

Erasing the nvram filesystem will remove all configuration files!

Continue? [confirm]

[enter]

[OK]

Erase of nvram: complete

*Mar 7 17:56:20.407: %SYS-7-NV_BLOCK_INIT: Initialized the geometry of nvram

Router#

reload

System configuration has been modified. Save? [yes/no]:

n

Proceed with reload? [confirm]

[enter]

*Mar 7 17:56:31.059: %SYS-5-RELOAD: Reload requested by console.

Reload Reason: Reload Command.

This command deletes the contents of NVRAM on the router. If you type

reload

at privileged mode and say no to saving changes, the router will reload and come up into setup mode.

Using the Cisco IOS File System to Manage Your Router’s Configuration (Cisco IFS)

Using the old, faithful copy

command is still useful and I recommend it. However, you still need to know about the Cisco IFS. The first thing we’ll do is use the

show file

command to see the contents of NVRAM and RAM:

R3#

show file information nvram:startup-config

nvram:startup-config:

type is config

R3#

cd nvram:

R3#

pwd

nvram:/

R3#

dir

Directory of nvram:/

190 -rw- 830 <no date> startup-config

191 ---- 5 <no date> private-config

192 -rw- 830 <no date> underlying-config

1 -rw- 0 <no date> ifIndex-table

196600 bytes total (194689 bytes free)

There really are no other commands that will actually show us the contents of NVRAM. However, I am not sure how helpful it is to see them either.

Let’s look at the contents of RAM:

R3#

cd system:

R3#

pwd

system:/

R3#

dir ?

/all List all files

/recursive List files recursively

all-filesystems List files on all filesystems

archive: Directory or file name

cns: Directory or file name

flash: Directory or file name

null: Directory or file name

nvram: Directory or file name

system: Directory or file name

xmodem: Directory or file name

ymodem: Directory or file name

<cr>

R3#

dir

Directory of system:/

3 dr-x 0 <no date> lib

33 dr-x 0 <no date> memory

1 -rw- 750 <no date> running-config

2 dr-x 0 <no date> vfiles

Again, not too exciting. Let’s use the copy

command with the Cisco IFS to copy a file from a TFTP host to RAM. First, let’s try the old command config net

that was used for the last 10 years or so to accomplish this same feat:

R3#

config net

Host or network configuration file [host]?

[enter]

This command has been replaced by the command:

'copy <url> system:/running-config'

Address or name of remote host [255.255.255.255]?

Although the output tells us that the old command has been replaced with the new URL command, the old command will still will work. Let’s try it with the Cisco IFS:

R3#

copy tftp://1.1.1.2/todd-confg system://running-config

Destination filename [running-config]?

[enter]

Accessing tftp://1.1.1.2/todd-confg...Loading todd-confg from 1.1.1.2

(via FastEthernet0/0): !

[OK - 776 bytes]

[OK]

776 bytes copied in 13.816 secs (56 bytes/sec)

R3#

*Mar 10 22:12:59.819: %SYS-5-CONFIG_I:

Configured from tftp://1.1.1.2/todd-confg by console

I guess we can say that this was easier than using the copy tftp run

command—Cisco says it is, so who am I to argue? Maybe it just takes some getting used to.

Using Cisco Discovery Protocol (CDP)

Cisco Discovery Protocol (CDP) is a proprietary protocol designed by Cisco to help administrators collect information about both locally attached and remote devices. By using CDP, you can gather hardware and protocol information about neighbor devices, which is useful info for troubleshooting and documenting the network.

In the following sections, I am going to discuss the CDP timer and CDP commands used to verify your network.

Getting CDP Timers and Holdtime Information

The show cdp

command ( sh cdp

for short) gives you information about two CDP global parameters that can be configured on Cisco devices:

CDP timer

is how often CDP packets are transmitted out all active interfaces.

CDP holdtime

is the amount of time that the device will hold packets received from neighbor devices.

Both Cisco routers and Cisco switches use the same parameters.

The output on the Corp router looks like this:

Corp#

sh cdp

Global CDP information:

Sending CDP packets every 60 seconds

Sending a holdtime value of 180 seconds

Sending CDPv2 advertisements is enabled

Use the global commands cdp holdtime

and cdp timer

to configure the CDP holdtime and timer on a router:

Corp(config)#

cdp ?

advertise-v2 CDP sends version-2 advertisements

holdtime Specify the holdtime (in sec) to be sent in packets

log Log messages generated by CDP

run Enable CDP

source-interface Insert the interface's IP in all CDP packets

timer Specify rate (in sec) at which CDP packets are sent run

Corp(config)#

cdp holdtime ?

<10-255> Length of time (in sec) that receiver must keep this packet

Corp(config)#

cdp timer ?

<5-254> Rate at which CDP packets are sent (in sec)

You can turn off CDP completely with the no cdp run

command from the global configuration mode of a router. To turn CDP off or on for an interface, use the no cdp enable

and cdp enable commands. Be patient—I’ll work through these with you in a second.

Gathering Neighbor Information

The show cdp neighbor

command ( sh cdp nei

for short) delivers information about directly connected devices. It’s important to remember that CDP packets aren’t passed through a Cisco switch and that you only see what’s directly attached. So this means that if your router is connected to a switch, you won’t see any of the devices hooked up to that switch.

The following output shows the show cdp neighbor

command used on my Corp 2811 router:

Corp#

sh cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID ap Fas 0/1 165 T I AIR-AP124 Fas 0

R2 Ser 0/1/0 140 R S I 2801 Ser 0/2/0

R3 Ser 0/0/1 157 R S I 1841 Ser 0/0/1

R1 Ser 0/2/0 154 R S I 1841 Ser 0/0/1

R1 Ser 0/0/0 154 R S I 1841 Ser 0/0/0

Corp#

Okay, we are directly connected with a console cable to the Corp router, and the router is directly connected to four devices. We have two connections to the R1 router. The device ID shows the configured hostname of the connected device, the local interface is our interface, and the port ID is the remote devices’ directly connected interface. All you get to view are directly connected devices.

Table 7-5 summarizes the information displayed by the show cdp neighbor

command for each device.

Table 7-5:

Output of the show cdp neighbor

command

Field Description

Device ID

The hostname of the device directly connected.

Local Interface

The port or interface on which you are receiving the CDP packet.

Holdtime

Capability

Platform

Port ID

The remaining amount of time the router will hold the information before discarding it if no more CDP packets are received.

The capability of the neighbor, such as the router, switch, or repeater. The capability codes are listed at the top of the command output.

The type of Cisco device directly connected. In the previous output, a 1240AP, 2801 router, and two 1841 routers are directly connected to the Corp router.

The neighbor device’s port or interface on which the CDP packets are multicast.

It is imperative that you can look at the output of a show cdp neighbors

command and decipher the neighbor’s device (capability, i.e., router or switch), model number

(platform), your port connecting to that device (local interface), and the port of the neighbor connecting to you (port ID).

Another command that’ll deliver the goods on neighbor information is the show cdp neighbors detail

command ( show cdp nei de

for short). This command can be run on both routers and switches, and it displays detailed information about each device connected to the device you’re running the command on. Check out this router output for an example:

Corp#

sh cdp neighbors detail

-------------------------

Device ID: ap

Entry address(es): 10.1.1.2

Platform: cisco AIR-AP1242AG-A-K9 , Capabilities: Trans-Bridge IGMP

Interface: FastEthernet0/1, Port ID (outgoing port): FastEthernet0

Holdtime : 122 sec

Version :

Cisco IOS Software, C1240 Software (C1240-K9W7-M), Version 12.3(8)JEA,

RELEASE SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by Cisco Systems, Inc.

Compiled Wed 23-Aug-06 16:45 by kellythw advertisement version: 2

Duplex: full

Power drawn: 15.000 Watts

-------------------------

Device ID: R2

Entry address(es):

IP address: 10.4.4.2

Platform: Cisco 2801, Capabilities: Router Switch IGMP

Interface: Serial0/1/0, Port ID (outgoing port): Serial0/2/0

Holdtime : 135 sec

Version :

Cisco IOS Software, 2801 Software (C2801-ADVENTERPRISEK9-M),

Experimental Version 12.4(20050525:193634) [jezhao-ani 145]

Copyright (c) 1986-2005 by Cisco Systems, Inc.

Compiled Fri 27-May-05 23:53 by jezhao

advertisement version: 2

VTP Management Domain: ''

-------------------------

Device ID: R3

Entry address(es):

IP address: 10.5.5.1

Platform: Cisco 1841, Capabilities: Router Switch IGMP

Interface: Serial0/0/1, Port ID (outgoing port): Serial0/0/1

Holdtime : 152 sec

Version :

Cisco IOS Software, 1841 Software (C1841-IPBASE-M), Version 12.4(1c),

RELEASE SOFTWARE (fc1)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2005 by Cisco Systems, Inc.

Compiled Tue 25-Oct-05 17:10 by evmiller advertisement version: 2

VTP Management Domain: ''

-------------------------

[output cut]

Corp#

What are we being shown here? First, we’re given the hostname and IP address of all directly connected devices. In addition to the same information displayed by the show cdp neighbor

command (see Table 7-5 ), the show cdp neighbor detail

command gives us the IOS version of the neighbor device.

Remember that you can see the IP address of only directly connected devices.

The

show cdp entry *

command displays the same information as the show cdp neighbors detail

command. Here’s an example of the router output using the show cdp entry *

command:

Corp#

sh cdp entry *

-------------------------

Device ID: ap

Entry address(es):

Platform: cisco AIR-AP1242AG-A-K9 , Capabilities: Trans-Bridge IGMP

Interface: FastEthernet0/1, Port ID (outgoing port): FastEthernet0

Holdtime : 160 sec

Version :

Cisco IOS Software, C1240 Software (C1240-K9W7-M), Version 12.3(8)JEA,

RELEASE SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by Cisco Systems, Inc.

Compiled Wed 23-Aug-06 16:45 by kellythw advertisement version: 2

Duplex: full

Power drawn: 15.000 Watts

-------------------------

Device ID: R2

Entry address(es):

IP address: 10.4.4.2

Platform: Cisco 2801, Capabilities: Router Switch IGMP

--More--

[output cut]

There isn’t any difference between the show cdp neighbors detail

and show cdp entry *

commands. However, the sh cdp entry *

command has two options that the show cdp neighbors detail

command does not:

Corp#

sh cdp entry * ?

protocol Protocol information

version Version information

| Output modifiers

<cr>

Corp#

show cdp entry * protocols

Protocol information for ap :

IP address: 10.1.1.2

Protocol information for R2 :

IP address: 10.4.4.2

Protocol information for R3 :

IP address: 10.5.5.1

Protocol information for R1 :

IP address: 10.3.3.2

Protocol information for R1 :

IP address: 10.2.2.2

The preceding output of the show cdp entry * protocols

command can show you just the IP addresses of each directly connected neighbor. The show cdp entry * version

will show you only the IOS version of your directly connected neighbors:

Corp#

show cdp entry * version

Version information for ap :

Cisco IOS Software, C1240 Software (C1240-K9W7-M), Version

12.3(8)JEA, RELEASE SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2006 by Cisco Systems, Inc.

Compiled Wed 23-Aug-06 16:45 by kellythw

Version information for R2 :

Cisco IOS Software, 2801 Software (C2801-ADVENTERPRISEK9-M),

Experimental Version 12.4(20050525:193634) [jezhao-ani 145]

Copyright (c) 1986-2005 by Cisco Systems, Inc.

Compiled Fri 27-May-05 23:53 by jezhao

Version information for R3 :

Cisco IOS Software, 1841 Software (C1841-IPBASE-M), Version 12.4(1c),

RELEASE SOFTWARE (fc1)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2005 by Cisco Systems, Inc.

Compiled Tue 25-Oct-05 17:10 by evmiller

--More--

[output cut]

Although the show cdp neighbors detail

and show cdp entry

commands are very similar, the show cdp entry

command allows you to display only one line of output for each directly connected neighbor, whereas the show cdp neighbor detail

command does not. Next, let’s look at the show cdp traffic

command.

Gathering Interface Traffic Information

The show cdp traffic

command displays information about interface traffic, including the number of CDP packets sent and received and the errors with CDP.

The following output shows the show cdp traffic

command used on the Corp router:

Corp#

sh cdp traffic

CDP counters :

Total packets output: 911, Input: 524

Hdr syntax: 0, Chksum error: 0, Encaps failed: 2

No memory: 0, Invalid packet: 0, Fragmented: 0

CDP version 1 advertisements output: 0, Input: 0

CDP version 2 advertisements output: 911, Input: 524

This is not really the most important information you can gather from a router, but it does show how many CDP packets are sent and received on a device.

Gathering Port and Interface Information

The show cdp interface

command gives you the CDP status on router interfaces or switch ports.

As I said earlier, you can turn off CDP completely on a router by using the no cdp run

command. But remember that you can also turn off CDP on a per-interface basis with the no cdp enable

command. You enable a port with the cdp enable

command. All ports and interfaces default to cdp enable

.

On a router, the show cdp interface

command displays information about each interface using CDP, including the encapsulation on the line, the timer, and the holdtime for each interface. Here’s an example of this command’s output on the ISR router:

Corp#

sh cdp interface

FastEthernet0/0 is administratively down, line protocol is down

Encapsulation ARPA

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

FastEthernet0/1 is up, line protocol is up

Encapsulation ARPA

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/0/0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/0/1 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/1/0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/2/0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

The preceding output is nice because it always tells you the interface’s status. To turn off CDP on one interface on a router, use the no cdp enable

command from interface configuration mode:

Corp#

config t

Corp(config)#

int s0/0/0

Corp(config-if)#

no cdp enable

Corp(config-if)#

do show cdp interface

FastEthernet0/0 is administratively down, line protocol is down

Encapsulation ARPA

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

FastEthernet0/1 is up, line protocol is up

Encapsulation ARPA

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/0/1 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/1/0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Serial0/2/0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

Corp(config-if)#

Notice that serial 0/0/0 isn’t listed in the router output. To get that output, you’d have to perform a cdp enable

on serial 0/0/0. It would then show up in the output:

Corp(config-if)#

cdp enable

Corp(config-if)#

^Z

Corp#

CDP Can Save Lives!

Karen has just been hired as a senior network consultant at a large hospital in Dallas, Texas. She is expected to be able to take care of any problem that comes up. No stress here—she only has to worry about people possibly not getting the right health care if the network goes down. Talk about a potential life-or-death situation!

Karen starts her job happily. Soon, of course, the network has some problems. She asks one of the junior administrators for a network map so she can troubleshoot the network. This person tells her that the old senior administrator (who just got fired) had them with him and now no one can find them—ouch!

Doctors are calling every couple of minutes because they can’t get the necessary information they need to take care of their patients. What should she do?

CDP to the rescue! Thank God this hospital has all Cisco routers and switches and that CDP is enabled by default on all Cisco devices. Also, luckily, the disgruntled administrator who just got fired didn’t turn off CDP on any devices before he left.

All Karen has to do now is to use the show cdp neighbor detail

command to find all the information she needs about each device to help draw out the hospital network and save lives!

The only snag for you nailing this in your own network is if you don’t know the passwords of all those devices. Your only hope then is to somehow find out the access passwords or to perform password recovery on them.

So, use CDP— you never know when you may end up saving someone’s life.

This is a true story.

Documenting a Network Topology Using CDP

As the title of this section implies, I’m now going to show you how to document a sample network by using CDP. You’ll learn to determine the appropriate router types, interface types, and IP addresses of various interfaces using only CDP commands and the show running-config

command.

And you can only console into the Lab_A router to document the network. You’ll have to assign any remote routers the next IP address in each range. Figure 7-2 is what you’ll use to complete the documentation.

Figure 7-2:

Documenting a network topology using CDP

In this output, you can see that you have a router with four interfaces: two FastEthernet and two serial. First, determine the IP addresses of each interface by using the show running-config

command:

Lab_A#

sh running-config

Building configuration...

Current configuration : 960 bytes

!

version 12.2

service timestamps debug uptime service timestamps log uptime no service password-encryption

!

hostname Lab_A

!

ip subnet-zero

!

!

interface FastEthernet0/0

ip address 192.168.21.1 255.255.255.0

duplex auto

!

interface FastEthernet0/1

ip address 192.168.18.1 255.255.255.0

duplex auto

!

interface Serial0/0 ip address 192.168.23.1 255.255.255.0

!

interface Serial0/1 ip address 192.168.28.1 255.255.255.0

!

ip classless

!

line con 0 line aux 0 line vty 0 4

!

end

With this step completed, you can now write down the IP addresses of the Lab_A router’s four interfaces. Next, you need to determine the type of device on the other end of each of these interfaces. It’s easy to do this—just use the show cdp neighbors

command:

Lab_A#

sh cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability Platform Port ID

Lab_B Fas 0/0 178 R 2501 E0

Lab_C Fas 0/1 137 R 2621 Fa0/0

Lab_D Ser 0/0 178 R 2514 S1

Lab_E Ser 0/1 137 R 2620 S0/1

Lab_A#

You’ve got a good deal of information now! By using both the show running-config

and show cdp neighbors

commands, you know about all the IP addresses of the Lab_A router plus the types of routers connected to each of the Lab_A router’s links and all the interfaces of the remote routers.

And by using all the information gathered from show running-config

and show cdp neighbors

, we can now create the topology in Figure 7-3 .

Figure 7-3:

Network topology documented

If we needed to, we could’ve also used the show cdp neighbors detail

command to view the neighbor’s IP addresses. But since we know the IP addresses of each link on the Lab_A router, we already know what the next available IP address is going to be.

Link Layer Discovery Protocol (LLDP)

Before I move away from CDP, I need to discuss a nonproprietary discovery protocol that provides pretty much the same information as CDP but works in multivendor networks.

The IEEE created a new standardized discovery protocol called 802.1AB for Station and Media Access Control Connectivity Discovery. We’ll just call it Link Layer Discovery Protocol (LLDP).

LLDP defines basic discovery capabilities, but it was also enhanced to specifically address the voice application, and this version is called

LLDP-MED (Media Endpoint Discovery). LLDP and LLDP-MED are not compatible.

More information can be found here: www.cisco.com/en/US/docs/ios/cether/configuration/guide/ce_lldp-med.html

And here: www.cisco.com/en/US/technologies/tk652/tk701/technologies_white_paper0900aecd804cd46d.html

Using Telnet

Telnet

, part of the TCP/IP protocol suite, is a virtual terminal protocol that allows you to make connections to remote devices, gather information, and run programs.

After your routers and switches are configured, you can use the Telnet program to reconfigure and/or check up on them without using a console cable. You run the Telnet program by typing

telnet

from any command prompt (DOS or Cisco). You need to have VTY passwords set on the routers for this to work.

Remember, you can’t use CDP to gather information about routers and switches that aren’t directly connected to your device. But you can use the Telnet application to connect to your neighbor devices and then run CDP on those remote devices to get information on them.

You can issue the telnet

command from any router prompt like this:

Corp#

telnet 10.2.2.2

Trying 10.2.2.2 ... Open

Password required, but none set

[Connection to 10.2.2.2 closed by foreign host]

Corp#

As you can see, I didn’t set my passwords—how embarrassing! Remember that the VTY ports on a router are configured as login

, meaning that we have to either set the VTY passwords or use the no login

command. (You can review setting passwords in Chapter 6, “Cisco’s Internetworking

Operating System (IOS),” if you need to.)

If you find you can’t telnet into a device, it could be that the password on the remote device hasn’t been set. It’s also possible that an access control list is filtering the Telnet session.

On a Cisco router, you don’t need to use the telnet

command; you can just type in an IP address from a command prompt and the router will assume that you want to telnet to the device. Here’s how that looks using just the IP address:

Corp#

10.2.2.2

Trying 10.2.2.2 ... Open

Password required, but none set

[Connection to 10.2.2.2 closed by foreign host]

Corp#

At this point, it would be a great idea to set those VTY passwords on the router I want to telnet into. Here’s what I did on the remote router named

R1:

R1#

config t

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#

line vty 0 ?

<1-807> Last Line number

<cr>

R1(config)#

line vty 0 807

R1(config-line)#

password telnet

R1(config-line)#

login

R1(config-line)#

^Z

Now let’s try this again. Here I’m connecting to the router from the Corp console:

Corp#

10.2.2.2

Trying 10.2.2.2 ... Open

User Access Verification

Password:

R1>

Remember that the VTY password is the user-mode password, not the enable-mode password. Watch what happens when I try to go into privileged mode after telnetting into router R1:

R1>

en

% No password set

R1>

It is basically saying, “No way!” This is a really good security feature because you don’t want anyone telnetting into your device and being able to just type the enable

command to get into privileged mode. You’ve got to set your enable-mode password or enable secret password to use Telnet to configure remote devices!

When you telnet into a remote device, you will not see console messages by default. For example, you will not see debugging output. To allow console messages to be sent to your Telnet session, use the terminal monitor

command.

In the following examples, I am going to show you how to telnet into multiple devices simultaneously and then show you how to use hostnames instead of IP addresses.

Telnetting into Multiple Devices Simultaneously

If you telnet to a router or switch, you can end the connection by typing

exit

at any time. But what if you want to keep your connection to a remote device but still come back to your original router console? To do that, you can press the Ctrl+Shift+6 key combination, release it, and then press X.

Here’s an example of connecting to multiple devices from my Corp router console:

Corp#

10.2.2.2

Trying 10.2.2.2 ... Open

User Access Verification

Password:

R1>

Ctrl+Shift+6

Corp#

In this example, I telnetted to the R1 router and then typed the password to enter user mode. I next pressed Ctrl+Shift+6, then X (but you can’t see any of that because it doesn’t show on the screen output). Notice that my command prompt is now back at the Corp router.

Let’s run through some verification commands.

Checking Telnet Connections

To see the connections made from your router to a remote device, use the show sessions

command:

Corp#

sh sessions

Conn Host Address Byte Idle Conn Name

1 10.2.2.2 10.2.2.2 0 0 10.2.2.2

* 2 10.1.1.2 10.1.1.2 0 0 10.1.1.2

Corp#

See that asterisk (

*

) next to connection 2? It means that session 2 was your last session. You can return to your last session by pressing Enter twice. You can also return to any session by typing the number of the connection and pressing Enter.

Checking Telnet Users

You can list all active consoles and VTY ports in use on your router with the show users

command:

Corp#

sh users

Line User Host(s) Idle Location

* 0 con 0 10.1.1.2 00:00:01

10.2.2.2 00:01:06

In the command’s output, con

represents the local console. In this example, the console session is connected to two remote IP addresses, or in other words, two devices. In the next example, I typed sh users

on the ap device that the Corp router had telnetted into and is connected to via line 1:

Corp#

sh sessions

Conn Host Address Byte Idle Conn Name

1 10.1.1.2 10.1.1.2 0 0 10.1.1.2

* 2 10.2.2.2 10.2.2.2 0 0 10.2.2.2

Corp#

1

[Resuming connection 1 to 10.1.1.2 ... ] ap>

sh users

Line User Host(s) Idle Location

* 1 vty 0 idle 00:00:00 10.1.1.1

ap>

This output shows that the console is active and that VTY router line 1 is being used. The asterisk represents the current terminal session from which the show user

command was entered.

Closing Telnet Sessions

You can end Telnet sessions a few different ways—typing exit

or disconnect

is probably the easiest and quickest.

To end a session from a remote device, use the exit

command: ap>

exit

[Connection to 10.1.1.2 closed by foreign host]

Corp#

To end a session from a local device, use the disconnect

command:

Corp#

sh session

Conn Host Address Byte Idle Conn Name

*2 10.2.2.2 10.2.2.2 0 0 10.2.2.2

Corp#

disconnect ?

<2-2> The number of an active network connection

qdm Disconnect QDM web-based clients

ssh Disconnect an active SSH connection

Corp#

disconnect 2

Closing connection to 10.2.2.2 [confirm]

[enter]

Corp#

In this example, I used session number 2 because that was the connection to the R1 router that I wanted to end. As I showed, you can use the show sessions

command to see the connection number.

Resolving Hostnames

If you want to use a hostname rather than an IP address to connect to a remote device, the device that you are using to make the connection must be able to translate the hostname to an IP address.

There are two ways to resolve hostnames to IP addresses: building a host table on each router or building a Domain Name System (DNS) server, which is similar to a dynamic host table (assuming dynamic DNS).

Building a Host Table

A host table provides name resolution only on the router that it was built upon. The command to build a host table on a router is as follows: ip host host_name [tcp_port_number] ip_address

The default is TCP port number 23, but you can create a session using Telnet with a different TCP port number if you want. You can also assign up to eight IP addresses to a hostname.

Here’s an example of configuring a host table on the Corp router with two entries to resolve the names for the R1 router and the ap device:

Corp#

config t

Corp(config)#

ip host R1 ?

<0-65535> Default telnet port number

A.B.C.D Host IP address

additional Append addresses

mx Configure a MX record

ns Configure an NS record

srv Configure a SRV record

Corp(config)#

ip host R1 10.2.2.2 ?

A.B.C.D Host IP address

<cr>

Corp(config)#

ip host R1 10.2.2.2

Corp(config)#

ip host ap 10.1.1.2

Notice in the preceding router configuration that I can just keep adding IP addresses to reference a host, one after another, up to eight IP addresses. And to see the newly built host table, just use the show hosts

command:

Corp(config)#

do show hosts

Default domain is not set

Name/address lookup uses domain service

Name servers are 255.255.255.255

Codes: UN - unknown, EX - expired, OK - OK, ?? - revalidate

temp - temporary, perm - permanent

NA - Not Applicable None - Not defined

Host Port Flags Age Type Address(es) ap None (perm, OK) 0 IP 10.1.1.2

R1 None (perm, OK) 0 IP 10.2.2.2

Corp(config)#

^Z

Corp#

You can see the two hostnames plus their associated IP addresses in the preceding router output. The perm

in the

Flags

column means that the entry is manually configured. If it said temp

, it would be an entry that was resolved by DNS.

The show hosts

command provides information on temporary DNS entries and permanent name-to-address mappings created using the ip host

command.

To verify that the host table resolves names, try typing the hostnames at a router prompt. Remember that if you don’t specify the command, the router assumes you want to telnet.

In the following example, I’ll use the hostnames to telnet into the remote devices and press Ctrl+Shift+6 and then X to return to the main console of the Corp router:

Corp#

r1

Trying R1 (10.2.2.2)... Open

User Access Verification

Password:

R1>

Ctrl+Shift+6

Corp#

ap

Trying ap (10.1.1.2)... Open

User Access Verification

Password: ap>

Ctrl+Shift+6

Corp#

I successfully used entries in the host table to create a session to two devices by using the names to telnet into both devices. Names in the host table are not case sensitive.

Notice that the entries in the following show sessions

output now display the hostnames and IP addresses instead of just the IP addresses:

Corp#

sh sessions

Conn Host Address Byte Idle Conn Name

1 r1 10.2.2.2 0 1 r1

* 2 ap 10.1.1.2 0 0 ap

Corp#

If you want to remove a hostname from the table, just use the no ip host

command like this:

Corp(config)#

no ip host R1

The problem with the host table method is that you would need to create a host table on each router to be able to resolve names. And if you have a whole bunch of routers and want to resolve names, using DNS is a much better choice!

Using DNS to Resolve Names

If you have a lot of devices and don’t want to create a host table in each device, you can use a DNS server to resolve hostnames.

Any time a Cisco device receives a command it doesn’t understand, it will try to resolve it through DNS by default. Watch what happens when I type the special command todd

at a Cisco router prompt:

Corp#

todd

Translating "todd"...domain server (255.255.255.255)

Translating "todd"...domain server (255.255.255.255)

Translating "todd"...domain server (255.255.255.255)

% Unknown command or computer name, or unable to find

computer address

Corp#

It doesn’t know my name or what command I am trying to type, so it tries to resolve this through DNS. This is really annoying for two reasons: first, because it doesn’t know my name <grin>, and second, because I need to hang out and wait for the name lookup to time out. You can get around this and prevent a time-consuming DNS lookup by using the no ip domain-lookup

command on your router from global configuration mode.

If you have a DNS server on your network, you need to add a few commands to make DNS name resolution work:

The first command is ip domain-lookup

, which is turned on by default. It needs to be entered only if you previously turned it off (with the no ip domain-lookup

command). The command can be used without the hyphen as well ( ip domain lookup

).

The second command is ip name-server

. This sets the IP address of the DNS server. You can enter the IP addresses of up to six servers.

The last command is ip domain-name

. Although this command is optional, it really should be set. It appends the domain name to the hostname you type in. Since DNS uses a fully qualified domain name (FQDN) system, you must have a second level DNS name, in the form domain.com

.

Here’s an example of using these three commands:

Corp#

config t

Corp(config)#

ip domain-lookup

Corp(config)#

ip name-server ?

A.B.C.D Domain server IP address (maximum of 6)

Corp(config)#

ip name-server 192.168.0.70

Corp(config)#

ip domain-name lammle.com

Corp(config)#

^Z

Corp#

After the DNS configurations are set, you can test the DNS server by using a hostname to ping or telnet a device like this:

Corp#

ping R1

Translating "R1"...domain server (192.168.0.70) [OK]

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is

2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max

= 28/31/32 ms

Notice that the router uses the DNS server to resolve the name.

After a name is resolved using DNS, use the show hosts

command to see that the device cached this information in the host table:

Corp#

sh hosts

Default domain is lammle.com

Name/address lookup uses domain service

Name servers are 192.168.0.70

Host Flags Age Type Address(es)

R1 (temp, OK) 0 IP 10.2.2.2

ap (perm, OK) 0 IP 10.1.1.2

Corp#

The entry that was resolved is shown as temp

, but the ap device is still perm

, meaning that it’s a static entry. Notice that the hostname is a full domain name. If I hadn’t used the ip domain-name lammle.com

command, I would have needed to type in ping r1.lammle.com

, which is a pain.

Should You Use a Host Table or a DNS Server?

Karen has finally finished drawing out her network by using CDP and the doctors are much happier. However, Karen is having a difficult time administering the network because she has to look at the network drawing to find an IP address every time she needs to telnet to a remote router.

Karen was thinking about putting host tables on each router, but with literally hundreds of routers, this is a daunting task.

Most networks have a DNS server now anyway, so adding a hundred or so hostnames into it would be an easy way to go—certainly easier then adding these hostnames to each and every router! She can just add the three commands on each router and blammo—she’s resolving names.

Using a DNS server makes it easy to update any old entries too—remember, even one little change and off she goes to each and every router to manually update its table if she’s using static host tables.

Keep in mind that this has nothing to do with name resolution on the network and nothing to do with what a host on the network is trying to accomplish. This is only used when you’re trying to resolve names from the router console.

Checking Network Connectivity and Troubleshooting

You can use the ping

and traceroute

commands to test connectivity to remote devices, and both of them can be used with many protocols, not just IP.

But don’t forget that the show ip route

command is a good troubleshooting command for verifying your routing table and the show interfaces

command will show you the status of each interface.

I’m not going to get into the show interfaces

commands here because we’ve already been over that in Chapter 6. But I am going to go over both the debug

command and the show processes

command you need to troubleshoot a router.

Using the ping Command

So far, you’ve seen many examples of pinging devices to test IP connectivity and name resolution using the DNS server. To see all the different

protocols that you can use with the

Ping

program, type

ping ?

:

Corp#

ping ?

WORD Ping destination address or hostname

clns CLNS echo

ip IP echo

srb srb echo

tag Tag encapsulated IP echo

<cr>

The ping

output displays the minimum, average, and maximum times it takes for a ping packet to find a specified system and return. Here’s an example:

Corp#

ping R1

Translating "R1"...domain server (192.168.0.70)[OK]

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout

is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max

= 1/2/4 ms

Corp#

You can see that the DNS server was used to resolve the name, and the device was pinged in a minimum of 1 ms (milliseconds), an average of

2 ms, and up to 4 ms.

The ping

command can be used in user and privileged mode but not configuration mode.

Using the traceroute Command

Traceroute

(the traceroute

command, or trace for short) shows the path a packet takes to get to a remote device. It uses time to live (TTL) time-outs and ICMP error messages to outline the path a packet takes through an internetwork to arrive at a remote host.

Trace

(the trace

command), which can be used from either user mode or privileged mode, allows you to figure out which router in the path to an unreachable network host should be examined more closely for the cause of the network’s failure.

To see the protocols that you can use with the traceroute

command, type

traceroute ?

:

Corp#

traceroute ?

WORD Trace route to destination address or hostname

appletalk AppleTalk Trace

clns ISO CLNS Trace

ip IP Trace

ipv6 IPv6 Trace

ipx IPX Trace

<cr>

The traceroute

command shows the hop or hops that a packet traverses on its way to a remote device. Here’s an example:

Corp#

traceroute r1

Type escape sequence to abort.

Tracing the route to R1 (10.2.2.2)

1 R1 (10.2.2.2) 4 msec * 0 msec

Corp#

You can see that the packet went to only one hop to find the destination.

Do not get confused! You can’t use the tracert

command—it’s a Windows command. For a router, use the traceroute

command!

Here’s an example of using tracert

from a Windows DOS prompt (notice the command tracert

!):

C:\>

tracert www.whitehouse.gov

Tracing route to a1289.g.akamai.net [69.8.201.107] over a maximum of 30 hops:

1 * * * Request timed out.

2 53 ms 61 ms 53 ms hlrn-dsl-gw15-207.hlrn.qwest.net

[207.225.112.207]

3 53 ms 55 ms 54 ms hlrn-agw1.inet.qwest.net [71.217.188.113]

4 54 ms 53 ms 54 ms hlr-core-01.inet.qwest.net [205.171.253.97]

5 54 ms 53 ms 54 ms apa-cntr-01.inet.qwest.net [205.171.253.26]

6 54 ms 53 ms 53 ms 63.150.160.34

7 54 ms 54 ms 53 ms www.whitehouse.gov [69.8.201.107]

Trace complete.

Okay, let’s move on now and talk about how to troubleshoot your network using the debug

command.

Debugging

Debug is a troubleshooting command that’s available from the privileged exec mode of Cisco IOS. It’s used to display information about various router operations and the related traffic generated or received by the router, plus any error messages.

It’s a useful and informative tool, but you really need to understand some important facts about its use. Debug is regarded as a very highoverhead task because it can consume a huge amount of resources and the router is forced to process-switch the packets being debugged. So you don’t just use debug as a monitoring tool—it’s meant to be used for a short period of time and only as a troubleshooting tool. By using it, you can really find out some truly significant facts about both working and faulty software and/or hardware components.

Because debugging output takes priority over other network traffic, and because the debug all

command generates more output than any other debug

command, it can severely diminish the router’s performance—even render it unusable. So in virtually all cases, it’s best to use more-specific debug

commands.

As you can see from the following output, you can’t enable debugging from user mode, only privileged mode:

Corp>

debug ?

% Unrecognized command

Corp>

en

Corp#

debug ?

aaa AAA Authentication, Authorization and Accounting

access-expression Boolean access expression

adjacency adjacency

all Enable all debugging

[output cut]

If you’ve got the freedom to pretty much take out a router and you really want to have some fun with debugging, use the debug all

command:

Corp#

debug all

This may severely impact network performance. Continue? (yes/[no]):

yes

All possible debugging has been turned on

2d20h: SNMP: HC Timer 824AE5CC fired

2d20h: SNMP: HC Timer 824AE5CC rearmed, delay = 20000

2d20h: Serial0/0: HDLC myseq 4, mineseen 0, yourseen 0, line down

2d20h:

2d20h: Rudpv1 Sent: Pkts 0, Data Bytes 0, Data Pkts 0

2d20h: Rudpv1 Rcvd: Pkts 0, Data Bytes 0, Data Pkts 0

2d20h: Rudpv1 Discarded: 0, Retransmitted 0

2d20h:

2d20h: RIP-TIMER: periodic timer expired

2d20h: Serial0/0: HDLC myseq 5, mineseen 0, yourseen 0, line down

2d20h: Serial0/0: attempting to restart

2d20h: PowerQUICC(0/0): DCD is up.

2d20h: is_up: 0 state: 4 sub state: 1 line: 0

2d20h:

2d20h: Rudpv1 Sent: Pkts 0, Data Bytes 0, Data Pkts 0

2d20h: Rudpv1 Rcvd: Pkts 0, Data Bytes 0, Data Pkts 0

2d20h: Rudpv1 Discarded: 0, Retransmitted 0

2d20h: un all

All possible debugging has been turned off

Corp#

To disable debugging on a router, just use the command no

in front of the debug command:

Corp#

no debug all

But I typically just use the undebug all

command since it is so easy when using the shortcut:

Corp#

un all

Remember that instead of using the debug all

command, it’s almost always better to use specific commands—and only for short periods of time.

Here’s an example of deploying debug ip rip

that will show you RIP updates being sent and received on a router:

Corp#

debug ip rip

RIP protocol debugging is on

Corp#

1w4d: RIP: sending v2 update to 224.0.0.9 via Serial0/0 (192.168.12.1)

1w4d: RIP: build update entries

1w4d: 10.10.10.0/24 via 0.0.0.0, metric 2, tag 0

1w4d: 171.16.125.0/24 via 0.0.0.0, metric 3, tag 0

1w4d: 172.16.12.0/24 via 0.0.0.0, metric 1, tag 0

1w4d: 172.16.125.0/24 via 0.0.0.0, metric 3, tag 0

1w4d: RIP: sending v2 update to 224.0.0.9 via Serial0/2 (172.16.12.1)

1w4d: RIP: build update entries

1w4d: 192.168.12.0/24 via 0.0.0.0, metric 1, tag 0

1w4d: 192.168.22.0/24 via 0.0.0.0, metric 2, tag 0

1w4d: RIP: received v2 update from 192.168.12.2 on Serial0/0

1w4d: 192.168.22.0/24 via 0.0.0.0 in 1 hops

Corp#

un all

I’m sure you can see that the debug

command is one powerful command. And because of this, I’m also sure you realize that before you use any of the debugging commands, you should make sure you check the utilization of your router. This is important because in most cases, you don’t want to negatively impact the device’s ability to process the packets through on your internetwork. You can determine a specific router’s utilization information by using the show processes

command.

Remember, when you telnet into a remote device, you will not see console messages by default! For example, you will not see debugging output. To allow console messages to be sent to your Telnet session, use the terminal monitor

command.

Using the show processes Command

As mentioned in the previous section, you’ve really got to be careful when using the debug

command on your devices. If your router’s CPU utilization is consistently at 50 percent or more, it’s probably not a good idea to type in the debug all

command unless you want to see what a router looks like when it crashes!

So what other approaches can you use? Well, the show processes

(or show processes cpu

) is a good tool for determining a given router’s CPU utilization. Plus, it’ll give you a list of active processes along with their corresponding process ID, priority, scheduler test (status), CPU time used, number of times invoked, and so on. Lots of great stuff! Plus, this command is super handy when you want to evaluate your router’s performance and CPU utilization—for instance, when you find yourself otherwise tempted to reach for the debug

command.

Okay—what do you see in the following output? The first line shows the CPU utilization output for the last 5 seconds, 1 minute, and 5 minutes.

The output provides 2%/0% in front of the CPU utilization for the last 5 seconds. The first number equals the total utilization and the second one indicates the utilization due to interrupt routines:

Corp#

sh processes

CPU utilization for five seconds: 2%/0%; one minute: 0%; five minutes: 0%

PID QTy PC Runtime (ms) Invoked uSecs Stacks TTY Process

1 Cwe 8034470C 0 1 0 5804/6000 0 Chunk Manager

2 Csp 80369A88 4 1856 2 2616/3000 0 Load Meter

3 M* 0 112 14 800010656/12000 0 Exec

5 Lst 8034FD9C 268246 52101 5148 5768/6000 0 Check heaps

6 Cwe 80355E5C 20 3 6666 5704/6000 0 Pool Manager

7 Mst 802AC3C4 0 2 0 5580/6000 0 Timers

[output cut]

So basically, the output from the show processes

command shows that our router is happily able to process debugging commands without being overloaded.

Summary

In this chapter, you learned how Cisco routers are configured and how to manage those configurations.

This chapter covered the internal components of a router, which included ROM, RAM, NVRAM, and flash.

In addition, I covered what happens when a router boots and which files are loaded. The configuration register tells the router how to boot and where to find files, and you learned how to change and verify the configuration register settings for password recovery purposes.

Next, you learned how to back up and restore a Cisco IOS image as well as how to back up and restore the configuration of a Cisco router. I showed you how to manage these files using the CLI, and IFS.

Then you learned how to use CDP and Telnet to gather information about remote devices. Finally, the chapter covered how to resolve hostnames and use the ping

and trace

commands to test network connectivity as well as how to use the debug

and show processes

commands.

Exam Essentials

Define the Cisco Router components.

Describe the functions of the bootstrap, POST, ROM monitor, mini-IOS, RAM, ROM, flash memory,

NVRAM and the configuration register.

Identify the steps in the router boot sequence.

The steps in the boot sequence are POST, loading the IOS, and copying the startup configuration from NVRAM to RAM.

Understand configuration register commands and settings.

The 0x2102 setting is the default on all Cisco routers and tells the router to look in NVRAM for the boot sequence. 0x2101 tells the router to boot from ROM, and 0x2142 tells the router to not load the startup-config in

NVRAM to provide password recovery.

Perform password recovery.

The steps in the password recovery process are interrupt the router boot sequence, change the configuration register, reload the router and enter privileged mode, change/set the password, save the new configuration, reset the configuration register, and reload the router.

Back up an IOS image.

By using the privileged-mode command copy flash tftp

, you can back up a file from flash memory to a TFTP (network) server.

Restore or upgrade an IOS image.

By using the privileged-mode command copy tftp flash

, you can restore or upgrade a file from a TFTP

(network) server to flash memory.

Describe best practices to prepare to back up an IOS image to a network server.

Make sure that you can access the network server,

ensure that the network server has adequate space for the code image, and verify the file naming and path requirement.

Save the configuration of a router.

There are a couple of ways to do this, but the most common, as well as most tested, method is copy running-config startup-config.

Erase the configuration of a router.

Type the privileged-mode command erase startup-config

and reload the router.

Understand and use Cisco IFS file system management commands.

The commands to use are dir

, copy

, more

, delete

, erase

or format

, cd

and pwd

, and mkdir

and rmdir

.

Describe the value of CDP.

Cisco Discovery Protocol can be used to help you document as well as troubleshoot your network.

List the information provided by the output of the

show cdp neighbors

command.

The show cdp neighbors

command provides the following information: device ID, local interface, holdtime, capability, platform, and port ID (remote interface).

Understand how to establish a Telnet session with multiple routers simultaneously.

If you telnet to a router or switch, you can end the connection by typing

exit

at any time. However, if you want to keep your connection to a remote device but still come back to your original router console, you can press the Ctrl+Shift+6 key combination, release it, and then press X.

Identify current Telnet sessions.

The command show sessions

will provide you with information about all the currently active sessions your router has with other routers.

Build a static host table on a router.

By using the global configuration command ip host

host_name ip_address

, you can build a static host table on your router. You can apply multiple IP addresses against the same host entry.

Verify the host table on a router.

You can verify the host table with the show hosts command.

Describe the function of the ping command.

Packet Internet Groper (Ping) uses ICMP echo request and ICMP echo replies to verify an active IP address on a network.

Ping a valid host ID from the correct prompt.

You can ping an IP address from a router’s user mode or privileged mode but not from configuration mode. You must ping a valid address, such as 1.1.1.1.

Written Lab 7

In this section, you’ll complete the following labs to make sure you’ve got the information and concepts contained within them fully dialed in:

Lab 7.1: IOS Management

Lab 7.2: Router Memory

(The answers to the written labs can be found following the answers to the review questions for this chapter.)

Written Lab 7.1

Write the answers to the following questions:

1.

What is the command to copy a Cisco IOS to a TFTP server?

2.

What is the command to copy a Cisco startup-config file to a TFTP server?

3.

What is the command to copy the startup-config file to DRAM?

4.

What is an older command that you can use to copy the startup-config file to DRAM?

5.

What command can you use to see the neighbor router’s IP address from your router prompt?

6.

What command can you use to see the hostname, local interface, platform, and remote port of a neighbor router?

7.

What keystrokes can you use to telnet into multiple devices simultaneously?

8.

What command will show you your active Telnet connections to neighbor and remote devices?

9.

What command can you use to upgrade a Cisco IOS?

10.

What command can you use to merge a backup configuration with the configuration in RAM?

Written Lab 7.2

Identify the location in a router where each of the following files is stored by default.

1.

Cisco IOS

2.

Bootstrap

3.

Startup configuration

4.

POST routine

5.

Running configuration

6.

ARP cache

7.

Mini IOS

8.

ROM Monitor

9.

Routing tables

10.

Packet buffers

Hands-on Labs

To complete the labs in this section, you need at least one router (three would be best) and at least one PC running as a TFTP server. TFTP server software must be installed and running on the PC. For this lab, it is also assumed that your PC and the router(s) are connected together with a switch or hub and that all interfaces (PC NIC and router interfaces) are in the same subnet. You can alternately connect the PC directly to the router or connect the routers directly to one another (use a crossover cable in that case). Remember that the labs listed here were created for use with real routers but can easily be used with Cisco’s Packet Tracer program.

Here is a list of the labs in this chapter:

Lab 7.1: Backing Up Your Router IOS

Lab 7.2: Upgrading or Restoring Your Router IOS

Lab 7.3: Backing Up the Router Configuration

Lab 7.4: Using the Cisco Discovery Protocol (CDP)

Lab 7.5: Using Telnet

Lab 7.6: Resolving Hostnames

Hands-on Lab 7.1: Backing Up Your Router IOS

1.

Log into your router and go into privileged mode by typing

en

or

enable

.

2.

Make sure you can connect to the TFTP server that is on your network by pinging the IP address from the router console.

3.

Type

show flash to see the contents of flash memory.

4.

Type

show version

at the router privileged-mode prompt to get the name of the IOS currently running on the router. If there is only one file in flash memory, the show flash

and show version

commands show the same file. Remember that the show version

command shows you the file that is currently running and the show flash

command shows you all of the files in flash memory.

5.

Once you know you have good Ethernet connectivity to the TFTP server and you also know the IOS filename, back up your IOS by typing

copy flash tftp

. This command tells the router to copy a specified file from flash memory (this is where the IOS is stored by default) to a TFTP server.

6.

Enter the IP address of the TFTP server and the source IOS filename. The file is now copied and stored in the TFTP server’s default directory.

Hands-on Lab 7.2: Upgrading or Restoring Your Router IOS

1.

Log into your router and go into privileged mode by typing

en

or

enable

.

2.

Make sure you can connect to the TFTP server by pinging the IP address of the server from the router console.

3.

Once you know you have good Ethernet connectivity to the TFTP server, issue the

copy tftp flash

command.

4.

Confirm that the router will not function during the restore or upgrade by following the prompts provided on the router console. It is possible this prompt may not occur.

5.

Enter the IP address of the TFTP server.

6.

Enter the name of the IOS filename you want to restore or upgrade.

7.

Confirm that you understand that the contents of flash memory will be erased if there is not enough room in flash to store the new image.

8.

Watch in amazement as your IOS is deleted out of flash memory and your new IOS is copied to flash memory.

If the file that was in flash memory is deleted but the new version wasn’t copied to flash memory, the router will boot from ROM monitor mode.

You’ll need to figure out why the copy operation did not take place.

Hands-on Lab 7.3: Backing Up the Router Configuration

1.

Log into your router and go into privileged mode by typing

en

or

enable

.

2.

Ping the TFTP server to make sure you have IP connectivity.

3.

From RouterB, type

copy run tftp

.

4.

When prompted, type the IP address of the TFTP server (for example,

172.16.30.2

) and press Enter.

5.

By default, the router will prompt you for a filename. The hostname of the router is followed by the suffix

-confg

(yes, I spelled that correctly). You can use any name you want.

Name of configuration file to write [RouterB-confg]?

Press Enter to accept the default name.

Write file RouterB-confg on host 172.16.30.2? [confirm]

Press Enter to confirm.

Hands-on Lab 7.4: Using the Cisco Discovery Protocol (CDP)

1.

Log into your router and go into privileged mode by typing

en

or

enable

.

2.

From the router, type

sh cdp

and press Enter. You should see that CDP packets are being sent out to all active interfaces every 60 seconds and the holdtime is 180 seconds (these are the defaults).

3.

To change the CDP update frequency to 90 seconds, type

cdp timer 90

in global configuration mode.

RouterC#

config t

Enter configuration commands, one per line. End with

CNTL/Z.

RouterC(config)#

cdp timer ?

<5-900> Rate at which CDP packets are sent (in sec)

RouterC(config)#

cdp timer 90

4.

Verify that your CDP timer frequency has changed by using the command

show cdp

in privileged mode.

RouterC#

sh cdp

Global CDP information:

Sending CDP packets every 90 seconds

Sending a holdtime value of 180 seconds

5.

Now use CDP to gather information about neighbor routers. You can get the list of available commands by typing

sh cdp ?

.

RouterC#

sh cdp ?

entry Information for specific neighbor entry

interface CDP interface status and configuration

neighbors CDP neighbor entries

traffic CDP statistics

<cr>

6.

Type

sh cdp int

to see the interface information plus the default encapsulation used by the interface. It also shows the CDP timer information.

7.

Type

sh cdp entry *

to see complete CDP information received from all devices.

8.

Type

show cdp neighbors

to gather information about all connected neighbors. (You should know the specific information output by this command.)

9.

Type

show cdp neighbors detail

. Notice that it produces the same output as show cdp entry *

.

Hands-on Lab 7.5: Using Telnet

1.

Log into your router and go into privileged mode by typing

en

or

enable

.

2.

From RouterA, telnet into your remote router (RouterB) by typing

telnet ip_address

from the command prompt. Type

exit

to disconnect.

3.

Now type in RouterB’s IP address from RouterA’s command prompt. Notice that the router automatically tries to telnet to the IP address you specified. You can use the telnet

command or just type in the IP address.

4.

From RouterB, press Ctrl+Shift+6 and then X to return to RouterA’s command prompt. Now telnet into your third router, RouterC. Press

Ctrl+Shift+6 and then X to return to RouterA.

5.

From RouterA, type

show sessions

. Notice your two sessions. You can press the number displayed to the left of the session and press

Enter twice to return to that session. The asterisk shows the default session. You can press Enter twice to return to that session.

6.

Go to the session for your RouterB. Type

show users

. This shows the console connection and the remote connection. You can use the disconnect

command to clear the session or just type

exit

from the prompt to close your session with RouterB.

7.

Go to RouterC’s console port by typing

show sessions

on the first router and using the connection number to return to RouterC. Type

show user

and notice the connection to your first router, RouterA.

8.

Type

clear line

line_number

to disconnect the Telnet session.

Hands-on Lab 7.6: Resolving Hostnames

1.

Log into your router and go into privileged mode by typing

en

or

enable

.

2.

From RouterA, type

todd

and press Enter at the command prompt. Notice the error you receive and the delay. The router is trying to resolve the hostname to an IP address by looking for a DNS server. You can turn this feature off by using the no ip domain-lookup

command from global configuration mode.

3.

To build a host table, you use the ip host

command. From RouterA, add a host table entry for RouterB and RouterC by entering the following commands:

ip host routerb

ip_address

ip host routerc

ip_address

Here is an example:

ip host routerb 172.16.20.2

ip host routerc 172.16.40.2

4.

Test your host table by typing

ping routerb

from the privileged mode prompt (not the config

prompt).

RouterA#

ping routerb

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.20.2, timeout

is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip

min/avg/max = 4/4/4 ms

5.

Test your host table by typing

ping routerc

.

RouterA#

ping routerc

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.16.40.2, timeout

is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip

min/avg/max = 4/6/8 ms

6.

Telnet to RouterB and keep your session to RouterB open to RouterA by pressing Ctrl+Shift+6, then X.

7.

Telnet to RouterC by typing

routerc

at the command prompt.

8.

Return to RouterA and keep the session to RouterC open by pressing Ctrl+Shift+6, then X.

9.

View the host table by typing

show hosts

and pressing Enter.

Default domain is not set

Name/address lookup uses domain service

Name servers are 255.255.255.255

Host Flags Age Type Address(es) routerb (perm, OK) 0 IP 172.16.20.2

routerc (perm, OK) 0 IP 172.16.40.2

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s

Introduction.

1. What does the command confreg 0x2142

provide?

A. It is used to restart the router.

B. It is used to bypass the configuration in NVRAM.

C. It is used to enter ROM monitor mode.

D. It is used to view the lost password.

2. Which command will copy the IOS to a backup host on your network?

A. transfer IOS to 172.16.10.1

B. copy run start

C. copy tftp flash

D. copy start tftp

E. copy flash tftp

3. You are troubleshooting a connectivity problem in your corporate network and want to isolate the problem. You suspect that a router on the route to an unreachable network is at fault. What IOS user exec command should you issue?

A.

Router>ping

B.

Router>trace

C.

Router>show ip route

D.

Router>show interface

E.

Router>show cdp neighbors

4. You copy a configuration from a network host to a router’s RAM. The configuration looks correct, yet it is not working at all. What could the problem be?

A. You copied the wrong configuration into RAM.

B. You copied the configuration into flash memory instead.

C. The copy did not override the shutdown

command in running-config.

D. The IOS became corrupted after the copy

command was initiated.

5. A network administrator wants to upgrade the IOS of a router without removing the image currently installed. What command will display the amount of memory consumed by the current IOS image and indicate whether there is enough room available to hold both the current and new images?

A. show version

B. show flash

C. show memory

D. show buffers

E. show running-config

6. The corporate office sends you a new router to connect, but upon connecting the console cable, you see that there is already a configuration on the router. What should be done before a new configuration is entered in the router?

A. RAM should be erased and the router restarted.

B. Flash should be erased and the router restarted.

C. NVRAM should be erased and the router restarted.

D. The new configuration should be entered and saved.

7. Which command loads a new version of the Cisco IOS into a router?

A.

copy flash ftp

B.

copy ftp flash

C.

copy flash tftp

D. copy tftp flash

8. Which command will show you the IOS version running on your router?

A. sh IOS

B. sh flash

C. sh version

D. sh running-config

9. What should the configuration register value be after you successfully complete the password recovery procedure and return the router to normal operation?

A. 0x2100

B. 0x2101

C. 0x2102

D. 0x2142

10. You save the configuration on a router with the copy running-config startup-config

command and reboot the router. The router, however, comes up with a blank configuration. What can the problem be?

A. You didn’t boot the router with the correct command.

B. NVRAM is corrupted.

C. The configuration register setting is incorrect.

D. The newly upgraded IOS is not compatible with the hardware of the router.

E. The configuration you save is not compatible with the hardware.

11. If you want to have more than one Telnet session open at the same time, what keystroke combination would you use?

A. Tab+spacebar

B. Ctrl+X, then 6

C. Ctrl+Shift+X, then 6

D. Ctrl+Shift+6, then X

12. You are unsuccessful in telnetting into a remote device, but you could telnet to the router earlier however, you can still ping the remote device.

What could the problem be? (Choose two.)

A. IP addresses are incorrect.

B. Access control list is filtering Telnet.

C. There is a defective serial cable.

D. The VTY password is missing.

13. What information is displayed by the show hosts

command? (Choose two.)

A. Temporary DNS entries

B. The names of the routers created using the hostname

command

C. The IP addresses of workstations allowed to access the router

D. Permanent name-to-address mappings created using the ip host

command

E. The length of time a host has been connected to the router via Telnet

14. Which three commands can be used to check LAN connectivity problems on a router? (Choose three.)

A. show interfaces

B. show ip route

C. tracert

D. ping

E. dns lookups

15. You telnet to a router and make your necessary changes; now you want to end the Telnet session. What command do you type in?

A. close

B. disable

C. disconnect

D. exit

16. You telnet into a remote device and type debug ip rip

, but no output from the debug

command is seen. What could the problem be?

A. You must type the show ip rip

command first.

B. IP addressing on the network is incorrect.

C. You must use the terminal monitor

command.

D. Debug output is sent only to the console.

17. Which command displays the configuration register setting?

A. show ip route

B. show boot version

C. show version

D. show flash

18. You need to gather the IP address of a remote switch that is located in Hawaii. What can you do to find the address?

A. Fly to Hawaii, console into the switch, then relax and have a drink with an umbrella in it.

B. Issue the show ip route

command on the router connected to the switch.

C. Issue the show cdp neighbor

command on the router connected to the switch.

D. Issue the show ip arp

command on the router connected to the switch.

E. Issue the show cdp neighbors detail

command on the router connected to the switch.

19. You have your laptop directly connected into a router’s Ethernet port. Which of the following are among the requirements for the copy flash tftp command to be successful? (Choose three.)

A. TFTP server software must be running on the router.

B. TFTP server software must be running on your laptop.

C. The Ethernet cable connecting the laptop directly into the router’s Ethernet port must be a straight-through cable.

D. The laptop must be on the same subnet as the router’s Ethernet interface.

E. The copy flash tftp

command must be supplied the IP address of the laptop.

F. There must be enough room in the flash memory of the router to accommodate the file to be copied.

20. The configuration register setting of 0x2102 provides what function to a router?

A. Tells the router to boot into ROM monitor mode

B. Provides password recovery

C. Tells the router to look in NVRAM for the boot sequence

D. Boots the IOS from a TFTP server

E. Boots an IOS image stored in ROM

Answers to Review Questions

1. B. The default configuration setting is 0x2102, which tells the router to load the IOS from flash and the configuration from NVRAM.

0x2142

tells the router to bypass the configuration in NVRAM so that you can perform password recovery.

2. E. To copy the IOS to a backup host, which is stored in flash memory by default, use the copy flash tftp

command.

3. B. The command traceroute

( trace

for short), which can be issued from user mode or privileged mode, is used to find the path a packet takes through an internetwork and will also show you where the packet stops because of an error on a router.

4. C. Since the configuration looks correct, you probably didn’t screw up the copy job. However, when you perform a copy from a network host to a router, the interfaces are automatically shut down and need to be manually enabled with the no shutdown

command.

5. B. The show flash

command will provide you with the current IOS name and size and the size of flash memory.

6. C. Before you start to configure the router, you should erase the NVRAM with the erase startup-config

command and then reload the router using the reload

command.

7. D. The command copy tftp flash

will allow you to copy a new IOS into flash memory on your router.

8. C. The best answer is show version

, which shows you the IOS file running currently on your router. The show flash

command shows you the contents of flash memory, not which file is running.

9. C. All Cisco routers have a default configuration register setting of 0x2102, which tells the router to load the IOS from flash memory and the configuration from NVRAM.

10. C. If you save a configuration and reload the router and it comes up either in setup mode or as a blank configuration, chances are you have the configuration register setting incorrect.

11. D. To keep open one or more Telnet sessions, use the Ctrl+Shift+6 and then X keystroke combination.

12. B, D. The best answers, the ones you need to remember, are that either an access control list is filtering the Telnet session or the VTY password is not set on the remote device.

13. A, D. The show hosts

command provides information on temporary DNS entries and permanent name-to-address mappings created using the ip host

command.

14. A, B, D. The tracert

command is a Windows command and will not work on a router! A router uses the traceroute

command.

15. D. Since the question never mentioned anything about a suspended session, you can assume that the Telnet session is still open, and you would just type

exit

to close the session.

16. C. To see console messages through your Telnet session, you must enter the terminal monitor

command.

17. C. The show version

command provides you with the current configuration register setting.

18. E. Although option A is certainly the “best” answer, unfortunately option E will work just fine and your boss would probably prefer you to use the show cdp neighbors detail

command.

19. B, D, E. Before you back up an IOS image to a laptop directly connected to a router’s Ethernet port, make sure the TFTP server software is running on your laptop, that the Ethernet cable is a “crossover,” and that the laptop is in the same subnet as the router’s Ethernet port, and then you can use the copy flash tftp

command from your laptop.

20. C. The default configuration setting of 0x2102 tells the router to look in NVRAM for the boot sequence.

Answers to Written Lab 7

Written Lab 7.1

1.

copy flash tftp

2.

copy start tftp

3.

copy start run

4.

config mem

5.

show cdp neighbor detail

or show cdp entry *

6.

show cdp neighbor

7.

Ctrl+Shift+6, then X

8.

show sessions

9.

copy tftp flash

10.

Either copy tftp run

or copy start run

1.

Flash memory

2.

ROM

3.

NVRAM

4.

ROM

5.

RAM

6.

RAM

7.

ROM

8.

ROM

9.

RAM

10.

RAM

Written Lab 7.2

Chapter 8

IP Routing

The CCNA exam topics covered in this chapter include the following:

Describe how a network works

Determine the path between two hosts across a network

Configure, verify, and troubleshoot basic router operation and routing on Cisco devices

Describe basic routing concepts (including: packet forwarding, router lookup process)

Configure, verify, and troubleshoot RIPv2

Access and utilize the router to set basic parameters (including: CLI/SDM)

Connect, configure, and verify operation status of a device interface

Verify device configuration and network connectivity using ping, traceroute, telnet, SSH. or other utilities

Perform and verify routing configuration tasks for a static or default route given specific routing requirements

Compare and contrast methods of routing and routing protocols

Configure, verify, and troubleshoot OSPF

Configure, verify, and troubleshoot EIGRP

Verify network connectivity (including: using ping, traceroute, and telnet or SSH)

Troubleshoot routing issues

Verify router hardware and software operation using SHOW and DEBUG commands

Implement basic router security

In this chapter, I’m going to discuss the IP routing process. This is an important subject to understand since it pertains to all routers and configurations that use IP. IP routing is the process of moving packets from one network to another network using routers. And as before, by routers

I mean Cisco routers, of course!

But before you read this chapter, you must understand the difference between a routing protocol and a routed protocol. A

routing protocol

is used by routers to dynamically find all the networks in the internetwork and to ensure that all routers have the same routing table. Basically, a routing protocol determines the path of a packet through an internetwork. Examples of routing protocols are RIP, RIPv2, EIGRP, and OSPF.

Once all routers know about all networks, a

routed protocol

can be used to send user data (packets) through the established enterprise. Routed protocols are assigned to an interface and determine the method of packet delivery. Examples of routed protocols are IP and IPv6.

I’m pretty sure that I don’t have to tell you that this is definitely important stuff to know. You most likely understand that from what I’ve said so far. IP routing is basically what Cisco routers do, and they do it very well. Again, this chapter is dealing with truly fundamental material—these are things you must know if you want to understand the objectives covered in this book!

In this chapter, I’m going to show you how to configure and verify IP routing with Cisco routers. I’ll be covering the following:

Routing basics

The IP routing process

Static routing

Default routing

Dynamic routing

In Chapter 9, “Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF),” I’ll be moving into more advanced, dynamic routing with EIGRP and OSPF. But first, you’ve really got to nail down the basics of how packets actually move through an internetwork, so let’s get started!

For up-to-the minute updates for this chapter, please see www.lammle.com

and/or www.sybex.com/go/ccna7e

.

Routing Basics

Once you create an internetwork by connecting your WANs and LANs to a router, you’ll need to configure logical network addresses, such as IP addresses, to all hosts on the internetwork so that they can communicate across that internetwork.

The term

routing

refers to taking a packet from one device and sending it through the network to another device on a different network. Routers don’t really care about hosts—they only care about networks and the best path to each network. The logical network address of the destination host

is used to get packets to a network through a routed network, and then the hardware address of the host is used to deliver the packet from a router to the correct destination host.

If your network has no routers, then it should be apparent that you are not routing. Routers route traffic to all the networks in your internetwork. To be able to route packets, a router must know, at a minimum, the following:

Destination address

Neighbor routers from which it can learn about remote networks

Possible routes to all remote networks

The best route to each remote network

How to maintain and verify routing information

The router learns about remote networks from neighboring routers or from an administrator. The router then builds a routing table (a map of the internetwork) that describes how to find the remote networks. If a network is directly connected, then the router already knows how to get to it.

If a network isn’t directly connected to the router, the router must use one of two ways to learn how to get to the remote network: static routing, meaning that someone must hand-type all network locations into the routing table, or something called dynamic routing.

In

dynamic routing

, a protocol on one router communicates with the same protocol running on neighboring routers. The routers then update each other about all the networks they know about and place this information into the routing table. If a change occurs in the network, the dynamic routing protocols automatically inform all routers about the event. If

static routing

is used, the administrator is responsible for updating all changes by hand onto all routers. Typically, in a large network, a combination of both dynamic and static routing is used.

Before we jump into the IP routing process, let’s take a look at a very simple example that demonstrates how a router uses the routing table to route packets out of an interface. We’ll be going into a more detailed study of the process in the next section, but what I am showing now is called the “longest match rule,” which means that IP will look through a routing table for the longest match compared to the destination address of a packet. Let’s take a look.

Figure 8-1 shows a simple two-router network. Lab_A has one serial interface and three LAN interfaces.

Looking at

Figure 8-1 , can you see which interface Lab_A will use to forward an IP datagram to a host with an IP address of 10.10.10.10?

Figure 8-1:

A simple routing example

By using the command show ip route

, we can see the routing table (map of the internetwork) that Lab_A uses to make forwarding decisions:

Lab_A#

sh ip route

[output cut]

Gateway of last resort is not set

C 10.10.10.0/24 is directly connected, FastEthernet0/0

C 10.10.20.0/24 is directly connected, FastEthernet0/1

C 10.10.30.0/24 is directly connected, FastEthernet0/2

C 10.10.40.0/24 is directly connected, Serial 0/0

The

C

in the routing table output means that the networks listed are “directly connected,” and until we add a routing protocol—something like RIP,

EIGRP, etc.—to the routers in our internetwork (or use static routes), we’ll have only directly connected networks in our routing table.

So let’s get back to the original question: By looking at the figure and the output of the routing table, can you tell what IP will do with a received packet that has a destination IP address of 10.10.10.10? The router will packet-switch the packet to interface FastEthernet 0/0, and this interface will frame the packet and then send it out on the network segment. To reiterate on the longest match rule, IP would look for 10.10.10.10 in this example, and if that is not found in the table, then IP would search for 10.10.10.0, then 10.10.0.0, and so on until a route is found.

Because we can, let’s do another example: Based on the output of the next routing table, which interface will a packet with a destination address of 10.10.10.14 be forwarded from?

Lab_A#

sh ip route

[output cut]

Gateway of last resort is not set

C 10.10.10.16/28 is directly connected, FastEthernet0/0

C 10.10.10.8/29 is directly connected, FastEthernet0/1

C 10.10.10.4/30 is directly connected, FastEthernet0/2

C 10.10.10.0/30 is directly connected, Serial 0/0

First, you can see that the network is subnetted and each interface has a different mask. And I have to tell you—you just can’t answer this question if you can’t subnet! 10.10.10.14 would be a host in the 10.10.10.8/29 subnet connected to the FastEthernet0/1 interface. Don’t freak out if you don’t get it. Just go back and reread Chapter 4 if you’re struggling, and this should make perfect sense to you afterward.

For everyone who’s ready to move on, let’s get into this process in more detail.

The IP Routing Process

The IP routing process is fairly simple and doesn’t change, regardless of the size of your network. For an example, we’ll use

Figure 8-2 to describe step-by-step what happens when Host_A wants to communicate with Host_B on a different network.

Figure 8-2:

IP routing example using two hosts and one router

In this example, a user on Host_A pings Host_B’s IP address. Routing doesn’t get simpler than this, but it still involves a lot of steps. Let’s work through them:

1.

Internet Control Message Protocol (ICMP) creates an echo request payload (which is just the alphabet in the data field).

2.

ICMP hands that payload to Internet Protocol (IP), which then creates a packet. At a minimum, this packet contains an IP source address, an IP destination address, and a Protocol field with 01h. (Remember that Cisco likes to use

0x

in front of hex characters, so this could look like 0x01.) All that tells the receiving host to whom it should hand the payload when the destination is reached—in this example,

ICMP.

3.

Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one.

4.

Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so it can be routed to the remote network. The Registry in Windows is parsed to find the configured default gateway.

5.

The default gateway of host 172.16.10.2 (Host_A) is configured to 172.16.10.1. For this packet to be sent to the default gateway, the hardware address of the router’s interface Ethernet 0 (configured with the IP address of 172.16.10.1) must be known. Why? So the packet can be handed down to the Data Link layer, framed, and sent to the router’s interface that’s connected to the 172.16.10.0 network.

Because hosts only communicate via hardware addresses on the local LAN, it’s important to recognize that for Host_A to communicate to

Host_B, it has to send packets to the Media Access Control (MAC) address of the default gateway on the local network.

MAC addresses are always local on the LAN and never go through and past a router.

6.

Next, the Address Resolution Protocol (ARP) cache of the host is checked to see if the IP address of the default gateway has already been resolved to a hardware address:

If it has, the packet is then free to be handed to the Data Link layer for framing. (The hardware destination address is also handed down with that packet.) To view the ARP cache on your host, use the following command:

C:\>

arp -a

Interface: 172.16.10.2 --- 0x3

Internet Address Physical Address Type

172.16.10.1 00-15-05-06-31-b0 dynamic

If the hardware address isn’t already in the ARP cache of the host, an ARP broadcast is sent out onto the local network to search for the hardware address of 172.16.10.1. The router responds to the request and provides the hardware address of Ethernet 0, and the host caches this address.

7.

Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used (in this example, Ethernet). A frame is then generated, encapsulating the packet with control information.

Within that frame are the hardware destination and source addresses plus, in this case, an Ether-Type field that describes the Network layer protocol that handed the packet to the Data Link layer—in this instance, IP. At the end of the frame is something called a Frame

Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC). The frame would look something like what I’ve detailed in Figure 8-3 . It contains Host_A’s hardware (MAC) address and the destination hardware address of the default gateway. It does not include the remote host’s MAC address—remember that!

Figure 8-3:

Frame used from Host_A to the Lab_A router when Host_B is pinged

8.

Once the frame is completed, it’s handed down to the Physical layer to be put on the physical medium (in this example, twisted-pair wire) one bit at a time.

9.

Every device in the collision domain receives these bits and builds the frame. They each run a CRC and check the answer in the FCS field. If the answers don’t match, the frame is discarded.

If the CRC matches, then the hardware destination address is checked to see if it matches too (which, in this example, is the router’s interface Ethernet 0).

If it’s a match, then the Ether-Type field is checked to find the protocol used at the Network layer.

10.

The packet is pulled from the frame, and what is left of the frame is discarded. The packet is handed to the protocol listed in the Ether-

Type field—it’s given to IP.

11.

IP receives the packet and checks the IP destination address. Since the packet’s destination address doesn’t match any of the addresses configured on the receiving router itself, the router will look up the destination IP network address in its routing table.

12.

The routing table must have an entry for the network 172.16.20.0 or the packet will be discarded immediately and an ICMP message will be sent back to the originating device with a destination network unreachable message.

13.

If the router does find an entry for the destination network in its table, the packet is switched to the exit interface—in this example, interface Ethernet 1. The output below displays the Lab_A router’s routing table. The

C

means “directly connected.” No routing protocols are needed in this network since all networks (all two of them) are directly connected.

Lab_A>

sh ip route

Codes:C - connected,S - static,I - IGRP,R - RIP,M - mobile,B –

[output cut]

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

C 172.16.10.0 is directly connected, Ethernet0

C 172.16.20.0 is directly connected, Ethernet1

14.

The router packet-switches the packet to the Ethernet 1 buffer.

15.

The Ethernet 1 buffer needs to know the hardware address of the destination host and first checks the ARP cache.

If the hardware address of Host_B has already been resolved and is in the router’s ARP cache, then the packet and the hardware address are handed down to the Data Link layer to be framed. Let’s take a look at the ARP cache on the Lab_A router by using the show ip arp

command:

Lab_A#

sh ip arp

Protocol Address Age(min) Hardware Addr Type Interface

Internet 172.16.20.1 - 00d0.58ad.05f4 ARPA Ethernet1

Internet 172.16.20.2 3 0030.9492.a5dd ARPA Ethernet1

Internet 172.16.10.1 - 00d0.58ad.06aa ARPA Ethernet0

Internet 172.16.10.2 12 0030.9492.a4ac ARPA Ethernet0

The dash (-) means that this is the physical interface on the router. From the output above, we can see that the router knows the

172.16.10.2 (Host_A) and 172.16.20.2 (Host_B) hardware addresses. Cisco routers will keep an entry in the ARP table for 4 hours.

If the hardware address has not already been resolved, the router sends an ARP request out E1 looking for the hardware address of 172.16.20.2. Host_B responds with its hardware address, and the packet and destination hardware addresses are both sent to the Data Link layer for framing.

16.

The Data Link layer creates a frame with the destination and source hardware address, Ether-Type field, and FCS field at the end.

The frame is handed to the Physical layer to be sent out on the physical medium one bit at a time.

17.

Host_B receives the frame and immediately runs a CRC. If the result matches what’s in the FCS field, the hardware destination address is then checked. If the host finds a match, the Ether-Type field is then checked to determine the protocol that the packet should be handed to at the Network layer—IP in this example.

18.

At the Network layer, IP receives the packet and runs a CRC on the IP header. If that passes, IP then checks the destination address.

Since there’s finally a match made, the Protocol field is checked to find out to whom the payload should be given.

19.

The payload is handed to ICMP, which understands that this is an echo request. ICMP responds to this by immediately discarding the packet and generating a new payload as an echo reply.

20.

A packet is then created including the source and destination addresses, Protocol field, and payload. The destination device is now

Host_A.

21.

IP then checks to see whether the destination IP address is a device on the local LAN or on a remote network. Since the destination device is on a remote network, the packet needs to be sent to the default gateway.

22.

The default gateway IP address is found in the Registry of the Windows device, and the ARP cache is checked to see if the hardware address has already been resolved from an IP address.

23.

Once the hardware address of the default gateway is found, the packet and destination hardware addresses are handed down to the

Data Link layer for framing.

24.

The Data Link layer frames the packet of information and includes the following in the header:

The destination and source hardware addresses

The Ether-Type field with 0x0800 (IP) in it

The FCS field with the CRC result in tow

25.

The frame is now handed down to the Physical layer to be sent out over the network medium one bit at a time.

26.

The router’s Ethernet 1 interface receives the bits and builds a frame. The CRC is run, and the FCS field is checked to make sure the answers match.

27.

Once the CRC is found to be okay, the hardware destination address is checked. Since the router’s interface is a match, the packet is pulled from the frame and the Ether-Type field is checked to see what protocol at the Network layer the packet should be delivered to.

28.

The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination

IP address.

IP does not run a complete CRC as the Data Link layer does—it only checks the header for errors.

Since the IP destination address doesn’t match any of the router’s interfaces, the routing table is checked to see whether it has a route to

172.16.10.0. If it doesn’t have a route over to the destination network, the packet will be discarded immediately. (This is the source point of confusion for a lot of administrators—when a ping fails, most people think the packet never reached the destination host. But as we see here, that’s not

always

the case. All it takes is for just one of the remote routers to be lacking a route back to the originating host’s network and—

poof!

—the packet is dropped on the

return trip

, not on its way to the host.)

Just a quick note to mention that when (if) the packet is lost on the way back to the originating host, you will typically see a request timed out message because it is an unknown error. If the error occurs because of a known issue, such as if a route is not in the routing table on the way to the destination device, you will see a destination unreachable message. This should help you determine if the problem occurred on the way to the destination or on the way back.

29.

In this case, the router does know how to get to network 172.16.10.0—the exit interface is Ethernet 0—so the packet is switched to interface Ethernet 0.

30.

The router checks the ARP cache to determine whether the hardware address for 172.16.10.2 has already been resolved.

31.

Since the hardware address to 172.16.10.2 is already cached from the originating trip to Host_B, the hardware address and packet are handed to the Data Link layer.

32.

The Data Link layer builds a frame with the destination hardware address and source hardware address and then puts IP in the Ether-

Type field. A CRC is run on the frame and the result is placed in the FCS field.

33.

The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time.

34.

The destination host receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out to whom to hand the packet.

35.

IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the Protocol field for further direction.

IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply.

36.

ICMP acknowledges that it has received the reply by sending an exclamation point (!) to the user interface. ICMP then attempts to send four more echo requests to the destination host.

You’ve just experienced Todd’s 36 easy steps to understanding IP routing. The key point to understand here is that if you had a much larger network, the process would be the

same

. In a really big internetwork, the packet just goes through more hops before it finds the destination host.

It’s super-important to remember that when Host_A sends a packet to Host_B, the destination hardware address used is the default gateway’s

Ethernet interface. Why? Because frames can’t be placed on remote networks—only local networks. So packets destined for remote networks must go through the default gateway.

Let’s take a look at Host_A’s ARP cache now:

C:\ >

arp -a

Interface: 172.16.10.2 --- 0x3

Internet Address Physical Address Type

172.16.10.1 00-15-05-06-31-b0 dynamic

172.16.20.1 00-15-05-06-31-b0 dynamic

Did you notice that the hardware (MAC) address that Host_A uses to get to Host_B is the Lab_A E0 interface? Hardware addresses are

always

local, and they never pass a router’s interface. Understanding this process is as important as air to you, so carve this into your memory!

Testing Your IP Routing Understanding

I really want to make sure you understand IP routing because it’s super-important. So I’m going to use this section to test your understanding of the

IP routing process by having you look at a couple of figures and answer some very basic IP routing questions.

Figure 8-4 shows a LAN connected to RouterA, which is, in turn, connected via a WAN link to RouterB. RouterB has a LAN connected with an

HTTP server attached.

The critical information you need to glean from this figure is exactly how IP routing will occur in this example. Okay—we’ll cheat a bit. I’ll give you the answer, but then you should go back over the figure and see if you can answer example 2 without looking at my answers.

1.

The destination address of a frame, from HostA, will be the MAC address of the Fa0/0 interface of the RouterA router.

2.

The destination address of a packet will be the IP address of the network interface card (NIC) of the HTTP server.

3.

The destination port number in the segment header will have a value of 80.

Figure 8-4:

IP routing example 1

That example was a pretty simple one, and it was also very to the point. One thing to remember is that if multiple hosts are communicating to the server using HTTP, they must all use a different source port number. That is how the server keeps the data separated at the Transport layer.

Let’s mix it up a little and add another internetworking device into the network and then see if you can find the answers.

Figure 8-5 shows a network with only one router but two switches.

Figure 8-5:

IP routing example 2

What you want to understand about the IP routing process here is what happens when HostA sends data to the HTTPS server:

1.

The destination address of a frame, from HostA, will be the MAC address of the Fa0/0 interface of the RouterA router.

2.

The destination address of a packet will be the IP address of the network interface card (NIC) of the HTTPS server.

3.

The destination port number in the segment header will have a value of 443.

Notice that the switches weren’t used as either a default gateway or another destination. That’s because switches have nothing to do with routing.

I wonder how many of you chose the switch as the default gateway (destination) MAC address for HostA? If you did, don’t feel bad—just take another look with that fact in mind. It’s very important to remember that the destination MAC address will always be the router’s interface—if your packets are destined for outside the LAN, as they were in these last two examples.

Before we move into some of the more advanced aspects of IP routing, let’s discuss ICMP in more detail, as well as how ICMP is used in an internetwork. Take a look at the network shown in Figure 8-6 . Ask yourself what will happen if the LAN interface of Lab_C goes down.

Figure 8-6:

ICMP error example

Lab_C will use ICMP to inform Host A that Host B can’t be reached, and it will do this by sending an ICMP destination unreachable message.

The point of this figure is to help you visualize how ICMP data is routed via IP back to the originating station.

Let’s look at another problem: Look at the output of a corporate router’s routing table:

Corp#

sh ip route

[output cut]

R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0

R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0

R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0

C 192.168.20.0 is directly connected, Serial0/0

C 192.168.214.0 is directly connected, FastEthernet0/0

What do we see here? If I were to tell you that the corporate router received an IP packet with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3, what do you think the Corp router will do with this packet?

If you said, “The packet came in on the FastEthernet 0/0 interface, but since the routing table doesn’t show a route to network 192.168.22.0 (or a default route), the router will discard the packet and send an ICMP destination unreachable message back out to interface FastEthernet 0/0,” you’re a genius! The reason it does this is because that’s the source LAN where the packet originated from.

Now, let’s check out another figure and talk about the frames and packets in detail. Really, we’re not exactly chatting about anything new; I’m just making sure that you totally, completely, fully understand basic IP routing. That’s because this book, and the exam objectives it’s geared toward, are all about IP routing, which means you need to be all over this stuff! We’ll use Figure 8-7 for the next few questions.

Figure 8-7:

Basic IP routing using MAC and IP addresses

Referring to Figure 8-7 , here’s a list of all the questions you need the answers to emblazoned in your brain:

1.

In order to begin communicating with the Sales server, Host 4 sends out an ARP request. How will the devices exhibited in the topology respond to this request?

2.

Host 4 has received an ARP reply. Host 4 will now build a packet, then place this packet in the frame. What information will be placed in the header of the packet that leaves Host 4 if Host 4 is going to communicate to the Sales server?

3.

At last, the Lab_A router has received the packet and will send it out Fa0/0 onto the LAN toward the server. What will the frame have in the header as the source and destination addresses?

4.

Host 4 is displaying two web documents from the Sales server in two browser windows at the same time. How did the data find its way to the correct browser windows?

I probably should write the following in a teensy font and put them upside down in another part of the book so it would be really hard for you to cheat and peek, but since it’s actually you who’s going to lose out if you peek, here are your answers:

1.

In order to begin communicating with the server, Host 4 sends out an ARP request. How will the devices exhibited in the topology respond to this request? Since MAC addresses must stay on the local network, the Lab_B router will respond with the MAC address of the Fa0/0 interface and Host 4 will send all frames to the MAC address of the Lab_B Fa0/0 interface when sending packets to the Sales server.

2.

Host 4 has received an ARP reply. Host 4 will now build a packet, then place this packet in the frame. What information will be placed in the header of the packet that leaves Host 4 if Host 4 is going to communicate to the Sales server? Since we’re now talking about packets, not frames, the source address will be the IP address of Host 4 and the destination address will be the IP address of the Sales server.

3.

Finally, the Lab_A router has received the packet and will send it out Fa0/0 onto the LAN toward the server. What will the frame have in the header as the source and destination addresses? The source MAC address will be the Lab_A router’s Fa0/0 interface, and the destination MAC address will be the Sales server’s MAC address. (All MAC addresses must be local on the LAN.)

4.

Host 4 is displaying two web documents from the Sales server in two different browser windows at the same time. How did the data

find its way to the correct browser windows? TCP port numbers are used to direct the data to the correct application window.

Great! But we’re not quite done yet. I’ve got a few more questions for you before you actually get to configure routing in a real network. Ready?

Figure 8-8 shows a basic network, and Host 4 needs to get email. Which address will be placed in the destination address field of the frame when it leaves Host 4?

Figure 8-8:

Testing basic routing knowledge

The answer is that Host 4 will use the destination MAC address of the Fa0/0 interface of the Lab_B router—which I’m so sure you knew, right?

Look at Figure 8-8 again: Host 4 needs to communicate with Host 1. Which OSI layer 3 source address will be found in the packet header when it reaches Host 1?

Hopefully you know this: At layer 3, the source IP address will be Host 4 and the destination address in the packet will be the IP address of Host

1. Of course, the destination MAC address from Host 4 will always be the Fa0/0 address of the Lab_B router, right? And since we have more than one router, we’ll need a routing protocol that communicates between both of them so that traffic can be forwarded in the right direction to reach the network in which Host 1 is attached.

Okay—one more question and you’re on your way to being an IP routing genius! Again, using

Figure 8-8 ., Host 4 is transferring a file to the email server connected to the Lab_A router. What would be the layer 2 destination address leaving Host 4? Yes, I’ve asked this question more than once.

But not this one: What will be the source MAC address when the frame is received at the email server?

Hopefully, you answered that the layer 2 destination address leaving Host 4 will be the MAC address of the Fa0/0 interface of the Lab_B router and that the source layer 2 address that the email server will receive will be the Fa0/0 interface of the Lab_A router.

If you did, you’re all set to get the skinny on how IP routing is handled in a larger network.

Configuring IP Routing

It’s time to get serious and configure a real network!

Figure 8-9 shows four routers: Corp, Remote1, Remote2, and Remote3. Remember that, by default, these routers only know about networks that are directly connected to them. I’ll continue to use this figure and network throughout the rest of the chapters in this book.

Figure 8-9:

Configuring IP routing

As you might guess, I’ve got quite a nice collection of routers for us to play with. The Corp router is a 2811 with four serial interfaces and a switch module, and remote routers 1 and 2 are 1841 routers. Remote 3 is another 2811 with a wireless interface card. I’m simply going to call the remote routers R1, R2, and R3. (Understand that you can still perform most of the commands I use in this book with older routers or with a router simulator)

The first step for this project is to correctly configure each router with an IP address on each interface.

Table 8-1 shows the IP address scheme

I’m going to use to configure the network. After we go over how the network is configured, I’ll cover how to configure IP routing. Each network in the following table has a 24-bit subnet mask (255.255.255.0), which makes the interesting (subnet) octet the third one.

Table 8-1:

Network addressing for the IP network

The router configuration is really a pretty straightforward process since you just need to add IP addresses to your interfaces and then perform a no shutdown

on those same interfaces. It gets a tad more complex later on, but for right now, let’s configure the IP addresses in the network.

Corp Configuration

We need to configure five interfaces to configure the Corp router. And configuring the hostnames of each router will make identification much easier. While we’re at it, why not set the interface descriptions, banner, and router passwords too? It’s a really good idea to make a habit of configuring these commands on every router.

To get started, I performed an erase startup-config

on the router and reloaded, so we’ll start in setup mode. I choose no

to entering setup mode, which will get us straight to the username prompt of the console. I’m going to configure all my routers this same way.

I need to mention one small issue before I configure the Corp router and that is the switch card configuration. The IP address is configured on a logical interface on a switch, not a physical interface, and that interface by default is named vlan 1. Also, unlike with standalone switches, the interfaces on my switch card installed in the router are not enabled by default, so you’ll see that I enable the ports we are using in this lab.

Here’s how I did all that:

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]:

n

Press RETURN to get started!

Router>en

Router#

config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

hostname Corp

Corp(config)#

enable secret todd

Corp(config)#

interface vlan 1

Corp(config-if)#

description Switch Card to Core Network

Corp(config-if)#

ip address 10.1.1.1 255.255.255.0

Corp(config-if)#

no shutdown

Corp(config-if)#int f1/0

Corp(config-if)#description Switch Port connection to WWW Server

Corp(config-if)#no shutdown

Corp(config-if)#int f1/1

Corp(config-if)#description Switch port connection to Email Server

Corp(config-if)#no shut

Corp(config-if)#int f1/2

Corp(config-if)#description Switch port connection to DNS Server

Corp(config-if)#no shut

Corp(config-if)#

int s0/0/0

Corp(config-if)#

description 1st Connection to R1

Corp(config-if)#

ip address 10.1.2.1 255.255.255.0

Corp(config-if)#

no shut

Corp(config-if)#

int s0/0/1

Corp(config-if)#

description 2nd Connection to R1

Corp(config-if)#

ip address 10.1.3.1 255.255.255.0

Corp(config-if)#

no shut

Corp(config-if)#

int s0/1/0

Corp(config-if)#

description Connection to R2

Corp(config-if)#

ip address 10.1.4.1 255.255.255.0

Corp(config-if)#

no shut

Corp(config-if)#

int fa0/0

Corp(config-if)#

description Connection to R3

Corp(config-if)#

ip address 10.1.5.1 255.255.255.0

Corp(config-if)#

no shut

Corp(config-if)#

line con 0

Corp(config-line)#

password console

Corp(config-line)#

login

Corp(config-line)#

logging synchronous

Corp(config-line)#

exec-timeout 0 0

Corp(config-line)#

line aux 0

Corp(config-line)#

password aux

Corp(config-line)#

login

Corp(config-line)#

exit

Corp(config)#

line vty 0 ?

<1-15> Last Line number

<cr>

Corp(config)#

line vty 0 15

Corp(config-line)#

password telnet

Corp(config-line)#

login

Corp(config-line)#

exit

Corp(config)#

no ip domain lookup

Corp(config)#

banner motd # This is my Corp 2811 ISR Router #

Corp(config-if)#

^Z

Corp#

copy running-config startup-config

Destination filename [startup-config]?

[enter]

Building configuration...

[OK]

Corp#

If you have a hard time understanding this configuration process, refer back to Chapter 6, “Cisco’s Internetworking Operating System (IOS).”

To view the IP routing tables created on a Cisco router, use the command show ip route

. The command output is shown as follows:

Corp#

sh ip route

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS

level-2, ia - IS-IS inter area, * - candidate default, U - per-user

static route, o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets

C 10.1.1.0 is directly connected, Vlan1

Corp#

It’s important to remember that only configured, directly connected networks are going to show up in the routing table. So why is it that I only see the Vlan1 interface in the routing table? No worries—that’s just because you won’t see the serial interfaces come up until the other side of the links are operational. As soon as we configure our R1, R2, and R3 routers, all those interfaces should pop right up.

But did you notice the

C

on the left side of the output of the routing table? When you see that there, it means that the network is directly connected.

The codes for each type of connection are listed at the top of the show ip route

command, along with their descriptions.

In the interest of brevity, the codes will be cut in the rest of this chapter.

R1 Configuration

Now we’re ready to configure the next router—R1. To make that happen correctly, keep in mind that we have four interfaces to deal with: serial

0/0/0, serial 0/0/1, FastEthernet 0/0, and FastEthernet 0/1. So let’s make sure we don’t forget to add the hostname, passwords, interface descriptions, and banner to the router configuration. As I did with the Corp router, I erased the configuration and reloaded.

Here’s the configuration I used:

R1#

erase start

% Incomplete command.

R1#

erase startup-config

Erasing the nvram filesystem will remove all configuration files!

Continue? [confirm]

[enter]

[OK]

Erase of nvram: complete

R1#

reload

Proceed with reload? [confirm]

[enter]

[output cut]

%Error opening tftp://255.255.255.255/network-confg (Timed out)

%Error opening tftp://255.255.255.255/cisconet.cfg (Timed out)

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]:

n

Before we move on, I really want to discuss the preceding output with you. First, notice that the new 12.4 ISR routers will no longer take the command erase start

. The router has only one command after erase

that starts with

s

, as shown here:

Router#

erase s?

startup-config

I know, you’d think that the IOS would continue to accept the command, but nope—sorry! The second thing I want to point out is that the output tells us the router is looking for a TFTP host to see if it can download a configuration. When that fails, it goes straight into setup mode. This gives you a great picture of the Cisco router default boot sequence we talked about in Chapter 7.

Okay, let’s get back to configuring our router:

Press RETURN to get started!

Router>

en

Router#

config t

Router(config)#

hostname R1

R1(config)#

enable secret todd

R1(config)#

int s0/0/0

R1(config-if)#

ip address 10.1.2.2 255.255.255.0

R1(config-if)#

Description 1st Connection to Corp Router

R1(config-if)#

no shut

R1(config-if)#

int s0/0/1

R1(config-if)#

ip address 10.1.3.2 255.255.255.0

R1(config-if)#

no shut

R1(config-if)#

description 2nd connection to Corp Router

R1(config-if)#

int f0/0

R1(config-if)#

ip address 192.168.10.1 255.255.255.0

R1(config-if)#

description Connection to Finance PC

R1(config-if)#

no shut

R1(config-if)#

int f0/1

R1(config-if)#

ip address 192.168.20.1 255.255.255.0

R1(config-if)#

description Connection to Marketing PC

R1(config-if)#

no shut

R1(config-if)#

line con 0

R1(config-line)#

password console

R1(config-line)#

login

R1(config-line)#

logging synchronous

R1(config-line)#

exec-timeout 0 0

R1(config-line)#

line aux 0

R1(config-line)#

password aux

R1(config-line)#

login

R1(config-line)#

exit

R1(config)#

line vty 0 ?

<1-807> Last Line number

<cr>

R1(config)#

line vty 0 807

R1(config-line)#

password telnet

R1(config-line)#

login

R1(config-line)#

banner motd # This is my R1 Router #

R1(config)#

no ip domain-lookup

R1(config)#

exit

R1#

copy run start

Destination filename [startup-config]?

[enter]

Building configuration...

[OK]

R1#

Let’s take a look at our configuration of the interfaces:

R1#

sh run | begin interface

interface FastEthernet0/0

description Connection to Finance PC

ip address 192.168.10.1 255.255.255.0

duplex auto

speed auto

!

interface FastEthernet0/1

description Connection to Marketing PC

ip address 192.168.20.1 255.255.255.0

duplex auto

speed auto

!

interface Serial0/0/0

description 1st Connection to Corp Router

ip address 10.1.2.2 255.255.255.0

!

interface Serial0/0/1

description 2nd connection to Corp Router

ip address 10.1.3.2 255.255.255.0

!

The show ip route

command displays the following:

R1#

show ip route

10.0.0.0/24 is subnetted, 4 subnets

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.2.0 is directly connected, Serial0/0/0

C 192.168.20.0 is directly connected, FastEthernet0/1

C 192.168.10.0 is directly connected, FastEthernet0/0

R1#

Notice that router R1 knows how to get to networks 10.1.3.0, 10.1.2.0, 192.168.20.0, and 192.168.10.0. We can now ping to the Corp router from R1:

R1#

10.1.2.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.2.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

R1#

Now let’s go back to the Corp router and look at the routing table:

Corp#

sh ip route

[output cut]

10.0.0.0/24 is subnetted, 4 subnets

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.1.0 is directly connected, Vlan1

Corp#

The R1 serial interface 0/0/0 and 0/0/1 are DCE connections, which means a clock rate

needs to be set on the interface. Remember that you don’t need to use the clock rate

command in production. Even though this is very true, it’s still imperative that you know how/when you can use it and that you understand it really well when studying for your CCNA exam!

We can see our clocking with the show controllers

command:

R1#

sh controllers s0/0/1

Interface Serial0/0/1

Hardware is GT96K

DCE V.35, clock rate 2000000

One last thing before we get into configuring the other remote routers: Did you notice the clock rate is 2000000 under the serial interfaces of the

R1 router? That’s important because if you think back to when we were configuring the R1 router, you’ll recall that I didn’t set the clock rate. The reason I didn’t is because ISR routers will auto-detect a DCE-type cable and automatically configure the clock rate—a really sweet feature!

Since the serial links are showing up, we can now see three networks in the Corp routing table. And once we configure R2 and R3, we’ll see two more networks in the routing table of the Corp router. The Corp router can’t see either the 192.168.10.0 or 192.168.20.0 networks because we don’t have any routing configured yet—routers see only directly connected networks by default.

R2 Configuration

To configure R2, we’re going to do pretty much the same thing we did with the other two routers. There are three interfaces: serial 0/0/0,

FastEthernet 0/0, and FastEthernet 0/1 to deal with, and again, we’ll be sure to add the hostname, passwords, interface descriptions, and a banner to the router configuration:

Router>

en

Router#

config t

Router(config)#

hostname R2

R2(config)#

enable secret todd

R2(config)#

int s0/0/0

R2(config-if)#

ip address 10.1.4.2 255.255.255.0

R2(config-if)#

description Connection to Corp Router

R2(config-if)#

no shut

R2(config-if)#

int f0/0

R2(config-if)#

ip address 192.168.30.1 255.255.255.0

R2(config-if)#

description Connection to Sales PC

R2(config-if)#

no shut

R2(config-if)#

int f0/1

R2(config-if)#

ip address 192.168.40.1 255.255.255.0

R2(config-if)#

description Connection to HR PC

R2(config-if)#

no shut

R2(config-if)#

line con 0

R2(config-line)#

password console

R2(config-line)#

login

R2(config-line)#

logging sync

R2(config-line)#

exec-timeout 0 0

R2(config-line)#

line aux 0

R2(config-line)#

password aux

R2(config-line)#

login

R2(config-line)#

exit

R2(config)#

line vty 0 ?

<1-807> Last Line number

<cr>

R2(config)#

line vty 0 807

R2(config-line)#

password telnet

R2(config-line)#

login

R2(config-line)#

exit

R2(config)#

banner motd # This is my R2 Router #

R2(config)#

no ip domain-lookup

R2(config)#

^Z

R2#

copy run start

Destination filename [startup-config]?

[enter]

Building configuration...

[OK]

R2#

Nice—everything was pretty straightforward. The output of the following show ip route

command displays the directly connected networks of

192.168.30.0, and 192.168.40.0 and 10.1.4.0, as you can see here:

R2#

sh ip route

10.0.0.0/24 is subnetted, 3 subnets

C 192.168.30.0 is directly connected, FastEthernet0/0

C 192.168.40.0 is directly connected, FastEthernet0/1

C 10.1.4.0 is directly connected, Serial0/0/0

R2#

The Corp, R1, and R2 routers now have all their directly connected links up. But we still need to configure the R3 router.

R3 Configuration

To configure R3, we’re going to do pretty much the same thing we did with the other routers. However, there are only two interfaces: FastEthernet

0/0, and Dot11Radio0/0/0 to deal with, and again, we’ll be sure to add the hostname, passwords, interface descriptions, and a banner to the router configuration:

Router>

en

Router#

config t

Router(config)#

hostname R3

R3(config)#

enable secret todd

R3(config)#

int f0/0

R3(config-if)#

ip address 10.1.5.2 255.255.255.0

R3(config-if)#

description Connection to Corp Router

R3(config-if)#

no shut

R3(config-if)#

int dot11radio0/0/0

R3(config-if)#

ip address 172.16.10.1 255.255.255.0

R3(config-if)#

description WLAN for Mobile User

R3(config-if)#

no shut

R3(config-if)#

ssid ADMIN

R3(config-if-ssid)#

guest-mode

R3(config-if-ssid)#

authentication open

R3(config-if-ssid)#

infrastructure-ssid

R3(config-if-ssid)#

exit

R3(config-line)#

line con 0

R3(config-line)#

password console

R3(config-line)#

login

R3(config-line)#

logging sync

R3(config-line)#

exec-timeout 0 0

R3(config-line)#

line aux 0

R3(config-line)#

password aux

R3(config-line)#

login

R3(config-line)#

exit

R3(config)#

line vty 0 ?

<1-807> Last Line number

<cr>

R3(config)#

line vty 0 807

R3(config-line)#

password telnet

R3(config-line)#

login

R3(config-line)#

exit

R3(config)#

banner motd # This is my R3 Router #

R3(config)#

no ip domain-lookup

R3(config)#

^Z

R3#

copy run start

Destination filename [startup-config]?

[enter]

Building configuration...

[OK]

R3#

Nice—everything again was pretty straightforward…except for that wireless interface. It’s true, the wireless interface is really just another interface on a router, and it looks just like that in the routing table as well. But, in order to bring up the wireless interface, more configurations are needed than for a simple FastEthernet interface. So check out the following output, and then I’ll tell you about the special configuration needs for this wireless interface:

R3(config-if)#

int dot11radio0/0/0

R3(config-if)#

ip address 172.16.10.1 255.255.255.0

R3(config-if)#

description WLAN for Mobile User

R3(config-if)#

no shut

R3(config-if)#

ssid ADMIN

R3(config-if-ssid)#

guest-mode

R3(config-if-ssid)#

authentication open

R3(config-if-ssid)#

infrastructure-ssid

So, what we see here is that everything is pretty commonplace until we get to the SSID configuration. This is the Service Set Identifier that creates a wireless network that hosts can connect to. Unlike access points, the interface on the R3 router is actually a routed interface, which is the reason the IP address is placed under the physical interface—typically, if this was an access point only and not a router, the IP address would be placed under the Bridge-Group Virtual Interface (BVI), which is a logical management interface.

That guest-mode

line means that the interface will broadcast the SSID so wireless hosts will understand that they can connect to this interface.

Authentication open

means just that. . . no authentication. (Even so, you still have to type that command in at minimum to make the wireless interface work.) Last, the infrastructure-ssid

indicates that this interface can be used to communicate to other access points, or other devices on the infrastructure—to the actual wired network itself.

Configuring DHCP on Our Router

But wait, we’re not done yet—we still need to configure the DHCP pool for the wireless clients connecting to the Dot11Radio0/0/0 interface, so let’s do that now:

R3#

config t

R3(config)#

ip dhcp pool Admin

R3(dhcp-config)#

network 172.16.10.0 255.255.255.0

R3(dhcp-config)#

default-router 172.16.10.1

R3(dhcp-config)#

ip name-server 172.16.10.4

R3(dhcp-config)#

exit

R3(config)#

ip dhcp excluded-address 172.16.10.1 172.16.10.10

R3(config)#

Creating DHCP pools on a router is actually a pretty simple process, and this would be the same configuration for any router you need to add a

DHCP pool to. To create the DHCP server on a router, you just create the pool name, add the network/subnet and the default gateway, and exclude any addresses you don’t want handed out (like the default gateway address), and you’d usually add a DNS server as well. Don’t forget to add your exclusions, addresses you don’t want the DHCP server handing out as valid host IPs. These exclusions are configured from global config mode, not within the DHCP pool config. Notice, also, that you can exclude a range of addresses on one line—very convenient. In the preceding example, I excluded 172.16.10.1 through 172.16.10.10 from being assigned by the DHCP server as valid IP addresses to DHCP clients. You can verify the

DHCP pool with the show ip dhcp binding

command:

R3#sh ip dhcp binding

IP address Client-ID/ Lease expiration Type

Hardware address

172.16.10.11 0001.96AB.8538 -- Automatic

R3#

And of course, you can verify the client with the ipconfig

command.

PC>ipconfig /all

Physical Address................: 0001.96AB.8538

IP Address......................: 172.16.10.11

Subnet Mask.....................: 255.255.255.0

Default Gateway.................: 172.16.10.1

DNS Servers.....................: 172.16.10.2

Now that we did a basic WLAN configuration, our mobile user is connected to the wireless network. The user just can’t get anywhere else yet in our internetwork! Let’s fix that.

Wireless networks will be discussed in detail in Chapter 14, “Cisco’s Wireless Technologies.”

Configuring IP Routing in Our Network

Our network is good to go—right? After all, it’s been correctly configured with IP addressing, administrative functions, and even clocking

(automatically on the ISR routers). But how does a router send packets to remote networks when the only way it can send them is by looking at the routing table to find out how to get to the remote networks? Our configured routers only have information about directly connected networks in each routing table. And what happens when a router receives a packet for a network that isn’t listed in the routing table? It doesn’t send a broadcast looking for the remote network—the router just discards it. Period.

So we’re not exactly ready to rock after all. But no worries—there are several ways to configure the routing tables to include all the networks in our little internetwork so that packets will be forwarded. And what’s best for one network isn’t necessarily what’s best for another. Understanding the different types of routing will really help you come up with the best solution for your specific environment and business requirements.

You’ll learn about the following types of routing in the following sections:

Static routing

Default routing

Dynamic routing

I’m going to start off by describing and implementing static routing on our network because if you can implement static routing

and

make it work, it means you have a solid understanding of the internetwork. So let’s get started.

Static Routing

Static routing occurs when you manually add routes in each router’s routing table. There are pros and cons to static routing, but that’s true for all routing processes.

Static routing has the following benefits:

There is no overhead on the router CPU, which means you could possibly buy a cheaper router than you would use if you were using dynamic routing.

There is no bandwidth usage between routers, which means you could possibly save money on WAN links.

It adds security because the administrator can choose to allow routing access to certain networks only.

Static routing has the following disadvantages:

The administrator must really understand the internetwork and how each router is connected in order to configure routes correctly.

If a network is added to the internetwork, the administrator has to add a route to it on all routers—by hand.

It’s not feasible in large networks because maintaining it would be a full-time job in itself.

Okay—that said, here’s the command syntax you use to add a static route to a routing table: ip route [destination_network] [mask] [next-hop_address or

exitinterface] [administrative_distance] [permanent]

This list describes each command in the string: ip route

The command used to create the static route.

destination_network

The network you’re placing in the routing table.

mask

The subnet mask being used on the network.

next-hop_address

The address of the next-hop router that will receive the packet and forward it to the remote network. This is the IP address of a router interface that’s on a directly connected network. You must be able to ping the router interface before you can successfully add the route.

If you type in the wrong next-hop address or the interface to that router is down, the static route will show up in the router’s configuration but not in the routing table.

exitinterface

Used in place of the next-hop address if you want, and shows up as a directly connected route.

administrative_distance

By default, static routes have an administrative distance of 1 (or even 0 if you use an exit interface instead of a next-hop address). You can change the default value by adding an administrative weight at the end of the command. I’ll talk a lot more about this subject later in the chapter when we get to the section on dynamic routing.

permanent

If the interface is shut down or the router can’t communicate to the next-hop router, the route will automatically be discarded from the routing table by default. Choosing the permanent

option keeps the entry in the routing table no matter what happens.

Before we dive into configuring static routes, let’s take a look at a sample static route and see what we can find out about it.

Router(config)#

ip route 172.16.3.0 255.255.255.0 192.168.2.4

The ip route

command tells us simply that it is a static route.

172.16.3.0 is the remote network we want to send packets to.

255.255.255.0 is the mask of the remote network.

192.168.2.4 is the next hop, or router, we will send packets to.

However, suppose the static route looked like this:

Router(config)#

ip route 172.16.3.0 255.255.255.0 192.168.2.4 150

The 150 at the end changes the default administrative distance (AD) of 1 to 150. No worries—I’ll talk much more about AD when we get into dynamic routing. For now, just remember that the AD is the trustworthiness of a route, where 0 is best and 255 is worst.

One more example, then we’ll start configuring:

Router(config)#

ip route 172.16.3.0 255.255.255.0 s0/0/0

Instead of using a next-hop address, we can use an exit interface that will make the route show up as a directly connected network. Functionally, the next hop and exit interface work exactly the same.

To help you understand how static routes work, I’ll demonstrate the configuration on the internetwork shown previously in

Figure 8-9 . I have shown this internetwork again here, in Figure 8-10 , to save you from having to go back many pages to view the same figure when needed.

Figure 8-10:

Our internetwork

Corp

Each routing table automatically includes directly connected networks. To be able to route to all indirectly connected networks within the internetwork, the routing table must include information that describes where these other networks are located and how to get to them.

The Corp router is connected to five networks. For the Corp router to be able to route to all networks, the following networks have to be configured into its routing table:

192.168.10.0

192.168.20.0

192.168.30.0

192.168.40.0

172.16.10.0

The following router output shows the static routes on the Corp router and the routing table after the configuration. For the Corp router to find the remote networks, I had to place an entry into the routing table describing the remote network, the remote mask, and where to send the packets. I am going to add a 150 at the end of each line to raise the administrative distance. (When we get to dynamic routing, you’ll see why I did it this way.)

Corp(config)#

ip route 192.168.10.0 255.255.255.0 10.1.2.2 150

Corp(config)#

ip route 192.168.20.0 255.255.255.0 10.1.3.2 150

Corp(config)#

ip route 192.168.30.0 255.255.255.0 10.1.4.2 150

Corp(config)#

ip route 192.168.40.0 255.255.255.0 10.1.4.2 150

Corp(config)#

ip route 172.16.10.0 255.255.255.0 10.1.5.2 150

Corp(config)#

do show run | begin ip route ip route 192.168.10.0 255.255.255.0 10.1.2.2 150 ip route 192.168.20.0 255.255.255.0 10.1.3.2 150 ip route 192.168.30.0 255.255.255.0 10.1.4.2 150 ip route 192.168.40.0 255.255.255.0 10.1.4.2 150 ip route 172.16.10.0 255.255.255.0 10.1.5.2 150

For networks 192.168.10.0 and 192.168.20.0, I used a different path for each network, although I could have used just one. After the router is configured, you can type

show ip route

to see the static routes:

Corp(config)#

do show ip route

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.5.2

S 192.168.10.0/24 [150/0] via 10.1.2.2

S 192.168.20.0/24 [150/0] via 10.1.3.2

S 192.168.30.0/24 [150/0] via 10.1.4.2

S 192.168.40.0/24 [150/0] via 10.1.4.2

The Corp router is configured to route and know about all routes to all networks.

I want you to understand that if the routes don’t appear in the routing table, it’s because the router can’t communicate with the next-hop address you’ve configured. You can use the permanent

parameter to keep the route in the routing table even if the next-hop device can’t be contacted.

The

S

in the preceding routing table entries means that the route is a static entry. The

[150/0]

is the administrative distance and metric (something we’ll cover later) to the remote network.

Okay—we’re good. The Corp router now has all the information it needs to communicate with the other remote networks. But keep in mind that if the R1, R2, and R3 routers aren’t configured with all the same information, the packets will simply be discarded. We’ll need to fix this by configuring static routes.

Don’t stress about the 150 at the end of the static route configuration. I promise I will discuss the topic really soon in this chapter, not a later one! Be assured that you don’t need to worry about it at this point.

R1

The R1 router is directly connected to the networks 10.1.2.0, 10.1.3.0, 192.168.10.0, and 192.168.20.0, so we’ve got to configure the following static routes on the R1 router:

10.1.1.0

10.1.4.0

10.1.5.0

192.168.30.0

192.168.40.0

172.16.10.0

Here’s the configuration for the R1 router. Remember, we’ll never create a static route to any network we’re directly connected to, and we can use the next hop of either 10.1.2.1 or 10.1.3.1 since we have two links between the Corp and R1 router. I’ll change between next hops so all data doesn’t go down one link. It really doesn’t matter which link I use at this point. Let’s check out the commands:

R1(config)#

ip route 10.1.1.0 255.255.255.0 10.1.2.1 150

R1(config)#

ip route 10.1.4.0 255.255.255.0 10.1.3.1 150

R1(config)#

ip route 10.1.5.0 255.255.255.0 10.1.2.1 150

R1(config)#

ip route 192.168.30.0 255.255.255.0 10.1.3.1 150

R1(config)#

ip route 192.168.40.0 255.255.255.0 10.1.2.1 150

R1(config)#

ip route 172.16.10.0 255.255.255.0 10.1.3.1 150

R1(config)#

do show run | begin ip route

ip route 10.1.1.0 255.255.255.0 10.1.2.1 150 ip route 10.1.4.0 255.255.255.0 10.1.3.1 150 ip route 10.1.5.0 255.255.255.0 10.1.2.1 150 ip route 192.168.30.0 255.255.255.0 10.1.3.1 150 ip route 192.168.40.0 255.255.255.0 10.1.2.1 150 ip route 172.16.10.0 255.255.255.0 10.1.3.1 150

By looking at the routing table, you can see that the R1 router now understands how to find each network:

R1(config)#

do show ip route

10.0.0.0/24 is subnetted, 5 subnets

S 10.1.1.0 [150/0] via 10.1.2.1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

S 10.1.4.0 [150/0] via 10.1.3.1

S 10.1.5.0 [150/0] via 10.1.2.1

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.3.1

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

S 192.168.30.0/24 [150/0] via 10.1.3.1

S 192.168.40.0/24 [150/0] via 10.1.2.1

The R1 router now has a complete routing table. As soon as the other routers in the internetwork have all the networks in their routing table, R1 will be able to communicate with all remote networks.

R2

The R2 router is directly connected to three networks 10.1.4.0, 192.168.30.0, and 192.168.40.0, so these are the routes that need to be added:

10.1.1.0

10.1.2.0

10.1.3.0

10.1.5.0

192.168.10.0

192.168.20.0

172.16.10.0

Here’s the configuration for the R2 router:

R2(config)#

ip route 10.1.1.0 255.255.255.0 10.1.4.1 150

R2(config)#

ip route 10.1.2.0 255.255.255.0 10.1.4.1 150

R2(config)#

ip route 10.1.3.0 255.255.255.0 10.1.4.1 150

R2(config)#

ip route 10.1.5.0 255.255.255.0 10.1.4.1 150

R2(config)#

ip route 192.168.10.0 255.255.255.0 10.1.4.1 150

R2(config)#

ip route 192.168.20.0 255.255.255.0 10.1.4.1 150

R2(config)#

ip route 172.16.10.0 255.255.255.0 10.1.4.1 150

R2(config)#

do show run | begin ip route

ip route 10.1.1.0 255.255.255.0 10.1.4.1 150 ip route 10.1.2.0 255.255.255.0 10.1.4.1 150 ip route 10.1.3.0 255.255.255.0 10.1.4.1 150 ip route 10.1.5.0 255.255.255.0 10.1.4.1 150 ip route 192.168.10.0 255.255.255.0 10.1.4.1 150 ip route 192.168.20.0 255.255.255.0 10.1.4.1 150 ip route 172.16.10.0 255.255.255.0 10.1.4.1 150

The following output shows the routing table on the R2 router:

R2(config)#

do show ip route

10.0.0.0/24 is subnetted, 5 subnets

S 10.1.1.0 [150/0] via 10.1.4.1

S 10.1.2.0 [150/0] via 10.1.4.1

S 10.1.3.0 [150/0] via 10.1.4.1

C 10.1.4.0 is directly connected, Serial0/0/0

S 10.1.5.0 [150/0] via 10.1.4.1

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.4.1

S 192.168.10.0/24 [150/0] via 10.1.4.1

S 192.168.20.0/24 [150/0] via 10.1.4.1

C 192.168.30.0/24 is directly connected, FastEthernet0/0

C 192.168.40.0/24 is directly connected, FastEthernet0/1

R2 now shows all 10 networks in the internetwork, so it too can now communicate with all routers and networks (that are configured so far).

R3

The R3 router is directly connected to networks 10.1.5.0 and 172.16.10.0, but we need to add all these routes, eight in total:

10.1.1.0

10.1.2.0

10.1.3.0

10.1.4.0

192.168.10.0

192.168.20.0

192.168.30.0

192.168.40.0

Here’s the configuration for the R3 router; however, I am going to use the exit interface instead of the next hop address for this router:

R3#

show run | begin ip route

R3(config)#

ip route 10.1.1.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 10.1.2.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 10.1.3.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 10.1.4.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 192.168.10.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 192.168.20.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 192.168.30.0 255.255.255.0 fastethernet 0/0 150

R3(config)#

ip route 192.168.40.0 255.255.255.0 fastethernet 0/0 150

R3#

show ip route

10.0.0.0/24 is subnetted, 5 subnets

S 10.1.1.0 is directly connected, FastEthernet0/0

S 10.1.2.0 is directly connected, FastEthernet0/0

S 10.1.3.0 is directly connected, FastEthernet0/0

S 10.1.4.0 is directly connected, FastEthernet0/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

C 172.16.10.0 is directly connected, Dot11Radio0/0/0

S 192.168.10.0/24 is directly connected, FastEthernet0/0

S 192.168.20.0/24 is directly connected, FastEthernet0/0

S 192.168.30.0/24 is directly connected, FastEthernet0/0

S 192.168.40.0/24 is directly connected, FastEthernet0/0

R3#

Looking at the show ip route

command output, you can see that the static routes are listed as directly connected. Strange? Not really, because I used the exit interface instead of the next-hop address, and functionally, there’s no difference, only how they display in the routing table. However, now that I showed you what using an exit interface displays in the routing table instead of using a next hop with static routing, let me show you an easier way for the R3 router.

Default Routing

For the R2 and R3 routers that I have connected to the Corp router, they are considered stub routers. A stub indicates that the networks in this design have only one way out to reach all other networks. I’ll show you the configuration, verify the network in the next section, and then I’ll discuss default routing in detail. Here’s the configuration I could have done on the R3 router instead of typing in eight static routes due to its stub status:

R3(config)#

no ip route 10.1.1.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 10.1.2.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 10.1.3.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 10.1.4.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 192.168.10.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 192.168.20.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 192.168.30.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

no ip route 192.168.40.0 255.255.255.0 FastEthernet0/0 150

R3(config)#

ip route 0.0.0.0 0.0.0.0 10.1.5.1

R3(config)#

ip classless

R3(config)#

do show ip route

10.0.0.0/24 is subnetted, 1 subnets

C 10.1.5.0 is directly connected, Vlan1

172.16.0.0/24 is subnetted, 1 subnets

C 172.16.10.0 is directly connected, Dot11Radio0

S* 0.0.0.0/0 [1/0] via 10.1.5.1

Okay—once I removed all the initial static routes I configured, this seems a lot easier than typing eight static routes, doesn’t it? And it is, but there’s a catch—you can’t do things like this on all routers, only on stub routers. I could’ve used default routing on the R2 as well since that router is considered a stub, and I didn’t add the 150 to this default route even though I easily could have. I didn’t do that because it’s just really simple to just remove the route if we need to when we get to dynamic routing later.

So we’re there—we’ve done it! All the routers have the correct routing table, so all routers and hosts should be able to communicate without a hitch—for now. But if you add even one more network or another router to the internetwork, you’ll have to update each and every router’s routing tables by hand—yikes! This isn’t a problem at all if you’ve got a small network, but it’s obviously extremely time-consuming if you’re dealing with a large internetwork!

Verifying Your Configuration

We’re not done yet—once all the routers’ routing tables are configured, they need to be verified. The best way to do this, besides using the show ip route

command, is with the Ping program. I’ll start by pinging from the R3 router to the R1 router.

Here’s the output:

R3#

ping 10.1.2.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.2.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

From router R3, a ping to the Corp backbone, the servers WWW, Email, and DNS would be a good test as well. Here’s the router output:

R3#

ping 10.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/5 ms

R3#

ping 10.1.1.2

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.1.2, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/7/10 ms

R3# ping 10.1.1.3

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.1.3, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 5/7/10 ms

R3#

ping 10.1.1.4

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 10.1.1.4, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 3/5/10 ms

Also, we can trace from the Mobile User wireless host to the Finance host connected to the R2 router to see the hops the packet takes to get to the Finance host, but first we have to make sure the Mobile User host received a DHCP server address from the R3 router:

PC>

ipconfig

IP Address......................: 172.16.10.2

Subnet Mask.....................: 255.255.255.0

Default Gateway.................: 172.16.10.1

PC>

ping 192.168.10.2

Pinging 192.168.10.2 with 32 bytes of data:

Reply from 192.168.10.2: bytes=32 time=17ms TTL=125

Reply from 192.168.10.2: bytes=32 time=21ms TTL=125

Reply from 192.168.10.2: bytes=32 time=19ms TTL=125

Reply from 192.168.10.2: bytes=32 time=17ms TTL=125

Ping statistics for 192.168.10.2:

Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),

Approximate round trip times in milli-seconds:

Minimum = 17ms, Maximum = 21ms, Average = 18ms

PC>

tracert 192.168.10.2

Tracing route to 192.168.10.2 over a maximum of 30 hops:

1 15 ms 11 ms 14 ms 172.16.10.1

2 13 ms 13 ms 8 ms 10.1.5.1

3 12 ms 14 ms 15 ms 10.1.2.2

4 16 ms 14 ms 15 ms 192.168.10.2

Trace complete.

Notice I used a “tracert” command because I am on a Windows host. Remember, tracert

is not a valid Cisco command; we must use the command traceroute

from a router prompt.

Okay, since we can communicate from end to end and to each host without a problem, our static and default route configurations have been a success!

Default Routing

We use

default routing

to send packets with a remote destination network not in the routing table to the next-hop router. You should only use default routing on stub networks—those with only one exit path out of the network, although there are exceptions to this statement, and default routing is configured on a case-by-case basis when a network is designed. This is a rule of thumb to keep in mind.

If you tried to put a default route on a router that isn’t a stub, it is possible that packets wouldn’t be forwarded to the correct networks because they have more than one interface routing to other routers. You can easily create loops with default routing, so be careful!

To configure a default route, you use wildcards in both the network address and mask locations of a static route (as I demonstrated in the R3 configuration). In fact, you can just think of a default route as a static route that uses wildcards instead of network and mask information.

By using a default route, you can just create one static route entry instead. This sure is easier then typing in all those routes!

R3(config)#

ip route 0.0.0.0 0.0.0.0 10.1.5.1

R3(config)#

ip classless

R3(config)#

do show ip route

Gateway of last resort is 10.1.5.1 to network 0.0.0.0

10.0.0.0/24 is subnetted, 1 subnets

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

C 172.16.10.0 is directly connected, Dot11Radio0/0/0

S* 0.0.0.0/0 [1/0] via 10.1.5.1

If you look at the routing table, you’ll see only the two directly connected networks plus an

S*

, which indicates that this entry is a candidate for a default route. So instead of configuring eight static routes on R3, I could also have completed the default route command another way:

R3(config)#

ip route 0.0.0.0 0.0.0.0 Fa0/0

What this is telling us is that if you don’t have an entry for a network in the routing table, just forward it out Fa0/0. You can choose the IP address of the next-hop router or the exit interface—either way, it will work the same. Remember, I used the exit interface configuration with the R3 static route configs, which showed as directly connected in the router table. However, when I configured the default route on R3, I used the next-hop functionally; there is no difference.

Notice also on the first line in the routing table that the gateway of last resort is now set. Even so, there’s one more command you must be aware of when using default routes: the ip classless

command.

All Cisco routers are classful routers, meaning they expect a default subnet mask on each interface of the router. When a router receives a

packet for a destination subnet that’s not in the routing table, it will drop the packet by default. If you’re using default routing, you must use the ip classless

command because it is possible that no remote subnets will be in the routing table. Why? Because a configured default route will be ignored for subnets that are members of the same classful network as the other routes in the router’s routing table and this command basically says, “Hey, IP! Before you discard that packet, check to see if a gateway of last resort is set!”

Since I have version 12.4 of the IOS on my routers, the ip classless

command is on by default and it doesn’t even show in the configuration. If you’re using default routing and this command isn’t in your configuration, you will need to add it if you have subnetted networks on your routers. The command is shown here:

R3(config)#ip classless

There’s another command you can use to help you in your internetwork if you have configured a gateway of last resort—the ip default-network command, and I’ll use this in a configuration example at the end of the chapter. Figure 8-11 shows a network that needs to have a gateway of last resort statement configured.

Here are three commands (all providing a default route solution) for adding a gateway of last resort on the router to the ISP.

Gateway(config)#

ip route 0.0.0.0 0.0.0.0 217.124.6.1

Gateway(config)#

ip route 0.0.0.0 0.0.0.0 s0/0

Gateway(config)#

ip default-network

network

Figure 8-11:

Configuring a gateway of last resort

The first two are the same command—one just uses the next hop and one the exit interface. You will find no difference in this configuration, as I’ve already discussed. However, if you set them both for some reason, the exit interface would be used. Do you know why? Directly connected routes have an administrative distance of 0, but in this example, you’d see absolutely no functional difference between the two commands.

The ip default-network command would advertise the default network you configured on your border router when you configure an Interior Gateway

Protocol (IGP), like RIP (like RIP) on the router. This is so other routers in your internetwork will receive this route as a default route automatically.

Again, I’ll configure this in our network at the end of the chapter, so now you have something pretty exciting to look forward to!

But what happens if you misconfigured a default route? Let’s take a look at the output of a show ip route

command and compare that to the network in Figure 8-12 and see if you can find a problem:

Router#

sh ip route

[output cut]

Gateway of last resort is 172.19.22.2 to network 0.0.0.0

C 172.17.22.0 is directly connected, FastEthernet0/0

C 172.18.22.0 is directly connected, Serial0/0

S* 0.0.0.0/0 [1/0] via 172.19.22.2

Find anything? You can see by looking at the figure and the directly connected routes in the routing table that the WAN link is on network

172.18.22.0 and that the default route is forwarding all packets to the 172.19.22.0 network. This is just bad—it will never work, so the problem is a misconfigured static (default) route.

Figure 8-12:

Misconfigured default route

One last thing before moving on to dynamic routing: if you have the routing table output as shown in the following lines, what happens if the router receives a packet from 10.1.6.100 destined for host 10.1.8.5?

Router#

sh ip route

[output cut]

Gateway of last resort is 10.1.5.5 to network 0.0.0.0

R 10.1.3.0 [120/1] via 101.2.2, 00:00:00, Serial 0/0

C 10.1.2.0 is directly connected, Serial0/0

C 10.1.5.0 is directly connected, Serial0/1

C 10.1.6.0 is directly connected, Fastethernet0/0

R* 0.0.0.0/0 [120/0] via 10.1.5.5, 00:00:00 Serial 0/1

This is a tad different than what I’ve shown you up until now because the default route is listed as

R*

, which means it’s a RIP-injected route. This is because someone configured the ip default-network

command on a remote router as well as configuring RIP, causing RIP to advertise this route through the internetwork as a default route. Since the destination address is 10.1.8.5 and there is no route to network 10.1.8.0, the router would use the default route and send the packet out serial 0/1.

Dynamic Routing

Dynamic routing is when protocols are used to find networks and update routing tables on routers. True, this is easier than using static or default routing, but it’ll cost you in terms of router CPU processing and bandwidth on the network links. A routing protocol defines the set of rules used by a router when it communicates routing information between neighboring routers.

The routing protocol I’m going to talk about in this chapter is Routing Information Protocol (RIP) versions 1 and 2.

Two types of routing protocols are used in internetworks: interior gateway protocols (IGPs) and exterior gateway protocols (EGPs). IGPs are used to exchange routing information with routers in the same autonomous system (AS). An AS is a collection of networks under a common administrative domain, which basically means that all routers sharing the same routing table information are in the same AS. EGPs are used to communicate between ASes. An example of an EGP is Border Gateway Protocol (BGP), which is beyond the scope of this book.

Since routing protocols are so essential to dynamic routing, I’m going to give you the basic information you need to know about them next. Later on in this chapter, we’ll focus on configuration.

Routing Protocol Basics

There are some important things you should know about routing protocols before getting deeper into RIP. Specifically, you need to understand administrative distances, the three different kinds of routing protocols, and routing loops. We will look at each of these in more detail in the following sections.

Administrative Distances

The

administrative distance (AD)

is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.

If both advertised routes to the same network have the same AD, then routing protocol metrics (such as

hop count

or bandwidth of the lines) will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network (which means that it sends packets down each link).

Table 8-2 shows the default administrative distances that a Cisco router uses to decide which route to take to a remote network.

Table 8-2:

Default administrative distances

Route Source Default AD

Connected interface 0

Static route 1

EIGRP

IGRP

90

100

OSPF

RIP

External EIGRP

Unknown

110

120

170

255 (This route will never be used.)

If a network is directly connected, the router will always use the interface connected to the network. If you configure a static route, the router will then believe that route over any other learned routes. You can change the administrative distance of static routes, but by default, they have an AD of

1. In our previous static route configuration, the AD of each route is set at 150. This lets us configure routing protocols without having to remove the static routes. They’ll be used as backup routes in case the routing protocol experiences a failure of some type.

For example, if you have a static route, a RIP-advertised route, and an EIGRP-advertised route listing the same network, then by default, the router will always use the static route unless you change the AD of the static route—which we did.

Routing Protocols

There are three classes of routing protocols:

Distance vector

The

distance-vector protocols

in use today find the best path to a remote network by judging distance. For example, in the case of RIP routing, each time a packet goes through a router, that’s called a

hop

. The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and IGRP are distance-vector routing protocols. They periodically send the entire routing table to directly connected neighbors.

Link state

In

link-state protocols

, also called

shortest-path-first protocols

, the routers each create three separate tables. One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table. Linkstate routers know more about the internetwork than any distance-vector routing protocol. OSPF is an IP routing protocol that is completely link state. Link-state protocols send updates containing the state of their own links to all other directly connected routers on the network, which is then propagated to their neighbors.

Hybrid Hybrid protocols

use aspects of both distance vector and link state—for example, EIGRP.

There’s no set way of configuring routing protocols for use with every business. This is something you really have to do on a case-by-case basis.

If you understand how the different routing protocols work, you can make good, solid decisions that truly meet the individual needs of any business.

Distance-Vector Routing Protocols

The distance-vector routing algorithm passes complete routing table contents to neighboring routers, which then combine the received routing table entries with their own routing tables to complete the router’s routing table. This is called routing by rumor because a router receiving an update from a neighbor router believes the information about remote networks without actually finding out for itself.

It’s possible to have a network that has multiple links to the same remote network, and if that’s the case, the administrative distance of each received update is checked first. If the AD is the same, the protocol will have to use metrics to determine the best path to use to that remote network.

RIP uses only hop count to determine the best path to a network. If RIP finds more than one link with the same hop count to the same remote network, it will automatically perform a round-robin load balancing. RIP can perform load balancing for up to six equal-cost links (four by default).

However, a problem with this type of routing metric arises when the two links to a remote network are different bandwidths but the same hop count. Figure 8-13 , for example, shows two links to remote network 172.16.10.0.

Figure 8-13:

Pinhole congestion

Since network 172.16.30.0 is a T1 link with a bandwidth of 1.544Mbps and network 172.16.20.0 is a 56K link, you’d want the router to choose the T1 over the 56K link, right? But because hop count is the only metric used with RIP routing, the two links would be seen as being of equal cost.

This little snag is called

pinhole congestion

.

It’s important to understand what a distance-vector routing protocol does when it starts up. In

Figure 8-14 , the four routers start off with only their directly connected networks in their routing tables. After a distance-vector routing protocol is started on each router, the routing tables are updated with all route information gathered from neighbor routers.

As shown in

Figure 8-14 , each router has only the directly connected networks in each routing table. Each router sends its complete routing table out to each active interface. The routing table of each router includes the network number, exit interface, and hop count to the network.

In

Figure 8-15 , the routing tables are complete because they include information about all the networks in the internetwork. They are considered

converged

. When the routers are converging, it is possible that no data will be passed. That’s why fast convergence time is a serious plus. In fact, that’s one of the problems with RIP—its slow convergence time.

The routing table in each router keeps information regarding the remote network number, the interface to which the router will send packets to reach that network, and the hop count or metric to the network.

Figure 8-14:

The internetwork with distance-vector routing

Figure 8-15:

Converged routing tables

Routing Loops

Distance-vector routing protocols keep track of any changes to the internetwork by broadcasting periodic routing updates out all active interfaces.

This broadcast includes the complete routing table. This works just fine, but it’s expensive in terms of CPU processing and link bandwidth. And if a network outage happens, real problems can occur. Plus, the slow convergence of distance-vector routing protocols can result in inconsistent routing tables and routing loops.

Routing loops can occur because every router isn’t updated simultaneously, or even close to it. Here’s an example—let’s say that the interface to

Network 5 in Figure 8-16 fails. All routers know about Network 5 from RouterE. RouterA, in its tables, has a path to Network 5 through RouterB.

Figure 8-16:

Routing loop example

When Network 5 fails, RouterE tells RouterC. This causes RouterC to stop routing to Network 5 through RouterE. But routers A, B, and D don’t know about Network 5 yet, so they keep sending out update information. RouterC will eventually send out its update and cause B to stop routing to

Network 5, but routers A and D are still not updated. To them, it appears that Network 5 is still available through RouterB with a metric of 3.

The problem occurs when RouterA sends out its regular 30-second “Hello, I’m still here—these are the links I know about” message, which includes the ability to reach Network 5, and now routers B and D receive the wonderful news that Network 5 can be reached from RouterA, so routers B and D then send out the information that Network 5 is available. Any packet destined for Network 5 will go to RouterA, to RouterB, and then back to RouterA. This is a routing loop—how do you stop it?

Maximum Hop Count

The routing loop problem just described can create an issue called

counting to infinity

, and it’s caused by gossip (broadcasts) and wrong information being communicated and propagated throughout the internetwork. Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router.

One way of solving this problem is to define a

maximum hop count

. RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable. In other words, after a loop of 15 hops, Network 5 will be considered down. Thus, the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable.

Split Horizon

Another solution to the routing loop problem is called

split horizon

. This reduces incorrect routing information and routing overhead in a distancevector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received.

In other words, the routing protocol differentiates which interface a network route was learned on, and once this is determined, it won’t advertise the route back out that same interface. This would have prevented RouterA from sending the update information it received from RouterB back to

RouterB.

Route Poisoning

Another way to avoid problems caused by inconsistent updates and stop network loops is

route poisoning

. For example, when Network 5 goes down, RouterE initiates route poisoning by advertising Network 5 with a hop count of 16, or unreachable (sometimes referred to as

infinite

).

This poisoning of the route to Network 5 keeps RouterC from being susceptible to incorrect updates about the route to Network 5. When RouterC receives a route poisoning from RouterE, it sends an update, called a

poison reverse

, back to RouterE. This ensures that all routers on the segment have received the poisoned route information.

Holddowns

A

holddown

prevents regular update messages from reinstating a route that is going up and down (called

flapping

). Typically, this happens on a serial link that’s losing connectivity and then coming back up. If there wasn’t a way to stabilize this, the network would never converge and that one flapping interface could bring the entire network down!

Holddowns prevent routes from changing too rapidly by allowing time for either the downed route to come back up or the network to stabilize somewhat before changing to the next best route. These also tell routers to restrict, for a specific time period, changes that might affect recently removed routes. This prevents inoperative routes from being prematurely restored to other routers’ tables.

Routing Information Protocol (RIP)

Routing Information Protocol (RIP) is a true distance-vector routing protocol. RIP sends the complete routing table out to all active interfaces every

30 seconds. RIP only uses hop count to determine the best way to a remote network, but it has a maximum allowable hop count of 15 by default, meaning that 16 is deemed unreachable. RIP works well in small networks, but it’s inefficient on large networks with slow WAN links or on networks with a large number of routers installed.

RIP version 1 uses only

classful routing

, which means that all devices in the network must use the same subnet mask. This is because RIP version 1 doesn’t send updates with subnet mask information in tow. RIP version 2 provides something called

prefix routing

and does send subnet mask information with the route updates. This is called

classless routing

.

In the following sections, we will discuss the RIP timers and then RIP configuration.

RIP Timers

RIP uses four different kinds of timers to regulate its performance:

Route update timer

Sets the interval (typically 30 seconds) between periodic routing updates in which the router sends a complete copy of its routing table out to all neighbors.

Route invalid timer

Determines the length of time that must elapse (180 seconds) before a router determines that a route has become invalid. It will come to this conclusion if it hasn’t heard any updates about a particular route for that period. When that happens, the router will send out updates to all its neighbors letting them know that the route is invalid.

Holddown timer

This sets the amount of time during which routing information is suppressed. Routes will enter into the holddown state when an update packet is received that indicates the route is unreachable. This continues either until an update packet is received with a better metric, the original route comes back up, or the holddown timer expires. The default is 180 seconds.

Route flush timer

Sets the time between a route becoming invalid and its removal from the routing table (240 seconds). Before it’s removed from the table, the router notifies its neighbors of that route’s impending demise. The value of the route invalid timer must be less than that of the route flush timer. This gives the router enough time to tell its neighbors about the invalid route before the local routing table is updated.

Configuring RIP Routing

To configure RIP routing, just turn on the protocol with the router rip

command and tell the RIP routing protocol which networks to advertise. That’s it.

Let’s configure our four-router internetwork ( Figure 8-10 ) with RIP routing.

Corp

RIP has an administrative distance of 120. Static routes have an administrative distance of 1 by default, and since we currently have static routes configured, the routing tables won’t be populated with RIP information. However, because I added the 150 to the end of each static route, we’re good to go.

You can add the RIP routing protocol by using the router rip

command and the network

command. The network

command tells the routing protocol which classful network to advertise. By doing this process, you activate the RIP routing process on the interfaces whose addressing falls within the specified classful networks configured with the network command under the RIP routing process.

Look at the Corp router configuration and see how easy this is:

Corp#

config t

Corp(config)#

router rip

Corp(config-router)#

network 10.0.0.0

That’s it. Typically just two or three commands and you’re done—sure makes your job a lot easier than when using static routes, doesn’t it?

However, keep in mind the extra router CPU process and bandwidth that you’re consuming.

Notice I didn’t type in subnets, only the classful network address (all subnet bits and host bits off!). It is the job of the routing protocol to find the subnets and populate the routing tables. Since we have no router buddies running RIP, we won’t see any RIP routes in the routing table yet.

R1

Remember that RIP uses the classful address when configuring the network address. Because of this, all subnet masks of any particular classful network must be the same on all devices in the network (this is called classful routing). To clarify this, let’s say you’re using a Class B network address of 172.16.0.0/24 with subnets 172.16.10.0,

172.16.20.0, and 172.16.30.0. You would only type in the classful network address of 172.16.0.0 and let RIP find the subnets and place them in the routing table.

Let’s configure our R1 router, which is connected to three networks, and we need to configure all directly connected classful network (not subnets) :

R1#

config t

R1(config)#

router rip

R1(config-router)#

network 10.0.0.0

R1(config-router)#

network 192.168.10.0

R1(config-router)#

network 192.168.20.0

R1(config-router)#

do show ip route

10.0.0.0/24 is subnetted, 5 subnets

R 10.1.1.0 [120/1] via 10.1.2.1, 00:00:15, Serial0/0/0

[120/1] via 10.1.3.1, 00:00:15, Serial0/0/1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

R 10.1.4.0 [120/1] via 10.1.2.1, 00:00:15, Serial0/0/0

[120/1] via 10.1.3.1, 00:00:15, Serial0/0/1

R 10.1.5.0 [120/1] via 10.1.2.1, 00:00:15, Serial0/0/0

[120/1] via 10.1.3.1, 00:00:15, Serial0/0/1

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.3.1

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

S 192.168.30.0/24 [150/0] via 10.1.3.1

S 192.168.40.0/24 [150/0] via 10.1.2.1

R1(config-router)#

That was pretty straightforward. Let’s talk about this routing table. Since we have one RIP buddy out there that we are exchanging routing tables with, we can see the RIP networks coming from the Corp router. (All the other routes still show up as static.) RIP also found both connections to the

Corp router and will load-balance between them for each network that is advertised as a RIP injected route since the hop count is being advertised as 1 to each network. Luckily for us they are all the same bandwidth or we’d have pinhole congestion!

R2

Let’s configure our R2 router with RIP:

R2#

config t

R2(config)#

router rip

R2(config-router)#

network 10.0.0.0

R2(config-router)#

network 192.168.30.0

R2(config-router)#

network 192.168.40.0

R2(config-router)#

do show ip route

10.0.0.0/24 is subnetted, 5 subnets

R 10.1.1.0 [120/1] via 10.1.4.1, 00:00:17, Serial0/0/0

R 10.1.2.0 [120/1] via 10.1.4.1, 00:00:17, Serial0/0/0

R 10.1.3.0 [120/1] via 10.1.4.1, 00:00:17, Serial0/0/0

C 10.1.4.0 is directly connected, Serial0/0/0

R 10.1.5.0 [120/1] via 10.1.4.1, 00:00:17, Serial0/0/0

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.4.1

R 192.168.10.0/24 [120/2] via 10.1.4.1, 00:00:17, Serial0/0/0

R 192.168.20.0/24 [120/2] via 10.1.4.1, 00:00:17, Serial0/0/0

C 192.168.30.0/24 is directly connected, FastEthernet0/0

C 192.168.40.0/24 is directly connected, FastEthernet0/1

R2(config-router)#

The routing table is growing

R s as we add RIP buddies! We can still see that all routes are in the routing table; only one is still a static route—just one more router to go.

R3

Let’s configure our R3 router with RIP—here is the last router’s RIP configuration:

R3#

config t

R3(config)#

router rip

R3(config-router)#

network 10.0.0.0

R3(config-router)#

network 172.16.0.0

R3(config-router)#

do sh ip route

10.0.0.0/24 is subnetted, 5 subnets

R 10.1.1.0 [120/1] via 10.1.5.1, 00:00:15, FastEthernet0/0

R 10.1.2.0 [120/1] via 10.1.5.1, 00:00:15, FastEthernet0/0

R 10.1.3.0 [120/1] via 10.1.5.1, 00:00:15, FastEthernet0/0

R 10.1.4.0 [120/1] via 10.1.5.1, 00:00:15, FastEthernet0/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

C 172.16.10.0 is directly connected, Dot11Radio0/0/0

R 192.168.10.0/24 [120/2] via 10.1.5.1, 00:00:15, FastEthernet0/0

R 192.168.20.0/24 [120/2] via 10.1.5.1, 00:00:15, FastEthernet0/0

R 192.168.30.0/24 [120/2] via 10.1.5.1, 00:00:15, FastEthernet0/0

R 192.168.40.0/24 [120/2] via 10.1.5.1, 00:00:15, FastEthernet0/0

R3#

Finally, all routes showing in the routing table are RIP injected routes. Notice that since we are configuring classful network statements that the

WLAN network is 172.16.0.0, not 172.16.10.0!

It’s also to important to remember administrative distances and why we needed to either remove the static routes before we added RIP routing or set them higher than 120 as we did.

By default, directly connected routes have an administrative distance of 0, static routes have an administrative distance of 1, and RIP has an administrative distance of 120. I call RIP the “gossip protocol” because it reminds me of junior high school, where if you hear a rumor (advertised route), it just has to be true without exception. And that pretty much sums up how RIP behaves on an internetwork—rumor mill as protocol!

Verifying the RIP Routing Tables

Each routing table should now have all directly connected routes as well as RIP-injected routes received from neighboring routers. Now we can go back to the Corp router and check it out.

This output shows us the contents of the Corp routing table:

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

R 172.16.0.0/16 [120/1] via 10.1.5.2, 00:00:19, FastEthernet0/0

S 172.16.10.0/24 [150/0] via 10.1.5.2

R 192.168.10.0/24 [120/1] via 10.1.2.2, 00:00:19, Serial0/0/0

[120/1] via 10.1.3.2, 00:00:19, Serial0/0/1

R 192.168.20.0/24 [120/1] via 10.1.2.2, 00:00:19, Serial0/0/0

[120/1] via 10.1.3.2, 00:00:19, Serial0/0/1

R 192.168.30.0/24 [120/1] via 10.1.4.2, 00:00:19, Serial0/1/0

R 192.168.40.0/24 [120/1] via 10.1.4.2, 00:00:19, Serial0/1/0

Corp#

This output shows us basically the same routing table has the same entries that it had when we were using static routes—except for that

R

. The

R means that the networks were added dynamically using the RIP routing protocol. The

[120/1]

is the administrative distance of the route (120) along with the number of hops to that remote network (1). From the Corp router, all networks are one hop away. There is one odd entry in this table, and you may have noticed this: The 172.16.10.0 network is listed twice, once as a /16 and once as a /24. One route is listed as a static route and one is listed as a RIP injected route. This route should not be in the table twice, especially since the static route even has

[150/0]

, which is a high administrative distance.

Let’s take a look at R2’s routing table as well:

10.0.0.0/24 is subnetted, 5 subnets

R 10.1.1.0 [120/1] via 10.1.4.1, 00:00:21, Serial0/0/0

R 10.1.2.0 [120/1] via 10.1.4.1, 00:00:21, Serial0/0/0

R 10.1.3.0 [120/1] via 10.1.4.1, 00:00:21, Serial0/0/0

C 10.1.4.0 is directly connected, Serial0/0/0

R 10.1.5.0 [120/1] via 10.1.4.1, 00:00:21, Serial0/0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

R 172.16.0.0/16 [120/2] via 10.1.4.1, 00:00:21, Serial0/0/0

S 172.16.10.0/24 [150/0] via 10.1.4.1

R 192.168.10.0/24 [120/2] via 10.1.4.1, 00:00:21, Serial0/0/0

R 192.168.20.0/24 [120/2] via 10.1.4.1, 00:00:21, Serial0/0/0

C 192.168.30.0/24 is directly connected, FastEthernet0/0

C 192.168.40.0/24 is directly connected, FastEthernet0/1

R2#

Notice the same issue. RIPv1 doesn’t work with discontiguous networks, and that is what we have here. Keep this thought in mind and I’ll tell you why this is happening later in this chapter, and what must be done to fix it in Chapter 9.

So, while yes, it’s true that RIP has worked in our little internetwork, it’s not the solution for every enterprise. That’s because this technique has a maximum hop count of only 15 (16 is deemed unreachable). Plus, it performs full routing-table updates every 30 seconds, which would bring a larger internetwork to a painful crawl pretty quick!

There’s one more thing I want to show you about RIP routing tables and the parameters used to advertise remote networks. Notice, using as an example a different router on a different network for a second, that the following routing table shows

[120/15]

in the 10.1.3.0 network metric. This means that the administrative distance is 120, the default for RIP, but the hop count is 15. Remember that each time a router sends out an update to a neighbor router, it increments the hop count by one for each route.

Router#

sh ip route

10.0.0.0/24 is subnetted, 12 subnets

C 10.1.11.0 is directly connected, FastEthernet0/1

C 10.1.10.0 is directly connected, FastEthernet0/0

R 10.1.9.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1

R 10.1.8.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1

R 10.1.12.0 [120/1] via 10.1.11.2, 00:00:00, FastEthernet0/1

R 10.1.3.0 [120/15] via 10.1.5.1, 00:00:15, Serial0/0/1

R 10.1.2.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1

R 10.1.1.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1

R 10.1.7.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1

R 10.1.6.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1

C 10.1.5.0 is directly connected, Serial0/0/1

R 10.1.4.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1

So this

[120/15]

is really bad because the next router that receives the table from router R3 will just discard the route to network 10.1.3.0 since the hop count would then be 16, which is invalid.

If a router receives a routing update that contains a higher-cost path to a network that’s already in its routing table, the update will be ignored.

Configuring RIP Routing Example 2

Before we move onto learning more about RIP configurations, let’s take a look at

Figure 8-17 . In this example, we first will find and implement our subnets and then add the RIP configuration to the router.

For this configuration, we are going to assume that the Lab_B and Lab_C routers are already configured and we just need to configure the

Lab_A router. We will use the network ID of 192.168.164.0/28. The s0/0 interface of Lab_A will use the last available IP address in the eighth subnet and the fa0/0 will use the last available IP address in the second subnet. Do not consider the zero subnet valid.

Figure 8-17:

RIP routing example 2

Before we start, you do know that /28 is a 255.255.255.240 mask, right? And that we have a block size of 16 in the fourth octet? It is very important that you know this, and if you need another review of Chapters 3 and 4, that’s okay! Reviewing subnetting will never hurt you.

Since we have a block size of 16, our subnets are 16 (remember we are not starting at zero for this example), 32, 48, 64, 80, 96, 112, 128, 144, etc. The eighth subnet (which we will use for the s0/0 interface) is subnet 128. The valid host range for the 128 subnet is 129 through 142, and 143 is the broadcast address of the 128 subnet. The second subnet (which we will use for the fa0/0 interface) is the 32 subnet. The valid hosts are 33 through 46, and 47 is the broadcast address of the 32 subnet.

So, here is what our configuration on the Lab_A router will look like:

Lab_A(config)#

interface s0/0

Lab_A(config-if)#

ip address 192.168.164.142 255.255.255.240

Lab_A(config-if)#

no shutdown

Lab_A(config-if)#

interface fa0/0

Lab_A(config-if)#

ip address 192.168.164.46 255.255.255.240

Lab_A(config-if)#

no shutdown

Lab_A(config-if)#

router rip

Lab_A(config-router)#

network 192.168.164.0

Lab_A(config-router)#

^Z

Lab_A#

Finding the subnets and configuring the last valid host should be pretty straightforward. If not, head back to Chapter 4. However, what I really want you to notice is that although we added two subnets to the Lab_A router, we only had one network statement under RIP. Sometimes it is hard to remember that you configure only the classful network statement, which means you turn all host bits off.

This was the real purpose of this second RIP configuration example—to remind you of classful network addressing. And it never hurts to practice subnetting, right?

Holding Down RIP Propagations

You probably don’t want your RIP network advertised everywhere on your LAN and WAN. There’s not a whole lot to be gained by advertising your

RIP network to the Internet, now, is there?

There are a few different ways to stop unwanted RIP updates from propagating across your LANs and WANs, and the easiest one is through the passive-interface

command. This command prevents RIP update broadcasts from being sent out a specified interface, yet that same interface can still receive RIP updates.

Here’s an example of how to configure a passive-interface

on a router using the CLI:

Lab_A#

config t

Lab_A(config)#

router rip

Lab_A(config-router)#

network 192.168.10.0

Lab_A(config-router)#

passive-interface serial 0/0

This command will stop RIP updates from being propagated out serial interface 0/0, but serial interface 0/0 can still receive RIP updates.

RIP Version 2 (RIPv2)

Let’s spend a couple of minutes discussing RIPv2, and although I don’t solve our little routing table mystery of two routes to the same network in the

Corp and R2 routing table until Chapter 9, the answer lies within this section, and we’ll advertise the routes on R3 to the other routers in the internetwork.

Should We Really Use RIP in an Internetwork?

You have been hired as a consultant to install a couple of Cisco routers into a growing network. They have a couple of old Unix routers that they want to keep in the network.

These routers do not support any routing protocol except RIP. I guess this means you just have to run RIP on the entire network.

Well, yes and no. You can run RIP on a router connecting that old network, but you certainly don’t need to run RIP throughout the whole internetwork!

You can do what is called redistribution, which is basically translating from one type of routing protocol to another. This means that you can support those old routers using

RIP but use Enhanced IGRP, for example, on the rest of your network.

This will stop RIP routes from being sent all over the internetwork and eating up all that precious bandwidth.

RIP version 2 is mostly the same as RIP version 1. Both RIPv1 and RIPv2 are distance-vector protocols, which means that each router running

RIP sends its complete routing table out all active interfaces at periodic time intervals. Also, the timers and loop-avoidance schemes are the same in both RIP versions (i.e., holddown timers and split horizon rule). Both RIPv1 and RIPv2 are configured using classful addressing (but RIPv2 is considered classless because subnet information is sent with each route update), and both have the same administrative distance (120).

But there are some important differences that make RIPv2 more scalable than RIPv1. And I’ve got to add a word of advice here before we move on: I’m definitely not advocating using RIP of either version in your network. But since RIP is an open standard, you can use it with any brand of router. You can also use OSPF (discussed in Chapter 9) since OSPF is an open standard as well. RIP just requires too much bandwidth, making it pretty intensive to use in your network. Why go there when you have other, more elegant options?

Table 8-3 discusses the differences between RIPv1 and RIPv2.

Table 8-3:

RIPv1 vs. RIPv2

RIPv1 RIPv2

Distance vector Distance vector

Maximum hop count of 15

Classful

Broadcast based

No support for VLSM

Maximum hop count of 15

Classless

Uses multicast 224.0.0.9

Supports VLSM networks

No authentication Allows for MD5 authentication

No support for discontiguous networks Supports discontiguous networks

RIPv2, unlike RIPv1, is a classless routing protocol (even though it is configured as classful, like RIPv1), which means that it sends subnet mask information along with the route updates. By sending the subnet mask information with the updates, RIPv2 can support Variable Length Subnet

Masks (VLSMs) as well as the summarization of network boundaries, which cause more harm than good at times in our current network designs. In addition, RIPv2 can support discontiguous networking, which I’ll go over more in Chapter 9 and finally solve our routing table mystery!

Configuring RIPv2 is pretty straightforward. Here’s an example:

Lab_C(config)#

router rip

Lab_C(config-router)#

network 192.168.40.0

Lab_C(config-router)#

network 192.168.50.0

Lab_C(config-router)#

version 2

That’s it; just add the command

version 2

under the

(config-router)#

prompt and you are now running RIPv2. I am going to go through the RIP verification commands and then configure RIPv2 on our internetwork.

RIPv2 is classless and supports VLSM and discontiguous networks.

Verifying Your Configurations

It’s important to verify your configurations once you’ve completed them, or at least once you

think

you’ve completed them. The following list includes the commands you can use to verify the routed and routing protocols configured on your Cisco routers: show ip route show ip protocols debug ip rip

The first command was covered in the previous section—I’ll go over the others in the sections that follow.

The show ip protocols Command

The show ip protocols

command shows you the routing protocols that are configured on your router. Looking at the following output, you can see that

RIP is running on the router and the timers that RIP uses:

Corp#

sh ip protocols

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 23 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: rip

Default version control: send version 1, receive any version

Interface Send Recv Triggered RIP Key-chain

Vlan1 1 2 1

FastEthernet0/0 1 2 1

Serial0/0/0 1 2 1

Serial0/0/1 1 2 1

Serial0/1/0 1 2 1

Automatic network summarization is in effect

Maximum path: 4

Routing for Networks:

10.0.0.0

Passive Interface(s):

Routing Information Sources:

Gateway Distance Last Update

10.1.5.2 120 00:00:28

10.1.2.2 120 00:00:21

10.1.3.2 120 00:00:21

10.1.4.2 120 00:00:12

Distance: (default is 120)

Notice in this output that RIP is sending updates every 30 seconds, which is the default. The other timers used in distance vector are also shown.

Notice further down that RIP is routing for directly connected interfaces f0/0, S0/0/0, s0/0/1, and s0/1/0. The send and receive versions are listed to the right of the interfaces—RIPv1 and v2. This is an important troubleshooting section. If the interface you need is not listed in this section, you did not type the correct network statements in and this information can be found under the heading Routing for Networks.

Under the Gateway heading, the neighbors it found and the last entry is the default AD for RIP (120).

Troubleshooting with the show ip protocols Command

Let’s use a sample router and use the show ip protocols

command to see what we can determine about routing by looking at this output from a router on another network:

Router#

sh ip protocols

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 6 seconds

Invalid after 180 seconds, hold down 180, flushed afteR340

Outgoing update filter list for all interfaces is

Incoming update filter list for all interfaces is

Redistributing: rip

Default version control: send version 1, receive any version

Interface Send Recv Key-chain

Serial0/0 1 1 2

Serial0/1 1 1 2

Routing for Networks:

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.168.11.14

120 00:00:21

Distance: (default is 120)

Let’s also look at the show ip interface brief

command from the same router and see what we find out:

Router#

sh ip interface brief

Interface IP-Address OK? Method Status

FastEthernet0/0 192.168.18.1 YES manual up

Serial0/0 10.168.11.17 YES manual up

FastEthernet0/1 unassigned YES NRAM Administratively down

Serial0/1 192.168.11.21 YES manual up

Under the show ip protocols

output, you can see that we’re using RIP routing for network 10.0.0.0, which means our configuration would look like this:

Router(config)#

router rip

Router(config-router)#

network 10.0.0.0

Also, only serial 0/0 and serial 0/1 are participating in the RIP network. And last, our neighbor router is 10.168.11.14.

From the output of the show ip interface brief

command, you can see that only serial 0/0 is in the 10.0.0.0 network. This means that the router will only send and receive routing updates with the 10.0.0.0 network and not advertise the 192.168.0.0 networks out any interface. To fix this, you would need to add the 192.168.11.0 and 192.168.18.0 networks under the router rip

global command.

The debug ip rip Command

The debug ip rip

command displays routing updates as they are sent and received on the router to the console session. If you are telnetted into the

router, you’ll need to use the terminal monitor

command to be able to receive the output from the debug

commands.

We can see in this output that RIP is both sending and receiving (the metric is the hop count):

R3#

debug ip rip

RIP protocol debugging is on

RIP: received v1 update from 10.1.5.1 on FastEthernet0/0

10.1.1.0 in 1 hops

10.1.2.0 in 1 hops

10.1.3.0 in 1 hops

10.1.4.0 in 1 hops

192.168.10.0 in 2 hops

192.168.20.0 in 2 hops

192.168.30.0 in 2 hops

192.168.40.0 in 2 hops

RIP: sending v1 update to 255.255.255.255 via Dot11Radio0/0/0(172.16.10.1)

RIP: build update entries

network 10.0.0.0 metric 1

network 192.168.10.0 metric 3

network 192.168.20.0 metric 3

network 192.168.30.0 metric 3

network 192.168.40.0 metric 3

RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (10.1.5.2)

RIP: build update entries

network 172.16.0.0 metric 1)

Let’s talk about the output for a minute. First, R3 received all the routes that the Corp router has, and RIP is sending v1 packets to

255.255.255.255—an “all-hands” broadcast—out interface Dot11Radio0/0/0/0 via 172.16.10.1. This is where RIPv2 will come in handy. Why?

Because RIPv2 doesn’t send broadcasts; it used the multicast 224.0.0.9. So even though the RIP packets could be transmitted onto a network with no routers, all hosts would just ignore them, making RIPv2 a bit of an improvement over RIPv1.

Okay—now check out the fact that RIP is sending advertisements for all networks out Dot11Radio0/0/0/0, yet the last advertisement out

FastEthernet 0/0 on R3 is only advertising 172.16.0.0. Why? If you answered the split horizon rule, you nailed it! The R3 router in this example will not advertise all those networks received from a neighbor router back to the same router.

If the metric of a route shows 16, this is a route poison, and the network being advertised is unreachable.

Troubleshooting with the debug ip rip Command

Now let’s use the debug ip rip

command to both discover a problem and figure out how RIP was configured on a router from a different sample network:

07:12:58: RIP: sending v1 update to 255.255.255.255 via

FastEthernet0/0 (172.16.1.1)

07:12:58: network 10.0.0.0, metric 1

07:12:58: network 192.168.1.0, metric 2

07:12:58: RIP: sending v1 update to 255.255.255.255 via

Serial0/0 (10.0.8.1)

07:12:58: network 172.16.0.0, metric 1

07:12:58: RIP: Received v1 update from 10.0.15.2 n Serial0/0

07:12:58: 192.168.1.0 in one hop

07:12:58: 192.168.168.0 in 16 hops (inaccessible)

You can see from the updates that we’re sending out information about networks 10.0.0.0, 192.168.1.0, and 172.16.0.0. But both the 10.0.0.0

network and the 172.16.0.0 network are being advertised with a hop count (metric) of 1, meaning that these networks are directly connected. The

192.168.1.0 is being advertised as a metric of 2, which means that it is not directly connected.

For this to be happening, our configuration would have to look like this:

Router(config)#

router rip

Router(config-router)#

network 10.0.0.0

Router(config-router)#

network 172.16.0.0

And there’s something else you can find out by looking at this: There are at least two routers participating in the RIP network because we’re sending out two interfaces but only receiving RIP updates on one interface. Also, notice that the network 192.168.168.0 is being advertised as 16 hops away. RIP has a maximum hop count of 15, so 16 is considered unreachable, making this network inaccessible. So what will happen if you try to ping to a host on network 192.168.168.0? You just will not be successful, that’s what! But if you try any pings to network 10.0.0.0, you should be successful.

I have one more output I want to show you—see if you can find the problem. Both a debug ip rip

and a show ip route

output are shown from our sample router:

07:12:56: RIP: received v1 update from 172.16.100.2 on Serial0/0

07:12:56: 172.16.10.0 in 1 hops

07:12:56: 172.16.20.0 in 1 hops

07:12:56: 172.16.30.0 in 1 hops

Router#

sh ip route

[output cut]

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 8 subnets

C 172.16.150.0 is directly connected, FastEthernet0/0

C 172.16.220.0 is directly connected, Loopback2

R 172.16.210.0 is directly connected, Loopback1

R 172.16.200.0 is directly connected, Loopback0

R 172.16.30.0 [120/2] via 172.16.100.2, 00:00:04, Serial0/0

S 172.16.20.0 [120/2] via 172.16.150.15

R 172.16.10.0 [120/2] via 172.16.100.2, 00:00:04, Serial0/0

R 172.16.100.0 [120/2] is directly connected, Serial0/0

Looking at the two outputs, can you tell why users can’t access 172.16.20.0?

The debug output shows that network 172.16.20.0 is one hop away and being received on serial0/0 from 172.16.100.2. By viewing the show ip route

output, you can see that packets with a destination of 172.16.20.0 are being sent to 172.16.150.15 because of a static route entry. The output also shows that 172.16.150.0 is directly connected to FastEthernet 0/0 and network 172.16.20.0 is really out serial 0/0, so packets with a destination of 172.16.20.0 are being sent out the wrong interface because of a mis-configured static route.

Enabling RIPv2 on Our Internetwork

Before we move on to Chapter 9 and configure EIGRP and OSPF, I want to enable RIPv2 on our routers. It’ll only take a second. Here are my configurations:

Corp#

config t

Corp(config)#

router rip

Corp(config-router)#

version 2

Corp(config-router)#

^Z

R1#

config t

R1(config)#

router rip

R1(config-router)#

version 2

R1(config-router)#

^Z

R2#

config t

Enter configuration commands, one per line. End with CNTL/Z.

R2(config)#

router rip

R2(config-router)#

version 2

R2(config-router)#

^Z

R3#

config t

R3#(config)#

router rip

R3#(config-router)#

version 2

R3#(config-router)#

^Z

This was probably the easiest configuration we have done in the book so far. Let’s see if we can find a difference in our routing tables. Here’s the

Corp router’s routing table now:

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

R 172.16.0.0/16 [120/1] via 10.1.5.2, 00:00:18, FastEthernet0/0

S 172.16.10.0/24 [150/0] via 10.1.5.2

R 192.168.10.0/24 [120/1] via 10.1.2.2, 00:00:04, Serial0/0/0

[120/1] via 10.1.3.2, 00:00:04, Serial0/0/1

R 192.168.20.0/24 [120/1] via 10.1.2.2, 00:00:04, Serial0/0/0

[120/1] via 10.1.3.2, 00:00:04, Serial0/0/1

R 192.168.30.0/24 [120/1] via 10.1.4.2, 00:00:06, Serial0/1/0

R 192.168.40.0/24 [120/1] via 10.1.4.2, 00:00:06, Serial0/1/0

Corp#

Well—looks the same to me, and it still didn’t fix my double entry for the 172.16.0.0 network. I’m going to turn on debugging and see if that shows us anything new:

Corp#

debug ip rip

RIP protocol debugging is on

Corp#RIP: sending v2 update to 224.0.0.9 via Vlan1 (10.1.1.1)

RIP: build update entries

10.1.2.0/24 via 0.0.0.0, metric 1, tag 0

10.1.3.0/24 via 0.0.0.0, metric 1, tag 0

10.1.4.0/24 via 0.0.0.0, metric 1, tag 0

10.1.5.0/24 via 0.0.0.0, metric 1, tag 0

172.16.0.0/16 via 0.0.0.0, metric 2, tag 0

192.168.10.0/24 via 0.0.0.0, metric 2, tag 0

192.168.20.0/24 via 0.0.0.0, metric 2, tag 0

192.168.30.0/24 via 0.0.0.0, metric 2, tag 0

192.168.40.0/24 via 0.0.0.0, metric 2, tag 0

RIP: sending v2 update to 224.0.0.9 via FastEthernet0/0 (10.1.5.1

)

[output cut]

Bingo! Look at that! The networks are still being advertised every 30 seconds, but they’re now sending the advertisements as v2 and as a multicast address of 224.0.0.9. Let’s take a look at the show ip protocols

output:

Corp#

sh ip protocols

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 20 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: rip

Default version control: send version 2, receive 2

Interface Send Recv Triggered RIP Key-chain

Vlan1 2 2

FastEthernet0/0 2 2

Serial0/0/0 2 2

Serial0/0/1 2 2

Serial0/1/0 2 2

Automatic network summarization is in effect

Maximum path: 4

Routing for Networks:

10.0.0.0

Passive Interface(s):

Routing Information Sources:

Gateway Distance Last Update

10.1.5.2 120 00:00:09

10.1.2.2 120 00:00:20

10.1.3.2 120 00:00:20

10.1.4.2 120 00:00:23

Distance: (default is 120)

We are now sending and receiving RIPv2. Nice when things work out well, huh? However, I never did fix that double entry for the 172.16.0.0

network in the Corp and R2 routing tables, even though I could have using RIPv2, with an additional configuration entry, I want to save that example for EIGRP. But the answer for this problem was previously shown in Table 8-3 .

Advertising a Default Route Using RIP

I want to show you how to advertise a way out of your autonomous system. Imagine that you were to look at our network diagram and that instead of having our wireless network connected to R3, we could use a serial interface and configure our little internetwork to the Internet from R3.

If we do add an Internet connection to R3, all routers in our AS need to know where to send packets that are destined for networks on the Internet, or they’ll just drop the packets if they get a packet with a remote request. One solution would be to put a default route on every router and funnel the information to R3, which in turn would have a default route to the ISP. Most people do this type of configuration in small to medium size networks.

However, since I am running RIPv2 on all routers including R3, I’ll just add a default route on R3 to the ISP, as I would normally, but then add another command to advertise my network to the other routers in the AS as the default route.

Here would be an example of my new R3 configuration:

R3(config)#

interface s0/0

R3(config-if)#

ip address 172.16.10.5 255.255.255.252

R3(config-if)#

exit

R3(config)#

ip route 0.0.0.0 0.0.0.0 s0/0

R3(config)#

ip default-network 172.16.0.0

Now, let’s see what the Corp and R2 routers’ routing tables see:

Corp#

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

R 172.16.0.0/16 [120/1] via 10.1.5.2, 00:00:16, FastEthernet0/0

S 172.16.10.0/24 [150/0] via 10.1.5.2

R 192.168.10.0/24 [120/1] via 10.1.2.2, 00:00:16, Serial0/0/0

[120/1] via 10.1.3.2, 00:00:16, Serial0/0/1

R 192.168.20.0/24 [120/1] via 10.1.2.2, 00:00:16, Serial0/0/0

[120/1] via 10.1.3.2, 00:00:16, Serial0/0/1

R 192.168.30.0/24 [120/1] via 10.1.4.2, 00:00:02, Serial0/1/0

R 192.168.40.0/24 [120/1] via 10.1.4.2, 00:00:02, Serial0/1/0

R* 0.0.0.0/0 [120/1] via 10.1.5.2, 00:00:16, FastEthernet0/0

Corp#

Nice—look at the last entry: R3 is advertising to the Corp router that “Hey, I am the way to the Internet!” or “I am the way out of the AS!” Let’s see if

R2 can see this same entry:

R2#

10.0.0.0/24 is subnetted, 5 subnets

R 10.1.1.0 [120/1] via 10.1.4.1, 00:00:29, Serial0/0/0

R 10.1.2.0 [120/1] via 10.1.4.1, 00:00:29, Serial0/0/0

R 10.1.3.0 [120/1] via 10.1.4.1, 00:00:29, Serial0/0/0

C 10.1.4.0 is directly connected, Serial0/0/0

R 10.1.5.0 [120/1] via 10.1.4.1, 00:00:29, Serial0/0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

R 172.16.0.0/16 [120/2] via 10.1.4.1, 00:00:29, Serial0/0/0

S 172.16.10.0/24 [150/0] via 10.1.4.1

R 192.168.10.0/24 [120/2] via 10.1.4.1, 00:00:29, Serial0/0/0

R 192.168.20.0/24 [120/2] via 10.1.4.1, 00:00:29, Serial0/0/0

C 192.168.30.0/24 is directly connected, FastEthernet0/0

C 192.168.40.0/24 is directly connected, FastEthernet0/1

R* 0.0.0.0/0 [120/2] via 10.1.4.1, 00:00:29, Serial0/0/0

R2#

R2 is seeing it as well, so our ip default-network

command is working and advertising with RIP, and in addition, I verified that R1 is receiving the default route as well. This command would work with either RIP or RIPv2.

You’re ready now to move on to the next chapter!

Summary

This chapter covered IP routing in detail. It’s extremely important that you really understand the basics we covered in this chapter because everything that’s done on a Cisco router typically will have some type of IP routing configured and running.

You learned in this chapter how IP routing uses frames to transport packets between routers and to the destination host. From there, we configured static routing on our routers and discussed the administrative distance used by IP to determine the best route to a destination network. If you have a stub network, you can configure default routing, which sets the gateway of last resort on a router.

We then discussed dynamic routing in detail, specifically RIP and how it works on an internetwork (not well). We finished by verifying RIP and then adding RIPv2 to our little internetwork, and also advertising a default route throughout the AS.

In the next chapter, we’ll continue on with dynamic routing by discussing EIGRP and OSPF.

Exam Essentials

Describe the basic IP routing process.

You need to remember that the frame changes at each hop but that the packet is never changed or manipulated in any way until it reaches the destination device (the TTL field in the IP header is decremented for each hop, but that’s it!).

List the information required by a router to successfully route packets.

To be able to route packets, a router must know, at a minimum, the destination address, the location of neighboring routers through which it can reach remote networks, possible routes to all remote networks, the best route to each remote network, and how to maintain and verify routing information.

Describe how MAC addresses are used during the routing process.

A MAC (hardware) address will only be used on a local LAN. It will never pass a router’s interface. A frame uses MAC (hardware) addresses to send a packet on a LAN. The frame will take the packet to either a host on the LAN or a router’s interface (if the packet is destined for a remote network). As packets move from one router to another, the

MAC addresses used will change but normally the original source and destination IP addresses within the packet will not.

View and interpret the routing table of a router.

Use the show ip route

command to view the routing table. Each route will be listed along with the source of the routing information. A

C

to the left of the route will indicate directly connected routes, and other letters next to the route can also indicate a particular routing protocol that provided the information, such as, for example,

R

for RIP.

Differentiate the three types of routing.

The three types of routing are static (in which routes are manually configured at the CLI), dynamic

(in which the routers share routing information via a routing protocol), and default routing (in which a special route is configured for all traffic without a more specific destination network found in the table).

Compare and contrast static and dynamic routing.

Static routing creates no routing update traffic and creates less overhead on the router and network links, but it must be configured manually and does not have the ability to react to link outages. Dynamic routing creates routing update traffic and uses more overhead on the router and network links, but it can both react to link outages and choose the best route when multiple routes exist to the same network.

Configure static routes at the CLI.

The command syntax to add a route is ip route [destination_network] [mask] [next-hop_address or exitinterface] [administrative_distance] [permanent]

.

Create a default route.

To add a default route, use the command syntax ip route 0.0.0.0 0.0.0.0 ip-address or exit interface type and number

.

Understand administrative distance and its role in the selection of the best route.

Administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. Administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. All routing protocols are assigned a default AD, but it can be changed at the CLI.

Differentiate distance-vector, link-state and hybrid routing protocols.

Distance-vector routing protocols make routing decisions based on hop count (think RIP), while link-state routing protocols are able to consider multiple factors such as bandwidth available and delay when selecting the best route. Hybrid routing protocols exhibit characteristics of both types.

List mechanisms used to prevent routing loops in the network.

Maximum hop count, split horizon, route poisoning, and holddown counters all play roles in preventing routing loops.

Describe the counters used in the operation of RIP.

The route update timer is the interval between routing updates, the route invalid timer

determines the length of time that must elapse (180 seconds) before a router determines that a route has become invalid, the holddown timer sets the amount of time during which routing information is suppressed (when a link is lost), and the route flush timer sets the time between a route becoming invalid and its removal from the routing table (240 seconds).

Configure RIP routing.

To configure RIP routing, first you must be in global configuration mode and then you type the command

router rip

.

Then you add all directly connected networks, making sure to use the classful address.

Identify commands used to verify RIP routing.

The show ip route

command will provide you with the contents of the routing table. An

R

on the left side of the table indicates a RIP-found route. The debug ip rip

command will show you RIP updates being sent and received on your router. If you see a route with a metric of 16, that route is considered down.

Describe the differences between RIPv1 and RIPv2.

RIPv1 sends broadcasts every 30 seconds and has an AD of 120. RIPv2 sends multicasts (224.0.0.9) every 30 seconds and also has an AD of 120. RIPv2 sends subnet mask information with the route updates, which allows it to support classless networking and discontiguous networks. RIPv2 also supports authentication between routers and RIPv1 does not.

Written Lab 8

Write the answers to the following questions:

1.

At the appropriate command prompt, create a static route to network 172.16.10.0/24 with a next-hop gateway of 172.16.20.1 and an administrative distance of 150.

2.

When a PC sends a packet to another PC in a remote network, what destination IP address and MAC address will be in the frame that it sends to its default gateway?

3.

At the appropriate command prompt, create a default route to 172.16.40.1.

4.

If you are using default routing in a classless environment, what command must also be used?

5.

On which type of network is a default route most beneficial?

6.

At the appropriate command prompt, display the routing table on your router.

7.

When creating a static or default route, you don’t have to use the next-hop IP address; you can use the ___________________.

8.

True/False: To reach a destination host, you must know the MAC address of the remote host.

9.

True/False: To reach a destination host, you must know the IP address of the remote host.

10.

At the appropriate command prompt, execute the command required on a DCE serial interface that is not required on a DTE serial interface.

11.

At the appropriate command prompt(s), enable RIP routing on the interface with the IP address 10.0.0.1/24.

12.

At the appropriate command prompt(s), prevent a router from propagating RIP information out serial 1.

13.

What routing loop prevention mechanism sends out a maximum hop count as soon as a link fails?

14.

What routing loop prevention mechanism suppresses the resending of routing information to an interface through which it was received?

15.

At the appropriate command prompt, display RIP routing updates as they are sent and received on the router to the console session.

(The answers to Written Lab 8 can be found following the answers to the review questions for this chapter.)

Hands-on Labs

In the following hands-on labs, you will configure a network with three routers. These exercises assume all the same setup requirements as the labs found in earlier chapters.

This chapter includes the following labs:

Lab 8.1: Creating Static Routes

Lab 8.2: Configuring RIP Routing

The internetwork shown in the following graphic will be used to configure all routers.

Table 8-4 shows our IP addresses for each router (each interface uses a /24 mask).

Table 8-4:

Our IP addresses

Router Interface IP Address

Lab_A F0/0

Lab_A S0/0

Lab_B S0/0

172.16.10.1

172.16.20.1

172.16.20.2

Lab_B S0/1

Lab_C S0/0

172.16.30.1

172.16.30.2

Lab_C Fa0/0 172.16.40.1

These labs were written without using the LAN interface on the Lab_B router. You can choose to add that LAN into the labs if necessary.

Hands-on Lab 8.1: Creating Static Routes

In this lab, you will create a static route in all three routers so that the routers see all networks. Verify with the Ping program when complete.

1.

The Lab_A router is connected to two networks, 172.16.10.0 and 172.16.20.0. You need to add routes to networks 172.16.30.0 and

172.16.40.0. Use the following commands to add the static routes.

Lab_A#

config t

Lab_A(config)#

ip route 172.16.30.0 255.255.255.0

172.16.20.2

Lab_A(config)#

ip route 172.16.40.0 255.255.255.0

172.16.20.2

2.

Save the current configuration for the Lab_A router by going to the privileged mode, typing

copy run start

, and pressing Enter.

3.

On the Lab_B router, you have direct connections to networks 172.16.20.0 and 172.16.30.0. You need to add routes to networks

172.16.10.0 and 172.16.40.0. Use the following commands to add the static routes.

Lab_B#

config t

Lab_B(config)#

ip route 172.16.10.0 255.255.255.0

172.16.20.1

Lab_B(config)#

ip route 172.16.40.0 255.255.255.0

172.16.30.2

4.

Save the current configuration for router Lab_B by going to the enabled mode, typing

copy run start

, and pressing Enter.

5.

On router Lab_C, create a static route to networks 172.16.10.0 and 172.16.20.0, which are not directly connected. Create static routes so that router Lab_C can see all networks, using the commands shown here:

Lab_C#

config t

172.16.30.1

172.16.30.1

6.

Save the current configuration for router Lab_C by going to the enable mode, typing

copy run start

, and pressing Enter.

7.

Check your routing tables to make sure all four networks show up by executing the

show ip route

command.

8.

Now ping from each router to your hosts and from each router to each router. If it is set up correctly, it will work.

Hands-on Lab 8.2: Configuring RIP Routing

In this lab, we will use the dynamic routing protocol RIP instead of static routing.

1.

Remove any static routes or default routes configured on your routers by using the no ip route

command. For example, here is how you would remove the static routes on the Lab_A router:

Lab_A#

config t

Lab_A(config)#

no ip route 172.16.30.0 255.255.255.0

Lab_C(config)#

ip route 172.16.20.0 255.255.255.0

172.16.20.2

Lab_A(config)#

no ip route 172.16.40.0 255.255.255.0

Lab_C(config)#

ip route 172.16.10.0 255.255.255.0

172.16.20.2

Do the same thing for routers Lab_B and Lab_C. Verify that only your directly connected networks are in the routing tables.

2.

After your static and default routes are clear, go into configuration mode on router Lab_A by typing

config t

.

3.

Tell your router to use RIP routing by typing

router rip

and pressing Enter, as shown here:

config t router rip

4.

Add the network number for the networks you want to advertise. Since router Lab_A has two interfaces that are in two different

networks you must enter a network statement using the network ID of the network in which each interface resides. Alternately, you could use a summarization of these networks and use a single statement, minimizing the size of the routing table. Since the two networks are

172.16.10.0/24 and 172.16.20.0/24, the network summarization 172.16.0.0 would include both subnets. Do this by typing

network 172.16.0.0

and pressing Enter.

5.

Press Ctrl+Z to get out of configuration mode.

6.

The interfaces on Lab_B and Lab_C are in the 172.16.20.0/24 and 172.16.30.0/24 networks; therefore, the same summarized network statement will work there as well. Type the same commands, as shown here:

Config t

Router rip network 172.16.0.0

7.

Verify that RIP is running at each router by typing the following commands at each router:

show ip protocols

(Should indicate to you that RIP is present on the router.

show ip route

(Should have routes present with an

R

to the left of them)

show running-config or show run

(Should indicate that RIP is present and the networks are being advertised)

8.

Save your configurations by typing

copy run start

or

copy running-config startup-config

and pressing Enter at each router.

9.

Verify the network by pinging all remote networks and hosts.

Review Questions

The following questions are designed to test your understanding of this chapter’s material. For more information on how to get additional questions, please see this book’s

Introduction.

1. The Acme Company uses a router named Gateway to connect to its ISP. The address of the ISP router is 206.143.5.2. Which commands could be configured on the Gateway router to allow Internet access to the entire network? (Choose two.)

A.

Gateway(config)#

ip route 0.0.0.0 0.0.0.0 206.143.5.2

B.

Gateway(config)#

router rip

Gateway(config-router)#

network 206.143.5.0

C.

Gateway(config)#

router rip

Gateway(config-router)#

network 206.143.5.0 default

D.

Gateway(config)#

ip route 206.143.5.0 255.255.255.0 default

E.

Gateway(config)#

ip default-network 206.143.5.0

2. What command will prevent RIP routing updates from exiting an interface but will still allow the interface to receive RIP route updates?

A.

Router(config-if)#

no routing

B.

Router(config-if)#

passive-interface

C.

Router(config-router)#

passive-interface s0

D.

Router(config-router)#

no routing updates

3. Which of the following statements are true regarding the command ip route 172.16.4.0 255.255.255.0 192.168.4.2

? (Choose two.)

A. The command is used to establish a static route.

B. The default administrative distance is used.

C. The command is used to configure the default route.

D. The subnet mask for the source address is 255.255.255.0.

E. The command is used to establish a stub network.

4. What destination addresses will be used by HostA to send data to the HTTPS server as shown in the following network? (Choose two.)

A. The IP address of the switch

B. The MAC address of the remote switch

C. The IP address of the HTTPS server

D. The MAC address of the HTTPS server

E. The IP address of RouterA’s Fa0/0 interface

F. The MAC address of RouterA’s Fa0/0 interface

5. Which of the following is true regarding the following output? (Choose two.)

04:06:16: RIP: received v1 update from 192.168.40.2 on Serial0/1

04:06:16: 192.168.50.0 in 16 hops (inaccessible)

04:06:40: RIP: sending v1 update to 255.255.255.255 via

FastEthernet0/0 (192.168.30.1)

04:06:40: RIP: build update entries

04:06:40: network 192.168.20.0 metric 1

04:06:40: network 192.168.40.0 metric 1

04:06:40: network 192.168.50.0 metric 16

04:06:40: RIP: sending v1 update to 255.255.255.255 via Serial0/1

(192.168.40.1)

A. There are three interfaces on the router participating in this update.

B. A ping to 192.168.50.1 will be successful.

C. There are at least two routers exchanging information.

D. A ping to 192.168.40.2 will be successful.

6. Which of the following is the best description of the operation of split horizon?

A. Information about a route should not be sent back in the direction from which the original update came.

B. It splits the traffic when you have a large bus (horizon) physical network.

C. It holds the regular updates from broadcasting to a downed link.

D. It prevents regular update messages from reinstating a route that has gone down.

7. Which of the following would be true if HostA is trying to communicate to HostB and interface F0/0 of RouterC goes down, as shown in the following graphic? (Choose two.)

A. RouterC will use an ICMP to inform HostA that HostB cannot be reached.

B. RouterC will use ICMP to inform RouterB that HostB cannot be reached.

C. RouterC will use ICMP to inform HostA, RouterA, and RouterB that HostB cannot be reached.

D. RouterC will send a destination unreachable message type.

E. RouterC will send a router selection message type.

F. RouterC will send a source quench message type.

8. Which statement is true regarding classless routing protocols? (Choose two.)

A. The use of discontiguous networks is not allowed.

B. The use of Variable Length Subnet Masks is permitted.

C. RIPv1 is a classless routing protocol.

D. IGRP supports classless routing within the same autonomous system.

E. RIPv2 supports classless routing.

9. Which two of the following are true regarding the distance-vector and link-state routing protocols?

A. Link state sends its complete routing table out all active interfaces at periodic time intervals.

B. Distance vector sends its complete routing table out all active interfaces at periodic time intervals.

C. Link state sends updates containing the state of its own links to all routers in the internetwork.

D. Distance vector sends updates containing the state of its own links to all routers in the internetwork.

10. Which command displays RIP routing updates?

A. show ip route

B. debug ip rip

C. show protocols

D. debug ip route

11. What does RIPv2 use to prevent routing loops? (Choose two.)

A. CIDR

B. Split horizon

C. Authentication

D. Classless masking

E. Holddown timers

12. A network administrator views the output from the show ip route

command. A network that is advertised by both RIP and EIGRP appears in the routing table flagged as an EIGRP route. Why is the RIP route to this network not used in the routing table?

A. EIGRP has a faster update timer.

B. EIGRP has a lower administrative distance.

C. RIP has a higher metric value for that route.

D. The EIGRP route has fewer hops.

E. The RIP path has a routing loop.

13. You type

debug ip rip

on your router console and see that 172.16.10.0 is being advertised to you with a metric of 16. What does this mean?

A. The route is 16 hops away.

B. The route has a delay of 16 microseconds.

C. The route is inaccessible.

D. The route is queued at 16 messages a second.

14. What metric does RIPv2 use to find the best path to a remote network?

A. Hop count

B. MTU

C. Cumulative interface delay

D. Load

E. Path bandwidth value

15. The Corporate router receives an IP packet with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3. Looking at the output from the Corporate router, what will the router do with this packet?

Corp#

sh ip route

[output cut]

R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0

R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0

R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0

C 192.168.20.0 is directly connected, Serial0/0

C 192.168.214.0 is directly connected, FastEthernet0/0

A. The packet will be discarded.

B. The packet will be routed out the S0/0 interface.

C. The router will broadcast looking for the destination.

D. The packet will be routed out the Fa0/0 interface.

16. If your routing table has a static, a RIP, and an EIGRP route to the same network, which route will be used to route packets by default?

A. Any available route

B. RIP route

C. Static route

D. EIGRP route

E. They will all load-balance.

17. You have the following routing table. Which of the following networks will not be placed in the neighbor routing table?

R 192.168.30.0/24 [120/1] via 192.168.40.1, 00:00:12, Serial0

C 192.168.40.0/24 is directly connected, Serial0

172.16.0.0/24 is subnetted, 1 subnets

C 172.16.30.0 is directly connected, Loopback0

R 192.168.20.0/24 [120/1] via 192.168.40.1, 00:00:12, Serial0

R 10.0.0.0/8 [120/15] via 192.168.40.1, 00:00:07, Serial0

C 192.168.50.0/24 is directly connected, Ethernet0

A. 172.16.30.0

B. 192.168.30.0

C. 10.0.0.0

D. All of them will be placed in the neighbor routing table.

18. Two connected routers are configured only with RIP routing. What will be the result when a router receives a routing update that contains a higher-cost path to a network already in its routing table?

A. The updated information will be added to the existing routing table.

B. The update will be ignored and no further action will occur.

C. The updated information will replace the existing routing table entry.

D. The existing routing table entry will be deleted from the routing table and all routers will exchange routing updates to reach convergence.

19. Which of the following is true about route poisoning?

A. It sends back the protocol received from a router as a poison pill, which stops the regular updates.

B. It is information received from a router that can’t be sent back to the originating router.

C. It prevents regular update messages from reinstating a route that has just come up.

D. It describes when a router sets the metric for a downed link to infinity.

20. Which of the following is true regarding RIPv2?

A. It has a lower administrative distance than RIPv1.

B. It converges faster than RIPv1.

C. It has the same timers as RIPv1.

D. It is harder to configure than RIPv1.

Answers to Review Questions

1. A, E. There are actually three different ways to configure the same default route, but only two are shown in the answer. First, you can set a default route with the 0.0.0.0 0.0.0.0 mask and then specify the next hop, as in option A. Or you can use 0.0.0.0 0.0.0.0 and use the exit interface instead of the next hop. Finally, you can use option E with the ip default-network

command.

2. C. The

(config-router)#

passive-interface

command stops updates from being sent out an interface, but route updates are still received. It is not executed in interface configuration mode, but in RIP configuration mode (accessed by typing

router rip

) and the interface is specified at the end of the command in the form

interface_type number

.

3. A, B. Although option D almost seems right, it is not; the mask is the mask used on the remote network, not the source network. Since there is no number at the end of the static route, it is using the default administrative distance of 1.

4. C, F. The switches are not used as either a default gateway or other destination. Switches have nothing to do with routing. It is very important to remember that the destination MAC address will always be the router’s interface. The destination address of a frame, from HostA, will be the MAC address of the Fa0/0 interface of RouterA. The destination address of a packet will be the IP address of the network interface card (NIC) of the

HTTPS server. The destination port number in the segment header will have a value of 443 (HTTPS).

5. C, D. The route to 192.168.50.0 is unreachable (a metric of 16 for RIP means the same thing) and only interfaces s0/1 and FastEthernet 0/0 are participating in the RIP update. Since a route update was received, at least two routers are participating in the RIP routing process. Since a route update for network 192.168.40.0 is being sent out Fa0/0 and a route was received from 192.168.40.2, we can assume a ping to that address will be successful.

6. A. A split horizon will not advertise a route back to the same router it learned the route from.

7. A, D. RouterC will use ICMP to inform HostA that HostB cannot be reached. It will perform this by sending a destination unreachable ICMP message type.

8. B, E. Classful routing means that all hosts in the internetwork use the same mask and that only default masks are in use. Classless routing means that you can use Variable Length Subnet Masks (VLSMs) and can also support discontiguous networking.

9. B, C. The distance-vector routing protocol sends its complete routing table out all active interfaces at periodic time intervals. Link-state routing protocols send updates containing the state of its own links to all routers in the internetwork.

10. B.

Debug ip rip

is used to show the Internet Protocol (IP) Routing Information Protocol (RIP) updates being sent and received on the router.

11. B, E. RIPv2 uses the same timers and loop-avoidance schemes as RIPv1. Split horizon is used to stop an update from being sent out the same interface it was received on. Holddown timers allow time for a network to become stable in the case of a flapping link.

12. B. RIP has an administrative distance (AD) of 120, while EIGRP has an administrative distance of 90, so the router will discard any route with a higher AD than 90 to that same network.

13. C. You cannot have 16 hops on a RIP network by default. If you receive a route advertised with a metric of 16, this means it is inaccessible.

14. A. RIPv1 and RIPv2 only use the lowest hop count to determine the best path to a remote network.

15. A. Since the routing table shows no route to the 192.168.22.0 network, the router will discard the packet and send an ICMP destination unreachable message out interface FastEthernet 0/0, which is the source LAN from which the packet originated.

16. C. Static routes have an administrative distance of 1 by default. Unless you change this, a static route will always be used over any other dynamically learned route. EIGRP has an administrative distance of 90, and RIP has an administrative distance of 120, by default.

17. C. The network 10.0.0.0 cannot be placed in the next router’s routing table because it already is at 15 hops. One more hop would make the route 16 hops, and that is not valid in RIP networking.

18. B. When a routing update is received by a router, the router first checks the administrative distance (AD) and always chooses the route with the lowest AD. However, if two routes are received and they both have the same AD and differing metrics, then the router will choose the one route with the lowest metrics or, in RIP’s case, hop count.

19. D. Another way to avoid problems caused by inconsistent updates and to stop network loops is route poisoning. When a network goes down, the distance-vector routing protocol initiates route poisoning by advertising the network with an unreachable metric of 16 (for RIP), sometimes referred to as

infinite

.

20. C. RIPv2 is pretty much just like RIPv1. It has the same administrative distance and timers and is configured similarly.

Answers to Written Lab 8

1.

router(config)#ip route 172.16.10.0 255.255.255.0 150

2.

It will use the gateway interface MAC at L2 and the actual destination IP at L3.

3.

router(config)#ip route 0.0.0.0 0.0.0.0 172.16.40.1

4.

Router(config)#

ip classless

5.

Stub network

6.

Router#

show ip route

7.

Exit interface

8.

False. The MAC address would be the router interface, not the remote host.

9.

True

10.

Router(config-if)#

clock rate speed

11.

router(config)#router rip routre(config-router)#network 10.0.0.0

12.

router(config)#router rip router(config-router)#passive-interface S1

13.

Route poisoning

14.

Split horizon

15.

debug ip rip

Chapter 9

Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF)

The CCNA exam topics covered in this chapter include the following:

Configure, verify, and troubleshoot basic router operation and routing on Cisco devices

Access and utilize the router to set basic parameters (including CLI/SDM)

Connect, configure, and verify operation status of a device interface

Verify device configuration and network connectivity using ping, traceroute, telnet, SSH, or other utilities

Perform and verify routing configuration tasks for a static or default route given specific routing requirements

Compare and contrast methods of routing and routing protocols

Configure, verify, and troubleshoot OSPF

Configure, verify, and troubleshoot EIGRP

Verify network connectivity (including: using ping, traceroute, and telnet or SSH)

Troubleshoot routing issues

Verify router hardware and software operation using SHOW and DEBUG commands

Implement basic router security

Enhanced Interior Gateway Routing Protocol (EIGRP) is a proprietary Cisco protocol that runs on Cisco routers. It is important for you to understand

EIGRP because it is probably one of the two most popular routing protocols in use today. In this chapter, I’ll show you the many features of EIGRP and describe how it works, with particular focus on the unique way it discovers, selects, and advertises routes.

I’m also going to introduce you to the Open Shortest Path First (OSPF) routing protocol, which is the other popular routing protocol in use today.

You’ll build a solid foundation for understanding OSPF by first becoming familiar with the terminology and internal operation of it and then learning about OSPF’s advantages over RIP. Next, we’ll explore the issues surrounding implementations of OSPF in broadcast and non-broadcast networks of various types. I’ll explain how to implement single-area OSPF in different and specific networking environments and demonstrate how to verify that everything is running smoothly.

For up-to-the minute updates for this chapter, please see www.lammle.com

and/or www.sybex.com/go/ccna7e

.

EIGRP Features and Operation

Enhanced IGRP (EIGRP)

is a classless, enhanced distance-vector protocol that uses the concept of an autonomous system to describe the set of contiguous routers that run the same routing protocol and share routing information. EIGRP includes the subnet mask in its route updates because it is considered classless. And as you now know, the advertisement of subnet information allows us to use Variable Length Subnet Masks (VLSMs) and manual summarization when designing our networks!

EIGRP is sometimes referred to as a

hybrid routing protocol

because it has characteristics of both distance-vector and link-state protocols. For example, EIGRP doesn’t send link-state packets as OSPF does; instead, it sends traditional distance-vector updates containing information about networks plus the cost of reaching them from the perspective of the advertising router. And EIGRP has link-state characteristics as well—it synchronizes routing tables between neighbors at startup and then sends specific updates only when topology changes occur. This makes EIGRP suitable for very large networks. EIGRP has a maximum hop count of 255 (the default is set to 100). Don’t get confused by what I just said. EIGRP does not use hop count as a metric as RIP does; what hop count means with EIGRP is how many routers an EIGRP route update packet can go through before it is discarded. This limits the size of the AS and, again, has no bearing on how metrics are calculated.

There are a number of powerful features that make EIGRP a real standout from other protocols. The main ones are listed here:

Support for IP and IPv6 (and some other useless routed protocols) via protocol-dependent modules

Considered classless (same as RIPv2 and OSPF)

Support for VLSM/CIDR

Support for summaries and discontiguous networks

Efficient neighbor discovery

Communication via Reliable Transport Protocol (RTP)

Best path selection via Diffusing Update Algorithm (DUAL)

Cisco calls EIGRP a distance-vector routing protocol or sometimes an advanced distance-vector or even a hybrid routing protocol.

Protocol-Dependent Modules

One of the most interesting features of EIGRP is that it provides routing support for multiple Network layer protocols: IP, IPX, AppleTalk, and now

IPv6. (Obviously we won’t use IPX and AppleTalk, but EIGRP does support them.) The only other routing protocol that comes close and supports multiple network layer protocols is

Intermediate System-to-Intermediate System (IS-IS)

.

EIGRP supports different Network layer protocols through the use of

protocol-dependent modules (PDMs)

. Each EIGRP PDM will maintain a separate series of tables containing the routing information that applies to a specific protocol. What this means to you is that there will be IP/EIGRP routing tables and IPv6/EIGRP routing tables, for example.

Neighbor Discovery

Before EIGRP routers are willing to exchange routes with each other, they must become neighbors. There are three conditions that must be met for neighborship establishment:

Hellos received

AS numbers match

Identical metrics (K values)

Link-state protocols tend to use Hello messages to establish neighborship (also called adjacency) because they normally do not send out periodic route updates and there has to be some mechanism to help neighbors realize when a new peer has moved in or an old one has left or gone down. To maintain the neighborship relationship, EIGRP routers must also continue receiving Hellos from their neighbors.

EIGRP routers that belong to different autonomous systems (ASs) don’t automatically share routing information and they don’t become neighbors. This behavior can be a real benefit when used in larger networks to reduce the amount of route information propagated through a specific AS. The only catch is that you might have to take care of redistribution between the different ASs manually.

The only time EIGRP advertises its complete information is when it discovers a new neighbor and forms an adjacency with it through the exchange of Hello packets. When this happens, both neighbors advertise all their information to one another. After each has learned its neighbor’s routes, only changes to the routing table are propagated from then on.

When EIGRP routers receive their neighbors’ updates, they store them in a local topology table. This table contains all known routes from all known neighbors and serves as the raw material from which the best routes are selected and placed into the routing table.

Let’s define some terms before we move on:

Feasible distance (FD)

This is the best metric among all paths to a remote network, including the metric to the neighbor that is advertising that remote network. The route with the lowest FD is the route that you will find in the routing table because it is considered the best path. The metric of a feasible distance is the metric reported by the neighbor (called reported or advertised distance) plus the metric to the neighbor reporting the route.

Reported/advertised distance (AD)

This is the metric of a remote network, as reported by a neighbor. It is also the routing table metric of the neighbor and is the same as the second number in parentheses as displayed in the topology table, the first number being the feasible distance.

Neighbor table

Each router keeps state information about adjacent neighbors. When a newly discovered neighbor is learned, the address and interface of the neighbor are recorded, and this information is held in the neighbor table, stored in RAM. There is one neighbor table for each protocol-dependent module. Sequence numbers are used to match acknowledgments with update packets. The last sequence number received from the neighbor is recorded so that out-of-order packets can be detected.

Topology table

The topology table is populated by the protocol-dependent modules and acted upon by the Diffusing Update Algorithm

(DUAL). It contains all destinations advertised by neighboring routers, holding each destination address and a list of neighbors that have advertised the destination. For each neighbor, the advertised metric (distance), which comes only from the neighbor’s routing table, is recorded as well as the FD. If the neighbor is advertising this destination, it must be using the route to forward packets.

The neighbor and topology tables are stored in RAM and maintained through the use of Hello and update packets. Yes, the routing table is also stored in RAM, but the information stored in the routing table is gathered only from the topology table.

Feasible successor

A feasible successor is a path whose advertised distance is less than the feasible distance of the current successor, and it is considered a backup route. EIGRP will keep up to 16 feasible successors in the topology table. Only the one with the best metric (the successor) is copied and placed in the routing table. The show ip eigrp topology

command will display all the EIGRP feasible successor routes known to a router.

A feasible successor is a backup route and is stored in the topology table. A successor route is stored in the topology table and is copied and placed in the routing table.

Successor

A successor route (think successful!) is the best route to a remote network. A successor route is used by EIGRP to forward traffic

to a destination and is stored in the routing table. It is backed up by a feasible successor route that is stored in the topology table—if one is available.

By using the successor, and having feasible successors in the topology table as backup links, the network can converge instantly, and updates to any neighbor make up the only traffic sent from EIGRP.

Reliable Transport Protocol (RTP)

EIGRP uses a proprietary protocol called

Reliable Transport Protocol (RTP)

to manage the communication of messages between EIGRPspeaking routers. And as the name suggests, reliability is a key concern of this protocol. Cisco has designed a mechanism that leverages multicasts and unicasts to deliver updates quickly and to track the receipt of the data.

When EIGRP sends multicast traffic, it uses the Class D address 224.0.0.10. As I said, each EIGRP router is aware of who its neighbors are, and for each multicast it sends out, it maintains a list of the neighbors who have replied. If EIGRP doesn’t get a reply from a neighbor, it will switch to using unicasts to resend the same data. If it still doesn’t get a reply after 16 unicast attempts, the neighbor is declared dead. People often refer to this process as

reliable multicast

.

Routers keep track of the information they send by assigning a sequence number to each packet. With this technique, it’s possible for them to detect the arrival of old, redundant, or out-of-sequence information.

Being able to do these things is highly important because EIGRP is a quiet protocol. It depends upon its ability to synchronize routing databases at startup time and then maintain the consistency of databases over time by communicating only changes. So the permanent loss of any packets, or the out-of-order execution of packets, can result in corruption of the routing database.

Diffusing Update Algorithm (DUAL)

EIGRP uses

Diffusing Update Algorithm (DUAL)

for selecting and maintaining the best path to each remote network. This algorithm allows for the following:

Backup route determination if one is available

Support of VLSMs

Dynamic route recoveries

Queries for an alternate route if no feasible successor route can be found

DUAL provides EIGRP with possibly the fastest route convergence time among all protocols. The key to EIGRP’s speedy convergence is twofold: First, EIGRP routers maintain a copy of all of their neighbors’ routes, which they use to calculate their own cost to each remote network—if the best path goes down, it may be as simple as examining the contents of the topology table to select the best replacement route. Second, if there isn’t a good alternative in the local topology table, EIGRP routers very quickly ask their neighbors for help finding one—they aren’t afraid to ask directions! Relying on other routers and leveraging the information, they provide accounts for the “diffusing” character of DUAL.

And as I said, the whole idea of the Hello protocol is to enable the rapid detection of new or dead neighbors. RTP answers this call by providing a reliable mechanism for conveying and sequencing update, query, and query response messages. Building upon this solid foundation, DUAL is responsible for selecting and maintaining information about the best paths.

Using EIGRP to Support Large Networks

EIGRP includes a bunch of cool features that make it suitable for use in large networks:

Support for multiple ASs on a single router

Support for VLSM and summarization

Route discovery and maintenance

Each of these capabilities adds one small piece to the complex puzzle of supporting a huge number of routers and multiple networks.

Multiple ASs

EIGRP uses autonomous system numbers to identify the collection of routers that share route information. Only routers that have the same autonomous system numbers share routes. In large networks, you can easily end up with really complicated topology and route tables, and that can markedly slow convergence during diffusing computation operations.

So what’s an administrator to do to mitigate the impact of managing really big networks? Well, it’s possible to divide the network into multiple distinct EIGRP autonomous systems, or ASs. Each AS is populated by a contiguous series of routers, and route information can be shared among the different ASs via redistribution.

The use of redistribution within EIGRP leads us to another interesting feature. Normally, the administrative distance (AD) of an EIGRP route is

90, but this is true only for what is known as an

internal EIGRP route

. These are routes originated within a specific autonomous system by EIGRP routers that are members of the same autonomous system. The other type of route is called an

external EIGRP route

and has an AD of 170, which is not so good. These routes appear within EIGRP route tables courtesy of manual redistribution, and they represent networks that originated outside of the EIGRP autonomous system. And it doesn’t matter if the routes originated from another EIGRP autonomous system or from another routing protocol such as OSPF—they’re all considered to be external routes when redistributed within EIGRP.

VLSM Support and Summarization

As one of the more sophisticated classless routing protocols, EIGRP supports the use of Variable Length Subnet Masks. This is really important because it allows for the conservation of address space through the use of subnet masks that more closely fit the host requirements, such as using

30-bit subnet masks for point-to-point networks. And because the subnet mask is propagated with every route update, EIGRP also supports the use of discontiguous subnets, something that gives us a lot more flexibility when designing the network’s IP address plan.

What’s a discontiguous network? I mentioned this term many times in Chapter 8 and it’s now time to get to the answer! It’s one internetwork that has two or more subnetworks of a classful network connected together by different classful networks. Sounds complicated, but it’s not. Let’s take a look. Figure 9-1 displays a typical discontiguous network.

Figure 9-1:

A discontiguous network

The subnets 172.16.10.0 and 172.16.20.0 are connected together with a 10.3.1.0 network. By default, for the purpose of route advertising, each router thinks it has the only 172.16.0.0 classful network.

It’s important to understand that discontiguous networks just won’t work with RIPv1 or Cisco’s old IGRP at all. And they don’t work by default on

RIPv2 or EIGRP either, but discontiguous networks do work on OSPF networks by default because OSPF does not auto-summarize like EIGRP.

Ah ha! So that must be the answer we’ve been looking for since Chapter 8. RIP, RIPv2, and EIGRP auto-summarize classful boundaries by default!

But no worries—there are ways to make this work; it just doesn’t work by default. I’ll show you how to fix this when we configure EIGRP.

EIGRP also supports the manual creation of summaries at any and all EIGRP routers on a per-interface basis, which can substantially reduce the size of the routing table because EIGRP automatically summarizes networks at their classful boundaries. Figure 9-2 shows how a router running

EIGRP would see the network plus the boundaries that it would auto-summarize.

Figure 9-2:

EIGRP auto-summarization

Obviously, this would never work by default! Make a note to yourself that RIPv1 and RIPv2 would also auto-summarize these same classful boundaries by default, but OSPF won’t.

Route Discovery and Maintenance

The hybrid nature of EIGRP is fully revealed in its approach to route discovery and maintenance. Like many link-state protocols, EIGRP supports the concept of neighbors that are discovered via a Hello process and whose states are monitored. Like many distance-vector protocols, EIGRP uses the routing-by-rumor mechanism I talked about earlier that implies that many routers never hear about a route update firsthand. Instead, they hear about it from another router that may also have heard about it from another one, and so on.

Given the huge amount of information that EIGRP routers have to collect, it makes sense that they have a place to store it, right? Well they do—

EIGRP uses a series of tables to store important information about its environment:

Neighborship table

The

neighborship table

(usually referred to as the neighbor table) records information about routers with whom neighborship relationships have been formed.

Topology table

The

topology table

stores the route advertisements received from each neighbor about every route in the internetwork.

Route table

The

route table

stores the routes that are currently used to make routing decisions. There would be separate copies (instances) of each of these tables for each protocol that is actively being supported by EIGRP, whether it’s IP or IPv6.

I am now going to discuss the EIGRP metrics and then move right into the

easy

configuration of EIGRP.

EIGRP Metrics

Another really sweet thing about EIGRP is that unlike many other protocols that use a single factor to compare routes and select the best possible path, EIGRP can use a combination of four, called a composite metric:

Bandwidth

Delay

Reliability

Load

EIGRP uses only bandwidth and delay of the line to determine the best path to a remote network by default. Cisco sometimes likes to call these

path bandwidth value

and

cumulative line delay

—go figure.

And it’s worth noting that there’s a fifth element,

maximum transmission unit (MTU)

size. This element has never been used in EIGRP calculations, but it’s a required parameter in some EIGRP-related commands, especially those involving redistribution. The value of the MTU element represents the smallest MTU value encountered along the path to the destination network.

Maximum Paths and Hop Count

By default, EIGRP can provide equal-cost load balancing across up to 4 links (RIP does this as well). However, you can have EIGRP actually loadbalance across up to 16 links (equal or unequal) by using the following command:

R1(config)#

router eigrp 10

R1(config-router)#

maximum-paths ?

<1-16> Number of paths

In addition, EIGRP has a default maximum hop count of 100, but it can be set up to 255. Chances are you wouldn’t want to ever change this, but if you did, here is how you would do it:

R1(config)#

router eigrp 10

R1(config-router)#

metric maximum-hops ?

<1-255> Hop count

As you can see from this router output, EIGRP can be set to a maximum of 255 hops, and even though it doesn’t use hop count in the path metric calculation, it still uses the maximum hop count to limit the scope of the AS.

Configuring EIGRP

Although EIGRP can be configured for IP, IPv6, IPX, and AppleTalk, as a future Cisco Certified Network Associate, you really only need to focus on the configuration of IP for now.

There are two modes from which EIGRP commands are entered: router configuration mode and interface configuration mode. Router configuration mode enables the protocol, determines which networks will run EIGRP, and sets global characteristics. Interface configuration mode allows customization of summaries and bandwidth.

To start an EIGRP session on a router, use the router eigrp

command followed by the autonomous system number of your network. You then enter the network numbers connected to the router using the network

command followed by the network number.

Let’s look at an example of enabling EIGRP for autonomous system 20 on a router connected to two networks, with the network numbers being

10.3.1.0/24 and 172.16.10.0/24:

Router#

config t

Router(config)#

router eigrp 20

Router(config-router)#

network 172.16.0.0

Router(config-router)#

network 10.0.0.0

Remember—as with RIP, you use the classful network address, which is all subnet and host bits turned off. This is why EIGRP is so great—it has the complexity of a link-state protocol running in the background, with the same easy configuration of RIP.

Understand that the AS number is irrelevant—that is, as long as all routers use the same number! You can use any number from 1 to 65,535.

Say you need to stop EIGRP from working on a specific interface, such as a FastEthernet interface or a serial connection to the Internet. To do that, you would flag the interface as passive using the passive-interface interface

command, as discussed in Chapter 8 with RIP. The following command shows you how to make interface serial 0/1 a passive interface:

Router(config)#

router eigrp 20

Router(config-router)#

passive-interface serial 0/1

Doing this will prohibit the interface from sending or receiving Hello packets and, as a result, stop it from forming adjacencies. This means that it won’t send or receive route information on this interface.

Okay, let’s configure the same network that we configured in the last chapter with RIP and RIPv2. It doesn’t matter that RIPv2 (as well as our static routes) are already running—unless you’re worried about bandwidth consumption and CPU cycles, of course, because EIGRP has an AD of 90.

Remember that our static routes were changed to an AD of 150, and RIP is 120, so only EIGRP routes will populate the routing tables, even if RIP and static routing are enabled.

The impact of the

passive-interface

command depends upon the routing protocol under which the command is issued. For example, on an interface running RIP, the passive-interface

command will prohibit the sending of route updates but allow their receipt. Thus, a RIP router with a passive interface will still learn about the networks advertised by other routers. This is different from EIGRP, where a passive-interface

will neither send nor receive updates.

Figure 9-3 shows the network that we’ve been working with—the same one we’re going to use to configure with EIGRP.

Figure 9-3:

Our internetwork

So you can use it as a reminder, Table 9-1 contains the IP addresses we’ve been using on each interface.

Table 9-1:

Network addressing for the IP network

It’s actually really easy to add EIGRP to our internetwork—this is the beauty of EIGRP.

Corp

The AS number, as shown in the following router output, can be any number from 1 to 65,535. A router can be a member of as many ASs as you want it to be, but for this book’s purposes, we’re just going to configure a single AS:

Corp#

config t

Corp(config)#

router eigrp ?

<1-65535> Autonomous system number

Corp(config)#

router eigrp 10

Corp(config-router)#

network 10.0.0.0

The router eigrp [as]

command turns EIGRP routing on in the router. As with RIPv1, you still need to add the classful network numbers you want to advertise. But unlike RIP, EIGRP uses classless routing—but you still configure it as classful. Classless, which I’m sure you remember, means that the subnet mask information is sent along with routing protocol updates (RIPv2 is classless).

R1

To configure the R1 router, all you need to do is turn on EIGRP routing using AS 10 and then add the network number like this:

R1#

config t

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#

router eigrp 10

R1(config-router)#

network 10.0.0.0

R1(config-router)#

%DUAL-5-NBRCHANGE: IP-EIGRP 10: Neighbor 10.1.2.1 (Serial0/0/0) is up: new adjacency

%DUAL-5-NBRCHANGE: IP-EIGRP 10: Neighbor 10.1.3.1 (Serial0/0/1) is up: new adjacency

R1(config-router)#

network 192.168.10.0

R1(config-router)#

network 192.168.20.0

The R1 router found the Corp neighbor—the two routers are adjacent! Notice that it found both links connected between the routers. This is a good thing.

R2

To configure the R2 router, all I need to do is again turn on EIGRP using AS 10:

R2#

config t

Enter configuration commands, one per line. End with CNTL/Z.

R2(config)#

router eigrp 10

R2(config-router)#

network 10.0.0.0

R2(config-router)#

%DUAL-5-NBRCHANGE: IP-EIGRP 10: Neighbor 10.1.4.1 (Serial0/0/0) is up: new adjacency

R2(config-router)#

network 192.168.30.0

R2(config-router)#

network 192.168.40.0

That’s it—really! Most routing protocols are pretty simple to set up, and EIGRP is no exception. But that’s only for the basic configuration, of course.

R3

To configure the R3 router, all I need to do is turn on EIGRP using AS 10:

R3#

config t

Enter configuration commands, one per line. End with CNTL/Z.

R3(config)#

router eigrp 10

R3(config-router)#

network 10.0.0.0

R3(config-router)#

%DUAL-5-NBRCHANGE: IP-EIGRP 10: Neighbor 10.1.5.1 (FastEthernet0/0) is up: new adjacency

R3(config-router)#

network 172.16.0.0

That’s it, done.

Our configuration seems pretty solid, but remember—only our directly connected and EIGRP routes are going to wind up in the routing table because EIGRP has the lowest AD. So by having RIP running in the background, we’re not only using more memory and CPU cycles on the router, we’re sucking up precious bandwidth across every one of our links! This is definitely not good, and it’s something you’ll really want to keep in mind.

Let’s check out the Corp’s routing table:

Corp#

sh ip route

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

D 172.16.0.0/16 [90/28160] via 10.1.5.2, 00:01:48, FastEthernet0/0

S 172.16.10.0/24 [150/0] via 10.1.5.2

D 192.168.10.0/24 [90/2172416] via 10.1.3.2, 00:05:07, Serial0/0/1

[90/2172416] via 10.1.2.2, 00:05:07, Serial0/0/0

D 192.168.20.0/24 [90/2172416] via 10.1.2.2, 00:05:04, Serial0/0/0

[90/2172416] via 10.1.3.2, 00:05:04, Serial0/0/1

D 192.168.30.0/24 [90/20514560] via 10.1.4.2, 00:03:32, Serial0/1/0

D 192.168.40.0/24 [90/20514560] via 10.1.4.2, 00:03:29, Serial0/1/0

Corp#

Okay, cool—all routes are showing up as “D” for DUAL. Let’s take a look at R2’s routing table:

R2#

sh ip route

[output cut]

10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks

D 10.0.0.0/8 is a summary, 00:02:27, Null0

D 10.1.1.0/24 [90/27769856] via 10.1.4.1, 00:02:31, Serial0/0/0

D 10.1.2.0/24 [90/2681856] via 10.1.4.1, 00:02:31, Serial0/0/0

D 10.1.3.0/24 [90/2681856] via 10.1.4.1, 00:02:31, Serial0/0/0

C 10.1.4.0/24 is directly connected, Serial0/0/0

D 10.1.5.0/24 [90/2172416] via 10.1.4.1, 00:02:31, Serial0/0/0

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

D 172.16.0.0/16 [90/2172416] via 10.1.4.1, 00:00:42, Serial0/0/0

S 172.16.10.0/24 [150/0] via 10.1.4.1

D 192.168.10.0/24 [90/2684416] via 10.1.4.1, 00:02:31, Serial0/0/0

D 192.168.20.0/24 [90/2684416] via 10.1.4.1, 00:02:31, Serial0/0/0

C 192.168.30.0/24 is directly connected, FastEthernet0/0

C 192.168.40.0/24 is directly connected, FastEthernet0/1

R2#

We can see all the networks in the routing table, including our extra route to the 172.16.10.0 network still. Finally! Let’s fix this!

Configuring Discontiguous Networks

There’s one more configuration that you need to be aware of that has to do with auto-summarization. Remember

Figure 9-2 and how it demonstrated how EIGRP would auto-summarize the boundaries on a discontiguous network? Take a look at that figure again, and then I’ll provide a sample configuration on both routers with EIGRP.

In the

Figure 9-1 , the Lab_A router is connected to a 172.16.10.0/24 network and the 10.3.1.0/24 backbone. The Lab_B router is connected to the 172.16.20.0/24 network and the 10.3.1.0/24 backbone. Both routers, by default, would automatically summarize across classful boundaries and routing would not work. Here’s the configuration that would make this network work:

Lab_A#

config t

Lab_A(config)#

router eigrp 100

Lab_A(config-router)#

network 172.16.0.0

Lab_A(config-router)#

network 10.0.0.0

Lab_A(config-router)#

no auto-summary

Lab_B#

config t

Lab_B(config)#

router eigrp 100

Lab_B(config-router)#

network 172.16.0.0

Lab_B(config-router)#

network 10.0.0.0

Lab_B(config-router)#

no auto-summary

Because I used the no auto-summary

command, EIGRP will advertise all the subnets between the two routers. If the networks were larger, you could then provide manual summarization on these same boundaries.

So, with this in mind, why is our Corp router showing the extra route to 172.16.0.0 network? The configuration on R3 is this:

R3(config)#

router eigrp 10

R3(config-router)#

network 10.0.0.0

R3(config-router)#

network 172.16.0.0

There are actually two answers to our 172.16.0.0 mystery. R3 has a pretty solid classful boundary from the 10.0.0.0 network to the 172.16.0.0

network and will auto-summarize. But so will our other routers in our internetwork. Take a look at R1:

R1#

sh ip route

10.0.0.0/8 is variably subnetted, 6 subnets, 2 masks

D 10.0.0.0/8 is a summary, 00:10:14, Null0

D 10.1.1.0/24 [90/27769856] via 10.1.2.1, 00:10:18, Serial0/0/0

[90/27769856] via 10.1.3.1, 00:10:18, Serial0/0/1

C 10.1.2.0/24 is directly connected, Serial0/0/0

C 10.1.3.0/24 is directly connected, Serial0/0/1

D 10.1.4.0/24 [90/21024000] via 10.1.2.1, 00:10:18, Serial0/0/0

[90/21024000] via 10.1.3.1, 00:10:18, Serial0/0/1

D 10.1.5.0/24 [90/2172416] via 10.1.2.1, 00:10:18, Serial0/0/0

[90/2172416] via 10.1.3.1, 00:10:18, Serial0/0/1

172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

D 172.16.0.0/16 [90/2172416] via 10.1.3.1, 00:06:54, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:06:54, Serial0/0/0

S 172.16.10.0/24 [150/0] via 10.1.3.1

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

D 192.168.30.0/24 [90/21026560] via 10.1.2.1, 00:08:38, Serial0/0/0

[90/21026560] via 10.1.3.1, 00:08:38, Serial0/0/1

D 192.168.40.0/24 [90/21026560] via 10.1.3.1, 00:08:35, Serial0/0/1

[90/21026560] via 10.1.2.1, 00:08:35, Serial0/0/0

R1#

Okay, we’re still seeing the 172.16.0.0 issue, but the R1 router is summarizing the 10.0.0.0 network out the FastEthernet links, which isn’t necessarily a problem as our internetwork does not have discontiguous networking, but let’s turn off auto-summary on our network:

Corp#

config t

Corp(config)#

router eigrp 10

Corp(config-router)#

no auto-summary

R1#

config t

R1(config)#

router eigrp 10

R1(config-router)#

no auto-summary

R2#

config t

R2(config)#

router eigrp 10

R2(config-router)#

no auto-summary

R3#

config t

R3(config)#

router eigrp 10

R3(config-router)#

no auto-summary

Let’s take a look at our routing Corp routing table now:

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

D 172.16.10.0 [90/28160] via 10.1.5.2, 00:03:18, FastEthernet0/0

D 192.168.10.0/24 [90/2172416] via 10.1.3.2, 00:03:19, Serial0/0/1

[90/2172416] via 10.1.2.2, 00:03:19, Serial0/0/0

D 192.168.20.0/24 [90/2172416] via 10.1.3.2, 00:03:19, Serial0/0/1

[90/2172416] via 10.1.2.2, 00:03:19, Serial0/0/0

D 192.168.30.0/24 [90/20514560] via 10.1.4.2, 00:03:19, Serial0/1/0

D 192.168.40.0/24 [90/20514560] via 10.1.4.2, 00:03:19, Serial0/1/0

Corp#

Notice that our mystery link is gone, and now let’s take a look at our R1 table:

10.0.0.0/24 is subnetted, 5 subnets

D 10.1.1.0 [90/27769856] via 10.1.3.1, 00:03:50, Serial0/0/1

[90/27769856] via 10.1.2.1, 00:03:50, Serial0/0/0

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

D 10.1.4.0 [90/21024000] via 10.1.3.1, 00:03:50, Serial0/0/1

[90/21024000] via 10.1.2.1, 00:03:50, Serial0/0/0

D 10.1.5.0 [90/2172416] via 10.1.3.1, 00:03:50, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:03:50, Serial0/0/0

172.16.0.0/24 is subnetted, 1 subnets

D 172.16.10.0 [90/2172416] via 10.1.3.1, 00:03:49, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:03:49, Serial0/0/0

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

D 192.168.30.0/24 [90/21026560] via 10.1.2.1, 00:03:50, Serial0/0/0

[90/21026560] via 10.1.3.1, 00:03:50, Serial0/0/1

D 192.168.40.0/24 [90/21026560] via 10.1.2.1, 00:03:50, Serial0/0/0

[90/21026560] via 10.1.3.1, 00:03:50, Serial0/0/1

R1#

No more auto-summarizing the 10.0.0.0 network out for R1. Okay, if we don’t have discontiguous networking, why did this solve our problem?

Well, we didn’t necessarily have a problem; we had a mystery route in our Corp router but our network was still working, and auto-summarizing in our network was fine as long as we didn’t scale incorrectly in the future and create a discontiguous network. Remember back to the beginning of

Chapter 8 where I discussed the longest match rule? Let’s discuss this.

172.16.10.0 is a better match than 172.16.0.0, and the Corp, R1, and R2 each had a static route with an AD of 150 to R3, with the longest match of 172.16.10.0. However, R3 was advertising with a summary that 172.16.0.0 was directly connected, so the routing tables inserted both. Once auto-summary was turned off, the route of 172.16.10.0 with a lower AD was advertised and the static route disappeared from the routing table. If our internetwork had an actual discontiguous network, RIPv2 and EIGRP would not have worked at all until we used the no auto-summary

command.

Load Balancing with EIGRP

You might know that by default, EIGRP can load-balance up to 4 equal-cost links. But did you remember that we can configure EIGRP to loadbalance across up to 16 equal-/unequal-cost links to a remote network? Well, we can, so let’s take a look at the load balancing that our Corp and

R1 routers have running. First, let’s take a look at the R1 routing table and make sure that EIGRP has already found both links between the routers:

R1#

sh ip route

10.0.0.0/24 is subnetted, 5 subnets

D 10.1.1.0 [90/27769856] via 10.1.3.1, 00:21:30, Serial0/0/1

[90/27769856] via 10.1.2.1, 00:21:30, Serial0/0/0

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

D 10.1.4.0 [90/21024000] via 10.1.3.1, 00:21:30, Serial0/0/1

[90/21024000] via 10.1.2.1, 00:21:30, Serial0/0/0

D 10.1.5.0 [90/2172416] via 10.1.3.1, 00:21:30, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:21:30, Serial0/0/0

172.16.0.0/24 is subnetted, 1 subnets

D 172.16.10.0 [90/2172416] via 10.1.3.1, 00:21:29, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:21:29, Serial0/0/0

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

D 192.168.30.0/24 [90/21026560] via 10.1.2.1, 00:21:30, Serial0/0/0

[90/21026560] via 10.1.3.1, 00:21:30, Serial0/0/1

D 192.168.40.0/24 [90/21026560] via 10.1.2.1, 00:21:30, Serial0/0/0

[90/21026560] via 10.1.3.1, 00:21:30, Serial0/0/1

R1#

You can see that we have two links to every remote route in our internetwork, and again, EIGRP will load-balance across the s0/0/0 and S0/0/1 links by default because they’re the same metric.

EIGRP really does offer some pretty cool features, and one of them is automatic load balancing. But how about bundling links? Well, EIGRP can allow us to do this too—even with no extra configuration! Let me show you how this works. I’m going to configure the links between our Corp and R1 routers with the same subnet, meaning both links will have all interfaces within the same subnet.

Check out my configuration as I configure the s0/0/1 on each router to be in the 10.1.2.0 network, which will place these interfaces in the same subnet as the S0/0/0 interfaces of both routers:

Corp#

config t

Corp(config)#

int s0/0/1

Corp(config-if)#

ip address 10.1.2.4 255.255.255.0

R1#

config t

R1(config)#

int s0/0/1

R1(config-if)#

ip address 10.1.2.3 255.255.255.0

R1(config-if)#

do show run | begin interface

interface Serial0/0/0

description 1st Connection to Corp Router

ip address 10.1.2.2 255.255.255.0

!

interface Serial0/0/1

description 2nd connection to Corp Router

ip address 10.1.2.3 255.255.255.0

Now both links have all four interfaces in the same subnet.

R1#

sh ip route

10.0.0.0/24 is subnetted, 5 subnets

D 10.1.1.0 [90/27769856] via 10.1.2.3, 00:21:30, Serial0/0/1

[90/27769856] via 10.1.2.1, 00:21:30, Serial0/0/0

C 10.1.2.0 is directly connected, Serial0/0/0

is directly connected, Serial0/0/1

D 10.1.4.0 [90/21024000] via 10.1.2.3, 00:21:30, Serial0/0/1

[90/21024000] via 10.1.2.1, 00:21:30, Serial0/0/0

D 10.1.5.0 [90/2172416] via 10.1.2.3, 00:21:30, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:21:30, Serial0/0/0

172.16.0.0/24 is subnetted, 1 subnets

D 172.16.10.0 [90/2172416] via 10.1.2.3, 00:21:29, Serial0/0/1

[90/2172416] via 10.1.2.1, 00:21:29, Serial0/0/0

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

D 192.168.30.0/24 [90/21026560] via 10.1.2.1, 00:21:30, Serial0/0/0

[90/21026560] via 10.1.2.3, 00:21:30, Serial0/0/1

D 192.168.40.0/24 [90/21026560] via 10.1.2.1, 00:21:30, Serial0/0/0

[90/21026560] via 10.1.2.3, 00:21:30, Serial0/0/1

R1#

To make this fabulous configuration work, EIGRP positively must be enabled first. If not, you’ll get an error on your router that the addresses overlap!

Did you notice there’s a subtle change or two in the routing table now? Networks 10.1.2.0 and 10.1.3.0 used to show up as individual, directly connected interfaces, but not anymore. Now only the 10.1.2.0 network shows up as two directly connected interfaces, and the router now has a

3MB pipe through that line instead of just two 1.5Mbps T1 links. And just because these changes are subtle doesn’t make them any less cool!

I am going to add subnet 10.1.3.0 back into the network so we can have some more fun with these dual links. I’ll go to the Corp and R1 s0/0/1 interfaces and conFigure 10-1.3.1/24 and 10.1.3.2/24. Now 10.1.3.0 is being advertised again, but let’s mix things up a bit and change the metric of the 10.1.3.0 link and see what happens:

R1#

config t

R1(config)#

int s0/0/1

R1(config-if)#

bandwidth 256

R1(config-if)#

delay 300000

Corp#

config t

Corp(config)#

int s0/0/1

Corp(config-if)#

bandwidth 256

Corp(config-if)#

delay 300000

Since by default EIGRP uses bandwidth and delay of the line to determine the best path to each network, I lowered the bandwidth and raised the delay of the s0/0/1 interfaces of the both the R1 and Corp routers. Now, let’s verify EIGRP on our network, plus check out what our dual links are up to now between the R1 and Corp routers.

Verifying EIGRP

There are several commands that can be used on a router to help you troubleshoot and verify the EIGRP configuration.

Table 9-2 contains all of the most important commands that are used in conjunction with verifying EIGRP operation and offers a brief description of what each command does.

Table 9-2:

EIGRP troubleshooting commands

Command Description/Function

show ip route show ip route eigrp

Shows the entire routing table

Shows only EIGRP entries in the routing table show ip eigrp neighbors

Shows all EIGRP neighbors show ip eigrp topology show ip protocols debug eigrp packet debug ip eigrp events

Shows entries in the EIGRP topology table

Shows routing protocols configuration

Shows Hello packets sent/received between adjacent routers

Shows EIGRP changes and updates as they occur on your network

Since EIGRP is pretty simple to configure, you’d be wise to study the verification and troubleshooting of EIGRP. I can’t remind you enough throughout this book that the CCENT/CCNA is a routing and switching course and exam. Twenty-five percent of the objectives are routing and 25 percent of the objectives are switching, which we’ll start working on in the next chapter. But we still have another 50 percent to cover: 25 percent is verification and troubleshooting routing, and the last 25 percent is verification and troubleshooting switching. So, with this in mind, let’s concentrate hard on this next section.

I’ll demonstrate how you would use the commands in

Table 9-2 by using them on our internetwork.

The following router output is from the Corp router in our example:

Corp#

sh ip route

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

D 172.16.10.0 [90/28160] via 10.1.5.2, 01:00:11, FastEthernet0/0

D 192.168.10.0/24 [90/2172416] via 10.1.2.2, 01:00:12, Serial0/0/0

D 192.168.20.0/24 [90/2172416] via 10.1.2.2, 01:00:12, Serial0/0/0

D 192.168.30.0/24 [90/20514560] via 10.1.4.2, 01:00:12, Serial0/1/0

D 192.168.40.0/24 [90/20514560] via 10.1.4.2, 01:00:12, Serial0/1/0

Corp#

You can see that all routes are there in the routing table (10.1.3.0 shows that it’s directly connected again) and we have only one link to each of the remote networks now! Notice that EIGRP routes are indicated with simply a

D

designation (DUAL) and that the default AD of these routes is 90.

This represents internal EIGRP routes. Let’s take a look at the R1 router table now that we’ve changed the metrics:

R1#

sh ip route

10.0.0.0/24 is subnetted, 5 subnets

D 10.1.1.0 [90/27769856] via 10.1.2.1, 00:59:38, Serial0/0/0

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

D 10.1.4.0 [90/21024000] via 10.1.2.1, 00:59:38, Serial0/0/0

D 10.1.5.0 [90/2172416] via 10.1.2.1, 00:59:38, Serial0/0/0

172.16.0.0/24 is subnetted, 1 subnets

D 172.16.10.0 [90/2172416] via 10.1.2.1, 00:59:37, Serial0/0/0

C 192.168.10.0/24 is directly connected, FastEthernet0/0

C 192.168.20.0/24 is directly connected, FastEthernet0/1

D 192.168.30.0/24 [90/21026560] via 10.1.2.1, 00:59:38, Serial0/0/0

D 192.168.40.0/24 [90/21026560] via 10.1.2.1, 00:59:38, Serial0/0/0

R1#

Again, in the R1 router we have only one route to each remote network, and the 10.1.3.0 network is our backup link. Obviously, it would be better

if we could use both links at the same time, but in my example, I made the 10.1.3.0 network a backup link.

Let’s go back to the Corp router and see what it shows us in the neighbor table:

Corp#

sh ip eigrp neighbors

IP-EIGRP neighbors for process 10

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 10.1.5.2 Fa0/0 14 01:02:00 40 1000 0 143

1 10.1.4.2 Se0/1/0 12 01:02:00 40 1000 0 114

2 10.1.2.2 Se0/0/0 11 01:02:00 40 1000 0 131

3 10.1.3.2 Se0/0/1 11 00:33:37 40 1000 0 132

We read the information in this output like this:

The H field indicates the order in which the neighbor was discovered.

The hold time is how long this router will wait for a Hello packet to arrive from a specific neighbor.

The uptime indicates how long the neighborship has been established.

The SRTT field is the smooth round-trip timer—an indication of the time it takes for a round-trip from this router to its neighbor and back. This value is used to determine how long to wait after a multicast for a reply from this neighbor. If a reply isn’t received in time, the router will switch to using unicasts in an attempt to complete the communication. The time between multicast attempts is specified by:

The Retransmission Time Out (RTO) field, which is the amount of time EIGRP waits before retransmitting a packet from the retransmission queue to a neighbor.

The Q value, which indicates whether there are any outstanding messages in the queue—consistently large values would indicate a problem.

The Seq field, which indicates the sequence number of the last update from that neighbor—something that’s used to maintain synchronization and avoid duplicate or out-of-sequence processing of messages.

The show ip eigrp neighbors

command allows you to check the IP addresses as well as the retransmit interval and queue counts for the neighbors that have established an adjacency—remember this.

Now let’s see what’s in the Corp topology table by using the show ip eigrp topology

command—this should be interesting!

Corp#

sh ip eigrp topology

IP-EIGRP Topology Table for AS 10

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - Reply status

P 10.1.1.0/24, 1 successors, FD is 25625600

via Connected, Vlan1

P 10.1.5.0/24, 1 successors, FD is 28160

via Connected, FastEthernet0/0

P 10.1.4.0/24, 1 successors, FD is 20512000

via Connected, Serial0/1/0

P 10.1.3.0/24, 1 successors, FD is 76809984

via Connected, Serial0/0/1

P 10.1.2.0/24, 1 successors, FD is 2169856

via Connected, Serial0/0/0

P 192.168.10.0/24, 1 successors, FD is 2172416

via 10.1.2.2 (2172416/28160), Serial0/0/0

via 10.1.3.2 (76828160/28160), Serial0/0/1

P 192.168.20.0/24, 1 successors, FD is 2172416

via 10.1.2.2 (2172416/28160), Serial0/0/0

via 10.1.3.2 (76828160/28160), Serial0/0/1

P 192.168.30.0/24, 1 successors, FD is 20514560

via 10.1.4.2 (20514560/28160), Serial0/1/0

P 192.168.40.0/24, 1 successors, FD is 20514560

via 10.1.4.2 (20514560/28160), Serial0/1/0

P 172.16.10.0/24, 1 successors, FD is 28160

via 10.1.5.2 (28160/25600), FastEthernet0/0

Notice that every route is preceded by a

P

. This means that the route is in the

passive state

, which is a good thing because routes in the

active state

(

A

) indicate that the router has lost its path to this network and is searching for a replacement. Each entry also indicates the feasible distance, or FD, to each remote network plus the next-hop neighbor through which packets will travel to their destination. Plus, each entry also has two numbers in parentheses. The first indicates the feasible distance (FD) and the second the advertised distance (AD) to a remote network.

Now here’s where things get interesting—notice that under the 192.168.10.0 and 192.168.20.0 outputs there are two links to each network and that the feasible distance of each are different. What this means is that we have one successor to the networks and one feasible successor—a backup route! So very cool!

Let’s take a closer look:

P 192.168.10.0/24, 1 successors, FD is 2172416

via 10.1.2.2 (2172416/28160), Serial0/0/0

via 10.1.3.2 (76828160/28160), Serial0/0/1

P 192.168.20.0/24, 1 successors, FD is 2172416

via 10.1.2.2 (2172416/28160), Serial0/0/0

via 10.1.3.2 (76828160/28160), Serial0/0/1

The FD is the feasible distance—the cost from the Corp router to get to that network. But we also need to look at the AD, or advertised distance.

For the 192.168.10.0, we’re seeing this in the table: via 10.1.2.2 (2172416/28160), Serial0/0/0 via 10.1.3.2 (76828160/28160), Serial0/0/1

For the s0/0/0 link, we see (2172416/28160); the first number is the FD and the second number is the AD. The R1 router is advertising the same cost of 28160 to get to the 192.168.10.0 and 192.168.20.0 network. However, the Corp router needs to add the cost of what it will take to get to each network, and this is where we get our FD. Since the s0/0/1 link has a lower bandwidth and higher delay, we can see that the s0/0/0 has a lower FD and that is the path placed in the routing table.

You need to remember that even though both routes are in the topology table, only the successor route (the ones with the lowest metrics) will be copied and placed into the routing table.

In order for the route to be a feasible successor, its advertised distance must be less than the feasible distance of the successor route.

EIGRP will load-balance across both links automatically when they are of equal variance (equal cost), but EIGRP can load-balance across unequal-cost links as well if we use the variance

command. The variance metric is set to 1 by default, meaning that only equal-cost links will loadbalance. You can change the variance anywhere up to 128. Changing a variance value enables EIGRP to install multiple, loop-free routes with unequal cost in a local routing table.

So basically, if the variance is set to 1, only routes with the same metric as the successor will be installed in the local routing table. And, for example, if the variance is set to 2, any EIGRP-learned route with a metric less than or equal to two times the successor metric will be installed in the local routing table (if it is already a feasible successor). This is a complicated configuration and you need to be careful before you start configuring the variance

command.

Let’s check out one last show

command before we look at a debugging output, the show ip protocols

command. We can get information about all routing protocols configured on our router, but let’s see what EIGRP shows us:

Corp#

sh ip protocols

Routing Protocol is "eigrp 10 "

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp 10

Automatic network summarization is in effect

Automatic address summarization:

Maximum path: 4

Routing for Networks:

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.1.5.2 90 40

10.1.3.2 90 6867

10.1.2.2 90 6916

10.1.4.2 90 8722

Distance: internal 90 external 170

Corp#

From the output of the show ip protocols

command, we can see the AS number and the metric weights called the “k” values, where bandwidth and delay of the line are used and enabled by default. Also the maximum hop count for a route update packet is shown (100 by default) as well as the variance, which is set to 1, meaning equal-cost load balancing. Maximum path 4 means that four equal-cost paths will load-balance by default.

Now’s a great time for us to check out some debugging outputs. First, let’s use the debug eigrp packet

command that will show our Hello packets being sent between neighbor routers:

Corp#

debug eigrp packet

EIGRP: Received HELLO on Serial0/1/0 nbr 10.1.4.2

AS 10, Flags 0x0, Seq 115/0 idbQ 0/0

EIGRP: Sending HELLO on Serial0/0/0

AS 10, Flags 0x0, Seq 148/0 idbQ 0/0 iidbQ un/rely 0/0

EIGRP: Received HELLO on Serial0/0/1 nbr 10.1.3.2

AS 10, Flags 0x0, Seq 133/0 idbQ 0/0

EIGRP: Received HELLO on Serial0/0/0 nbr 10.1.2.2

AS 10, Flags 0x0, Seq 133/0 idbQ 0/0

EIGRP: Received HELLO on FastEthernet0/0 nbr 10.1.5.2

AS 10, Flags 0x0, Seq 144/0 idbQ 0/0

EIGRP: Sending HELLO on Serial0/1/0

AS 10, Flags 0x0, Seq 148/0 idbQ 0/0 iidbQ un/rely 0/0

EIGRP: Sending HELLO on Serial0/0/1

AS 10, Flags 0x0, Seq 148/0 idbQ 0/0 iidbQ un/rely 0/0

EIGRP: Sending HELLO on FastEthernet0/0

AS 10, Flags 0x0, Seq 148/0 idbQ 0/0 iidbQ un/rely 0/0

EIGRP: Received HELLO on Serial0/1/0 nbr 10.1.4.2

AS 10, Flags 0x0, Seq 115/0 idbQ 0/0

EIGRP: Sending HELLO on Vlan1

AS 10, Flags 0x0, Seq 148/0 idbQ 0/0 iidbQ un/rely 0/0

EIGRP: Sending HELLO on Serial0/0/0

AS 10, Flags 0x0, Seq 148/0 idbQ 0/0 iidbQ un/rely 0/0

Since my Corp router is connected to three EIGRP neighbors, and because the 224.0.0.10 multicast is sent out every 5 seconds, I didn’t have any problem seeing the updates. The Hello packets are sent out every active interface as well as all the interfaces that we have neighbors connected to. Did you notice that the AS number is provided in the update? This is because if a neighbor doesn’t have the same AS number, the

Hello update would just be discarded.

I know you’ve learned a lot about EIGRP so far, but stick around because you’re not done with this chapter just yet! It’s now time to give you the skinny on OSPF!

Open Shortest Path First (OSPF) Basics

Open Shortest Path First (OSPF)

is an open standard routing protocol that’s been implemented by a wide variety of network vendors, including

Cisco. If you have multiple routers and not all of them are Cisco (what!), then you can’t use EIGRP, can you? So your remaining CCNA objective options are basically RIP, RIPv2, and OSPF. If it’s a large network, then, really, your only options are OSPF and something called route redistribution—a translation service between routing protocols.

OSPF works by using the Dijkstra algorithm. First, a shortest path tree is constructed, and then the routing table is populated with the resulting best paths. OSPF converges quickly, although perhaps not as quickly as EIGRP, and it supports multiple, equal-cost routes to the same destination. Like EIGRP, it does support both IP and IPv6 routed protocols.

OSPF provides the following features:

Consists of areas and autonomous systems

Minimizes routing update traffic

Allows scalability

Supports VLSM/CIDR

Has unlimited hop count

Allows multi-vendor deployment (open standard)

OSPF is the first link-state routing protocol that most people are introduced to, so it’s useful to see how it compares to more traditional distancevector protocols such as RIPv2 and RIPv1. Table 9-3 gives you a comparison of these three protocols.

Table 9-3:

OSPF and RIP comparison

OSPF has many features beyond the few I’ve listed in Table 9-3 , and all of them contribute to a fast, scalable, and robust protocol that can be actively deployed in thousands of production networks.

OSPF is supposed to be designed in a hierarchical fashion, which basically means that you can separate the larger internetwork into smaller internetworks called areas. This is the best design for OSPF.

The following are reasons for creating OSPF in a hierarchical design:

To decrease routing overhead

To speed up convergence

To confine network instability to single areas of the network

This does not make configuring OSPF easier, but more elaborate and difficult.

Figure 9-4 shows a typical OSPF simple design. Notice how some routers connect to the backbone—called area 0, or the backbone area.

OSPF must have an area 0, and all other areas should connect to this area. (Areas that do not connect directly to area 0 can be connected by using virtual links, which are beyond the scope of this book.) Routers that connect other areas to the backbone area within an AS are called Area Border

Routers (ABRs). Still, at least one interface of the ABR must be in area 0.

Figure 9-4:

OSPF design example

OSPF runs inside an autonomous system, but it can also connect multiple autonomous systems together. The router that connects these ASs is called an Autonomous System Boundary Router (ASBR).

Ideally, you would create other areas of networks to help keep route updates to a minimum in larger networks and to keep problems from propagating throughout the network, basically isolating them to a single area.

As in the sections on EIGRP, I’ll first cover the essential terminology you need to understand OSPF.

OSPF Terminology

Imagine how challenging it would be if you were given a map and compass but had no knowledge of east or west, north or south, river or mountain, lake or desert. You’d probably not get very far putting your new tools to good use without knowing about this stuff. For this reason, you’ll begin your exploration of OSPF with a long list of terms that will prevent you from getting lost in the later sections. The following are important OSPF terms to familiarize yourself with before you proceed:

Link

A

link

is a network or router interface assigned to any given network. When an interface is added to the OSPF process, it’s considered by OSPF to be a link. This link, or interface, will have state information associated with it (up or down) as well as one or more IP addresses.

Router ID

The

Router ID (RID)

is an IP address used to identify the router. Cisco chooses the Router ID by using the highest IP address of all configured loopback interfaces. If no loopback interfaces are configured with addresses, OSPF will choose the highest IP address of all active physical interfaces.

Neighbor Neighbors

are two or more routers that have an interface on a common network, such as two routers connected on a point-to-point serial link.

Adjacency

An

adjacency

is a relationship between two OSPF routers that permits the direct exchange of route updates. OSPF is really picky about sharing routing information—unlike EIGRP, which directly shares routes with all of its neighbors. Instead, OSPF directly shares routes only with neighbors that have also established adjacencies. And not all neighbors will become adjacent—this depends upon both the type of network and the configuration of the routers.

Hello protocol

The OSPF Hello protocol provides dynamic neighbor discovery and maintains neighbor relationships. Hello packets and Link

State Advertisements (LSAs) build and maintain the topological database. Hello packets are addressed to multicast address 224.0.0.5.

Neighborship database

The

neighborship database

is a list of all OSPF routers for which Hello packets have been seen. A variety of details, including the Router ID and state, are maintained on each router in the neighborship database.

Topological database

The

topological database

contains information from all of the Link State Advertisement packets that have been received for an area. The router uses the information from the topology database as input into the Dijkstra algorithm that computes the shortest path to every network.

LSA packets are used to update and maintain the topological database.

Link State Advertisement

A

Link State Advertisement (LSA)

is an OSPF data packet containing link-state and routing information that’s shared among OSPF routers. There are different types of LSA packets, and I’ll go into these shortly. An OSPF router will exchange LSA packets only with routers to which it has established adjacencies.

Designated router

A

designated router (DR)

is elected whenever OSPF routers are connected to the same multi-access network. Cisco likes to call these “broadcast” networks, but really, they are networks that have multiple recipients. Try not to confuse multi-access with multipoint, which can be easy to do sometimes.

A prime example is an Ethernet LAN. To minimize the number of adjacencies formed, a DR is chosen (elected) to disseminate/receive routing information to/from the remaining routers on the broadcast network or link. This ensures that their topology tables are synchronized. All routers on the shared network will establish adjacencies with the DR and backup designated router (BDR)—I’ll define this next. The election is won by the router with the highest priority, and the highest Router ID is used as a tiebreaker if the priority of more than one router turns out to be the

same.

Backup designated router

A

backup designated router (BDR)

is a hot standby for the DR on multi-access links (remember that Cisco sometimes likes to call these “broadcast” networks). The BDR receives all routing updates from OSPF adjacent routers but doesn’t flood LSA updates.

OSPF areas

A n

OSPF area

is a grouping of contiguous networks and routers. All routers in the same area share a common Area ID.

Because a router can be a member of more than one area at a time, the Area ID is associated with specific interfaces on the router. This would allow some interfaces to belong to area 1 while the remaining interfaces can belong to area 0. All of the routers within the same area have the same topology table. When configuring OSPF, you’ve got to remember that there must be an area 0 and that this is typically considered the backbone area. Areas also play a role in establishing a hierarchical network organization—something that really enhances the scalability of OSPF!

Broadcast (multi-access) Broadcast (multi-access) networks

such as Ethernet allow multiple devices to connect to (or access) the same network as well as provide a

broadcast

ability in which a single packet is delivered to all nodes on the network. In OSPF, a DR and a BDR must be elected for each broadcast multi-access network.

Non-broadcast multi-access Non-broadcast multi-access (NBMA) networks

are types such as Frame Relay, X.25, and Asynchronous

Transfer Mode (ATM). These networks allow for multi-access but have no broadcast ability like Ethernet. So, NBMA networks require special

OSPF configuration to function properly and neighbor relationships must be defined.

DR and BDR are elected on broadcast and non-broadcast multi-access networks. Elections are covered in detail later in this chapter.

Point-to-point Point-to-point

refers to a type of network topology consisting of a direct connection between two routers that provides a single communication path. The point-to-point connection can be physical, as in a serial cable directly connecting two routers, or it can be logical, as in two routers that are thousands of miles apart yet connected by a circuit in a Frame Relay network. In either case, this type of configuration eliminates the need for DRs or BDRs—but neighbors are discovered automatically.

Point-to-multipoint Point-to-multipoint refers to a type of network topology consisting of a series of connections between a single interface on one router and multiple destination routers. All of the interfaces on all of the routers sharing the point-to-multipoint connection belong to the same network. As with point-to-point, no DRs or BDRs are needed.

All of these terms play an important part in understanding the operation of OSPF, so again, make sure you’re familiar with each of them. Reading through the rest of this chapter will help you to place the terms within their proper context.

SPF Tree Calculation

Within an area, each router calculates the best/shortest path to every network in that same area. This calculation is based upon the information collected in the topology database and an algorithm called

shortest path first (SPF)

. Picture each router in an area constructing a tree—much like a family tree—where the router is the root and all other networks are arranged along the branches and leaves. This is the shortest path tree used by the router to insert OSPF routes into the routing table.

It’s important to understand that this tree contains only networks that exist in the same area as the router itself does. If a router has interfaces in multiple areas, then separate trees will be constructed for each area. One of the key criteria considered during the route selection process of the

SPF algorithm is the metric or cost of each potential path to a network. But this SPF calculation doesn’t apply to routes from other areas.

OSPF uses a metric referred to as

cost

. A cost is associated with every outgoing interface included in an SPF tree. The cost of the entire path is the sum of the costs of the outgoing interfaces along the path. Because cost is an arbitrary value as defined in RFC 2338, Cisco had to implement its own method of calculating the cost for each OSPF-enabled interface. Cisco uses a simple equation of 10

8

/bandwidth. The bandwidth is the configured bandwidth for the interface. Using this rule, a 100Mbps Fast Ethernet interface would have a default OSPF cost of 1 and a 10Mbps

Ethernet interface would have a cost of 10.

An interface set with a bandwidth of 64,000 would have a default cost of 1,563.

This value may be overridden by using the ip ospf cost

command. The cost is manipulated by changing the value to a number within the range of 1 to 65,535. Because the cost is assigned to each link, the value must be changed on the interface for which you want to change the cost.

Cisco bases link cost on bandwidth. Other vendors may use other metrics to calculate a given link’s cost. When connecting links between routers from different vendors, you may have to adjust the cost to match another vendor’s router. Both routers must assign the same cost to the link for OSPF to work properly.

Configuring OSPF

Configuring basic OSPF isn’t as simple as configuring RIP and EIGRP, and it can get really complex once the many options that are allowed within

OSPF are factored in. But that’s okay—for your studies, you should be interested in the basic single-area OSPF configuration. The following sections describe how to configure single-area OSPF.

These two elements are the basic elements of OSPF configuration:

Enabling OSPF

Configuring OSPF areas

Enabling OSPF

The easiest and also least scalable way to configure OSPF is to just use a single area. Doing this requires a minimum of two commands.

The command you use to activate the OSPF routing process is as follows:

Router(config)#

router ospf ?

<1-65535>

A value in the range from 1 to 65,535 identifies the OSPF Process ID. It’s a unique number on this router that groups a series of OSPF configuration commands under a specific running process. Different OSPF routers don’t have to use the same Process ID to communicate. It’s purely a local value that essentially has little meaning, but it cannot start at 0; it has to start at a minimum of 1.

You can have more than one OSPF process running simultaneously on the same router if you want, but this isn’t the same as running multi-area

OSPF. The second process will maintain an entirely separate copy of its topology table and manage its communications independently of the first process. And because the CCNA objectives only cover single-area OSPF with each router running a single OSPF process, that’s what I’m going to focus on in this book.

The OSPF Process ID is needed to identify a unique instance of an OSPF database and is locally significant.

Configuring OSPF Areas

After identifying the OSPF process, you need to identify the interfaces that you want to activate OSPF communications on as well as the area in which each resides. This will also configure the networks you’re going to advertise to others. OSPF uses wildcards in the configuration—which are also used in access list configurations (covered in Chapter 13).

Here’s an OSPF basic configuration example for you:

Router#

config t

Router(config)#

router ospf 1

Router(config-router)#

network 10.0.0.0 0.255.255.255

area ?

<0-4294967295> OSPF area ID as a decimal value

A.B.C.D OSPF area ID in IP address format

Router(config-router)#

network 10.0.0.0 0.255.255.255

area 0

The areas can be any number from 0 to 4.2 billion. Don’t get these numbers confused with the Process ID, which is from 1 to 65,535.

Remember, the OSPF Process ID number is irrelevant. It can be the same on every router on the network, or it can be different—doesn’t matter.

It’s locally significant and just enables the OSPF routing on the router.

The arguments of the network

command are the network number (10.0.0.0) and the wildcard mask (0.255.255.255). The combination of these two numbers identifies the interfaces that OSPF will operate on and will also be included in its OSPF LSA advertisements. Based on my sample configuration, OSPF will use this command to find any interface on the router configured in the 10.0.0.0 network, and it will place any interface it finds into area 0. Notice that you can create about 4.2 billion areas. (A router wouldn’t let you actually create that many, but you can certainly name them using the numbers up to 4.2 billion.) You can also label an area using an IP address format.

A quick explanation of wildcards: A 0 octet in the wildcard mask indicates that the corresponding octet in the network must match exactly. On the other hand, a 255 indicates that you don’t care what the corresponding octet is in the network number. A network and wildcard mask combination of 1.1.1.1 0.0.0.0 would match an interface configured exactly with 1.1.1.1 only, and nothing else. This is really useful if you want to activate OSPF on a specific interface in a very clear and simple way. If you insist on matching a range of networks, the network and wildcard mask combination of

1.1.0.0 0.0.255.255 would match any interface in the range of 1.1.0.0 to 1.1.255.255. Because of this, it’s simpler and safer to stick to using wildcard masks of 0.0.0.0 and identify each OSPF interface individually, but once configured, they function exactly the same—one way is not better than the other.

The final argument is the area number. It indicates the area to which the interfaces identified in the network and wildcard mask portion belong.

Remember that OSPF routers will become neighbors only if their interfaces share a network that’s configured to belong to the same area number.

The format of the area number is either a decimal value from the range 1 to 4,294,967,295 or a value represented in standard dotted-decimal notation. For example, area 0.0.0.0 is a legitimate area and is identical to area 0.

Wildcard Example

Before getting down to configuring our network, let’s take a quick peek at a harder OSPF network configuration to find out what our OSPF network statements would be if we were using subnets and wildcards.

You have a router with these four subnets connected to four different interfaces:

192.168.10.64/28

192.168.10.80/28

192.168.10.96/28

192.168.10.8/30

All interfaces need to be in area 0. Seems to me, the easiest configuration would be this:

Test#

config t

Test(config)#

router ospf 1

Test(config-router)#

network 192.168.10.0 0.0.0.255 area 0

The preceding example is pretty simple, but easy isn’t always best, so although this example is an easy way to configure OSPF, what fun is that?

And worse yet, the CCNA objectives are not likely to cover something so simple for you! So let’s create a separate network statement for each interface using the subnet numbers and wildcards. That would look something like this:

Test#

config t

Test(config)#

router ospf 1

Test(config-router)#

network 192.168.10.64 0.0.0.15 area 0

Test(config-router)#

network 192.168.10.80 0.0.0.15 area 0

Test(config-router)#

network 192.168.10.96 0.0.0.15 area 0

Test(config-router)#

network 192.168.10.8 0.0.0.3 area 0

Wow, now that’s a different looking config! Truthfully, OSPF would work exactly the same way as in the easy configuration I showed you first—but unlike the easy configuration, this one covers the CCNA objectives!

Although this looks complicated, trust me, it is not. All you need to do is understand your block sizes! So all you have to do is just remember when configuring wildcards that they’re always one less than the block size. A /28 is a block size of 16, so we’d add our network statement using the subnet number and then add a wildcard of 15 in the interesting octet. For the /30, which is a block size of 4, we’d use a wildcard of 3. Once you practice this a few times, it’s pretty simple, and we’ll see them again when we get to access lists.

Let’s use

Figure 9-5 as an example and configure that network with OSPF using wildcards to make sure you have a solid grip on this. Figure 9-5 shows a three-router network with the IP addresses of each interface.

Figure 9-5:

Sample OSPF wildcard configuration

The very first thing you need to be able to do is to look at each interface and determine the subnet that the addresses are in. Hold on, I know what you’re thinking: “Why don’t I just use the exact IP addresses of the interface with the 0.0.0.0 wildcard?” Well, you can, but we’re paying attention to

CCNA objectives here, not just what’s easiest, remember?

The IP addresses for each interface are shown in the figure. The Lab_A router has two directly connected subnets: 192.168.10.64/29 and

10.255.255.80/30. Here’s the OSPF configuration using wildcards:

Lab_A#

config t

Lab_A(config)#

router ospf 1

Lab_A(config-router)#

network 192.168.10.64 0.0.0.7 area 0

Lab_A(config-router)#

network 10.255.255.80 0.0.0.3 area 0

The Lab_A router is using a /29, or 255.255.255.248, mask on the ethernet0 interface. This is a block size of 8, which is a wildcard of 7. The s0 interface is a mask of 255.255.255.252—block size of 4, with a wildcard of 3. You can’t configure OSPF this way if you can’t look at the IP address and slash notation and then figure out the subnet, mask, and wildcard, can you? So don’t take your exam until you can do this.

Here are our other two configurations to help you practice:

Lab_B#

config t

Lab_B(config)#

router ospf 1

Lab_B(config-router)#

network 192.168.10.48 0.0.0.7 area 0

Lab_B(config-router)#

network 10.255.255.80 0.0.0.3 area 0

Lab_B(config-router)#

network 10.255.255.8 0.0.0.3 area 0

Lab_C#

config t

Lab_C(config)#

router ospf 1

Lab_C(config-router)#

network 192.168.10.16 0.0.0.7 area 0

Lab_C(config-router)#

network 10.255.255.8 0.0.0.3 area 0

As I mentioned with the Lab_A configuration, you’ve got to be able to determine the subnet, mask, and wildcard just by looking at the IP address and mask of an interface. If you can’t do that, you won’t be able to configure OSPF using wildcards as I just demonstrated. So go over this until you’re really comfortable with it!

Configuring Our Network with OSPF

Okay—now we get to have some fun! Let’s configure our internetwork with OSPF using just area 0. Before we do that, we’ve got to remove EIGRP because OSPF has an administrative distance of 110. (EIGRP is 90—but you already knew that, right?). Let’s remove RIP while we’re at it, just because I don’t want you to get in the habit of having RIP running on your network.

There’s a bunch of different ways to configure OSPF, and as I said, the simplest and easiest is to use the wildcard mask of 0.0.0.0. But I want to demonstrate that we can configure each router differently with OSPF and still come up with the exact same result. This is one reason why OSPF is more fun than other routing protocols—it gives us all a lot more ways to screw things up, which provides a troubleshooting opportunity! We’ll use our network as shown in Figure 9-3 to configure OSPF.

Corp

Here’s the Corp router’s configuration:

Corp#

config t

Corp(config)#

no router eigrp 10

Corp(config)#

no router rip

Corp(config)#

router ospf 132

Corp(config-router)#

network 10.1.1.1 0.0.0.0 area 0

Corp(config-router)#

network 10.1.2.1 0.0.0.0 area 0

Corp(config-router)#

network 10.1.3.1 0.0.0.0 area 0

Corp(config-router)#

network 10.1.4.1 0.0.0.0 area 0

Corp(config-router)#

network 10.1.5.1 0.0.0.0 area 0

Hmmmm—it seems we have a few things to discuss here. First, I removed EIGRP and RIP and then added OSPF. So why did I use OSPF 132?

It really doesn’t matter—the number is irrelevant. I guess it just felt good to use 132!

The network commands are pretty straightforward. I typed in the IP address of each interface and used the wildcard mask of 0.0.0.0, which means that the IP address must match each octet exactly. But if this is one of those times where easier is better—just do this:

Corp(config)#

router ospf 132

Corp(config-router)#

network 10.1.0.0 0.0.255.255 area 0

One line instead of five! I really want you to understand that no matter which way you configure the network statement, OSPF will work the same here. Now, let’s move on to R1. To keep things simple, we’re going to use our same sample configuration.

R1

The R1 router has four directly connected networks. Instead of typing in each interface, I can use the one network command example and still make it work exactly the same:

R1#

config t

R1(config)#

no router eigrp 10

R1(config)#

no router rip

R1(config)#

router ospf 1

R1(config-router)#

network 10.1.0.0 0.0.255.255 area0

^

% Invalid input detected at '^' marker.

R1(config-router)#

network 10.1.0.0 0.0.255.255 area 0

R1(config-router)#

14:12:39: %OSPF-5-ADJCHG: Process 1, Nbr 10.1.5.1 on Serial0/0/0 from LOADING to FULL, Loading Done

R1(config-router)#

14:12:43: %OSPF-5-ADJCHG: Process 1, Nbr 10.1.5.1 on Serial0/0/1 from LOADING to FULL, Loading Done

R1(config-router)#

network 192.168.0.0 0.0.255.255 area 0

Okay—other than my little typo, where I forgot to place a space between the area command and the area number, this is truly a fast and efficient configuration.

All I did was to first disable EIGRP, and then I turned on OSPF routing process 1 and added the network command 10.1.0.0 with a wildcard of

0.0.255.255. What this did is basically say, “Find any interface that starts with 10.1, and place those interfaces into area 0.” Last, I added both

192.168.10.0 and 192.168.20.0 with one configuration line. Quick, easy, and slick!

R2

Let’s give the R2 router that’s directly connected to three networks some attention:

R2#

config t

R2(config)#

no router eigrp 10

R2(config)#

no router rip

R2(config)#

router ospf 45678

R2(config-router)#

network 10.0.0.0 0.0.0.255 area 0

R2(config-router)#

network 192.168.30.1 0.0.0.0 area 0

R2(config-router)#

network 192.168.40.1 0.0.0.0 area

I can use any process ID I want—as long as it’s a value from 1 to 65,535. And notice I used the 10.0.0.0 with wildcard 0.255.255.255 and then I used the 0.0.0.0 wildcard configuration for my 192.168.30 and 40.0 networks. This works well too.

R3

Finally, our last router! For the R3 router, we need to turn off RIP and EIGRP, and then configure OSPF.

R3(config)#

no router eigrp 10

R3(config)#

no router rip

R3(config)#

router ospf 1

R3(config-router)#

network 10.1.5.1 0.0.0.0 area 0

R3(config-router)#

network 172.16.10.0 0.0.0.255 area 0

Cool! Now that we’ve configured all the routers with OSPF, what’s next? Miller Time? Nope—not yet. It’s that verification thing again. We still have to make sure that OSPF is really working. That’s exactly what we’re going to do next.

Verifying OSPF Configuration

There are several ways to verify proper OSPF configuration and operation, and in the following sections I’ll show you the OSPF show

commands you need to know in order to do this. We’re going to start by taking a quick look at the routing table of the Corp router.

So, let’s issue a show ip route

command on the Corp router see if there is a troubleshooting opportunity for us:

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.5.2

O 192.168.10.0/24 [110/65] via 10.1.2.2, 00:01:55, Serial0/0/0

O 192.168.20.0/24 [110/65] via 10.1.2.2, 00:01:55, Serial0/0/0

S 192.168.30.0/24 [150/0] via 10.1.4.2

S 192.168.40.0/24 [150/0] via 10.1.4.2

The Corp router shows only two dynamic routes for internetwork, with the

O

representing OSPF internal routes (the

C

s are obviously our directly connected networks), but what’s with the

S

in the routing table?

So, unlike EIGRP, our little internetwork just didn’t “work” the first time I configured the routers. Let’s look at the problems and fix them. Let’s start with the 192.168.30.0 and 40.0; those should not be showing up as static. Let’s run over to R2 and take a look at what the configuration is:

!

router ospf 45678

log-adjacency-changes

network 10.0.0.0 0.0.0.255 area 0

network 192.168.30.1 0.0.0.0 area 0

network 192.168.40.1 0.0.0.0 area 0

!

The 192.168.30.0 and 40.0 looks correct, but I see a mistake in my first line for the 10.0.0.0 network. Do you see it? That’s right, my wildcards are telling OSPF to match the first three octets exactly, and I don’t have any interface that starts with 10.0.0, so I need to redo that network statement:

R2(config)#

router ospf 45678

R2(config-router)#

no network 10.0.0.0 0.0.0.255 area 0

R2(config-router)#

network 10.1.4.0 0.0.0.255 area 0

So, I took out the wrong statement and then configured a correct network statement. Let’s take a look at the Corp routing table now:

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

S 172.16.10.0 [150/0] via 10.1.5.2

O 192.168.10.0/24 [110/65] via 10.1.2.2, 00:09:50, Serial0/0/0

O 192.168.20.0/24 [110/65] via 10.1.2.2, 00:09:50, Serial0/0/0

O 192.168.30.0/24 [110/782] via 10.1.4.2, 00:00:02, Serial0/1/0

O 192.168.40.0/24 [110/782] via 10.1.4.2, 00:00:02, Serial0/1/0

Corp#

Okay, that’s better…but we’re still seeing that mystery 172.16.0.0 network that’s been popping in and out of our Corp routing table. Let’s take a look at R3 and see what I did wrong with the configuration: router ospf 1

log-adjacency-changes

network 10.1.5.1 0.0.0.0 area 0

network 172.16.10.0 0.0.0.255 area 0

Ah, another typo. See how easy this is to do with OSPF? Do you see my error? Typing show ip interface brief

should help you see it:

R3#

sh ip int brief

Interface IP-Address OK? Method Status Protocol

FastEthernet0/0 10.1.5.2 YES manual up up

Dot11Radio0/0/0 172.16.10.1 YES manual up up

You should see that my FastEthernet is 10.1.5.2, not the 10.1.5.1 that is in my OSPF configuration. Remember, in your network statements always type in your directly connected interfaces or networks, not the remote router’s networks. Here’s how I’ll fix it and then we should be up:

R3(config)#

router ospf 1

R3(config-router)#

no network 10.1.5.1 0.0.0.0 area 0

R3(config-router)#

network 10.1.5.2 0.0.0.0 area 0

Now let’s take a last look at our Corp routing table. We should be good to go now:

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.1.0 is directly connected, Vlan1

C 10.1.2.0 is directly connected, Serial0/0/0

C 10.1.3.0 is directly connected, Serial0/0/1

C 10.1.4.0 is directly connected, Serial0/1/0

C 10.1.5.0 is directly connected, FastEthernet0/0

172.16.0.0/24 is subnetted, 1 subnets

O 172.16.10.0 [110/2] via 10.1.5.2, 00:00:28, FastEthernet0/0

O 192.168.10.0/24 [110/65] via 10.1.2.2, 00:15:34, Serial0/0/0

O 192.168.20.0/24 [110/65] via 10.1.2.2, 00:15:34, Serial0/0/0

O 192.168.30.0/24 [110/782] via 10.1.4.2, 00:05:47, Serial0/1/0

O 192.168.40.0/24 [110/782] via 10.1.4.2, 00:05:47, Serial0/1/0

Now that’s a nice-looking OSPF routing table. It is important that you can troubleshoot and fix an OSPF network as I showed in my example here.

It is very easy to make little mistakes with OSPF, so watch for the little details.

It’s time to show you all the OSPF verification commands that you need to know.

The show ip ospf Command

The show ip ospf

command is used to display OSPF information for one or all OSPF processes running on the router. Information contained therein includes the Router ID, area information, SPF statistics, and LSA timer information. Let’s check out the output from the Corp router:

Corp#

sh ip ospf

Routing Process "ospf 132" with ID 10.1.5.1

Supports only single TOS(TOS0) routes

Supports opaque LSA

SPF schedule delay 5 secs, Hold time between two SPFs 10 secs

Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs

Number of external LSA 0. Checksum Sum 0x000000

Number of opaque AS LSA 0. Checksum Sum 0x000000

Number of DCbitless external and opaque AS LSA 0

Number of DoNotAge external and opaque AS LSA 0

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

External flood list length 0

Area BACKBONE(0)

Number of interfaces in this area is 5

Area has no authentication

SPF algorithm executed 5 times

Area ranges are

Number of LSA 5. Checksum Sum 0x0283f4

Number of opaque link LSA 0. Checksum Sum 0x000000

Number of DCbitless LSA 0

Number of indication LSA 0

Number of DoNotAge LSA 0

Flood list length 0

Notice the Router ID (RID) of 10.1.5.1, which is the highest IP address configured on the router.

The show ip ospf database Command

Using the show ip ospf database

command will give you information about the number of routers in the internetwork (AS) plus the neighboring router’s

ID (this is the topology database I mentioned earlier). Unlike the show ip eigrp topology

command, this command shows the OSPF routers, not each and every link in the AS as EIGRP does.

The output is broken down by area. Here’s a sample output, again from Corp:

Corp#

sh ip ospf database

OSPF Router with ID (10.1.5.1) (Process ID 132)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count

192.168.20.1 192.168.20.1 1585 0x80000006 0x00ae08 6

192.168.40.1 192.168.40.1 1005 0x80000005 0x0069c7 4

10.1.5.1 10.1.5.1 688 0x80000009 0x008108 8

172.16.10.1 172.16.10.1 688 0x80000004 0x0021a6 2

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

10.1.5.1 10.1.5.1 688 0x80000001 0x00c977

You can see all four routers and the RID of each router (the highest IP address on each router). The router output shows the link ID—remember that an interface is also a link—and the RID of the router on that link under the ADV router, or advertising router.

The show ip ospf interface Command

The show ip ospf interface

command displays all interface-related OSPF information. Data is displayed about OSPF information for all OSPFenabled interfaces or for specified interfaces. (I’ll bold some of the important things.)

Corp#sh ip ospf int f0/0

FastEthernet0/0 is up, line protocol is up

Internet address is 10.1.5.1/24, Area 0

Process ID 132, Router ID 10.1.5.1, Network Type BROADCAST, Cost: 1

Transmit Delay is 1 sec,

State DR

,

Priority 1

Designated Router (ID) 10.1.5.1

, Interface address 10.1.5.1

Backup Designated Router (ID) 172.16.10.1

, Interface address 10.1.5.2

Timer intervals configured,

Hello 10, Dead 40

, Wait 40, Retransmit 5

Hello due in 00:00:04

Index 5/5, flood queue length 0

Next 0x0(0)/0x0(0)

Last flood scan length is 1, maximum is 1

Last flood scan time is 0 msec, maximum is 0 msec

Neighbor Count is 1, Adjacent neighbor count is 1

Adjacent with neighbor 172.16.10.1 (Backup Designated Router)

Suppress hello for 0 neighbor(s)

The following information is displayed by this command:

Interface IP address

Area assignment

Process ID

Router ID

Network type

Cost

Priority

DR/BDR election information (if applicable)

Hello and Dead timer intervals

Adjacent neighbor information

The reason I used the show ip ospf interface f0/0 command is because I knew that there would be a designated router elected on the FastEthernet broadcast multi-access network between our Corp and R3 routers. We’ll get into DR and BDR elections in detail in a minute, as well as the other information that I bolded—all very important! We’ll especially come back to the timers shown in the show ip ospf interface

command output later.

Okay, so as an example you type in the show ip ospf interface

command and receive this response:

Corp#

sh ip ospf int f0/0

%OSPF: OSPF not enabled on FastEthernet0/0

This error occurs when OSPF is enabled on the router, but not the interface. You need to check your network statements because the interface you are trying to verify is not in your OSPF process.

The show ip ospf neighbor Command

The show ip ospf neighbor

command is super-useful because it summarizes the pertinent OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that information will also be displayed. Here’s a sample:

Corp#

sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

172.16.10.1 1 FULL/DR 00:00:39 10.1.5.2 FastEthernet0/0

192.168.20.1 0 FULL/ - 00:00:38 10.1.2.2 Serial0/0/0

192.168.20.1 0 FULL/ - 00:00:38 10.1.3.2 Serial0/0/1

192.168.40.1 0 FULL/ - 00:00:36 10.1.4.2 Serial0/1/0

This is a super-important command to understand because it’s extremely useful in production networks. Let’s take a look at the R3 router output:

R3#

sh ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

10.1.5.1 1 FULL/BDR 00:00:31 10.1.5.1 FastEthernet0/0

Since there’s an Ethernet link (broadcast multi-access) on the link between the R3 and the Corp router, there’s going to be an election to determine who will be the designated router (DR) and who will be the backup designated router (BDR). We can see that the R3 became the designated router, and it won because it had the highest IP address on the network. You can change this, but that’s the default.

The reason that the Corp connections to R1 and R2 don’t have a DR or BDR listed in the output is that by default, elections don’t happen on point-to-point links and they show FULL/ - .But we can see that the Corp router is fully adjacent to all three routers from its output.

The show ip protocols Command

The show ip protocols

command is also useful, whether you’re running OSPF, EIGRP, IGRP, RIP, BGP, IS-IS, or any other routing protocol that can be configured on your router. It provides an excellent overview of the actual operation of all currently running protocols.

Check out the output from the Corp router:

Corp#

sh ip protocols

Routing Protocol is "ospf 132"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 10.1.5.1

Number of areas in this router is 1. 1 normal 0 stub 0 nssa

Maximum path: 4

Routing for Networks:

10.1.1.1 0.0.0.0 area 0

10.1.2.1 0.0.0.0 area 0

10.1.3.1 0.0.0.0 area 0

10.1.4.1 0.0.0.0 area 0

10.1.5.1 0.0.0.0 area 0

Routing Information Sources:

Gateway Distance Last Update

10.1.5.1 110 00:05:16

172.16.10.1 110 00:05:16

192.168.20.1 110 00:16:36

192.168.40.1 110 00:06:55

Distance: (default is 110)

From looking at this output, you can determine the OSPF Process ID, OSPF Router ID, type of OSPF area, networks and areas configured for

OSPF, and the OSPF Router IDs of neighbors—that’s a lot. Read efficient! And hold on a second. Did you notice the absence of timers like the ones we were shown before in the RIP outputs from this command? That’s because link-state routing protocols don’t use timers to keep the network stable like distance-vector routing algorithms do.

Debugging OSPF

Debugging is a great tool for any protocol, so let’s take a look in

Table 9-4 at a few debugging commands for troubleshooting OSPF.

Table 9-4:

Debugging commands for troubleshooting OSPF

Command Description/Function

debug ip ospf packet

Shows Hello packets being sent and received on your router.

debug ip ospf hello debug ip ospf adj

Shows Hello packets being sent and received on your router. Shows more detail than the debug ip ospf packet

output.

Shows DR and BDR elections on a broadcast or non-broadcast multi-access network.

I’ll start by showing you the output from the Corp router I got using the debug ip ospf packet

command:

Corp#

debug ip ospf packet

OSPF packet debugging is on

*Mar 23 01:20:45.507: OSPF: rcv. v:2 t:1 l:48 rid:10.1.2.2

aid:0.0.0.0 chk:8076 aut:0 auk: from Serial0/0/0

*Mar 23 01:20:45.531: OSPF: rcv. v:2 t:1 l:48 rid: 10.1.4.2

aid:0.0.0.0 chk:8076 aut:0 auk: from Serial0/1/0

*Mar 23 01:20:45.531: OSPF: rcv. v:2 t:1 l:48 rid: 10.1.5.2

aid:0.0.0.0 chk:8074 aut:0 auk: from FastEthernet0/0

In the preceding output, we can see that our router is receiving Hello packets from neighbor (adjacent) routers. OSPF sends Hello packets every

10 seconds.

The next debug command I’m going show you is the debug ip ospf adj

command that will show us elections as they occur on broadcast and nonbroadcast multi-access networks, an important command for our next section. To get output, I’ll shut down F0/0 on R3 and then enable it again:

Corp#

debug ip ospf adj

OSPF adjacency events debugging is on

05:32:12: %OSPF-5-ADJCHG: Process 132, Nbr 172.16.10.1 on

FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

05:32:12: OSPF: Build router LSA for area 0, router ID 10.1.5.1, seq 0x80000016

05:32:12: OSPF: DR/BDR election on FastEthernet0/0

05:32:12: OSPF: Elect BDR 0.0.0.0

05:32:12: OSPF: Elect DR 0.0.0.0

05:32:12: OSPF: Elect BDR 0.0.0.0

05:32:12: OSPF: Elect DR 0.0.0.0

05:32:12: DR: none BDR: none

05:32:12: OSPF: Build router LSA for area 0, router ID 10.1.5.1, seq 0x80000017

05:32:12: OSPF: Build router LSA for area 0, router ID 10.1.5.1, seq 0x80000017

Corp#

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

05:33:57: OSPF: end of Wait on interface FastEthernet0/0

05:33:57: OSPF: DR/BDR election on FastEthernet0/0

05:33:57: OSPF: Elect BDR 172.16.10.1

05:33:57: OSPF: Elect DR 172.16.10.1

05:33:57: DR: 172.16.10.1 (Id) BDR: 172.16.10.1 (Id)

05:33:57: OSPF: Send DBD to 172.16.10.1 on FastEthernet0/0 seq 0x2d9e opt 0x00 flag 0x7 len 32

05:33:57: OSPF: Build router LSA for area 0, router ID 10.1.5.1, seq 0x80000018

05:33:57: OSPF: DR/BDR election on FastEthernet0/0

05:33:57: OSPF: Elect BDR 10.1.5.1

05:33:57: OSPF: Elect DR 172.16.10.1

05:33:57: OSPF: Elect BDR 10.1.5.1

05:33:57: OSPF: Elect DR 172.16.10.1

05:33:57: DR: 172.16.10.1 (Id) BDR: 10.1.5.1 (Id)

05:33:57: OSPF: Build router LSA for area 0, router ID 10.1.5.1, seq 0x80000018

All right—let’s move on and discover how elections occur in an OSPF network.

OSPF DR and BDR Elections

In this chapter, I have discussed OSPF in detail; however, I need to expand the section on designated routers and backup designated routers that

I’ve only briefly touched on so far. I’m also going to delve deeper into verifying the election process as well as provide you with a hands-on lab at the end of the chapter to help you understand that process even better.

To start with, I need to make sure you fully understand the terms

neighbors

and

adjacencies

again because they’re really crucial to the DR and

BDR election process. The election process happens when a broadcast or non-broadcast multi-access network is connected to a router and the link comes up. (Think Ethernet or Frame Relay.)

Neighbors

Routers that share a common segment become neighbors on that segment. These neighbors are elected via the Hello protocol. Hello packets are sent periodically out of each interface using IP multicast.

Two routers won’t become neighbors unless they agree on the following:

Area ID

The idea here is that the two routers’ interfaces have to belong to the same area on a particular segment. And of course, those interfaces have to belong to the same subnet.

Authentication

OSPF allows for the configuration of a password for a specific area. Although authentication between routers isn’t required, you have the option to set it if you need to do so. Also, keep in mind that in order for routers to become neighbors, they need to have the same password on a segment if you’re using authentication.

Hello and Dead intervals

OSPF exchanges Hello packets on each segment. This is a keepalive system used by routers to acknowledge their existence on a segment and for electing a designated router (DR) and backup designated router on both broadcast and non-broadcast multi-access segments.

The Hello interval specifies the number of seconds between Hello packets. The Dead interval is the number of seconds that a router’s Hello packets can go without being seen before its neighbors declare the OSPF router dead (down). OSPF requires these intervals to be exactly the same between two neighbors. If either of these intervals is different, the routers won’t become neighbors on that segment. You can see these timers with the show ip ospf interface

command.

Adjacencies

In the election process, adjacency is the next step after the neighboring process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process. In order to minimize the amount of information exchanged on a particular segment,

OSPF elects one router to be a designated router (DR) and one router to be a backup designated router (BDR) on each multi-access segment.

The BDR is elected as a backup router in case the DR goes down. The idea behind this is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router sends its information to the

DR and BDR. The DR then relays the information to everybody else.

DR and BDR Elections

DR and BDR election is accomplished via the Hello protocol. Hello packets are exchanged via IP multicast packets on each segment. However, only segments that are broadcast and non-broadcast multi-access networks (such as Ethernet and Frame Relay) will perform DR and BDR

elections. Point-to-point links, like a serial WAN for example, will not have a DR/BDR election process.

On a broadcast or non-broadcast multi-access network, the router with the highest OSPF priority on a segment will become the DR for that segment. This priority is shown with the show ip ospf interface

command and is set to 1 by default. If all routers have the default priority set, the router with the highest Router ID (RID) will win.

As you know, the RID is determined by the highest IP address on any interface at the moment of OSPF startup. This can be overridden with a loopback (logical) interface, which I’ll talk about in the next section.

If you set a router’s interface to a priority value of zero, that router won’t participate in the DR or BDR election on that interface. The state of the interface with priority zero will then be DROTHER.

Now let’s play with the RID on an OSPF router.

OSPF and Loopback Interfaces

Configuring loopback interfaces when using the OSPF routing protocol is important, and Cisco suggests using them whenever you configure

OSPF on a router.

Loopback interfaces

are logical interfaces, which are virtual, software-only interfaces; they are not real router interfaces. Using loopback interfaces with your OSPF configuration ensures that an interface is always active for OSPF processes.

They can be used for diagnostic purposes as well as OSPF configuration. The reason you want to configure a loopback interface on a router is because if you don’t, the highest active IP address on a router at the time of bootup will become that router’s RID. The RID is used to advertise the routes as well as elect the DR and BDR.

By default, OSPF uses the highest IP address on any active interface at the moment of OSPF startup. However, this can be overridden by a logical interface. The highest IP address of any logical interface will always become a router’s RID.

In the following sections, you will see how to configure loopback interfaces and how to verify loopback addresses and RIDs.

Configuring Loopback Interfaces

Configuring loopback interfaces rocks mostly because it’s the easiest part of OSPF configuration, and we all need a break about now—right? So hang on—we’re in the home stretch!

First, let’s see what the RID is on the Corp router with the show ip ospf

command:

Corp#

sh ip ospf

Routing Process "ospf 132" with ID 10.1.5.1

[output cut]

We can see that the RID is 10.1.5.1, or the FastEthernet0/0 interface of the router. So let’s configure a loopback interface using a completely different IP addressing scheme:

Corp(config)#

int loopback 0

*Mar 22 01:23:14.206: %LINEPROTO-5-UPDOWN: Line protocol on Interface

Loopback0, changed state to up

Corp(config-if)#

ip address 172.31.1.1 255.255.255.255

The IP scheme really doesn’t matter here, but each has to be in a separate subnet. By using the /32 mask, we can use any IP address we want as long as the addresses are never the same on any two routers.

Let’s configure the other routers:

R1#

config t

R1(config)#

int loopback 0

*Mar 22 01:25:11.206: %LINEPROTO-5-UPDOWN: Line protocol on Interface

Loopback0, changed state to up

R1(config-if)#

ip address 172.31.1.2 255.255.255.255

Here’s the configuration of the loopback interface on R2:

R2#

config t

R2(config)#

int loopback 0

*Mar 22 02:21:59.686: %LINEPROTO-5-UPDOWN: Line protocol on Interface

Loopback0, changed state to up

R2(config-if)#

ip address 172.31.1.3 255.255.255.255

Here’s the configuration of the loopback interface on R3. I am going to use a different IP and I’ll explain why in a second:

R3#

config t

R3(config)#

int loopback 0

*Mar 22 02:01:49.686: %LINEPROTO-5-UPDOWN: Line protocol on Interface

Loopback0, changed state to up

R3(config-if)#

ip address 172.31.100.4 255.255.255.255

I’m pretty sure you’re wondering what the IP address mask of 255.255.255.255 (/32) means and why we don’t just use 255.255.255.0 instead.

Well, either mask works, but the /32 mask is called a host mask and works fine for loopback interfaces, and it allows us to save subnets. Notice how I was able to use 172.31.1.1, .2, .3, and .4? If I didn’t use the /32, I’d have to use a separate subnet for each and every router!

Now, before we move on, did we actually change the RIDs of our router by setting the loopback interfaces? Let’s check into that by taking a look

at the Corp’s RID:

Corp#

sh ip ospf

Routing Process "ospf 132" with ID 10.1.5.1

What happened? You’d think that because we set logical interfaces, the IP addresses under the logical interfaces automatically become the RID of the router, right? Well, sort of—but only if you do one of two things: either reboot the router or delete OSPF and re-create the database on your router. And neither is really that great an option.

I’m going with rebooting the Corp router because it’s the easier of the two.

Now let’s look and see what our RID is:

Corp#

sh ip ospf

Routing Process "ospf 132" with ID 172.31.1.1

Okay, that did it. The Corp router now has a new RID! So I guess I’ll just go ahead and reboot all my routers to get their RIDs reset to our logical addresses.

Or not—there is

one

other way. What would you say about adding a new RID for the router right under the router ospf process-id

command instead?

I’d say let’s give it a shot! Here’s an example of doing that on the R3 router:

R3#

sh ip ospf

Routing Process "ospf 1" with ID 10.1.12.1

R3#

config t

R3(config)#

router ospf 1

R3(config-router)#

router-id 172.31.1.4

R3(config-router)#Reload or use "clear ip ospf process" command, for this to take effect

R3(config-router)#

do clear ip ospf process

Reset ALL OSPF processes? [no]:

yes

20:16:35: %OSPF-5-ADJCHG: Process 1, Nbr 10.1.5.1 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Adjacency forced to reset

20:16:35: %OSPF-5-ADJCHG: Process 1, Nbr 10.1.5.1 on FastEthernet0/0 from FULL to DOWN, Neighbor Down: Interface down or detached

R3(config-router)#

do sh ip ospf

Routing Process "ospf 1" with ID 172.31.1.4

Take a look at that—it worked! We changed the RID without reloading the router! But wait—remember, we set a loopback (logical interface) earlier. So does the loopback interface win over the router-id

command? Well, we can see our answer. A logical (loopback) interface will override the router-id

command, and we don’t have to reboot the router to make it take effect as the RID.

not

So it goes in this order:

1.

Highest active interface by default.

2.

Highest logical interface overrides a physical interface.

3.

The router-id

overrides the interface and loopback interface.

The only thing left now is to decide whether you want to advertise the loopback interfaces under OSPF. There are pros and cons to using an address that won’t be advertised versus using an address that will be. Using an unadvertised address saves on real IP address space, but the address won’t appear in the OSPF table, which means you can’t ping it.

So basically, what you’re faced with here is a choice that equals a trade-off between the ease of debugging the network and conservation of address space—what to do? A really tight strategy is to use a private IP address scheme as I did. Do this, and all will be well!

OSPF Interface Priorities

Another way to configure DRs and BDRs in OSPF is to “fix” elections instead of using loopback interfaces. We can do this by configuring interfaces on our router to gain a better priority over another router when elections occur. In other words, we can use priorities instead of logical addresses to force a certain router to become the DR or BDR in a network.

Let’s use

Figure 9-6 as an example. Looking at Figure 9-6 , what options would you use to ensure that the R2 router will be elected the designated router (DR) for the LAN (broadcast multi-access) segment? The first thing you’d need to do is determine what the RID is of each router and which router is the default DR for the 172.16.1.0 LAN.

At this point, we can see that R3 will be the DR by default because it has the highest RID of 192.168.11.254. That gives us three options to ensure that R2 will be elected the DR for the LAN segment 172.16.1.0/24:

Configure the priority value of the Fa0/0 interface of the R2 router to a higher value than any other interface on the Ethernet network.

Configure a loopback interface on the R2 with an IP address higher than any IP address on the other routers.

Change the priority value of the Fa0/0 interface of R1 and R3 to zero.

If we set a priority of zero (0) on the R1 and R3 routers, they wouldn’t be allowed to participate in the election process. But that may not be the best way to go—we might just be better off choosing options one and two.

Since you already know how to configure a loopback (logical) interface, here’s how to set a priority on the Fa0/0 interface on the R2 router:

R2#

config t

R2(config)#

int f0/0

R2(config-if)#

ip ospf priority ?

<0-255> Priority

R2(config-if)#

ip ospf priority 2

Figure 9-6:

Ensuring your designated router

That’s it! All router interfaces default to a priority of 1, so by setting this interface to 2, I’ve ensured that it will automatically become the DR of the

LAN segment. Setting an interface to 255 means that no one can beat your router!

Hold on though. Even if you change the priority of the interface, the router will not become the DR of the LAN segment until both the existing DR and the BDR are shut down. Once an election occurs, that’s all she wrote, and the election won’t happen again until the DR and BDR are reloaded and/or shut down. Just having a router with a better RID come up on your network doesn’t mean your DR or BDR will change!

You can see your priority with the show ip ospf interface

command:

R2(config-if)#

do show ip ospf int f0/0

FastEthernet0/0 is up, line protocol is up

Internet Address 10.1.13.1/24, Area 0

Process ID 132, Router ID 172.16.30.1, Network Type BROADCAST,Cost:1

Transmit Delay is 1 sec, State UP,

Priority 2

Remember, you can see the elections occur on a broadcast or non-broadcast multi-access network with the debug ip ospf adj

command.

Troubleshooting OSPF

This section will have you verify sample OSPF configurations and configuration outputs in order to troubleshoot, maintain, and fix OSPF-related issues.

If you see a configuration as shown here, you must know that there is no way a router will accept this input because the wildcard is incorrect:

Router(config)#

router ospf 1

Router(config-router)#

network 10.0.0.0 255.0.0.0 area 0

This would be the correct statement:

Router(config)#

router ospf 1

Router(config-router)#

network 10.0.0.0 0.255.255.255 area 0

Next, let’s take a look at a figure and see if we can determine which of the routers will become the designated router of the area. Figure 9-7 shows a network with six routers connected by two switches and a WAN link.

Figure 9-7:

Designated router example

Looking at Figure 9-7 , which routers are likely to be elected as a designated router (DR)? All the router OSPF priorities are at the default.

Notice the RIDs of each router. The routers with th