CCNA: Training Guide

CCNA: Training Guide

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Introduction

CCNA Training Guide is designed for advanced end-

users, service technicians, and network administrators with the goal of certification as a Cisco Certified

Network Associate. The CCNA exam measures your ability to install and configure Cisco routers and switches in an internetwork in order to improve performance, bandwidth, security, and connectivity.

This book is your one-stop shop. Everything you need to know to prepare for the exam is in here. You do not have to take a class in addition to buying this book to pass the exam. However, depending on your personal study habits or learning style, you may benefit from buying this book and taking a class.

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This book conducts a tour of all the areas covered by the CCNA exam and teaches you the specific skills you need to achieve your Network Associate certification.

You’ll also find helpful hints, tips, real-world examples, exercises, and references to additional study materials.

Specifically, this book is set up to help you in the following ways:

á

Organization. This book is organized by major exam topics and individual exam objectives. Every objective you need to know for the CCNA exam is covered in this book. The objectives are not covered in exactly the same order as they are listed by Cisco. However, we have attempted to organize the topics in the most logical and accessible fashion to make it as easy as possible for you to learn the information. We have also attempted to make the information accessible in the following ways:

• The full list of exam topics and objectives is included in this introduction.

• Each chapter begins with a list of the objectives to be covered.

• Following the list of objectives, you’ll find an outline that provides you an overview of the material and the page numbers where particular topics can be found.

• The objectives are repeated in the text directly under a heading where the material most directly relevant to it is covered (unless the whole chapter addresses a single objective).

• Information about where the objectives are covered is also conveniently condensed in the tear card at the front of this book.

á

Instructional Features. This book has been designed to provide you with multiple ways to learn and reinforce the exam material. Following are some of the helpful methods:

Objective explanations. As mentioned previously, each chapter begins with a list of the objectives covered in the chapter. In addition, immediately following each objective is an explanation of it in a context that defines it more meaningfully.

Study strategies. Following the chapter outline, you’ll find strategies for how to approach studying and retaining the material in the chapter, particularly as it is addressed on the exam.

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2 C C N A T R A I N I N G G U I D E

Exam tips. Exam tips appear in the margins to provide specific exam-related advice. Such tips might address what material is covered

(or not covered) on the exam, how it is covered, mnemonic devices, or particular quirks of that exam.

Review breaks and summaries. Crucial information is summarized at various points in the book in lists or tables. Each chapter includes a summary section, as well.

Key terms. A list of key terms appears near the end of each chapter.

Notes. These appear in the margins and contain various kinds of useful information, such as tips on technology or administrative practices, historical background on terms and technologies, or side commentary on industry issues.

Warnings. When using sophisticated information technology, there is always the potential for mistakes or even catastrophes that can occur because of improper application of the technology. Warnings appear in the margins to alert you to such potential problems.

In-depths. These more extensive discussions cover material that may not be directly relevant to the exam, but which is useful as reference material or in everyday practice.

In-depth sidebars may also provide useful background or contextual information necessary for understanding the larger topic under consideration.

Step by Steps. These are hands-on, tutorial instructions that lead you through a particular task or function relevant to the exam objectives.

Exercises. Found near the end of each chapter in the “Apply Your Knowledge” section, exercises may include additional tutorial material as well as other types of problems and questions.

Case studies. Presented throughout the book, case studies provide you with a more conceptual opportunity to apply and reinforce the knowledge you are developing. Case studies include a description of a scenario, the essence of the case, and an extended analysis section.

They also reflect the real-world experiences of the authors in ways that prepare you not only for the exam but for actual network administration as well.

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Extensive practice test options. This book provides numerous opportunities for you to assess your knowledge and practice for the exam. The practice options include the following:

Review questions. These open-ended questions appear in the “Apply Your Knowledge” section that appears at the end of each chapter. These questions allow you to quickly assess your comprehension of what you just read in the chapter. Answers to the questions are provided later in the section.

Exam questions. These questions also appear in the “Apply Your Knowledge” section. They reflect the kinds of multiple-choice questions that appear on the Cisco Certification exams.

Use them to practice for the exam and to help you determine what you know and what you need to review or study further. Answers and explanations for them are provided.

Practice exam. A practice exam is included in the “Final Review” section of the book. The

“Final Review” section and the practice exam are discussed later in this Introduction.

Top Score software. The Top Score Test

Simulation Software Suite included on the

CD-ROM provides further self-evaluation opportunities, in the form of practice exams, study cards, flash cards, and product simulations.

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I N T R O D U C T I O N 3

For a complete description of the New

Riders Top Score test engine, please see Appendix D, “How to Use the Top

Score Software Suite.”

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Final Review. This part of the book provides you with three valuable tools for preparing for the exam.

Fast Facts. This condensed version of the information contained in the book will prove extremely useful for last-minute review.

Study and Exam Tips. Read this section early on to help you develop study strategies. It also provides you with valuable exam-day tips and information on new exam and question formats, such as adaptive tests and simulationbased questions.

Practice Exam. A full practice exam is included. Questions are written in the styles used on the actual exam. Use it to assess your readiness for the real thing.

The book includes other features, such as sections titled “Suggested Reading and Resources,” which direct you toward further information that could aid you in your exam preparation or your actual work. There are several valuable appendices as well, including a glossary

(Appendix A), an overview of the Cisco certification program (Appendix B), and a description of what is on the CD-ROM (Appendix C). These and all the other book features mentioned previously will provide you with thorough preparation for the exam.

For more information about the exam or the certification process, contact Cisco:

Cisco Education: (800) 829-NETS (6387) or (408)

525-NETS (6387)

World Wide Web: http://www.cisco.com/training

Email: [email protected]

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The CCNA exam covers the 640-407 main topic areas represented by the conceptual groupings of the test objectives: OSI Reference, WAN Protocols, IOS,

Network Protocols, Routing, Network Security, and

LAN Switching. Each chapter represents one or more of these main topic areas. The exam objectives are listed by topic area in the following sections.

OSI Reference Model

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Identify and describe the functions of each of the seven layers of the OSI reference model.

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Describe connection-oriented network service and connectionless network service, and identify the key differences between them.

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Describe data link addresses and network addresses, and identify the key differences between them.

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Identify at least three reasons why the industry uses a layered model.

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Define and explain the five conversion steps of data encapsulation.

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Define flow control and describe the three basic methods used in networking.

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List the key internetworking functions of the OSI

Network layer and how they are performed in a router.

WAN Protocols

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Differentiate between the following WAN services:

Frame Relay, ISDN/LAPD, HDLC, and PPP.

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4 C C N A T R A I N I N G G U I D E

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Recognize key Frame Relay terms and features.

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List commands to configure Frame Relay LMIs, maps, and subinterfaces.

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List commands to monitor Frame Relay operation in the router.

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Identify PPP operations to encapsulate WAN data on Cisco routers.

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State a relevant use and context for ISDN networking.

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Identify ISDN protocols, function groups, reference points, and channels.

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Describe Cisco’s implementation of ISDN BRI.

Internetworking Operating

Systems (IOS)

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Log in to a router in both user and privileged modes.

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Use the context-sensitive help facility.

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Use the command history and editing features.

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Examine router elements (RAM, ROM, CDP, and show).

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Manage configuration files from the privileged exec mode.

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Control router passwords, identification, and banner.

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Identify the main Cisco IOS commands for router startup.

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Enter an initial configuration using the setup command.

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Copy and manipulate configuration files.

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List the commands to load Cisco IOS software from flash memory, a TFTP server, or ROM.

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Prepare to back up, upgrade, and load a backup

Cisco IOS software image.

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Prepare the initial configuration of your router and enable IP.

Network Protocols

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Monitor Novell IPX operation on the router.

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Describe the two parts of network addressing; then identify the parts in specific protocol address examples.

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Create the different classes of IP addresses (and subnetting).

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Configure IP addresses.

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Verify IP addresses.

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List the required IPX address and encapsulation type.

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Enable the Novell IPX protocol and configure interfaces.

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Identify the functions of the TCP/IP Transport layer protocols.

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Identify the functions of the TCP/IP Network layer protocols.

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Identify the functions performed by ICMP.

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Configure IPX access lists and SAP filters to control basic Novell traffic.

Routing

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Add the RIP routing protocol to your configuration.

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Add the IGRP routing protocol to your configuration.

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Explain the services of separate and integrated multiprotocol routing.

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List problems that each routing type encounters when dealing with topology changes and describe techniques to reduce the number of these problems.

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Describe the benefits of network segmentation with routers.

I N T R O D U C T I O N 5

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Distinguish between cut-through and store-andforward LAN switching.

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Describe the operation of the Spanning Tree protocol and its benefits.

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Describe the benefits of virtual LANs.

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Define and describe the function of a MAC address.

Network Security

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Configure standard and extended access lists to filter IP traffic.

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Monitor and verify selected access list operations on the router.

LAN Switching

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Describe the advantages of LAN segmentation.

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Describe LAN segmentation using bridges.

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Describe LAN segmentation using routers.

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Describe LAN segmentation using switches.

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Name and describe two switching methods.

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Describe full- and half-duplex Ethernet operation.

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Describe network congestion problems in

Ethernet networks.

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Describe the benefits of network segmentation with bridges.

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Describe the benefits of network segmentation with switches.

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Describe the features and benefits of Fast

Ethernet.

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Describe the guidelines and distance limitations of Fast Ethernet.

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A self-paced study guide, this book was designed with the expectation that you will use Windows 95 or later with Hyperterm as you follow along through the exercises while you learn. However, the theory covered in

CCNA Training Guide is applicable to a wide range of

network systems in a wide range of actual situations, and the exercises in this book encompass that range.

Your computer should meet the following criteria:

á

486DX2 66Mhz (or better) processor

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3.5-inch 1.44MB floppy drive

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VGA (or Super VGA) video adapter

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VGA (or Super VGA) monitor

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Mouse or equivalent pointing device

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Double-speed (or faster) CD-ROM drive

(optional)

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Network Interface Card (NIC) (optional)

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Presence on an existing network, or use of a two-port (or more) miniport hub to create a test network (optional)

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Hyperterm or other terminal emulation software for connection to console ports

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6 C C N A T R A I N I N G G U I D E

It is also recommended that you have one or more

Cisco routers available in order to become familiar with

Cisco’s Internetwork Operating System and Command

Line Interface. Used Cisco 2500 series routers can be obtained through many used equipment dealers.

It is easier to access the necessary computer hardware and software in a corporate business environment. It can be difficult, however, to allocate enough time within the busy workday to complete a self-study program. Most of your study time will necessarily occur after normal working hours, away from the everyday interruptions and pressures of your regular job.

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Extensive tips can be found in “Study and Exam Prep

Tips,” located in the “Final Review” section of this book, but keep this advice in mind as you study:

á

Read all the material. This book has included additional information not reflected in the objectives in an effort to give you the best possible preparation for the examination—and for the real-world network experiences to come.

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Do the Step by Steps and complete the exer-

cises in each chapter. They will help you gain experience using the product and/or carrying out the tasks you need to be familiar with in order to pass the exam.

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Use the questions to assess your knowledge.

Don’t just read the chapter content; use the review and exam questions to find out what you know and what you don’t. Study some more, review, then assess your knowledge again.

á

Review the exam objectives. Develop your own questions and examples for each topic listed. If you can make and answer several questions for each topic, you should not find it difficult to pass the exam.

Exam-Taking Advice Although this book is designed to prepare you to take and pass the CCNA exam, there are no guarantees. Read this book, work through the questions and exercises, and when you feel confident, take the practice exam and additional exams using the test engine located on the CD included at the back of this book. Your results should indicate whether you are ready for the real thing.

When taking the actual certification exam, make sure you answer all the questions before your time limit expires. Don’t spend too much time on any one question. If you are unsure about a question, answer it to the best of your ability; then mark it and review it when you have finished the rest of the questions.

Remember that the primary object is not to pass the exam—it is to understand the material. After you understand the material, passing the exam should be simple. Knowledge is a pyramid; to build upward, you need a solid foundation. This book is designed to ensure that you have that solid foundation.

Good luck!

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I N T R O D U C T I O N 7

N

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The staff of New Riders Publishing is committed to bringing you the very best in computer reference material. Each New Riders book is the result of months of work by authors and staff who research and refine the information contained within its covers.

As part of this commitment to you, the NRP reader,

New Riders invites your input. Please let us know if you enjoy this book, if you have trouble with the information or examples presented, or if you have a suggestion for the next edition.

Please note, however, that New Riders staff cannot serve as a technical resource during your preparation for the Cisco certification exams or for questions about software- or hardware-related problems. Please refer instead to the documentation that accompanies the

Cisco products or to the applications’ Help systems.

If you have a question or comment about any New

Riders book, there are several ways to contact New

Riders Publishing. We will respond to as many readers as we can. Your name, address, or phone number will never become part of a mailing list or be used for any purpose other than to help us continue to bring you the best books possible. You can write to us at the following address:

New Riders Publishing

Attn: Mary Foote, Executive Editor

201 W. 103rd Street

Indianapolis, IN 46290

If you prefer, you can fax New Riders Publishing at

317-581-4663.

You also can send email to New Riders at the following

Internet address: [email protected]

Thank you for selecting CCNA Training Guide!

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03 051-6 Part 1 8/10/99 9:14 AM Page 9

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REPARATION

1 The OSI Reference Model

2 Internetworking Operating Systems

3 Understanding and Configuring TCP/IP

4 Understanding and Configuring IPX/SPX and Other Nework Protocols

5 WAN Protocols

6 Routing

7 Network Security

8 LAN Switching

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O

B J E C T I V E S

Cisco provides the following objectives for the OSI

Reference portion of the CCNA exam:

Identify and describe the functions of each of the seven layers of the OSI Reference Model.

.

Understanding the seven layers of the OSI

Reference Model is fundamental to understanding network functions and how they operate in routers, bridges, and network stations.

Describe connection-oriented network service and connectionless network service and identify the key differences between them.

.

An understanding of connection-oriented and connectionless services is required to diagnose and maintain internetworking systems.

Describe data link addresses and network addresses and identify the key differences between them.

.

Data link and network addresses work together in internetworking systems. An understanding of the differences between these addresses and how they are used is necessary for maintaining internetworking systems.

Identify at least three reasons why the industry uses a layered model.

.

The layered model is the foundation for networking and internetworking systems. It is important to have a thorough understanding of why the layered model is used.

C H A P T E R

1

The OSI Reference

Model

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O

B J E C T I V E S

( c o n t i n u e d

)

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U T L I N E

Define and explain the five conversion steps of data encapsulation.

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Data encapsulation is the process by which data passes from the source application through the OSI

Reference Model to the network and back up the model at the receiving station to the receiving application.

Define flow control and describe the three basic methods used in networking.

.

Flow control techniques are used by networking stations to avoid data congestion. A thorough understanding of how flow control works is necessary for the maintenance of internetworking systems.

Introduction 14

Understanding the OSI Reference Model 14

The Seven Layers

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

OSI Model Communication

Encapsulation

17

17

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18

16

16

16

16

18

19

Data Link and Network Addresses

MAC Addresses

Network Addresses

Address Resolution Protocol (ARP)

21

21

22

23

Connection-Oriented and Connectionless

Services 23

Flow Control

Source-Quench Message

Windowing

Buffering

24

25

25

26

Networking Topologies

Ethernet

Token Ring

Active Monitor

FDDI

Serial Ports

Chapter Summary

26

26

27

28

29

29

33

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Memorize the seven layers of the OSI Reference

Model. Make sure you fully understand the functions of each and know common examples of each layer. Not only will this ensure that you will be able to answer these questions on the CCNA exam, but it will also help you with daily design and diagnostics of internetworking systems.

.

Understand how data link addresses and network addresses work together to deliver network traffic.

.

Have a good understanding of the common networking technologies and how they work. A good way to do this might be to set up a study lab. In doing so, it is useful to connect two

Cisco routers to each other via the serial ports.

You can do this by using a DCE cable on one router and a DTE cable on the other and connecting the two cables. Cisco’s part numbers for the cables to accomplish this are CAB-V35MT

(DTE, male) and CAB-V35FC (DCE, female).

.

As you read this chapter, keep in mind the practical application of this knowledge. One good exercise for you to test yourself is to review networking systems in your company and identify each of the layers used. This exercise will help you understand where different functions fit into the scheme.

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14 Pa r t I E X A M P R E PA R AT I O N

I

NTRODUCTION

This chapter covers the building blocks that make up internetworking systems. These blocks include the OSI Reference Model, types of network connections, data link and networking addresses, and flow control. These concepts are the building blocks of networking systems.

The importance of understanding these basics and building on this knowledge cannot be overemphasized. Internetworking systems are complex. They involve hardware, protocols, and designs from many different vendors that must work together for successful communication. To design, maintain, or diagnose these systems, it is imperative to understand the basics around which these complex systems were built.

Many examples in this chapter that illustrate working protocols use

TCP/IP. Thanks to the Internet, this is one of the most prevalent protocols in use today. This makes it a natural choice for examples. An important fact to remember in these discussions is that TCP/IP is not one protocol but a group of protocols covering many networking functions.

Internetworking systems is a term used to describe an environment where different networks are connected. This could be as basic as two simple networks with a bridge between them, or it could be a complex enterprise network with frame relay, Ethernet, token ring, and a FDDI backbone.

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The Open Systems Interconnection (OSI) Reference Model provides the basis for almost everything in networking. The OSI Reference

Model is conceptual rather than factual since it doesn’t really describe one particular protocol. Developers of networking products use the

OSI Reference Model as a loose guide for design and implementation.

In 1984, the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) Reference Model.

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C h a p t e r 1 T H E O S I R E F E R E N C E M O D E L 15

This model divides the functions of networking into small, mostly self-contained functions called layers. Since the layers are modular, one layer can be changed or updated, and the remaining layers should still function properly. For example, you could replace the

Physical layer with another network topology and expect the rest of the networking pieces to function as they did before.

The Institute of Electrical and Electronic Engineers (IEEE) defines many of the standards used in today’s networking systems. The IEEE formed the 802 committee in February of 1980 (where 80 represents the year 1980 and 2 represents the second month of the year). The

802 committee standardizes many networking technologies. Its committee numbers represent these standards:

á 802.1 defines issues common to all LANs, such as the spanning tree protocol defined in 802.1D.

á

802.2 defines the logical link control (LLC) sublayer.

á 802.3 defines LANs based on Carrier Sense Multiple

Access/Collision Detection (CSMA/CD) such as Ethernet.

á

802.4 defines token bus networks. This networking topology isn’t used much anymore.

á

802.5 defines token ring networks. IBM’s TokenRing and

IEEE 802.5 are functionally the same.

The Internet Engineering Task Force (IETF) maintains these standards on its Web site at www.ietf.org

. Another good starting point for references is InterNIC’s Web site at www.internic.net

.

Networking functions are divided into layers for several reasons:

á It divides complex network operations into less-complex pieces.

á It keeps changes in one area from adversely affecting other layers. This facilitates the evolution of networking components, since companies can focus on improving one area without worrying about the functions handled at other layers.

á It allows different vendors to create products that will work together.

á

It makes understanding the complex concepts of networking easier by dividing the concepts into smaller, easier-to-understand pieces.

The OSI Reference Model Is a

Basic Concept A strong grasp of the OSI Reference Model for networking will make life much easier when you are diagnosing complex internetworks. Cisco expects anyone certified to have a strong understanding of the OSI Reference

Model and how it relates to the different aspects of internetworking.

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16 Pa r t I E X A M P R E PA R AT I O N

The Seven Layers

The seven layers of the OSI Reference Model are Application,

Presentation, Session, Transport, Network, Data Link, and Physical, as shown in Figure 1.1. The following sections describe each in detail.

F I G U R E 1 . 1

The Network layers define where networking functionality exists.

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network processes to applications

Data representation

Interhost communication

End-to-end connections

Addresses and best path

Access to media

Binary transmission

A Useful Mnemonic Device A good way to remember the seven layers and the order in which they occur is to create a sentence from the layers’ first letters. One common sentence is All People Seem

To Need Data Processing. This represents Application, Presentation,

Session, Transport, Network, Data

Link, and Physical.

Application Layer

The Application layer provides the interface from the OSI Reference

Model to the end user’s application. Some examples of Applicationlayer implementations include File Transfer Protocol (FTP), Simple

Mail Transport Protocol (SMTP), and telnet.

Presentation Layer

The Presentation layer defines how the data will be presented to the

Application layer. Some examples of Presentation-layer implementations include MIDI, JPEG, EBCDIC, ASCII, MPEG, and encryption.

Session Layer

Communication sessions between network stations are established, managed, and terminated at the Session layer. Sessions handle the requests and responses between applications on the network stations.

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C h a p t e r 1 T H E O S I R E F E R E N C E M O D E L 17

Transport Layer

The Transport layer accepts data from the Session layer and puts it in segments to pass down to the Network layer. It is responsible for making sure packets are delivered error-free, in sequence, and without any losses or duplications.

Acknowledgments, a technique used for flow control, are generally sent from the Transport layer.

Network Layer

The Network layer defines the network address, which is different from the media access control (MAC) address. An Internet Protocol

(IP) address is an example of a Network layer address. Segments of data passed down from the Transport layer are put into packets and are passed down to the Data Link layer.

The Network layer determines the path to the destination and forwards packets that are destined for remote networks. The Network layer evaluates the network address of the destination system and determines whether it is on the same network as the source system. If so, it obtains the destination system’s MAC address either from a table or through a resolution technique such as IP’s Address Resolution Protocol (ARP).

Once it has addressed the packet with the source and destination addresses, it passes the packet down to the Data Link layer for delivery.

If the Network layer determines that the destination system is not on the same network as the source system, it addresses the packet with the

MAC address of the router that can deliver the packet. It determines this by referencing routing tables that can be built either statically or dynamically. Once it addresses the packet with the source address and the address of the router that can deliver the packet, it passes the packet down to the Data Link layer for delivery to the router.

Data Link Layer

The Data Link layer provides an error-free link between two network devices. Packets passed down from the Network layer are packaged into frames compatible with the networking topology and are passed down to the Physical layer. Error checking and correction information is added to the frame. One error-correction technique that is used is cyclic redundancy check (CRC).

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18 Pa r t I E X A M P R E PA R AT I O N

It is important to realize that there are areas of redundancy in some networking standards. Error checking and correction can actually take place at multiple layers of the OSI Reference Model.

The IEEE 802 committee has further subdivided the Data Link layer into two sublayers: logical link control (LLC) and media access control

(MAC). This separation of functionality allows upper-layer protocols to operate without relying on the physical media.

á LLC sublayer.

The LLC sublayer provides the functionality required for connection-oriented or connectionless services at the Data Link layer. It provides the resources for multiple upper-layer protocols to share the physical media. This is performed via Destination Service Access Point (DSAP) and

Source Service Access Point (SSAP). It is beneficial to know that these services exist. However, further exploration of these functions is not required to pass the CCNA.

á MAC sublayer.

The MAC sublayer provides the functions necessary for network media access. On any network, stations must be addressed individually so that data can be delivered to the desired station. The MAC address provides this individuality.

Physical Layer

The Physical layer is responsible for placing the data on the network media. The Physical layer defines such things as voltage levels, wire specifications, maximum distances, and connector types.

The layers of the OSI Reference Model communicate with each other and with remote stations. A key concept within this communication is the encapsulation of data. The process of encapsulation takes the user’s data and prepares it for travel across the network and then places it on the network media.

OSI Reference Model Communication

Within the OSI Reference Model, each layer communicates with three other layers: the layer directly above, the layer directly below, and its peer layer on the other end of the communication link (see

Figure 1.2).

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C h a p t e r 1 T H E O S I R E F E R E N C E M O D E L 19

System A

2

1

4

3

7

6

5

Information Unit

.

.

.

Header 2

Header 3

Header 4 Data

Data

Data

Data

Network

System B

2

1

4

3

7

6

5

Encapsulation

Each layer uses control information to communicate with the corresponding layer at the other end of the link. This control data is added to the packet as it moves down the stack. This process is called encapsulation.

As a packet travels down through the seven layers, each layer adds control data and then passes the data down to the next layer until the data reaches the network. Once the packet is delivered to the receiving station, the process is reversed. Each layer strips off its control data and passes the packet up to the next layer until the packet reaches the application.

Encapsulation consists of five steps, beginning at the Transport layer:

1. Build the data.

2. Package the data for end-to-end transport.

3. Append the network address in the header.

4. Append the local address in the data link header.

5. Convert to bits for transmission.

For an example, follow some data through the process. When you visit Cisco’s Web page at http://www.cisco.com

, the process goes something like what is shown in Figure 1.3.

F I G U R E 1 . 2

As the data passes down the protocol stack, headers are added and the data is passed down to the next layer.

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20 Pa r t I E X A M P R E PA R AT I O N

F I G U R E 1 . 3

Data flowing through the OSI layers when a user makes an HTTP request.

Web request login:

Data

Transport

Header

Data

Data

Segment

Network

Header

Transport

Header

Data Packet

Frame

Header

Network

Header

Transport

Header

Data

Frame

Header

0101101010110001010110101011000101011010101100

Frame

(medium dependent)

Bits

á Build the data.

You enter the URL for Cisco’s Web site in your browser. The Web address you are requesting is converted to data that can travel over the Internet.

á Package the data for end-to-end transport.

The data is packaged for the Transport layer. A Transport layer header is added to the data. In this example, the header will be a TCP header. This header will include information specifying that this request is destined for an HTTP service.

á Append the network address in the header.

The data is now put in a packet or datagram so that it can be transmitted over the network. The source and destination network addresses are included in the packet header. In this example, these addresses would be IP addresses.

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á Append the local address in the data link header.

The packet is now put in a frame compatible with the network media. The data link addresses of the source and destination stations are placed in the frame header. Since our packet will be traveling through a router, the destination data link address is that of our default gateway router. (This will be explained more in later chapters.)

á Convert to bits for transmission.

The frame is converted to ones and zeros and placed on the network media.

This process is repeated many times during the connection to the

Web site. The process is reversed on the receiving side: The data travels back up the process until it reaches the application and is presented to the user. Networking functions generally happen so fast that most people never consider what happens after they press Enter.

As an internetworking expert, it is important that you consider what is happening at each level of the process and how it occurs.

D

ATA

L

INK AND

N

ETWORK

A

DDRESSES

Networking requires that each device have a unique address so that data can be delivered reliably. Within the OSI Reference Model are two definitions for station addresses—the network address and the data link address. Understanding the use of each of these addresses is crucial to following how data is delivered in an internetworking system.

MAC Addresses

The Media Access Control (MAC) address from the Data Link layer uniquely identifies each station on the network media. MAC addresses are 48 bits long and are usually represented as three groups of four hexadecimal digits:

00e0.a38d.0800

Start at the Bottom When internetworking systems break, the first thing you must figure out is where in the process the problem is occurring. You can waste much time trying to diagnose a routing problem when the problem actually exists at the physical or Data Link layer.

Generally, when you’re trying to resolve a problem, it’s a good idea to start at the bottom and work your way up the model.

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22 Pa r t I E X A M P R E PA R AT I O N

A Useful Reason to Alter the MAC

Address Changing the MAC address is used in Hot Standby Router

Protocol (HSRP), a method of using two routers in fail-safe mode. Both routers are connected to the network and configured similarly. One is the primary router, and the other is the secondary. The primary router services the network, and the secondary constantly makes sure that the primary is still operational. If the secondary router detects that the primary router has failed, the secondary router changes its interface’s MAC address to that of the primary router. This allows the stations on the network to still forward their data to the same

MAC address even though the data is now handled by the secondary router.

The first 24 bits of the MAC address make up the Organizational

Unique Identifier (OUI). The OUI is a number assigned to each vendor by the IEEE. In the example, the OUI is 00e0.a3.

The second 24 bits make up the unique address that the manufacturer assigned to this network interface. In the example, the unique address is 8d.0800.

The MAC address is usually burned into a ROM on the device when it is made. When the device is powered on, the address is copied from the ROM to RAM, where it identifies the network device on the network media. Some devices allow you to change the

MAC address of a networking device. In this case, the MAC address would be obtained from the driver for the device and copied to

RAM. This is not very common in Ethernet installations but is quite common on token ring networks where mainframes are involved.

The data link address identifies the network station on the physical network media. MAC addresses are generally flat, meaning that there is no structure to how they are assigned. In contrast, network addresses are usually hierarchical in structure.

Network Addresses

The network address identifies the station at the network level (layer 3) of the OSI Reference Model. In TCP/IP, the network address is the device’s IP address. Under Novell’s IPX, the network address is dynamically assigned as a combination of the IPX network number and the station’s MAC address. Network addresses are usually hierarchical, like the mailing address of a business or home.

The sorting of mail is hierarchical since the sorting equipment can separate mail first by the ZIP code to which it is addressed, then the state, then the city, then the street, and finally the house number.

This lets the sorting equipment worry about only a piece of the address as the letter makes its way through the post office. Each time the letter is sorted, the piece of the address used is more specific.

Networking addresses can be structured the same way. Routers can look at part of the network address and forward the packet. Each router along the path looks at more specific information in the network address until the packet is delivered to the end network station.

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This concept becomes more important later when you look at IP addresses and how they are structured. A well-designed network addressing scheme can reduce the burden placed on routers by making use of this hierarchical address structure.

Since routing occurs at the Network layer, you will be looking at network addresses and how they function in more depth in Chapter 6,

“Routing.”

For an internetworking system to operate, it must have a way to map network addresses to data link addresses. In the TCP/IP protocol suite, this is performed by Address Resolution Protocol (ARP).

Address Resolution Protocol (ARP)

Address Resolution Protocol is responsible for mapping IP addresses to MAC addresses. ARP builds a table of IP addresses and their corresponding MAC addresses. If the network device needs to communicate with a station not in the ARP table, ARP sends out a data link broadcast with the IP address it is looking for. The desired station will see the broadcast and will reply with a directed response that includes its MAC address. This new entry is added to the ARP table, and a timer is set. This timer is called the ARP cache timer. It is updated each time the entry is referenced. Once this timer expires, the entry will be removed from the table. This way, the ARP entry is available in the local cache during an entire network conversation.

After the two stations have not communicated for a period of time, the entry is removed from the cache. This keeps the table from growing too large and requiring large amounts of memory.

C

ONNECTION

-O

RIENTED AND

C

ONNECTIONLESS

S

ERVICES

Network protocols can generally be classified as either connectionoriented or connectionless.

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24 Pa r t I E X A M P R E PA R AT I O N

Sender

Synchronize

Synchronize + Acknowledge

Synchronize

Receiver

Connection Established

Data Transfer

(Send Segments)

F I G U R E 1 . 4

A connection must be established before data can be exchanged.

Connection-oriented protocols establish a connection with the other end before they begin sending data. The protocol makes a request to the station it wants to exchange data with. This request must be accepted before the exchange can occur (see Figure 1.4).

During the exchange of data, the two stations verify that the data was received error-free. After the data exchange is complete, the connection between the two stations is terminated.

One protocol that uses connection-oriented network services is

Transmission Control Protocol (TCP). TCP is one of the Transport layer protocols of the TCP/IP protocol suite.

In a connectionless exchange, the two stations don’t need to establish a connection. One station can just send data to the other.

Connectionless exchanges don’t verify that the data was received or that it is error-free. Generally in a connectionless exchange, layers above the Transport layer are responsible for making sure that all the data is exchanged completely and free of errors.

User Datagram Protocol (UDP) is an example of connectionless protocol from the TCP/IP protocol suite. UDP exchanges datagrams without acknowledgments or guaranteed delivery.

File Transfer Protocol (FTP) is an application to copy files across a network to another station. FTP makes use of TCP exchanges. This puts the burden of error correction and reliable delivery on the Transport layer of the OSI Reference Model. FTP can pass the data it wants to send to the Transport layer protocol, TCP, and know that the data will arrive at the destination station error-free. There is a similar application called Trivial File Transfer Protocol (TFTP). TFTP uses UDP for its exchanges. This places the burden of reliable delivery on the application, not on the Transport layer of the OSI Reference Model. TFTP becomes more important when maintaining routers since it is used to copy software and configurations to and from Cisco routers.

Flow Control

Flow control techniques prevent network congestion, which can be caused by several different factors. A high-speed server might be sending data faster than a workstation can process it. Many different workstations might be trying to send data through a router simultaneously, overflowing the router’s capability to deal with the traffic.

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Traffic from a high-speed network might need to be transferred over a slower serial link to a remote office. Whatever the case, flow control paces data transmission to meet the capabilities of the network and receiving station.

There are three common methods of flow control: using sourcequench messages, buffering, and windowing. These techniques are often used together to pace the flow of data.

Source-Quench Messages

Source-quench messages are used to signal the sending station to slow down. When packets are arriving at a receiving station faster than it can process them, it starts by dropping packets. It then sends a source-quench message for every packet dropped. The sending station receives the source-quench messages and slows sending until it stops getting source-quench messages. The sending station then gradually increases the sending rate as long as no sourcequench messages are received. One of the problems with sourcequench messages is if the network is overburdened, there is no guarantee that the source-quench packet will arrive at the sending station.

Windowing

Windowing relies on the receiving station’s acknowledging receipt of packets. This acknowledgment is commonly called an ACK. It would create a great deal of overhead if the receiving station had to acknowledge every packet individually, so a window is used. The sending and receiving stations agree on a window size. This window defines the number of packets they will exchange before an acknowledgment is required. If the window size is five packets, the sending station will send five packets and then wait for an acknowledgment from the receiving station. If the acknowledgment comes, the sending station sends the next five packets and waits again. If the sending station does not receive the acknowledgment within a set period of time, it resends all the packets in the window at a reduced speed.

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26 Pa r t I E X A M P R E PA R AT I O N

Buffering

Buffering allows the receiving station to temporarily store packets received until it can process them. As long as the bursts are short and don’t overflow the buffer, this technique works well. However, if bursts overrun the available memory for buffering, the station must drop packets. Buffering is commonly used in conjunction with source-quench messages and windowing.

N

ETWORKING

T

OPOLOGIES

A good understanding of different networking topologies is required for

Cisco certification. A description of Ethernet, token ring, FDDI, and

Cisco serial ports is included here. Chapter 5, “WAN Protocols,” discusses other internetworking topologies, such as frame relay and ISDN.

Ethernet History Xerox Corporation originally developed Ethernet in the

1970s. Ethernet was later modified and specified in the IEEE 802.3 specification. Ethernet is actually slightly different from the IEEE 802.3 specification, but it is common to refer to all

CSMA/CD network topologies as

Ethernet.

Ethernet

Ethernet is by far the most popular networking topology in use today.

Its ease of use, simplicity, and reliability have established it as the mainstream networking topology.

Ethernet operates at 10Mbps (megabits per second) and uses Carrier

Sense Multiple Access/Collision Detection (CSMA/CD) to access the media. Contention for the network media is handled by collision detection. When a network interface has data to place on the wire, it first listens to the network to see if another station is currently transmitting. If the network isn’t busy, it begins transmitting its data.

After the transmission, it listens again. If it detects another station transmitting right after it finishes, it assumes a collision occurred.

When this occurs, both network stations involved in the collision must retransmit their packets. Each backs off for a random amount of time and then retries its transmission.

Ethernet is a broadcast-based network, meaning that every station sees all the packets on the media. The individual stations look at the destination address in the packet and determine whether the packet is intended for them. If it is, they copy the packet off the media and process it. If not, they simply ignore the packet. This is true of nonswitched networks. In networks where layer 2 switches are used, this process is much more complex. Layer 2 switches are discussed in more depth in Chapter 8, “LAN Switching.”

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Ethernet can be implemented several different ways. The most common are 10Base5, 10Base2, 10BaseT, 100BaseTX, and 100BaseFX.

Each implementation has different characteristics. 10Base5 is implemented on a thick coaxial cable with an impedance of 50 ohms.

10Base5 is commonly called ThickNet. Since 10Base5 uses low-loss coaxial cable, it has a maximum segment length of 500 meters.

10Base2 is implemented on a thin 50-ohm coaxial cable. 10Base2 is commonly called ThinNet. ThinNet is easier to work with and cheaper to implement than ThickNet. However, it has some problems that make it unsuitable for large installations. Since the network is made up of one string of coaxial that runs from one end of the network to the other, if a user unplugs one of the wires from the back of his workstation, the entire network fails. The maximum segment length for 10Base2 is 185 meters.

10BaseT is an implementation of Ethernet on unshielded twisted pair wire. It is the most reliable method of implementing Ethernet since it doesn’t suffer from the frailty of ThinNet. 10BaseT is wired differently than 10Base5 or 10Base2. 10BaseT is still one wire in a logical networking sense, but the workstations are connected to hubs that are connected to each other. Since each station is wired to the hub separately, it is less likely that a problem at a workstation will cause the entire network to fail. More-advanced hubs include features to isolate stations that are transmitting garbage so that they don’t affect the rest of the network. 10BaseT has a maximum segment length of 100 meters.

Token Ring

IBM developed Token Ring in the 1970s. The IEEE 802.5 specification is almost identical to IBM’s Token Ring. Token Ring is arranged in a ring, where each station is a node on the ring. These stations pass traffic around the ring. A network station accepts a packet from its downstream neighbor and sends it to its upstream neighbor. Token

Ring supports multiple speeds; 4Mbps and 16Mbps are the most common. Newer token ring equipment is starting to support 100Mbps.

In token ring, contention for the media is handled via a token. The token is a special packet that is passed around the ring.

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28 Pa r t I E X A M P R E PA R AT I O N

If a station has traffic to place on the network, it must wait until it gets the token. It then changes the token into a regular packet header by changing one of the bits. The data it must transmit is added to the packet, and the packet is sent to the upstream neighbor. As each station on the network gets this packet, it checks the packet to see if it was destined for this station. If so, it processes the packet, marks it as read, and then sends it back onto the network.

The originating station waits for the packet it sent out to come back around the ring. Once it arrives back at the sending station, the sending station checks to make sure the packet it received matches the packet it sent. If not, a network error must have occurred. Once the packet is received back at the originating station, the originating station releases the token so that other stations can use the network.

Some token ring networks support a feature called early token release. This allows the originating station to release the token after putting its traffic on the network and before seeing it return. This allows multiple packets to traverse the network simultaneously. For a network to support early token release, every station on the network must support it and be configured for it.

Active Monitor

Token ring networks require someone to watch over the network.

The station that does this is called the active monitor. A station becomes the active monitor through a negotiation process. All other stations on the network monitor the active monitor, and if it exits the network, a new active monitor is elected.

The active monitor is responsible for making sure packets don’t circle the ring more than once. When a packet traveling around the ring reaches the active monitor, it sets a special bit in the packet called the monitor bit. If the active monitor sees this packet again, it means that the station that originated the packet must have exited the ring and couldn’t remove it. The active monitor removes this packet and releases a new token on the network.

If the token somehow becomes lost, the active monitor creates a new token and releases it on the network.

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FDDI

Fiber Distributed Data Interface (FDDI) is an American National

Standards Institute (ANSI) standard. FDDI operates a dual token ring

LAN at 100Mbps. Its most common use is for campus and backbone installations. An implementation of FDDI that operates over twisted pair wire is called Copper Distributed Data Interface (CDDI).

FDDI is immune to the electromagnetic interference that plagues wire-based networks. This makes it very useful in hostile environments such as factories and Navy ships. It supports networks that are spread out physically.

FDDI has the following limitations:

á

500 nodes per FDDI LAN

á 100 kilometer maximum ring circumference

á

2 kilometer distance between nodes using multimode fiber

Serial Ports

Cisco routers use serial ports to connect to many devices, including

Channel Service Unit/Data Service Unit (CSU/DSU). CSU/DSUs are similar to modems but are used for digital circuits such as 64K leased lines and T1s.

These serial ports are very versatile. They support several different standards from one port. The port configures itself for the different standards based on the cable that is plugged in.

The end of the cable that plugs into the router includes pins for configuration. When the cable is made, certain combinations of these configuration pins are connected. When this cable is plugged into the router’s serial port, the serial port detects the type of cable and configures itself accordingly.

Here are some of the types of interfaces supported by Cisco serial ports:

á V.35

á

EIA-232 (RS-232)

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30 Pa r t I E X A M P R E PA R AT I O N

á EIA-449

á

X.21

á

EIA-530

Serial devices are divided into two categories: Data Terminal

Equipment (DTE) and Data Communications Equipment (DCE).

The main difference between these two types of equipment is that

DCE devices supply the clock signal that synchronizes the communication. DTE devices use the clock signal generated by the DCE.

The serial ports built into Cisco routers support high speeds compared to PC serial ports. Most PC serial ports only support up to

115Kbps. Cisco’s serial ports can support varying speeds. Some serial interfaces can support up to 8Mbps. The platform and serial card determine the speed. Cisco’s documentation for your platform should be referenced.

Serial communication can be divided into two categories: synchronous and asynchronous. Asynchronous serial communication relies on start and stop bits in the data stream to signal clocking information. Synchronous serial communication relies on a clocking source outside the data stream.

C

A S E

S

T U D Y

: T

H E

S

C H U T Z H U N D

C

L U B

E S S E N C E O F T H E C A S E

.

Diagnosing network problems using the layered approach

.

Layer 3 routing problems

.

Ping as a diagnostic tool

S C E N A R I O

The Schutzhund Dog Club has automated its event registration process. It had custom software developed that allows handlers to register their dogs over the Web. The software also lets show workers access a secured Web site during the Schutzhund trials to update scores and show information.

The automated system consists of several servers connected via 100Mbps Ethernet to a layer 2 switch. Also connected to this switch is a

Cisco 2501 router. The router is connected to the Internet via a T1.

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C

A S E

S

T U D Y

: T

H E

S

C H U T Z H U N D

C

L U B

One of the older servers is replaced with a new

Pentium II server running Windows NT. After installing the server, it finds that although the server has no problem talking with other servers on the local network, it can’t communicate with the Internet.

The club president spends days trying to resolve the problem but can’t. A major trial is coming, and it needs a solution quickly. You are called in to find the problem and provide a fix.

A N A LY S I S

The club president explains the problem and what he has tried. He asks you to correct the problem as quickly as possible. The trial is in one week, and handlers are trying to register their dogs over the Web but can’t.

You start by identifying the troubled server and following its network patch cable to the switch.

You remember that when diagnosing a networking problem it is best to start at the bottom of the OSI Reference Model and work your way up.

You find that the server is plugged into port 3 of the switch. The server is up and running, but the link light on the switch is not on. You remember the club president telling you that he could ping the other servers at first, but after working with it for a few days, he couldn’t ping any computers. He mentioned that he had tried different cables and ports in his diagnostics.

After looking around, you find another patch cord and replace the one on the server. The link light on the switch is now active. You have resolved a layer 1 (Physical) networking problem.

You try pinging the other servers on the network and find that your pings are successful. This confirms that the OSI Reference Model is working up to layer 3 (Network)—at least locally. You ping the router’s Ethernet port and get successful replies. You now try to ping an outside Internet address, and the pings fail.

On the server, you open a command window and type route print

. This displays the server’s routing table. You notice that the table doesn’t have a default route. The default route would be shown by a network address of 0.0.0.0 and a netmask of 0.0.0.0. In the following example, this entry is missing:

C:>route print

Active Routes:

Network Address Netmask Gateway Address Interface Metric

127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1

192.168.137.0 255.255.255.0 192.168.137.37 192.168.137.37 1

192.168.137.37 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.38 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.39 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.41 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.42 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.51 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.255 255.255.255.255 192.168.137.37 192.168.137.37 1

224.0.0.0 224.0.0.0 192.168.137.37 192.168.137.37 1

255.255.255.255 255.255.255.255 192.168.137.37 192.168.137.37 1

continues

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C

A S E

S

T U D Y

: T

H E

S

C H U T Z H U N D

C

L U B

continued

In opening the network configuration dialog box, you discover there is no entry for the default gateway. That explains it. The server can communicate with the local computers since they are all on the same network and the packets never need to travel through a layer 3 router. When you try to reach an IP address on a remote network, however, the server doesn’t know where to send the packets for forwarding.

You enter the IP address of the router’s Ethernet port in the default gateway dialog box and reboot the server. You now can successfully ping remote stations on the Internet.

You check the routing tables again and find a route representing the default gateway as 0.0.0.0

subnet 0.0.0.0. This is shown in the first line of the following routing table. This is a common way to represent a route to any unknown route. This is discussed in more detail in Chapter 6, “Routing.”

C:>route print

Active Routes:

Network Address Netmask Gateway Address Interface Metric

0.0.0.0 0.0.0.0 192.168.137.1 192.168.137.37 1

127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1

192.168.137.0 255.255.255.0 192.168.137.37 192.168.137.37 1

192.168.137.37 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.38 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.39 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.41 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.42 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.51 255.255.255.255 127.0.0.1 127.0.0.1 1

192.168.137.255 255.255.255.255 192.168.137.37 192.168.137.37 1

224.0.0.0 224.0.0.0 192.168.137.37 192.168.137.37 1

255.255.255.255 255.255.255.255 192.168.137.37 192.168.137.37 1

The moment you get this working, the server becomes active with HTTP requests, and the database starts filling with registration information for the upcoming trial.

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C

H A P T E R

S

U M M A R Y

This chapter covered some of the building blocks that make up networking systems. The layers of the OSI Reference Model were explained, as well as the reasons the OSI Reference Model is used.

Understanding this model is imperative. Networking protocols and equipment use the model to build complex systems from relatively simple tasks.

You looked at each layer of the model and saw examples of each.

The types of network conversations were covered. Connectionoriented conversations require that the two stations establish a communication link before exchanging data. In a connectionless conversation, none of this preliminary setup is required.

These pieces of networking help make up complex enterprise networks. If you start with a strong foundation, it will be easier to understand more complex concepts later.

KEY TERMS

• OSI Reference Model

• MAC address

• Network address

• Connection-oriented service

• Connectionless service

• Flow control

• Windowing

• CSMA/CD

• Encapsulation

• Source-quench message

• Address Resolution Protocol (ARP)

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A

P P L Y

Y

O U R

K

N O W L E D G E

Review Questions

1. What are the seven layers of the OSI Reference

Model, listed in order from layer 7 to layer 1?

2. What are some examples of Presentation layer implementations?

3. What are some examples of network addresses?

4. How are data link addresses mapped to network addresses?

5. Which layer of the OSI Reference Model is responsible for routing?

6. What does CSMA/CD stand for, and which networking technology does it represent?

7. What is an active monitor, and with which networking topology is it associated?

8. List the five steps of data encapsulation.

9. List three reasons why the industry uses the OSI

Reference Model.

10. What are the differences between connection-oriented and connectionless network conversations?

Exam Questions

1. Which of the following is an example of a function handled at the Physical layer of the OSI

Reference Model?

A. Application data is encrypted for secure transport over the network.

B. A route for the packet is determined.

C. A CRC is calculated and added to the packet to ensure that it is delivered error-free.

D. The data is converted to voltage levels defined by the network standard and is placed on the network media.

2. List the levels of the OSI Reference Model in the correct order.

A. Application, Session, Presentation, Transport,

Network, Data Link, Physical

B. Application, Presentation, Session, Transport,

Network, Data Link, Physical

C. Application, Presentation, Session, Transfer,

Network, Data Link, Physical

D. Application, Physical, Session, Transport,

Network

3. A computer using TCP/IP can ping computers on the local network but is unable to ping computers on remote networks. Other computers on the same network have no problem reaching remote computers. At which layer of the OSI Reference

Model is this computer likely experiencing problems?

A. Network

B. Data Link

C. Physical

D. Transport

4. Which of the following statements is false regarding network addresses and data link addresses?

A. Data link addresses are usually burned into

ROM and are copied to RAM during initialization.

B. Data link addresses are made up of 48 bits, with 24 bits as the OUI and 24 bits as the station number.

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A

P P L Y

Y

O U R

K

N O W L E D G E

C. Under TCP/IP, network addresses are mapped to data link addresses using a protocol called RAP.

D. Network addresses are used to route packets.

5. The OSI Reference Model is used for the following reason(s):

A. It allows different layers of the protocol stack to be upgraded without adversely affecting the operation of the rest of the stack.

B. It divides complex network operations into less-complex pieces.

C. It defines a rigid standard for specific protocols that all networking manufacturers must design for.

D. It makes understanding the complex nature of networking easier by dividing the complex pieces into smaller, easier-to-understand subcomponents.

6. A LAN engineer comes to you and says he is seeing many acknowledgments (ACKs) on the network and wants you to correct the problem.

What do you do?

A. You adjust the number of buffers in the router to reduce the number of ACKs.

B. You ask the LAN engineer to provide you with a LAN diagram so that you can determine where the problem lies.

C. You tell the LAN engineer there is no problem with the network. ACKs are normal for protocols using windowing flow control.

D. You ask for the MAC address of the station sending the ACKs so that you can replace the

Network Interface Card (NIC).

7. Which of the following is/are example(s) of the

Presentation layer of the OSI Reference Model?

A. MPEG

B. FTP

C. Telnet

D. MIDI

8. Which of the following is/are example(s) of the

Application layer of the OSI Reference Model?

A. MPEG

B. FTP

C. Telnet

D. MIDI

9. Which of the following is an example of flow control?

A. Buffering

B. Active monitor

C. Cyclic Redundancy Check (CRC)

D. Packet header

10. Which of the following is/are example(s) of network address(es)?

A. IPX address

B. IP address

C. MAC address

D. ARP

11. The routing of packets occurs at which layer?

A. Data Link

B. Network

C. Session

D. Transport

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36 Pa r t I E X A M P R E PA R AT I O N

A

P P L Y

Y

O U R

K

N O W L E D G E

12. Which of the following is/are associated with

CSMA/CD networks such as IEEE 802.3 and

Ethernet?

A. Token passing

B. Collisions

C. Active monitor

D. ThinNet

13. Which of the following is/are example(s) of the

Transport layer?

A. TCP

B. UDP

C. ARP

D. IP

Answers to Review Questions

1. The seven layers of the OSI Reference Model are

Application, Presentation, Session, Transport,

Network, Data Link, and Physical. For more information, see “The Seven Layers.”

2. MIDI, MPEG, JPEG, and GIF are examples of

Presentation layer implementations. There are many others. For more information, see

“Presentation Layer.”

3. IP and IPX addresses are examples of network addresses. For more information, see “Network

Addresses.”

4. Data link addresses are mapped to network addresses using Address Resolution Protocol (ARP).

ARP uses data link broadcasts to resolve an IP address and then places the addresses in a table for future reference. After a timer expires, the ARP entry is deleted to keep required resources to a minimum.

For more information, see “Data Link and

Network Addresses.”

5. Layer 3, the Network layer, is responsible for routing. This will be examined in more detail later in this book. For more information, see

“Network Layer.”

6. CSMA/CD stands for Carrier Sense Multiple

Access/Collision Detection. It is associated with

IEEE 802.3 and Ethernet networks. For more information, see “Networking Topologies.”

7. An active monitor is a network station responsible for making sure the token doesn’t get lost. It is associated with token ring and IEEE 802.5 networks. For more information, see “Active

Monitor.”

8. The five steps of data encapsulation are build the data, package the data, append the network address, append the data link address, and convert to bits for transmission. For more information, see

“Encapsulation.”

9. The OSI Reference Model divides complex network operations into less-complex pieces, keeps changes in one area from adversely affecting other layers, and makes complex concepts easier to understand by dividing them into smaller, more comprehensible pieces. For more information, see

“Understanding the OSI Reference Model.”

10. A connection-oriented conversation must initially establish a connection with the network station with which it wants to exchange data. After data exchange, the connection is closed. In a connectionless conversation, this initial connection is not established. For more information, see

“Connection-Oriented and Connectionless

Services.”

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C h a p t e r 1 T H E O S I R E F E R E N C E M O D E L 37

A

P P L Y

Y

O U R

K

N O W L E D G E

Answers to Exam Questions

1. D. The Physical layer (layer 1) of the OSI

Reference Model is responsible for the network’s physical characteristics. Such things as connector type, voltage levels, and clocking rates are defined at the Physical layer. For more information, see

“Physical Layer.”

2. B. Remember the sentence All People Seem To

Need Data Processing. This is something you just need to memorize. Take the time to do so; it will save you a lot of headaches later. For more information, see “Understanding the OSI

Reference Model.”

3. A. Routing is handled by the Network layer. Since this computer can ping local computers, the

Physical and Data Link layers must be working properly. The Network layer is functioning properly for the local network where route determination is simple. However, when the computer tries to communicate with a remote computer, it fails.

This is most likely a configuration problem with the IP address or default gateway on this computer since other computers on the same network are not experiencing this problem. For more information, see “Network Layer.”

4. C. Although this answer is almost correct, the protocol used to map network addresses to data link addresses is Address Resolution Protocol

(ARP). For more information, see “Address

Resolution Protocol (ARP).”

5. A, B, D. Although the OSI Reference Model is a guide for networking, it is not a rigid standard for specific protocols. For more information, see

“Understanding the OSI Reference Model.”

6. C. Acknowledgments are normal for protocols that use windowing flow control. For more information, see “Windowing.”

7. A, D. MPEG and MIDI are Presentation layer examples. FTP and telnet are Application layer examples. For more information, see

“Understanding the OSI Reference Model.”

8. B, C. FTP and telnet are examples of Application layer implementations. MPEG and MIDI are

Presentation layer examples. For more information, see “Understanding the OSI Reference Model.”

9. A. Buffering is used in flow control to temporarily store packets until the network station can process them. For more information, see

“Buffering.”

10. A, B. IPX and IP addresses are examples of network addresses. The MAC address is a data link address. ARP is the protocol that maps between

IP addresses and MAC addresses. For more information, see “Network Addresses.”

11. B. The Network layer is responsible for routing packets. We will discuss this in more depth when we discuss how the Network layer is implemented in a router. For more information, see “Network

Layer.”

12. B, D. Collisions and ThinNet are associated with

Ethernet. Token passing and active monitor are associated with token ring. For more information, see “Networking Topologies.”

13. A, B. TCP and UDP are implementations of transport protocols in the TCP/IP suite. For more information, see “Transport Layer.”

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38 Pa r t I E X A M P R E PA R AT I O N

A

P P L Y

Y

O U R

K

N O W L E D G E

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration. Cisco Press, 1998.

2. Ford, Merilee, H. Kim Lew, Steve Spanier, and Tim Stevenson. Internetworking

Technologies Handbook. Cisco Press, 1997.

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O

B J E C T I V E S

Cisco provides the following objectives for the

Internetworking Operating System (IOS) portion of the CCNA exam.

Log in to a router in both user and privileged modes.

.

To be able to maintain and diagnose routers and switches, you will need to be able to access the routers’ different modes.

.

Use the context-sensitive help facility.

There are so many commands available for configuring routers that it is impossible to remember them all. By using the context-sensitive help, it is easier to continue working without having to reference a manual.

.

Use the command history and editing features.

Cisco routers and switches offer some advanced editing and command history features that can save time. These features can also aid in reducing errors.

Examine router elements (RAM, ROM, CDP, show)

.

It is important to be able to examine the elements of the router to diagnose internetworking problems.

Manage configuration files from the privileged exec mode.

.

The configuration files tell the router what to do and how to do it. Managing these files is imperative.

C H A P T E R

2

Internetworking

Operating System

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O

B J E C T I V E S

Control router passwords, identification, and banner.

.

Maintaining the router passwords, identification, and banners is important to the daily maintenance of routers and switches.

Identify the main Cisco IOS commands for router startup.

.

Understanding Cisco startup commands is important to maintain routers.

Enter an initial configuration using the setup command.

.

Cisco routers provide a setup routine that you need to be familiar with.

Copy and manipulate configuration files.

.

Copying and manipulating router configuration files is important for maintaining routers.

List the commands to load Cisco IOS software from flash memory, a TFTP server, or ROM.

.

The IOS software can be loaded from several sources. Being able to perform these techniques is important.

Prepare to back up, upgrade, and load a backup Cisco IOS software image.

.

Upgrading IOS software is a common occurrence.

The skills required to perform this are necessary.

Prepare the initial configuration of your router and enable IP.

.

Preparing an initial configuration with IP enabled is essential for an internetworking professional.

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O

U T L I N E

Introduction

Logging in to a Router in Both User and Privileged Modes

Authentication Servers

Normal Password Authentication

Entering Commands

Context-Sensitive Help

Configuring the Router

Hostname

Message of the Day (MOTD)

Configuring Interfaces

Saving Config Changes

Managing Passwords

Using the Command History and

Editing Features

Command History

Viewing the Contents of the

Command History Buffer

Changing the Size of the History

Buffer

Editing Features

Cursor Movement

Deleting

Pasting Buffers

Miscellaneous Editing Commands

Turning Off Editing Features

Terminal Settings

44

44

44

46

43 Understanding Router Elements

Random Access Memory (RAM)

Read-Only Memory (ROM)

Flash Memory

Network Interfaces

Examining the Components of a Router

Configuration Register

Processor

Interfaces

48

49

50

50

51

52

53

Cisco Discovery Protocol (CDP)

Enabling and Disabling CDP

Seeing CDP Neighbors

CDP Parameters

Managing Configuration Files from the Privileged Exec Mode

70

70

70

72

60

60

60

60

61

62

63

63

64

72

55

55

56

56

57

57

57

58

58

58

59

Cisco IOC Commands for Router

Startup

Configuring a Router with the Setup

Command

Back Up and Upgrade IOS

Chapter Summary

73

74

78

89

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S

T U D Y

S

T R AT E G I E S

.

It is important to have actual hands-on experience with Cisco router configuration and maintenance. Cisco certification tests are structured to require hands-on experience. Without this, it is difficult to pass the test. While studying for any Cisco certification, it is a good idea to build a small router lab for practice. The investment in used routers can be recouped after you have completed your certification by selling them.

There is always a market for used 2500-series routers.

.

This lab can be as small as one Cisco 2501 hooked to an Ethernet LAN. A more useful lab environment would consist of two Cisco 2500series routers that are connected via their serial ports and a small LAN. In order to connect two routers back-to-back via the serial ports, you will need a DTE cable on one router and a

DCE cable on the other. The router with the DCE cable must have the clockrate command added to it’s configuration. This provides the clocking signal a CSU/DSU would normally provide. More details on DCE and DTE devices are covered in

Chapter 5, “WAN Protocols.”

.

There are two choices for connecting the routers via the serial ports. One choice is to use two

Cisco cables—one DCE female and one DTE male. The other choice is to purchase a cable specifically made to connect two routers. Cisco doesn’t make a cable of this type but there are vendors that do. However, be prepared to do some homework because the cables are not always available from any one vendor.

.

Some of the procedures that should be practiced will cause connectivity outages and other problems on production networks. Having a router lab allows you to work the exercises and try different configurations without causing problems for an existing network.

.

After you have your routers in place, play with them. Create different configurations. Try variations of the commands and explore. Reference the context-sensitive help and Cisco’s documentation for explanations of the commands and their options. Documentation for Cisco routers is available to everyone from its Web site at http://www.cisco.com

. At first, it is difficult to learn to traverse the Web site and find what you are seeking, but in searching you will inevitably run across something else that may come in handy. Spend a good deal of time researching different subjects on Cisco’s Web site. It has a great deal of information available for free to those willing to take the time searching. Cisco also includes a documentation CD with each router and switch. This CD is helpful for reading up on the documentation when you do not have Internet access.

.

Practice working with a Cisco router. Practice using the editing commands. They will save you lots of time. Above all, practice. Experience with different configurations and commands will make the difference when it comes time for the test. Although you can learn by reading, you gain superior knowledge and understanding through experience.

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I

NTRODUCTION

This chapter discusses Cisco’s Internetworking Operating System

(IOS). The IOS is what makes the router work. An internetworking engineer interfaces with the IOS through a command line much like old interactive BASIC programming or the DOS operating system.

With most applications today using graphical user interfaces, it is hard to imagine using such a seemingly archaic interface. However, it is arguably the best way to configure and maintain the complex systems that make up our networks. Cisco has developed a program with a graphical interface for creating simple configurations and downloading to a router. This software is called ConfigMaker and is available from Cisco’s Web site. However, it is difficult to create complex configurations and use advanced features of the IOS with this program.

This chapter introduces you to the following:

á

The Command Line Interface (CLI). This chapter will cover some features for making life easier. The CLI includes advanced tools for editing. It also includes context-sensitive help for quick reference.

á

The basic components of a router and how they are used in configurations and daily operation. The procedures for updating the IOS and maintaining configuration files are also explained.

á Configuration commands for basic configuration of the router.

á

Managing configuration files using a TFTP server.

á

Upgrading and managing the IOS using a TFTP server.

á Cisco Discovery Protocol (CDP). A protocol used to discover neighboring devices and exchange capability information with them.

There are some tools you will need to gather to perform some of the exercises in this chapter. You will need a Trivial File Transfer Protocol

(TFTP) server and your router lab. A good TFTP server can be obtained from Cisco’s Web site at http://www.cisco.com/public/ sw-center/sw-other.shtml

.

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44 Pa r t I E X A M P R E PA R AT I O N

L

OGGING IN TO A

R

OUTER IN

B

OTH

U

SER AND

P

RIVILEGED

M

ODES

Cisco routers and switches provide password protection to prevent unauthorized access and administration. Several different methods are supported for authenticating users. Only one of these methods—normal passwords—is addressed on the CCNA exam.

However, it is useful to know about the other methods of authentication.

á

Normal passwords stored in the router’s configuration

á

TACACS+ (Terminal Access Controller Access Control System)

á RADIUS (Remote Access Dial-In User Service)

Authentication Servers

Authentication servers are applications that provide an authentication service to the router, switch, or access server. An authentication server can run under most operating systems including UNIX,

Linux, and Windows NT. There are two main protocols for authentication: RADIUS and TACACS+. Cisco routers support both.

Authentication servers store a database of usernames and levels of security these users are assigned. When a user tries to access a router, it accepts a username and password. It then sends this information to the authentication server where it is verified and usually logged.

The server will respond with either an authorization or rejection for the user. Depending on the implementation, the authentication packets can be encrypted so network sniffers cannot capture clear text passwords traversing the network. Some advanced features of authentication servers and Cisco’s IOS allow an administrator to control on a granular level what commands users have access to.

Normal Password Authentication

The type of user authentication that is addressed on the CCNA exam is password login authentication. This method of authentication simply uses a password stored in the configuration file of the router.

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 45

When a user tries to access the router, a password is requested. This password is compared to the one stored in the configuration file and if they match, the user is granted access. There are two types of passwords: login and enable. The login password grants the user access to the router’s user exec mode. The user exec mode allows you to view some of the components of the router that will be helpful for diagnosing problems. Configuration changes are not allowed in the user exec mode. More advanced diagnostic methods and configuration of the router are available from the privileged exec mode. The privileged exec mode is accessed with the enable password. When you attempt to access a router, either through the console port or telnet, the initial login will appear as follows:

User Access Verification

Password:

Router>

The router displays either the standard message “User Access

Verification” or a Message of the Day (MOTD). The MOTD comes from multiuser days where a system presents a message to each person logging in. This message can provide any information the administrator wants to tell the user. Sometimes this message is used for warnings, scheduled outages, or any other information the administrator would like users or potential hackers to have before logging in.

After the initial message, the router prompts you to enter your password. If the password you enter matches the router’s configuration, you are granted access. The normal user exec prompt of the router includes the router’s hostname and a prompt character. In our example, the router’s hostname is the default

Router

.

The prompt character is

>

. The

> character (prompt character) indicates you are now in the user exec mode. In this mode, you have access to commands for monitoring the router and its interfaces.

What can be performed at this level is restricted. To configure the router or perform advanced diagnostics, you will need to access the privileged exec mode. To access the privileged exec mode, enter the command enable

. Then, enter the enable password.

User Access Verification

Password:

Router>enable

Password:

Router#

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46 Pa r t I E X A M P R E PA R AT I O N

From the user exec mode, you enter the command enable

. This command enables the privileged exec mode. The router then prompts you to enter the enable password. After this is entered successfully, the router grants you access to the privileged exec mode.

The new command prompt character (

#

) indicates you are now in privileged exec mode.

If you want to return to user exec mode from privileged exec, enter the command disable

.

User Access Verification

Password:

Router>enable

Password:

Router#disable

Router>

Entering Commands

You interface with the router’s IOS through the command line interface (CLI). This CLI accepts commands from the user and executes them. When entering commands, you only need to enter enough of the command so the CLI can determine which command you want.

You could enter the following command: c2511r1#show interface serial0

Serial0 is administratively down, line protocol is down

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 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 00:07:03

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, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=down DSR=down DTR=down RTS=down CTS=down c2511r1#

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 47

But the same results can be obtained by entering this abbreviated command: c2511r1#sh int s0

Serial0 is administratively down, line protocol is down

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 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 00:07:55

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, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=down DSR=down DTR=down RTS=down CTS=down c2511r1#

If you abbreviate a command too much, the CLI will not be able to uniquely identify which command you are trying to execute.

Instead, it will respond as follows: c2511r1#te

% Ambiguous command: “te” c2511r1#

To find out which commands begin with “te”, type the following lines: c2511r1#te?

telnet terminal test c2511r1#te

If you add one more letter to the command, the CLI will be able to determine which of these three commands you intended.

c2511r1#tel

Host: 192.168.137.33

Trying 192.168.137.33 ... Open

User Access Verification

Password:

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48 Pa r t I E X A M P R E PA R AT I O N

The CLI will complete a command for you. By typing the unique portion of the command and then pressing the Tab key, the IOS will complete the command for you.

c2511r1#tel<Tab> c2511r1#telnet

With all the different versions of IOS deployed, you will find that what works in one router may not in another. Cisco is constantly upgrading the IOS and adding new features. The context-sensitive help can be handy for discovering what commands are available with the version of IOS you are currently configuring.

It is difficult to remember all of the commands and the parameters for each. Cisco added context-sensitive help to the CLI for this reason.

C

ONTEXT

-S

ENSITIVE

H

ELP

Cisco’s IOS provides help for command syntax and options. This help system is available from the user exec or privileged exec mode command prompts.

To see a list of commands available at any time, simply type a question mark at the prompt. This list provides the commands available from the current mode and a brief description of each command. c2511r1>?

Exec commands:

<1-99> Session number to resume access-enable Create a temporary Access-List entry clear Reset functions connect Open a terminal connection disable Turn off privileged commands disconnect Disconnect an existing network connection enable Turn on privileged commands exit Exit from the EXEC help Description of the interactive help system lock Lock the terminal login Log in as a particular user logout Exit from the EXEC mrinfo Request neighbor and version information from a multicastrouter mstat Show statistics after multiple multicast traceroutes mtrace Trace reverse multicast path from destination to source name-connection Name an existing network connection pad Open a X.29 PAD connection ping Send echo messages ppp Start IETF Point-to-Point Protocol (PPP) resume Resume an active network connection

—More—

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Notice that the last line listed is

—More—

. This is the IOS’ way of indicating that there is more information to display. To see the next line of text, press the Enter key. To display the entire next screen of information, press the spacebar key. The line-by-line display key will come in very handy when you start looking at interface parameters.

If the information for which you were searching is displayed on the screen and you want to end the listing and return to the exec prompt, press the Esc key.

During command entry, you can get more information about the command by entering a question mark after the command. Entering a command followed by a space and then the question mark will list the options for the command.

c1604>show ?

WORD Flash device information - format <dev:>[partition] clock Display the system clock dialer Dialer parameters and statistics history Display the session command history hosts IP domain-name, lookup style, nameservers, and host table isdn ISDN information location Display the system location modemcap Show Modem Capabilities database ppp PPP parameters and statistics rmon rmon statistics sessions Information about Telnet connections snmp snmp statistics tacacs Shows tacacs+ server statistics terminal Display terminal configuration parameters traffic-shape traffic rate shaping configuration users Display information about terminal lines version System hardware and software status c1604>show

C

ONFIGURING THE

R

OUTER

To configure the router, you must enter the configuration mode. The configuration can be set from several different sources: terminal, memory, or network. When you configure from the terminal, you are entering commands directly into the routers running configuration.

When you make a change, it takes effect immediately. When you configure from memory, the startup configuration is copied to the running configuration. When you configure from network, the router copies a new running configuration from a TFTP server. This section will address the interactive configuration mode first. This is where you enter configuration commands from a terminal. This mode is accessed using the configuration command.

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50 Pa r t I E X A M P R E PA R AT I O N

Router#configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#

Notice that the prompt changes to

Router(config)#

. This indicates that the config mode has been entered. A new set of commands is available. In this mode, you are configuring global settings. These are settings affecting the entire router. The first thing you might want to do is assign a hostname to the router.

Hostname

The hostname of a router is set by using the global config command.

c2513r1(config)#hostname Keyser

Keyser(config)#

Notice that, after setting the hostname, the prompt changes immediately. The hostname is used to identify the router. It should represent something about the router. Some examples of router hostnames might be the city in which the router is located: Atlanta, Chicago, or

Boston. Another feature of the IOS that can be used for identification is the Message of the Day (MOTD).

Message of the Day (MOTD)

The Message of the Day (MOTD) is displayed when someone accesses the router. The MOTD can be used to inform network personnel about network changes, planned outages, or upcoming upgrades. The

MOTD is displayed prior to prompting for a password.

To enter a MOTD, use the following command from the global config mode: c2511r1(config)#banner motd #This router was replaced 1/09/99# c2511r1(config)#

The

# before and after the message is simply a delimiter character and can be any character not included in the message. It simply identifies the beginning and end of the message. The preceding command will display the following lines the next time someone connects to the router.

This router was replaced 1/09/99

User Access Verification

Password:

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 51

Beginning Configuration Review

.

The IOS has two levels of access: user exec mode and privileged exec mode. Logging in to a router puts you in user exec mode.

Privileged exec mode is accessed by entering the command enable and then entering the enable password.

.

Commands are entered at the Command Line Interface (CLI) and are executed immediately. Configuration changes take effect when the change is made.

.

Commands can be abbreviated. As long as you enter enough of the command for the CLI to determine which command you intended, it will execute the command.

.

Configuration changes are made in the config mode, which is entered by executing the command config from the privileged exec mode.

.

Context-sensitive help is available by typing

?

at any prompt.

If you have started a command and then press

?

, the CLI will show you a list of parameters for that command.

.

Configuration changes can come from a terminal, network, or memory. Usually, you will make all changes from a terminal.

.

The hostname identifies the router. Choose a sensible one that describes something about the router’s location or function.

When your network grows to hundreds of routers, you will be thankful you did this.

.

The Message of the Day (MOTD) is displayed when a user accesses the router before the password prompt is displayed. It can be used to identify information about the router or any other information you might want someone to know before entering the router.

R E V I E W B R E A K

Configuring Interfaces

When configuring interface parameters such as bandwidth, IP address, media type, and most anything else specific to an interface, you use the interface configuration mode. To enter the interface configuration mode, enter the following command:

Router(config)#interface ethernet0/0

Router(config-if)#

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52 Pa r t I E X A M P R E PA R AT I O N

This places you in the interface configuration mode for Ethernet0/0.

Parameters specific to the interface are entered here. IP addresses and ring numbers are just two examples of interface-specific parameters.

Cisco routers represent interfaces differently depending on the platform. Cisco 2500-series routers simply list the interfaces by type and number. The first serial port on a Cisco 2501 is called Serial0; the second is called Serial1. Each interface type starts at number 0 and if there are multiples of that type, they increment. In routers supporting plug-in modules such as the 3600 series, the interfaces are identified by interface type, then by slot/port. In a 3600 with a dual Ethernet module in slot 0, the first interface is identified as Ethernet0/0, and the second is Ethernet0/1. The 4000 series is an exception to this rule. It simply identifies the ports by type/number. Larger routers like the 7500 series have slots, modules, and ports. Their interfaces are addressed as type slot/module/port, such as Ethernet4/0/1. As with commands, IOS will accept abbreviated references to interfaces as long as it is able to uniquely identify which interface you are referring to. The same results can be obtained using the abbreviated command: c3620r1(config)#in e0/0 c3620r1(config-if)#

To return to the global config mode after configuring the interface, enter the command exit or use the command sequence Ctrl+Z to return to the privileged exec command prompt.

When you make changes to a router’s configuration, you are changing the running config. This is the config file currently used by the router for operation. After you make a change, it takes effect immediately. If you shut down the port you are using to access the router, you will immediately lose connectivity. Some config changes require forethought as to how to effectively make the necessary changes without losing connectivity.

Saving Config Changes

When changes are made to the running configuration, they must be saved to the startup configuration or the changes will be lost the next time the router is rebooted. To save the changes, use the following command: c3620r1#copy running-config startup-config

Building configuration...

[OK] c3620r1#

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This is commonly abbreviated copy run start

.

A legacy command that is still used often ( write memory

) is commonly abbreviated wr

. It is unclear whether this command will still be available in future versions of IOS.

The copy run start and wr commands both copy the running configuration file to the NVRAM of the router so that the next time it is booted, this configuration will be used.

Managing Passwords

Cisco routers store passwords for login access in the configuration file. There are two types of passwords used in this manner for accessing a router: the login password and the enable password. The login

password is used to gain access to the router’s user exec mode. The

enable password is used to enter the privileged exec mode where

configuration changes can be made.

In the configuration file, the entries for passwords appear as follows: c2513r1#sh run

Building configuration...

Current configuration:

!

! Last configuration change at 00:17:36 EST Tue Dec 22 1998

!

version 11.2

.

.

hostname c2513r1

!

enable password khayman

!

.

.

partition flash 2 8 8

.

line con 0 password cisco login line aux 0 password cisco login line vty 0 4 password cisco login

.

.

end c2513r1#

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54 Pa r t I E X A M P R E PA R AT I O N

Choosing Passwords Although some testing situations use cisco as a password, as an administrator, it is never a good idea to use cisco as the password for one of your routers.

This book uses it as an example in some places, but in a real life situation, you don’t want to use simple or obvious passwords. Strong passwords include both upper- and lowercase letters and numbers or special characters. A good example of a strong password is jst4u2GuEs

.

The preceding example shows the enable password set to khayman

.

Lines console 0, auxiliary 0, and vty 0 through 4 all have a login password of cisco

.

In this example, when an attempt is made to access this router via any of the available terminals, the router will prompt for a password.

After the password is entered, it is compared to the password for the line the request is coming from. In our example, the password for all lines is cisco

. It is important to note that passwords for Cisco routers are case-sensitive. ( cisco is not the same as

Cisco

.)

To enter the privileged exec mode, we would enter the password khayman at the enable prompt.

Because it is possible that your configuration may be viewed by people who you do not want to have access to the passwords, a method to encrypt the passwords is built into Cisco’s IOS. This service is enabled by entering the global configuration command: c2513r1(config)#service password-encryption c2513r1(config)#

This service encrypts the passwords stored in the configuration file.

Now when the configuration is viewed, the passwords appear as follows: c2513r1#sh run

Building configuration...

Current configuration:

!

! Last configuration change at 00:28:43 EST Tue Dec 22 1998

! NVRAM config last updated at 00:21:21 EST Tue Dec 22 1998

.

.

!

version 11.2

service password-encryption no service udp-small-servers no service tcp-small-servers

!

hostname c2513r1

!

enable password 7 110211040E1F0A02

!

partition flash 2 8 8

.

.

.

line con 0 password 7 094F471A1A0A login line aux 0

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 55

.

.

password 7 094F471A1A0A login line vty 0 4 password 7 094F471A1A0A login c2513r1#

Someone viewing the configuration of this router will not be able to immediately identify the passwords. However, the algorithm used for encrypting these passwords is relatively easy to break. There are several programs available on the Internet that will reverse the encryption used by Cisco’s password-encryption service.

A more robust method of encryption is available for the enable password by using the enable secret command instead of enable password

.

The enable secret command encrypts the password using the MD5 algorithm. The config for an enable secret appears as follows:

!

enable secret 5 $1$er0I$Qe0nMh/.7T73WsTq31r0l1

!

enable secret always shows the password encrypted even if the password-encryption service is not enabled. There is no reason to use the enable password command if you are using enable secret

.

U

SING THE

C

OMMAND

H

ISTORY AND

E

DITING

F

EATURES

Cisco’s IOS provides some advanced features to make your job easier, such as command history and editing features. The following sections will discuss the command history as well as cursor movement, deletion, pasting buffers, and other miscellaneous commands.

Command History

The IOS exec keeps a number of the commands you type in a buffer called the command history buffer. The default size of this buffer is ten command lines. This command history buffer allows you to recall a previously typed command and execute it again.

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56 Pa r t I E X A M P R E PA R AT I O N

This is very helpful when trying to diagnose problems and you have to keep trying to ping a remote location. It is also not uncommon to have to enter the same line many times with just a small change, such as the last digit of the IP address. The command history used in conjunction with the advanced editing commands can be a real typing saver.

Viewing the Contents of the Command

History Buffer

The contents of the command history buffer can be viewed by entering the command show history

.

c2511r1#show history en sh interfaces sh log sh clock detail sh run clear counters sh history c2511r1#

Scroll through the commands by pressing the up-arrow or downarrow key. After the command you want to execute is displayed, you can press Enter to execute it or you can modify it and then press

Enter. Alternatives to the up and down arrows are Ctrl+P for previous and Ctrl+N for next. For the special editing keys such as up arrow or down arrow to work, your terminal program must support

VT100 emulation.

Changing the Size of the History Buffer

The default size of the command history buffer is ten commands. The history buffer size can be changed. The change can be for the current session or for every session connecting on a particular line. To change the size of the buffer for the current session, enter the following command, which would change the size of the history buffer to 25 lines: c2511r1#terminal history size 25 c2511r1#

The size of the history buffer can be changed for every connection via a particular line by adding the following commands to the line configuration: c2511r1#config terminal

Enter configuration commands, one per line. End with

➥CNTL/Z.

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 57 c2511r1(config)#line vty 1 c2511r1(config-line)#history size 25 c2511r1(config-line)#

Editing Features

The editing features built into Cisco’s IOS can make your job much easier. Often, you will be entering the same command multiple times with just one slight change. By using the command history to bring up the last entered command and then using the editing features, you can save yourself much typing. Another benefit is that, with less typing to do, there is less of a chance to make a typographical error.

Cursor Movement

Even though the IOS supports the entry of only one line of text at a time, it is very helpful to be able to move the cursor around on this line.

The cursor can be moved around by using the following keys:

á

Ctrl+A. Move the cursor to the beginning of the line.

á

Ctrl+E. Move the cursor to the end of the line.

á Esc+B. Move to the beginning of the previous word.

á

Ctrl+F. Move forward one character.

á Ctrl+B. Move back one character.

á Esc+F. Move forward one word.

If the line you are typing extends beyond the width of your terminal, the CLI will scroll the line left ten characters and display a dollar sign at the beginning of the line to indicate that there is more to the left.

c2511r1(config)#$estamps debug datetime localtime msec

➥show-timezone

Deleting

When you type, you must delete. It’s a law that seems unbreakable.

The IOS provides some sophisticated deletion commands for a CLI.

The following are the deletion commands available to the user:

á

Delete or Backspace. Erase the character to the left of the cursor.

á

Ctrl+D. Delete the character at the cursor.

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58 Pa r t I E X A M P R E PA R AT I O N

á Ctrl+K. Delete all characters from the cursor to the end of the command line.

á

Ctrl+U or Crtl+X. Delete all characters from the cursor to the beginning of the command line.

á

Ctrl+W. Delete the word to the left of the cursor.

á Esc+D. Delete from the cursor to the end of the word.

Each of these deletion commands places the deleted text in a buffer.

IOS keeps the last ten deletions in buffers. These buffers can be recalled and pasted back into a line.

Pasting Buffers

There are ten buffers holding the last ten deletions. These deletions can be recalled and pasted in the line at the cursor using the recall paste keys.

To paste the buffer in the line at the cursor, type one of the following commands:

á Ctrl+Y. Recall and paste the most recent deletion at the cursor.

á

Esc+Y. Recall the next buffer entry and paste at the cursor.

Miscellaneous Editing Commands

There are some miscellaneous commands that can also come in handy:

á

Esc+C. Capitalize the letter at the cursor.

á

Esc+L. Change the word at the cursor to lowercase.

á Esc+U. Capitalize letters from the cursor to the end of the word.

á

Ctrl+T. Transpose the character to the left of the cursor with the character at the cursor.

á Ctrl+R. Redisplay the current command line.

Turning Off Editing Features

The editing features are on by default. They can be turned off for the current session by entering the following command:

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 59 c2511r1#terminal no editing c2511r1#

The editing features can also be turned off for all sessions on a particular line with the following configuration command: c2511r1#config terminal

Enter configuration commands, one per line. End with

➥CNTL/Z.

c2511r1(config)#line vty 1 c2511r1(config-line)#no editing

This will disable the editing features for any connection via this line.

Terminal Settings

Editing features, command history, and several other options are known as terminal settings. Terminal settings define some parameters about how the CLI interfaces with your terminal.

This includes the type of terminal, screen length and width, and more. The current values of these parameters can be viewed using the IOS command.

c2511r1#show terminal

Line 18, Location: “”, Type: “ANSISYS”

Length: 24 lines, Width: 80 columns

Baud rate (TX/RX) is 9600/9600

Status: Ready, Active, No Exit Banner

Capabilities: none

Modem state: Ready

Special Chars: Escape Hold Stop Start Disconnect Activation

^^x none - - none

Timeouts: Idle EXEC Idle Session Modem Answer Session Dispatch

00:10:00 never none not set

Idle Session Disconnect Warning never

Modem type is unknown.

Session limit is not set.

Time since activation: 00:17:41

Editing is disabled.

History is enabled, history size is 25.

DNS resolution in show commands is enabled

Full user help is disabled

Allowed transports are pad v120 telnet rlogin. Preferred is telnet.

No output characters are padded

No special data dispatching characters c2511r1#

Here, you see that editing has been disabled and the history buffer has been set to 25 commands.

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60 Pa r t I E X A M P R E PA R AT I O N

U

NDERSTANDING

R

OUTER

E

LEMENTS

A router contains many of the same components as a computer.

There is a processor for executing the operating system and random access memory (RAM) for storing programs and storing data. There is read-only memory (ROM), where Cisco stores the self test program and the initial IOS that will get the router running, and Non-Volatile

RAM—memory that maintains its contents when power is lost.

Cisco routers also include a type of memory not commonly found in computers. Flash memory (discussed later) is used for storing the IOS the router will execute. The router also includes network interfaces for connecting to the networks it will be handling traffic for.

Random Access Memory (RAM)

Like a computer, the router needs RAM to store data, execute programs, and buffer data. Packets of data can be buffered in RAM until the router has time to process them. In some models of Cisco routers

(3600, 7200, 7500, and more), the IOS runs in RAM. Routers that run the IOS from RAM need enough RAM to store the version of

IOS they are running plus enough to store routing tables, ARP tables, buffer packets, and more. Some routers, like the 2500 series, run the

IOS from flash memory.

Read-Only Memory (ROM)

Cisco routers use read-only memory in much the same way a computer does. The instructions for performing a self-test of the device are stored in ROM. The ROM also contains a subset (some routers contain a full version) of the IOS. This subset is used to get the router up and running in case the normal version of IOS in the router becomes corrupt or missing.

Flash Memory

Flash memory is like ROM that can be erased and rewritten electroni-

cally. Cisco routers use flash memory much like personal computers use disk storage. The copy of IOS the router runs is stored in flash memory.

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In larger routers, when the router boots, this copy of IOS is copied to RAM where it is executed. The smaller routers like the 2500 series run the IOS directly from flash. This creates a challenge when you need to upgrade the IOS in the router. Because the router is actually running the copy of IOS in flash memory, you cannot copy a new version over it while it is running.

In routers in which the IOS is copied to RAM during the boot process, the IOS can be upgraded by copying a new version of IOS to flash while the router runs the copy stored in RAM. After the new version of IOS is stored in flash memory, the router can be rebooted and the new version will be loaded and executed.

This does not work when the router is running the IOS from flash.

In this case you have two options:

á You can partition the flash memory into multiple areas. Then, copy a new version of the IOS to the currently unused partition.

After the IOS is stored there, change the router’s config to boot from the partition with the new version of IOS. Reboot the router and the new version is executed. This method requires the router to have enough flash memory for two full copies of the IOS to be stored simultaneously.

á You can also use the flash load helper built into the IOS. The flash load helper will prompt you for the information needed to update the router’s IOS. It will then proceed to download the new version of IOS. After completing the download, it will reboot. After it reboots, it will be running the new version of IOS.

Network Interfaces

Serving as a network interface is secondary to a computer’s primary function. A desktop PC is used primarily for word processing, spreadsheets, and games. Its network interface is secondary to those functions. It simply provides connectivity for storing data, printing to shared printers, or joining in a multiuser game of Quake II.

The network interfaces are primary to a router. It does not do word processing or spreadsheets, and it only provides connectivity for the multiuser game of Quake II. The router’s primary function is to connect networks. Therefore, the network interfaces are very important for the router.

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Unlike desktop PCs, which usually have only one network interface, the router can have many network interfaces. These interfaces can be for different types of networks such as Ethernet, token ring, or FDDI.

They can also be for Wide Area Network connections via serial ports.

Within the router, these interfaces are connected and pass data between each other based on the configuration of the router. The components of a router work together to function as a network connection for many different networks.

Examining the Components of a Router

The router can provide much information about the components within. Information on the amount of memory, version of IOS, and interfaces installed can be viewed with the show version command.

c2511r1#show version

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-IS-L), Version 11.2(9),

➥RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems, Inc.

Compiled Mon 22-Sep-97 21:31 by ckralik

Image text-base: 0x0302EB70, data-base: 0x00001000

ROM: System Bootstrap, Version 5.2(8a), RELEASE SOFTWARE

BOOTFLASH: 3000 Bootstrap Software (IGS-RXBOOT), Version

➥10.2(8a), RELEASE SOFTW

ARE (fc1) c2511r1 uptime is 5 days, 1 hour, 49 minutes

System restarted by power-on at 21:31:57 EST Wed Dec 9 1998

System image file is “flash:25ipp12.bin”, booted via flash cisco 2511 (68030) processor (revision D) with 16384K/2048K

➥bytes of memory.

Processor board ID 02313574, with hardware revision

➥00000000

Bridging software.

X.25 software, Version 2.0, NET2, BFE and GOSIP compliant.

1 Ethernet/IEEE 802.3 interface(s)

2 Serial network interface(s)

16 terminal line(s)

32K bytes of non-volatile configuration memory.

8192K bytes of processor board System flash (read-only)

Configuration register is 0x2102 c2511r1#

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C h a p t e r 2 I N T E R N E T WO R K I N G O P E R AT I N G S Y S T E M 63

This information tells us quite a bit about this router. The router is running IOS version 11.2(9). The ROM bootstrap, the subset of

IOS stored in ROM, is version 10.2(8a). The router’s hostname is c2511r1 and it has been running for five days, one hour, and 49 minutes. The last time the router was restarted, it was due to a power-on.

This occurred at 21:31:57 EST on Wednesday, December 9, 1998.

The IOS file the router is executing is flash:25ippl2.bin. This router is a model 2511 and has 1,6384KB of RAM. There is one Ethernet/

IEEE 802.3 interface, as well as two serial interfaces and 16 terminal lines. There are 32KB of Non-Volatile RAM. The router has

8,192KB of ROM. Finally, the router’s configuration register is set to

0x2102. The 0x indicates that the following number is represented in the hexadecimal numbering system. This numbering system is explained in more detail in Chapter 4, “Understanding and

Configuring IPX/SPX and Other Network Protocols.”

Configuration Register

The last line in the show version information tells us the configuration register’s value. The configuration register is 16 bits wide and defines some important characteristics of the router.

The lowest four bits of the configuration register (bits 0–3), make up the boot field. The boot field tells the router how to start up the IOS.

For more information on the hexadecimal numbering system, see

Chapter 4. The following list shows the values and the function they perform for the first four bits.

á

00 – Stay at the boot prompt.

á

01 – Boot the system image stored in ROM.

á

02-0F – Specifies the default netboot filename.

Processor

The processor does much of the work in the router. It keeps routing tables updated by calculating paths, moving packets, and performing many other tasks. When diagnosing problems, it is often important to check on the load of the processor. To view statistics on the processor, enter the command show processes

.

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64 Pa r t I E X A M P R E PA R AT I O N

CPU utilization for five seconds: 15%/7%; one minute: 20%; five minutes: 24%

PID QTy PC Runtime (ms) Invoked uSecs Stacks TTY Process

1 Csp 316BB12 16540 157074 105 736/1000 0 Load Meter

2 M* 0 3768 178 21168 2216/4000 18 Virtual Exec

3 Lst 3159C24 1253740 13415 93458 1724/2000 0 Check heaps

4 Cwe 315F7A6 0 1 0 1728/2000 0 Pool Manager

5 Mst 30FF97C 8 2 4000 1692/2000 0 Timers

6 Lwe 31860BE 23996 29013 827 1292/2000 0 ARP Input

7 Mwe 30B959E 0 1 0 1740/2000 0 SERIAL A’detect

8 Hwe 31A575C 231024 226402 1020 3016/4000 0 IP Input

9 Mwe 3206206 55356 91643 604 1504/2000 0 CDP Protocol

10 Hwe 324F448 4 1 4000 1816/2000 0 Asy FS Helper

11 Mwe 3191136 9936 160567 61 1300/2000 0 TCP Timer

12 Lwe 3195CAC 104 34 3058 3176/4000 0 TCP Protocols

13 Lwe 31D0878 0 1 0 1724/2000 0 Probe Input

14 Mwe 31D1792 0 1 0 1736/2000 0 RARP Input

15 Mwe 318AB40 4 1 4000 1612/2000 0 BOOTP Server

16 Mwe 31DE68C 1116072 797761 1399 2496/3000 0 IP Background

17 Lsi 32007E2 1140 13080 87 1724/2000 0 IP Cache Ager

18 Cwe 3162C1E 0 1 0 1748/2000 0 Critical Bkgnd

19 Mwe 3144E58 28 7 4000 1168/2000 0 Net Background

20 Lwe 30F6F12 8 32 250 1644/2000 0 Logger

21 Msp 30EE7EE 927644 784718 1182 1512/2000 0 TTY Background

—More—

Here, we see that the processor of this router is running at an average load of 24% over the past five minutes. Over the last minute the processor averaged 20% utilization, and for the last five seconds it averaged 15%, and 7% of this time was spent at the interrupt level.

Following the statistics for the CPU is a table of information about processes the CPU is executing. Each process list details how the process is being executed.

Interfaces

The interfaces of a router connect it to the networks for which it will be handling traffic. These interfaces provide a great deal of information about the network and how the interface is performing.

To view information about an interface, use the command show interfaces

.

c2511r1#show interfaces

Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0000.0c75.9d72

➥(bia 0000.0c75.9d72)

Internet address is 192.168.137.50/24

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/

➥255, load 1/255

Encapsulation ARPA, loopback not set, keepalive set

➥(10 sec)

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

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Last clearing of “show interface” counters 1w0d

Queueing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

5 minute input rate 0 bits/sec, 1 packets/sec

5 minute output rate 0 bits/sec, 1 packets/sec

312235 packets input, 25029135 bytes, 0 no buffer

Received 289017 broadcasts, 0 runts, 0 giants, 0

➥throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

0 input packets with dribble condition detected

233453 packets output, 19662124 bytes, 0 underruns

0 output errors, 1 collisions, 1 interface resets

0 babbles, 0 late collision, 10 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

Serial0 is administratively down, line protocol is down

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 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 1w0d

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, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=down DSR=down DTR=down RTS=down CTS=down

Serial1 is administratively down, line protocol is down

Hardware is HD64570

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/

➥255, load 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 1w0d

Input queue: 0/75/0 (size/max/drops); Total output drops:

➥0

Queueing strategy: weighted fair

Output queue: 0/64/0 (size/threshold/drops)

Conversations 0/0 (active/max active)

Reserved Conversations 0/0 (allocated/max allocated)

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

continues

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66 Pa r t I E X A M P R E PA R AT I O N

continued

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, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=down DSR=down DTR=down RTS=down CTS=down c2511r1#

In the preceding command, you see the output from show interfaces on a Cisco 2511 router. The 2511 has one Ethernet port, two serial ports, and 16 RS/EIA-232 serial lines.

You can also view this information for a specific interface by specifying the interface as follows: c2511r1>show interfaces e0

Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0000.0c75.9d72

➥(bia 0000.0c75.9d72)

Internet address is 199.250.137.50/27

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely

255/255, load 1/255

Encapsulation ARPA, loopback not set, keepalive set

➥(10 sec)

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of “show interface” counters never

Queueing strategy: fifo

Output queue 0/40, 0 drops; input queue 1/75, 0 drops

5 minute input rate 3000 bits/sec, 4 packets/sec

5 minute output rate 2000 bits/sec, 4 packets/sec

1272652 packets input, 99574686 bytes, 0 no buffer

Received 1210186 broadcasts, 0 runts, 0 giants, 0

➥throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

0 input packets with dribble condition detected

913831 packets output, 74071677 bytes, 0 underruns

0 output errors, 4 collisions, 3 interface resets

0 babbles, 0 late collision, 35 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out c2511r1>

In reviewing the output from show interfaces

, we can see that

Ethernet0 is “up and up.” This is a term used when talking about an interface on a Cisco router. There are two parts to an interface being considered “up.” First, the interface must be administratively up.

To set an interface administratively down, the command shutdown is placed in the configuration for the interface. To set the interface in an administratively up state, the command no shutdown is added to the configuration for the interface.

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In looking at Ethernet0, it is administratively up and the line protocol is up. The line protocol reports if the interface is receiving link status.

On an Ethernet port, this would be the carrier signal. On a serial port, it means the router is receiving hello packets. When the line protocol is up, it means the port is able to pass packets to and from the network attached to this interface. A common state for an interface experiencing problems is “up and down.” The interface is administratively up, yet the link protocol is down. An Ethernet interface would display this state if the cable were unplugged from the hub. The port would still be administratively up, but would no longer be receiving a carrier signal from the hub, and thus, the link protocol would be down. In this state, the interface cannot pass traffic.

In the following lines, you see the MAC address for this interface is

0000.0c75.9d72.

Hardware is Lance, address is 0000.0c75.9d72

➥(bia 0000.0c75.9d72)

Internet address is 192.168.137.50/24

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/

➥255, load 1/255

The burned-in address (bia) is also 0000.0c75.9d72. If we set the

MAC address on an interface different than the bia, these would be different. The first would be the MAC address the router is using to talk on the network, and the bia would simply be the address the interface was assigned when it was manufactured.

The Internet address for this interface is 192.168.137.50/24. This is the IP address assigned to the interface. It is represented somewhat differently than most people are accustomed to seeing with an IP address. The number after the IP address /24 is the number of subnet bits assigned to the IP address. You will learn more about this in

Chapter 4. This is equivalent to a subnet mask of 255.255.255.0.

The maximum transmission unit (MTU) for this interface is 1,500 bytes.

This is the largest packet size this interface can transmit. This information is important when the router must move data from dissimilar networks.

Token ring has a much higher MTU than Ethernet. Therefore, when a router is routing data from a token ring network to an Ethernet network, it will need to fragment packets larger than the MTU for Ethernet so the data can be transmitted over the media. Fragmenting is the process of taking a large packet and dividing it into smaller packets. The smaller packets are marked as fragments and the last packet in the fragmented packet is marked as such. When the end station receives fragmented packets, it reassembles them as they are passed up the OSI model. The application never knows the data was separated during transmission.

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The bandwidth of this interface is 10,000 kilobits and the delay is

1,000 milliseconds. The reliability of this network is 255/255. This information is represented as a fraction. In this case the reliability is

100%, 255 of 255 parts. A 50% reliable network would be 127/255.

The current load on this network is 1/255. Again, this is a fraction showing a very light load on the network. This information is used by advanced routing protocols to determine the fastest and most reliable path between two end stations.

Encapsulation ARPA, loopback not set, keepalive set

➥(10 sec)

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of “show interface” counters 1w0d

Queueing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

The preceding lines of the output provide more information about the interfaces. We see the encapsulation used for this interface is ARPA, there is not a loopback set, and keepalive packets are sent and expected every ten seconds. The address resolution protocol (ARP) used on this network is ARPA and the timeout for ARP data is four hours. The last packet coming in this interface was zero seconds ago and the last packet sent out this interface was zero seconds ago. There has never been an output hang on this interface. The last time the statistic counters for this interface were reset was one week, zero days ago.

The queuing strategy for this interface is first in first out (fifo). The

queuing strategy is how the router decides which packets to send first,

in the event the network interface becomes backed up with data.

The output queue for this interface currently has zero packets in it and has room for 40 packets. The input queue currently has zero packets in it and has room for 75 packets. There have been no packets dropped by this interface

5 minute input rate 0 bits/sec, 1 packets/sec

5 minute output rate 0 bits/sec, 1 packets/sec

312235 packets input, 25029135 bytes, 0 no buffer

Received 289017 broadcasts, 0 runts, 0 giants, 0

➥throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

0 input packets with dribble condition detected

233453 packets output, 19662124 bytes, 0 underruns

0 output errors, 1 collisions, 1 interface resets

0 babbles, 0 late collision, 10 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

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This information gives us statistics about the interface. In the past five minutes, the average input and output rates have been zero bits per second and one packet per second. Because the counters were reset, the interface has had 312,235 packets input, which total

25,029,135 bytes. None of the input packets were dropped, due to no buffers being available. The interface has sent 233,453 packets, totaling 19,662,124 bytes. There has been one collision and one interface reset. TEN packets were deferred.

The 16 serial lines do not show up in the show interfaces because they are considered lines to the router. The command show line displays information about these lines along with the console and auxiliary line.

c2511r1#show line

Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns

0 CTY - - - - - 0 0 1/7983

1 TTY 9600/9600 - inout - - - 1 285833 0/2

2 TTY 9600/9600 - inout - - - 0 7870839 0/2

3 TTY 9600/9600 - inout - - - 0 2484165 0/0

4 TTY 9600/9600 - inout - - - 0 0 0/0

5 TTY 9600/9600 - - - - - 0 0 0/0

6 TTY 9600/9600 - - - - - 0 0 0/0

7 TTY 9600/9600 - - - - - 0 0 0/0

8 TTY 9600/9600 - - - - - 0 0 0/0

9 TTY 9600/9600 - - - - - 0 0 0/0

10 TTY 9600/9600 - - - - - 0 0 0/0

11 TTY 9600/9600 - - - - - 0 0 0/0

12 TTY 9600/9600 - - - - - 0 0 0/0

13 TTY 9600/9600 - - - - - 0 0 0/0

14 TTY 9600/9600 - - - - - 0 0 0/0

15 TTY 9600/9600 - - - - - 0 0 0/0

16 TTY 9600/9600 - - - - - 0 0 0/0

17 AUX 9600/9600 - - - - - 0 0 0/0

* 18 VTY - - - - - 28 0 0/0

19 VTY - - - - - 0 0 0/0

20 VTY - - - - - 0 0 0/0

21 VTY - - - - - 0 0 0/0

Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns

22 VTY - - - - - 0 0 0/0 c2511r1#

The console port is displayed as CTY and the auxiliary port as

AUX. The 16 lines between these two are the 16 RS/EIA-232 lines of the 2511. The last five entries are the virtual terminal connections for telnet sessions. Notice the asterisk next to number 18. This indicates this is the line via which we are currently accessing the router.

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C

ISCO

D

ISCOVERY

P

ROTOCOL

(CDP)

Cisco discovery protocol runs on Cisco-manufactured devices including routers, access servers, and workgroup switches. It starts automatically when the device is booted. CDP operates at the Data Link layer.

Because it runs at the Data Link layer, it is not necessary for the devices to be in common network address domains to exchange CDP data.

CDP provides a method for a Cisco device to discover its neighbor

Cisco devices and exchange information about the devices.

Enabling and Disabling CDP

CDP is enabled by default; it can be disabled on a router by entering the following global command: c2511r1(config)#no cdp run

You can also disable CDP on a particular interface by entering the following command: c2511r1(config-if)#no cdp enable

Seeing CDP Neighbors

You can view a router’s CDP neighbors by entering the following command: c3620r1#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

GSD0090F28CE000 Eth 0/0 167 T S 1900 24 c2513r1 Tok 0/0 155 R 2500 Tok 0 c1604 BRI1/0 147 R 1604 BRI0 c3620r1#

This router has three neighbors. The first is a Cisco 1900 switch attached to local interface Ethernet0/0. It is attached via port 24 in the 1900. This neighbor has transparent bridging and switching capability. Next, we have a Cisco 2500 attached via local interface

TokenRing0/0 and remote interface TokenRing0. The first two devices support routing across the link. There is also a Cisco 1604 router attached to local interface BRI1/0 and remote interface BRI0 supporting routing.

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More detailed information about the neighbors can be viewed with the following command: c3620r1#sh cdp neighbors detail

-------------------------

Device ID: GSD0090F28CE000

Entry address(es):

IP address: 192.168.137.40

Platform: cisco 1900, Capabilities: Trans-Bridge Switch

Interface: Ethernet0/0, Port ID (outgoing port): 24

Holdtime : 132 sec

Version :

V8.00

-------------------------

Device ID: c2513r1

Entry address(es):

IP address: 192.168.200.3

Platform: cisco 2500, Capabilities: Router

Interface: TokenRing0/0, Port ID (outgoing port):

➥TokenRing0

Holdtime : 123 sec

Version :

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-J-L), Version 11.2(14),

➥RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1998 by cisco Systems, Inc.

Compiled Mon 18-May-98 12:57 by tlane

-------------------------

Device ID: c1604

Entry address(es):

IP address: 192.168.99.1

Platform: cisco 1604, Capabilities: Router

Interface: BRI1/0, Port ID (outgoing port): BRI0

Holdtime : 139 sec

Version :

Cisco Internetwork Operating System Software

IOS (tm) 1600 Software (C1600-Y-L), Version 11.2(16)P,

➥RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1998 by cisco Systems, Inc.

Compiled Mon 19-Oct-98 13:49 by pwade c3620r1#

This information lists each neighbor and the version of IOS it is running. This is very helpful when trying to diagnose why a feature is not working. If the neighbor router is running an older version of

IOS, it may not support some newer features. In an environment where different groups support different routers, this can be helpful in finding information about the routers with which you may need to communicate.

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CDP Parameters

CDP has two timers you can adjust. The first determines how often the router sends a CDP packet. The default for this timer is every 60 seconds. The second timer is the holdtime—how long the device will wait for a CDP packet before removing the neighboring device from the CDP table. The default holdtime is 180 seconds.

The values of these timers can be viewed using the following command: c3620r1#sh cdp interface

Ethernet0/0 is up, line protocol is up

Encapsulation ARPA

Sending CDP packets every 60 seconds

Holdtime is 180 seconds

To alter how often the router sends a CDP packet, use the following global command: c3620r1(config)#cdp timer 90

This sets the time between CDP packets to 90 seconds. We can change the holdtimer to 120 seconds by using the following global command: c3620r1(config)#cdp holdtime 120

M

ANAGING

C

ONFIGURATION

F

ILES

FROM THE

P

RIVILEGED

E

XEC

M

ODE

The configuration file defines how the router will interact with the connected networks and the data on those networks. Network addresses, bridge numbers, and routing protocols are just some of the parameters stored in the configuration file. As with any important information, this should be backed up so it can be retrieved if the device should fail.

Earlier you used the command copy running-config startup-config

.

This is much like copying files on a computer using DOS or a DOS window on a Windows 95 computer. You are copying the file runningconfig to the file startup-config. The only difference is that in a router, these files have special meaning. The running-config specifies how the router will behave with the networks attached. The startupconfig will become the running-config the next time the router is rebooted either by power-on or by a software reload.

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These files can be copied to and from a TFTP server. To copy the running-config to a TFTP server, use the following command: c2511r1#copy running-config tftp

Remote host []? 192.169.169.140

Name of configuration file to write [c2511r1-confg]?

Write file c2511r1-confg on host 192.168.169.140? [confirm]y

Building configuration...

Writing c2511r1-confg !! [OK] c2511r1#

Each exclamation point (

!

) indicates that one packet has been successfully transferred.

The running-config file is copied via the TFTP protocol to the TFTP server at IP address 129.168.169.140. The filename used on the

TFTP server is c2511r1-config. Finally, the file is transferred to the server. The startup-config can be copied using the same technique.

To reverse the process, simply reverse the order of the devices. Copy from the TFTP server to the filename running-config.

c2511r1#copy tftp running-config

Host or network configuration file [host]?

Address of remote host [255.255.255.255]? 192.168.169.140

Name of configuration file [c2511r1-confg]?

Configure using c2511r1-confg from 192.168.169.140?

➥[confirm]y

Loading c2511r1-confg from 192.168.169.140 (via Ethernet0):

➥!

[OK - 1260/32723 bytes] c2511r1#

C

ISCO

IOS C

OMMANDS FOR

R

OUTER

S

TARTUP

As with any other computer, a Cisco router must boot an operating system to function fully. For a Cisco router, this operating system is

Internetworking Operating System (IOS). The IOS exists as a file that can be stored in Flash RAM, in ROM, or on a TFTP server.

The configuration file of the router can dictate from where the router loads the IOS. The commands to do this are referred to as

boot commands.

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74 Pa r t I E X A M P R E PA R AT I O N

Normally, the router will boot the first image of IOS found in flash memory. If there are multiple copies of IOS stored in flash, you can specify which copy to load using the following command: boot system flash ios.new

This tells the router to load the image ios.new from flash memory.

Multiple boot statements can be placed in the config file. They are executed in the order they appear in the config file.

To make your router config as robust as possible, it is a good idea to give your router other methods of booting should the flash memory become corrupt.

This can be achieved by adding the boot command for a TFTP server and finally telling the router to boot from ROM if the first two options fail.

The following command will instruct the router to boot from a

TFTP server: c2511r1(config)#boot system tftp ios.net 192.168.1.9

The router will boot the system file named ios.net from the TFTP server at IP address 192.168.1.9.

In the event of both the flash being corrupt and the TFTP server being unreachable, the router can finally boot from the system

ROM. In most of Cisco’s routers, the copy of IOS stored in ROM is not complete. It contains just enough features to start the router. It is, however, better than the router not booting at all. To instruct the router to boot from ROM, use the following command: c3620r1(config)#boot system rom

Note that not all router platforms support the command boot system rom

.

C

ONFIGURING A

R

OUTER WITH THE

S

ETUP

C

OMMAND

Cisco provides a script-like setup method for routers. This script will set up the basics of a configuration. The setup script can be entered two ways. A new router without a configuration stored in NVRAM will boot and prompt you to ask if you would like to enter the initial setup as follows:

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System Bootstrap, Version 5.2(5), RELEASE SOFTWARE

Copyright (c) 1986-1994 by cisco Systems

2500 processor with 4096 Kbytes of main memory

F3: 5292020+130664+266752 at 0x3000060

Restricted Rights Legend

Use, duplication, or disclosure by the Government is subject to restrictions as set forth in subparagraph

(c) of the Commercial Computer Software - Restricted

Rights clause at FAR sec. 52.227-19 and subparagraph

(c) (1) (ii) of the Rights in Technical Data and Computer

Software clause at DFARS sec. 252.227-7013.

cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134-1706

Cisco Internetwork Operating System Software

IOS (tm) 3000 Software (IGS-J-L), Version 10.3(3), RELEASE

➥SOFTWARE (fc4)

Copyright (c) 1986-1995 by cisco Systems, Inc.

Compiled Sat 20-May-95 01:17 by nitin

Image text-base: 0x0302D220, data-base: 0x00001000 cisco 2500 (68030) processor (revision D) with 4092K/2048K

➥bytes of memory.

Processor board serial number 02336429

SuperLAT software copyright 1990 by Meridian Technology

➥Corp).

TN3270 Emulation software (copyright 1994 by TGV Inc).

X.25 software, Version 2.0, NET2, BFE and GOSIP compliant.

Bridging software.

Authorized for Enterprise software set. (0x0)

1 Ethernet/IEEE 802.3 interface.

1 Token Ring/IEEE 802.5 interface.

2 Serial network interfaces.

32K bytes of non-volatile configuration memory.

8192K bytes of processor board System flash (read-only)

Notice: NVRAM invalid, possibly due to write erase.

—- System Configuration Dialog —-

At any point you may enter a question mark ‘?’ for help.

Refer to the ‘Getting Started’ Guide for additional help.

Use ctrl-c to abort configuration dialog at any prompt.

Default settings are in square brackets ‘[]’.

Would you like to enter the initial configuration dialog?

➥[yes]:

In the preceding example, the router states that the NVRAM is invalid, possibly due to write erase.

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Clearing the Configuration from a

Router

Write erase is an IOS command that can be used to wipe out the NVRAM. This is useful if you are taking a router out of service. You could erase the configuration. The router would then be ready to configure for a future implementation.

The router then prompts

Would you like to enter the initial configuration dialog? [yes]:

It will then guide you through building a configuration for the router.

The following example demonstrates building a configuration with the setup dialog:

Router#setup

. The answer between the brackets (

[]

) is the default answer. Simply pressing Enter will enter the setup dialog.

—- System Configuration Dialog —-

At any point you may enter a question mark ‘?’ for help.

Refer to the ‘Getting Started’ Guide for additional help.

Use ctrl-c to abort configuration dialog at any prompt.

Default settings are in square brackets ‘[]’.

Continue with configuration dialog? [yes]:

First, would you like to see the current interface summary? [yes]:

Interface IP-Address OK? Method Status Protocol

Ethernet0 unassigned YES not set administratively down down

Serial0 unassigned YES not set administratively down down

Serial1 unassigned YES not set administratively down down

TokenRing0 unassigned YES not set administratively down down

Prior to actually configuring the router, the setup dialog will ask if you would like to see the current interface summary. If you answer yes it will show you a table of the router’s interfaces.

Configuring global parameters:

Enter host name [Router]: c2513r4

The enable secret is a one-way cryptographic secret used instead of the enable password when it exists.

Enter enable secret: cisco

The enable password is used when there is no enable secret and when using older software and some boot images.

Enter enable password: cisco1

Enter virtual terminal password: cisco

Configure SNMP Network Management? [no]:

Configure IP? [yes]:

Configure IGRP routing? [yes]: n

Configure RIP routing? [no]:

Configure Vines? [no]:

Configure IPX? [no]:

Configure AppleTalk? [no]:

Configure Apollo? [no]:

Configure DECnet? [no]:

Configure XNS? [no]:

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Configure CLNS? [no]:

Configure bridging? [no]:

Configure LAT? [no]:

Configuring interface parameters:

Configuring interface Ethernet0:

Is this interface in use? [no]: y

Configure IP on this interface? [no]: y

IP address for this interface: 192.168.28.9

Number of bits in subnet field [0]:

Class C network is 192.168.28.0, 0 subnet bits; mask is

➥255.255.255.0

Notice that Cisco’s setup dialog refers to subnet bits in a non-intuitive way. Most people think of the number of subnet bits as the total number of bits in the subnet mask. In a Class C address, this would be 24 bits (255.255.255.0). In contrast, Cisco’s setup dialog refers to subnet bits from the normal boundary for the class of address. In the above example, a Class C address was entered and zero subnet bits were specified. This resulted in an IP address of 192.168.28.9 with a subnet mask of 255.255.255.0, which is normal for a Class C address.

Configuring interface Serial0:

Is this interface in use? [no]:

Configuring interface Serial1:

Is this interface in use? [no]:

Configuring interface TokenRing0:

Is this interface in use? [no]:

After completing settings for all of the interfaces, the setup dialog displays the configuration created.

The following configuration command script was created: hostname c2513r4 enable secret 5 $1$DkrH$hY5W7HTq/epbwshCPwhPf.

enable password cisco1 line vty 0 4 password cisco no snmp-server

!

ip routing no vines routing no ipx routing no appletalk routing no apollo routing no decnet routing no xns routing no clns routing no bridge 1

!

continues

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78 Pa r t I E X A M P R E PA R AT I O N

continued

interface Ethernet0 no shutdown ip address 192.168.28.9 255.255.255.0

no mop enabled

!

interface Serial0 shutdown no ip address

!

interface Serial1 shutdown no ip address

!

interface TokenRing0 shutdown no ip address

!

end

Use this configuration? [yes/no]: y

#######

%LINK-3-UPDOWN: Interface Ethernet0, changed state to

➥up[OK]

Use the enabled mode ‘configure’ command to modify this

➥configuration.

c2513r4#

After displaying the configuration created, the router asks if you want to

Use this configuration? [yes/no]:

If you answer yes, the configuration is saved and the router enters the enable mode.

For some reason, the setup dialog creates both an enable secret and an enable password. Both of these are not needed. Because the enable password uses a weak encryption algorithm, it is generally best to use enable secret instead of enable password. After using the setup dialog to create the configuration, the enable password statement can be removed by typing no enable password in configuration mode.

B

ACK

U

P AND

U

PGRADE

IOS

The IOS image resides in the router as a file stored in flash memory.

This file can become corrupt due to a power problem or hardware failure. This file may also need to be upgraded to a newer version to support new features or correct problems due to software bugs. The

TFTP server is used for this process.

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Some of Cisco’s routers run IOS directly from flash memory and some copy the IOS from flash to RAM and execute it there.

Generally, lower-end routers like the 1600s and 2500s run from flash. Higher-end routers like the 3600s, 4700s, 7200s, and 7500s run from RAM. Copying a new version of IOS to a run from flash router presents some challenges because you cannot overwrite the version currently executing from flash. There are two options available. If the router is equipped with enough flash memory, you can partition the flash memory. This divides the flash memory into logical partitions much like can be done to a hard disk in a PC. After the flash is partitioned, the router can execute a copy of IOS in one partition while another partition can be erased and a new copy of

IOS can be copied. After this is complete, the router’s configuration is changed to boot from the partition with the new IOS, and the router is rebooted.

If the router does not have enough flash memory to partition and hold two copies of IOS, you must upgrade it another way. Run from flash routers have a flash load helper that can help. The flash load helper will accept the copy command. It will then verify that the TFTP server can be reached and the file requested is available.

After this has been confirmed, the flash load helper will reboot the router using the ROM-based IOS and copy the new IOS into flash.

During this time, the router will not perform routing functions.

The following example demonstrates copying an IOS file from the

TFTP server to the router’s flash using the flash load helper: c2511r1#copy tftp flash

****NOTICE****

Flash load helper v1.0

This process will accept the copy options and then

➥terminate the current system image to use the ROM based image for the

➥copy.

Routing functionality will not be available during that

➥time.

If you are logged in via telnet, this connection will

➥terminate.

Users with console access can see the results of the copy

➥operation.

---- ******** ----

Proceed? [confirm]y

System flash directory:

FileLength Name/status

1 580636425ipp12.bin

[5806428 bytes used, 2582180 available, 8388608 total]

continues

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80 Pa r t I E X A M P R E PA R AT I O N

continued

Address or name of remote host [255.255.255.255]?

➥199.250.137.37

Source file name? c2500-js-l_112-16.bin

Destination file name [c2500-js-l_112-16.bin]?

Accessing file ‘c2500-js-l_112-16.bin’ on 199.250.137.37...

Loading c2500-js-l_112-16.bin from 199.250.137.37

➥(via Ethernet0): ! [OK]

Erase flash device before writing? [confirm]y

Flash contains files. Are you sure you want to erase?

➥[confirm]y

Copy ‘c2500-js-l_112-16.bin’ from server as ‘c2500-js-l_112-16.bin’ into Flash WITH erase? [yes/no]y

To make a copy of the IOS stored in flash on the TFTP server, use the copy command with the source and destination reversed.

c3620r1#copy flash tftp

System flash directory:

File Length Name/status

1 4201072 c3620-js-mz_112-16_P.bin

[4201136 bytes used, 4187472 available, 8388608 total]

Address or name of remote host [255.255.255.255]?

➥199.250.137.37

Source file name? c3620-js-mz_112-16_P.bin

Destination file name [c3620-js-mz_112-16_P.bin]?

Verifying checksum for ‘c3620-js-mz_112-16_P.bin’

➥(file # 1)... OK

Copy ‘c3620-js-mz_112-16_P.bin’ from Flash to server as ‘c3620-js-mz_112-16_P.bin’? [yes/no]y

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!

Upload to server done

Flash device copy took 00:01:23 [hh:mm:ss] c3620r1#

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E S S E N C E O F T H E C A S E

.

Erase the configuration from a router.

.

Upgrade the IOS.

.

Create a simple configuration.

This case study reviews some of the techniques you learned in this chapter. It deals with taking a previously used router and implementing it in a new environment.

S C E N A R I O

You have been asked to take a 2513 router that was once in service in a branch office and configure it for use as an Internet gateway router. The router was taken out of the office when the office was closed. The version of IOS is very old and will need to be upgraded. You will need to create a simple configuration for an Internet gateway.

In this case study, you are taking a router that has previously been in service and configuring it for a new use. The router has an older version of

IOS that you need to upgrade and the configuration in the router is for the old installation. That must be cleared before configuring it for the new use. After you have completed the upgrade of IOS and clearing of the config, you must configure the router for the new use as an Internet gateway.

A N A LY S I S

The Cisco 2513 you have been given for this task was previously installed in a branch office for its connection to the corporate Frame Relay WAN.

Your Internet provider has given you the following instructions for configuring the router:

The serial port IP address should be

155.175.34.35 255.255.255.0.

The subnet assigned to your office is

155.175.35.0.

continues

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First, you should power up the router and erase the configuration stored in the NVRAM. Connect your laptop to the console port with a Cisco cable and launch Hyperterminal. Then, configure

Hyperterminal for 9600 baud, No Parity.

In powering up the router you see the following:

System Bootstrap, Version 11.0(10c)XB1,

➥PLATFORM SPECIFIC RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems

2500 processor with 14336 Kbytes of main

➥memory

F3: 3522860+70900+216864 at 0x3000060

Restricted Rights Legend

Use, duplication, or disclosure by the

➥Government is subject to restrictions as set forth in

➥subparagraph

(c) of the Commercial Computer Software -

➥Restricted

Rights clause at FAR sec. 52.227-19 and

➥subparagraph

(c) (1) (ii) of the Rights in Technical Data

➥and Computer

Software clause at DFARS sec. 252.227-7013.

cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134-1706

Cisco Internetwork Operating System Software

IOS (tm) 3000 Software (IGS-IN-L), Version

➥10.3(19a), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems, Inc.

Compiled Thu 04-Sep-97 20:00 by phester

Image text-base: 0x0301EB58, data-base:

➥0x00001000 cisco 2500 (68030) processor (revision M)

➥with 14336K/2048K bytes of memory.

Processor board serial number 10187811

Bridging software.

X.25 software, Version 2.0, NET2, BFE and

➥GOSIP compliant.

1 Ethernet/IEEE 802.3 interface.

1 Token Ring/IEEE 802.5 interface.

2 Serial network interfaces.

32K bytes of non-volatile configuration memory.

16384K bytes of processor board System flash

➥(read-only)

Press RETURN to get started!

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Ethernet0, changed state to up

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Serial0, changed state to down

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Serial1, changed state to down

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol on

➥Interface TokenRing0, changed state to down

0:00:07: %LINK-3-UPDOWN: Interface Ethernet0,

➥changed state to up

0:00:07: %LINK-3-UPDOWN: Interface Serial0,

➥changed state to up

0:00:07: %LINK-3-UPDOWN: Interface Serial1,

➥changed state to up

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Loopback0, changed state to up

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Loopback1, changed state to up

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Serial0, changed state to up

0:00:07: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Serial1, changed state to up

0:00:08: %LINEPROTO-5-UPDOWN: Line protocol

➥on Interface Serial0, changed state to down

0:00:09: %SYS-5-CONFIG_I: Configured from

➥memory by console

0:00:09: %SYS-5-RESTART: System restarted —

Cisco Internetwork Operating System Software

IOS (tm) 3000 Software (IGS-IN-L), Version

➥10.3(19a), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems, Inc.

Compiled Thu 04-Sep-97 20:00 by phester

0:00:10: %LINK-5-CHANGED: Interface Serial0,

➥changed state to administratively down

0:00:10: %LINK-5-CHANGED: Interface

➥TokenRing0, changed state to administrativel y down

Log in to the router and perform a show run

.

c2513r1#sh run

Building configuration...

Current configuration:

!

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E R V I C E version 10.3

service timestamps debug uptime service timestamps log uptime service password-encryption service udp-small-servers service tcp-small-servers

!

hostname c2513r1

!

enable secret 5

$1$wEaA$FeD7UevtNmlnInde.YQUC1

!

!

interface Loopback0 ip address 10.10.43.1 255.255.255.0

!

interface Loopback1

!

ip address 10.10.70.109 255.255.255.0

interface Ethernet0 ip address 199.250.137.59 255.255.255.224

!

no cdp enable interface Serial0 ip address 192.168.220.2 255.255.255.0

ip access-group 1 out shutdown

!

interface Serial1 ip address 192.168.42.1 255.255.255.0

ip access-group 1 out no cdp enable

!

interface TokenRing0 no ip address shutdown

!

no cdp enable router eigrp 100 network 192.168.220.0

!

router rip network 10.0.0.0

network 192.168.42.0

!

access-list 1 permit 10.10.42.0

access-list 1 permit 192.168.200.0 0.0.0.255

!

line con 0 exec-timeout 0 0 password 7 0822455D0A16 login line aux 0 password 7 0822455D0A16 login transport input all line vty 0 4 password 7 0822455D0A16

!

login end c2513r1#

You see that the router already has a configuration stored in the NVRAM. To clear this configuration and prepare the router for a new configuration you perform a write erase.

c2513r1#write erase

[OK] c2513r1#sh run

Building configuration...

Current configuration:

!

version 10.3

service timestamps debug uptime service timestamps log uptime service password-encryption service udp-small-servers service tcp-small-servers

!

!

!

hostname c2513r1

!

enable secret 5

$1$wEaA$FeD7UevtNmlnInde.YQUC1

interface Loopback0 ip address 10.10.43.1 255.255.255.0

!

interface Loopback1

!

ip address 10.10.70.109 255.255.255.0

interface Ethernet0 ip address 199.250.137.59 255.255.255.224

no cdp enable

!

interface Serial0 ip address 192.168.220.2 255.255.255.0

continues

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continued

ip access-group 1 out shutdown

!

interface Serial1 ip address 192.168.42.1 255.255.255.0

ip access-group 1 out

!

no cdp enable interface TokenRing0 no ip address shutdown no cdp enable

!

router eigrp 100

!

network 192.168.220.0

router rip network 10.0.0.0

network 192.168.42.0

!

access-list 1 permit 10.10.42.0

access-list 1 permit 192.168.200.0 0.0.0.255

!

line con 0 exec-timeout 0 0 password 7 0822455D0A16 login line aux 0 password 7 0822455D0A16 login transport input all line vty 0 4 password 7 0822455D0A16

!

login end

Reboot the router. It states that the NVRAM is empty, possibly due to a write erase

—do you want to enter the setup dialog? You still need to upgrade the IOS, so answer “no” to the query about using the setup dialog. The router then proceeds to boot and presents you with the user exec prompt. Typing enable places you in the privileged exec mode where you can configure the Ethernet port with an IP address so you can access your TFTP server on the network to download the new version of IOS.

System Bootstrap, Version 11.0(10c)XB1,

➥PLATFORM SPECIFIC RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems

2500 processor with 14336 Kbytes of main

➥memory

Notice: NVRAM invalid, possibly due to write erase.

F3: 3522860+70900+216864 at 0x3000060

Restricted Rights Legend

Use, duplication, or disclosure by the

➥Government is subject to restrictions as set forth in

➥subparagraph

(c) of the Commercial Computer Software -

➥Restricted

Rights clause at FAR sec. 52.227-19 and

➥subparagraph

(c) (1) (ii) of the Rights in Technical Data

➥and Computer

Software clause at DFARS sec. 252.227-7013.

c2513r1#sh startup

%% Non-volatile configuration memory has not

➥been set up c2513r1#

As you can see in the preceding example, the running config has been unaffected by the write erase

. However, when you try to show startupconfig

, the router reports

%% Non-volatile configuration memory has not been set up

. cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134-1706

Cisco Internetwork Operating System Software

IOS (tm) 3000 Software (IGS-IN-L), Version

➥10.3(19a), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems,

➥Inc.

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Compiled Thu 04-Sep-97 20:00 by phester

Image text-base: 0x0301EB58, data-base:

➥0x00001000 cisco 2500 (68030) processor (revision M)

➥with 14336K/2048K bytes of memory.

Processor board serial number 10187811

Bridging software.

X.25 software, Version 2.0, NET2, BFE and

➥GOSIP compliant.

1 Ethernet/IEEE 802.3 interface.

1 Token Ring/IEEE 802.5 interface.

2 Serial network interfaces.

32K bytes of non-volatile configuration

➥memory.

16384K bytes of processor board System flash

➥(read-only)

Notice: NVRAM invalid, possibly due to ➥write erase.

—- System Configuration Dialog —-

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 ‘[]’.

➥Would you like to enter the initial

➥configuration dialog? [yes]: n

Press RETURN to get started!

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Ethernet0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Serial0, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Serial1, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface TokenRing0, changed state to down

%LINK-3-UPDOWN: Interface Ethernet0, changed

➥state to up

%LINK-3-UPDOWN: Interface Serial0, changed

➥state to down

%LINK-3-UPDOWN: Interface Serial1, changed

➥state to down

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Ethernet0, changed state to down

%LINK-5-CHANGED: Interface Ethernet0, changed

➥state to administratively down

%LINK-5-CHANGED: Interface Serial0, changed

➥state to administratively down

%LINK-5-CHANGED: Interface Serial1, changed

➥state to administratively down

%LINK-5-CHANGED: Interface TokenRing0,

➥changed state to administratively down

%SYS-5-RESTART: System restarted —

Cisco Internetwork Operating System Software

IOS (tm) 3000 Software (IGS-IN-L), Version

➥10.3(19a), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1997 by cisco Systems, Inc.

Compiled Thu 04-Sep-97 20:00 by phester

Router>en

Router#

After answering “no,” you are presented with the router’s prompt. You enter the privileged exec mode with the command enable

.

You now need to configure the Ethernet port with an IP address so you can download the new version of IOS to the router’s flash

Router>en

Router#config t

Enter configuration commands, one per line.

➥End with CNTL/Z.

Router(config)#int e0

Router(config-if)#ip address 199.250.137.59

➥255.255.255.224

Router(config-if)#no shut

Router(config-if)#exit

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Ethernet0, changed state to up

%LINK-3-UPDOWN: Interface Ethernet0, changed

➥state to up

Router(config)#exit

Router#

Now, confirm that you can reach your TFTP server with a ping.

Router#ping 54.250.137.37

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to

➥199.250.137.37, timeout is 2 seconds:

.!!!!

continues

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continued

Success rate is 80 percent (4/5), round-trip

➥min/avg/max = 1/2/4 ms

Router#

%SYS-5-RELOAD: Reload requested

%FLH: c2500-js-l_112-16.bin from

➥199.250.137.37 to flash ...

You can now upgrade the IOS stored in flash memory. Use the flash load helper to perform this task.

Router#copy tftp flash

**** NOTICE ****

Flash load helper v1.0

This process will accept the copy options and

➥then terminate the current system image to use the ROM based

➥image for the copy.

Routing functionality will not be available

➥during that time.

If you are logged in via telnet, this

➥connection will terminate.

Users with console access can see the results

➥of the copy operation.

—— ******** ——

➥Proceed? [confirm]y

System flash directory:

File Length Name/status

1 3593792 igs-in-l_103-19a.bin

[3593856 bytes used, 13183360 available,

➥16777216 total]

Address or name of remote host

➥[255.255.255.255]? 199.250.137.37

Source file name? c2500-js-l_112-16.bin

Destination file name [c2500-js-l_112-16.bin]?

Accessing file ‘c2500-js-l_112-16.bin’ on

➥www.kmahler.com...

Loading c2500-js-l_112-16.bin from

➥199.250.137.37 (via Ethernet0): ! [OK]

Erase flash device before writing? [confirm]y

Flash contains files. Are you sure you want to

➥erase? [confirm]y

System configuration has been modified. Save?

➥[yes/no]: y

Building configuration...

[OK]

Copy ‘c2500-js-l_112-16.bin’ from server as ‘c2500-js-l_112-16.bin’ into Flash WITH

➥erase? [yes/no]y

System flash directory:

File Length Name/status

1 3593792 igs-in-l_103-19a.bin

[3593856 bytes used, 13183360 available,

➥16777216 total]Domain server mapped ip address 199.250.137.37 to www.kmahler.com

Accessing file ‘c2500-js-l_112-16.bin’ on

➥www.kmahler.com...

Loading c2500-js-l_112-16.bin from

➥199.250.137.37 (via Ethernet0): ! [OK]

Erasing device...

➥eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

➥eeeeeeeeeeeeeeeeee ee ...erased

Loading c2500-js-l_112-16.bin from

➥199.250.137.37 (via Ethernet0):

➥!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

➥!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[OK - 8102652/16777216 bytes]

Verifying checksum... OK (0x8DCB)

Flash copy took 0:04:01 [hh:mm:ss]

%FLH: Re-booting system after download

%SYS-3-CPUHOG: Task ran for 2912 msec (4/0),

Process = Boot Load, PC = 1117942

-Traceback= 10E781A 10E7E24 10E5F80 1010080

➥10005DE

F3: 8004052+98568+315656 at 0x3000060

Restricted Rights Legend

Use, duplication, or disclosure by the

➥Government is subject to restrictions as set forth in

➥subparagraph

(c) of the Commercial Computer Software -

➥Restricted

Rights clause at FAR sec. 52.227-19 and

➥subparagraph

(c) (1) (ii) of the Rights in Technical Data

➥and Computer

Software clause at DFARS sec. 252.227-7013.

cisco Systems, Inc.

170 West Tasman Drive

San Jose, California 95134-1706

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-JS-L), Version

➥11.2(16), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1998 by cisco Systems, Inc.

Compiled Tue 06-Oct-98 11:54 by ashah

Image text-base: 0x030400AC, data-base:

➥0x00001000 cisco 2500 (68030) processor (revision M)

➥with 14336K/2048K bytes of memory.

Processor board ID 10187811, with hardware

➥revision 00000000

Bridging software.

SuperLAT software copyright 1990 by Meridian

Technology Corp).

X.25 software, Version 2.0, NET2, BFE and

➥GOSIP compliant.

TN3270 Emulation software.

1 Ethernet/IEEE 802.3 interface(s)

1 Token Ring/IEEE 802.5 interface(s)

2 Serial network interface(s)

32K bytes of non-volatile configuration

➥memory.

16384K bytes of processor board System flash

➥(read-only)

Press RETURN to get started!

%LINK-3-UPDOWN: Interface Ethernet0, changed

➥state to up

%LINK-3-UPDOWN: Interface Serial0, changed

➥state to down

%LINK-3-UPDOWN: Interface Serial1, changed

➥state to down

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Ethernet0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Serial0, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface Serial1, changed state to down

%LINEPROTO-5-UPDOWN: Line protocol on

➥Interface TokenRing0, changed state to down

%SYS-5-CONFIG_I: Configured from memory by

➥console

%SYS-5-RESTART: System restarted —

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-JS-L), Version

➥11.2(16), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1998 by cisco Systems, Inc.

➥Compiled Tue 06-Oct-98 11:54 by ashah

%LINK-5-CHANGED: Interface Serial0, changed

➥state to administratively down

continues

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continued

%LINK-5-CHANGED: Interface Serial1, changed

➥state to administratively down

%LINK-5-CHANGED: Interface TokenRing0,

➥changed state to administratively down

Router>sh ver

Cisco Internetwork Operating System Software

IOS (tm) 2500 Software (C2500-JS-L), Version

➥11.2(16), RELEASE SOFTWARE (fc1)

Copyright (c) 1986-1998 by cisco Systems, Inc.

➥Compiled Tue 06-Oct-98 11:54 by ashah

Image text-base: 0x030400AC, data-base:

➥0x00001000

32K bytes of non-volatile configuration

➥memory.

16384K bytes of processor board System flash

➥(read-only)

Configuration register is 0x2102

ROM: System Bootstrap, Version 11.0(10c)XB1,

➥PLATFORM SPECIFIC RELEASE SOFTWARE

(fc1)

BOOTFLASH: 3000 Bootstrap Software

➥(IGS-BOOT-R), Version 11.0(10c)XB1, PLATFORM

➥SPECIFIC RELEASE SOFTWARE (fc1)

Router uptime is 0 minutes

System restarted by reload

System image file is “flash:c2500-js-l_112-

➥16.bin”, booted via flash cisco 2500 (68030) processor (revision M)

➥with 14336K/2048K bytes of memory.

Processor board ID 10187811, with hardware

➥revision 00000000

Bridging software.

SuperLAT software copyright 1990 by Meridian

➥Technology Corp).

X.25 software, Version 2.0, NET2, BFE and

➥GOSIP compliant.

TN3270 Emulation software.

1 Ethernet/IEEE 802.3 interface(s)

1 Token Ring/IEEE 802.5 interface(s)

2 Serial network interface(s)

Router>

Now proceed to enter the configuration for the router.

Router#config terminal

Router(config)#interface serial0

Router(config-if)#ip address 155.175.34.35

➥255.255.255.0

Router(config-if)#no shutdown

Router(config-if)#exit

Router(config)#interface ethernet0

Router(config-if)#ip address 155.175.35.1

➥255.255.255.0

Router(config-if)#no shutdown

Router(config-if)#exit

Router(config)#ip route 0.0.0.0 0.0.0.0

➥155.175.34.1

Router(config)#exit

Router#copy running-config startup-config

You have now successfully cleared the configuration of a router’s NVRAM, upgraded the IOS, and configured it to be the Internet gateway for your company.

After installing the router, test the Internet connection by visiting Cisco’s Web site to register for your CCNA test.

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H A P T E R

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U M M A R Y

This chapter covered a substantial amount of information. You were introduced to logging in to a router and interacting with the CLI.

You saw how the context-sensitive help aids when configuring a router. The elements of a router and the methods used to monitor them were introduced. You saw how the password encryption service can make the configuration files less vulnerable to wandering eyes.

You also learned how to use a TFTP server to perform several tasks such as backing up configuration files and IOS images.

In preparation for your CCNA exam, it is important to practice the tasks of configuring a router often. The repetition of performing these tasks will firmly implant them in your mind.

KEY TERMS

• Internetworking Operating System

• Context-sensitive help

• TFTP server

• RAM

• ROM

• Flash memory

• CDP

• IOS image file

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Exercises

2.1

Viewing Neighboring Cisco Devices

Use CDP to view neighboring Cisco devices.

Estimated Time: 10 minutes

1. Log in to your router.

2. At the

Router> prompt, type show CDP neighbor

.

3. View the neighboring device information and make note of the versions of IOS that each neighboring device is running.

4. View the neighboring device information and make note of what capabilities each device provides. These can include routing, switching, bridging, and more.

5. Choose one of the neighbors and use the command show CDP neighbor detail Router1 to view more information about the device.

2.3

Using show interfaces to Discover

Information About Interfaces

Use the show command to see information about the interfaces in a router.

Estimated Time: 15 minutes

1. Log in to your router and enter the privileged exec mode with the enable command.

2. At the

Router# prompt, type show interfaces

.

3. View the interface information and make note of which interfaces are up and which are down. Of the interfaces that are down, note which ones are administratively down.

4. View the interface information and make note of the interface’s details such as bandwidth,

Maximum Transmission Unit (MTU) size, and the current load on the interface. The load on the interface is shown as a fraction of 255. (For example, 128/255 is 50% utilization, and 25/255 would be 10% utilization.)

2.2

Using show version to Discover

Information About a Router

Use the show command to see information about the router’s components.

Estimated Time: 10 minutes

1. Log in to your router and enter the privileged exec mode with the enable command.

2. At the

Router# prompt, type show version

.

3. View the version information and make note of the version of IOS the router is running.

4. View the version information and make note of how long the router has been running.

5. View the version information and make note of what interfaces are installed in the router.

Review Questions

1. Where are passwords stored, and what services does IOS provide for encrypting them?

2. List some of the prompts the CLI presents and what they indicate.

3. How is the running config saved to the startup config? Explain the function of each.

4. List the types of memory in a router and describe how each type is used by the router.

5. What does the command history feature of IOS do?

6. How can you find out how long a router has been up and running?

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7. What is CDP and what does it do?

8. What are boot commands?

9. How do you view statistics on a router interface?

10. What is a TFTP server, and how can it be useful when configuring and maintaining Cisco routers?

Exam Questions

1. What does the router prompt

Router> indicate?

A. You are logged in to the router at the privileged exec level.

B. You are logged in to the router at the user exec level.

C. The router is prompting you to log in.

D. You have entered the global configuration mode of the router.

2. What is the proper way to copy the IOS image from flash to a TFTP server?

A.

Router(config)#copy tftp flash

B.

Router#copy tftp flash

C.

Router#(config)#copy flash tftp

D.

Router#copy flash tftp

3. What is the proper method for creating a banner message for a router?

A.

Router#banner motd “This router replaced

1/3/99”

B.

Router>motd “This router replaced 1/3/99”

C.

Router(config)#motd “This router replaced

1/3/99”

D.

Router(config)#banner motd “This router replaced 1/3/99”

4. How would you view the contents of the configuration register?

A.

Router#show config-register

B.

Router>show config-register

C.

Router#show version

D.

Router#show running-config

5. How do you enter the privileged exec mode?

A.

Router>enable

B. Enter the enable password at the password prompt.

C.

Router>privileged-mode

D.

Router>config

6. How is the router’s name set?

A.

Router(config)#hostname ParrotHead

B.

Router(config)>hostname ParrotHead

C.

Router#hostname ParrotHead

D.

Router#set hostname ParrotHead

7. Which command will give you statistics on network interfaces?

A.

Router#show interface statistics

B.

Router#show statistics

C.

Router#show interfaces

D.

Router#list interfaces

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8. What does changing the CDP timer to 90 do?

A. Changes the Collision Detection Protocol backoff timer to 90 seconds

B. Causes the router to wait 90 clock ticks between routing updates

C. Causes the router to send CDP packets every

90 seconds

D. Changes the Congestion Detection Protocol update timer to 90 seconds

9. How would you find out whether an interface is up or down?

A.

Router#show interfaces

B.

Router#show running-config

C.

Router#show startup-config

D.

Router#show cdp neighbors

Answers to Review Questions

1. Passwords are stored in the router’s configuration file. IOS has a password encryption service that will encrypt the passwords, making them fairly secure from wondering eyes. For more information, see “Normal Password Authentication.”

2.

Router> indicates we are logged in, in user exec mode.

Router# indicates we are logged in, in privileged exec mode.

Router(config)# indicates we are in global config mode.

Router(conifg-if)# indicates we are in interface config mode.

For more information, see “Logging in to a

Router in Both User and Privileged Modes” and

“Configuring the Router.”

3.

copy running-config startup-config

The running-config is the configuration the router is currently executing. When changes are made to the running-config

, they take effect immediately.

The startup-config is stored in NVRAM and is the config that the router copies to the runningconfig when it boots up. For more information, see “Saving Config Changes.”

4. RAM is used by the router to store packets, data, and routing tables. Flash memory is used much like a disk drive in a PC to store the IOS. Some routers execute the IOS directly from flash and some copy the IOS to RAM before executing.

NVRAM is Non-Volatile RAM and is used to store the startup configuration file. ROM is readonly memory and is used to store self test programs and a minimal version of IOS. For more information, see “Understanding Router

Elements.”

5. The command history feature of IOS keeps a number of commands you have used in a buffer.

These commands can be recalled by using the up arrow. After being recalled to the command line, the command can be edited and executed, or executed as is. For more information, see “Using the

Command History and Editing Features.”

6. The

Router#show version command lists how long it has been since the router has been rebooted. It also lists the reason for the reboot.

For more information, see “Examining the

Components of a Router.”

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7. CDP is Cisco Discovery Protocol. It runs on

Cisco equipment at the Data Link layer. It is used to discover neighbor devices and exchange capability information. For more information, see “Cisco Discovery Protocol (CDP).”

8. Boot commands are IOS configuration commands that instruct the router how to load the

IOS. Some boot commands can instruct the router to load IOS from Flash, TFTP server, or

ROM. A fault-tolerant configuration includes several boot commands, so the router can fall back to another source for the IOS image should the primary source become corrupt or unreachable. For more information, see “Cisco IOS

Commands for Router Startup.”

9. Statistics for an interface can be viewed using the show interfaces command. This will list statistics including errors, utilization, and interface parameters. For more information, see

“Examining the Components of a Router.”

10. A TFTP server is a computer running Trivial File

Transfer Protocol server software. A free copy of

TFTP server software for Windows NT can be downloaded from Cisco’s Web site. The TFTP server is useful for copying files to and from a router. These files can be IOS images and configuration files. This is a good way to back up the configurations of a router and update IOS images. For more information, see “Backup and

Upgrade IOS.”

Answers to Exam Questions

1. B. The > prompt indicates you are currently at the user exec mode. For more information, see

“Logging in to a Router in Both User and

Privileged Modes.”

2. D. The copy command uses syntax much like

DOS’s copy command.

Copy <source> < destination

>

. The

copy command is executed from the normal privileged exec prompt, not from the config mode. For more information, see “Managing

Configuration File from the Privileged Exec

Mode.”

3. D. The MOTD is set from the config mode. This is a good example of why hands-on experience is important when studying for the CCNA exam.

The proper syntax is as follows:

Router(config)#banner motd “This router replaced 1/3/99”

For more information, see “Message of the Day

(MOTD).”

4. C. The show version command displays the contents of the configuration register. For more information, see “Examining the Components of a Router.”

5. A. The enable command enters privileged exec mode. For more information, see “Logging in to a Router in Both User and Privileged Modes.”

6. A. The hostname is part of the configuration file.

You must be in global configuration mode to set the hostname. For more information, see

“Configuring the Router.”

7. C. show interfaces gives you much information about the network interfaces. Become familiar with the information provided by this command.

You will use this command a great deal when diagnosing problems. For more information, see

“Examining the Components of a Router.”

8. C. The CDP timer sets how often the router will send out Cisco Discovery Protocol packets. For more information, see “Cisco Discovery Protocol.”

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9. A. The show interfaces command includes information about the state of an interface, including whether it is up or down. For more information, see “Examining the Components of a Router.”

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration. Cisco Press, 1998.

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B J E C T I V E S

This chapter covers the following objectives from the

Network Protocols portion of the CCNA exam. (Chapter

4, “Understanding and Configuring IPX/SPX and Other

Network Protocols,” will cover the remainder of the objectives for the Network Protocols section.)

Describe the two parts of network addressing, then identify the parts in specific protocol address examples.

.

To successfully maintain internetworking systems, you must understand the two parts of network addressing. This chapter addresses the IP addressing;

Chapter 4 addresses this in relation to IPX.

.

Create the different classes of IP addresses and subnetting.

Understanding IP addresses and subnetting is an absolute must to design, install, and maintain Cisco routers.

.

Configure IP addresses.

To maintain internetworking systems, you must be able to configure IP addresses.

.

Verify IP addresses.

To maintain internetworking systems, you must be able to verify IP addresses.

Identify the functions of the TCP/IP Transportlayer protocols.

.

A strong understanding of the TCP/IP protocol suite is necessary for internetworking engineers.

C H A P T E R

3

Understanding and

Configuring TCP/IP

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O

B J E C T I V E S

( c o n t i n u e d

)

Identify the functions of the TCP/IP Networklayer protocols.

.

A strong understanding of the TCP/IP protocol suite is necessary for internetworking engineers.

Identify the functions performed by ICMP.

.

ICMP can provide some very useful diagnostics.

Understanding these functions is imperative for an internetworking engineer, in relation to troubleshooting.

O

U T L I N E

Introduction 98

Networking Address Overview

Network Addresses

MAC Address

Binary Math

How Numbers Are Represented in

Binary

Decimal Numbers

Binary Numbers

Converting Numbers From Binary to

Decimal

Converting Numbers from Decimal to

Binary

98

98

99

100

100

101

101

102

102

IP Addresses

Network Mask

Understanding IP Address Classes

Understanding Networks and

Broadcasts

Special IP Address Ranges

Subnetting IP Address Ranges 111

Subnetting a Class A address

Subnetting a Class B address

Subnetting a Class C address

112

113

114

Good Design Principles When Assigning

IP Addresses 118

Variable-Length Subnet Masking (VLSM) 118

Classless InterDomain Routing (CIDR)

Configuring IP Addresses

119

120

103

104

108

109

110

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TCP/IP Overview

TCP Segment Format

TCP/IP Services

Host Name-to-IP Address Mapping

Domain Name Server (DNS)

Static Name Mappings

Host Name-to-IP Address Mapping

Review

Internet Control Message Protocol

(ICMP)

Using the ping

Command

Using the traceroute

Command

Remote Access Using Telnet

Chapter Summary

S

T U D Y

S

T R AT E G I E S

127

127

131

132

133

133

135

136

121

123

125

.

You should spend a good deal of time studying for the TCP/IP portion of the CCNA exam. TCP/IP is the foundation on which many of the other concepts and objectives are based. TCP/IP is quickly becoming the standard of many companies; it scales well. This is proven by the success of the largest WAN to date, the Internet.

Be sure you can subnet IP addresses. Being able to properly subnet IP address ranges and verify that ranges are valid is extremely important in the day-to-day tasks as an internetworking engineer. You must learn to properly subnet

IP addresses and know what IP addresses are valid and invalid.

Understand how each of these network protocols is configured on the router and how to perform simple diagnostics.

Spend time working with the protocols on real networks. Learn to solve real problems with

TCP/IP networks.

140

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I

NTRODUCTION

This is what it’s all about. Internetworking engineering is about making networks communicate properly. Network protocols create the platform for making it all work together.

Network protocols are born, evolve, and die. Understanding a networking protocol today does not ensure your understanding of it tomorrow. The key to staying current and keeping sharp is reading.

Read everything you can get your hands on about networking. Visit

Cisco’s Web site regularly. Read RFCs regularly. Even reading computer magazines and newspapers can offer a great deal of insight into what is happening in the industry. The computer industry is unlike any other. In six months, more advances can be made in the computer industry than other industries make in years.

As an internetworking engineer, you are responsible for creating ways to deliver network traffic. This network traffic is delivered by networking protocols. Understanding these protocols is absolutely necessary for success. It’s also necessary to pass the CCNA test.

N

ETWORKING

A

DDRESS

O

VERVIEW

Network addresses are the addresses used at Layer 3 of the OSI reference model. The network address is used by routers to determine where to send the data. Routable network addresses differ from the

MAC addresses in that routable network addresses contain a hierarchical portion of the address, whereas MAC addresses are flat in structure.

Network Addresses

There are two network address types you need to understand for the

CCNA exam: IP addresses and IPX addresses (Novell). Each address type includes two parts a network number and a host address. The network number portion of the address is used by routers to determine how to forward traffic. The router compares the network number to its routing table and determines how to forward the traffic.

After the traffic reaches the destination network, the second part of the address, the host address, is used to deliver the data to the end station.

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You must be able to recognize IPX and IP addresses, and be able to determine which part of the address is the network and which part is the host address.

For IPX addresses it is relatively simple to determine the network and host portion of the address. The following is an example IPX address:

A31B3AF5.0000.0000.0001

In this example “A31B3AF5” is the network address portion. The host portion is “0000.0000.0001”. IPX addresses are represented in hexadecimal notation. The network address is up to 8 hexadecimal digits long and the host address is always three groups of 4 hexadecimal digits.

IP addresses are recognizable by their dotted decimal notation. IP addresses are represented as four decimal numbers no larger than

255 separated by periods. The following is an example IP address:

199.250.138.99 netmask 255.255.255.0

With IP addresses, the separation of the network address and the host address is determined by the netmask. Because this topic is more complex, it will be explained in full later in this chapter.

MAC Address

The MAC address identifies a device on the local network. A MAC address is 48 bits long and is usually represented as three groups of four hexadecimal digits, such as in the following example:

00e0.a38d.0800

It is mapped in some way to the network address. For Novell’s IPX protocol, the MAC address is used to create the network address.

Because it is made part of the network address, there is no need to create a map between the two addresses. TCP/IP, however, requires a method of mapping an IP address to a MAC address. Address

Resolution Protocol (ARP) performs this mapping. MAC addresses are discussed in more detail in Chapter 1, “The OSI Reference

Model.”

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B

INARY

M

ATH

To work with IP addresses, you must be able to convert binary numbers to decimal and back again. Most of this math is performed with a single byte of data and thus is rather simple. You learned the decimal system in school. The decimal system represents numbers as a power of 10. Each numbering system works the same way, only with different values for each position. The binary numbering system uses powers of 2 for each digit position.

How Numbers Are Represented in

Binary

Computers use the byte as a unit of storage. A byte holds 8 bits. The largest number that can be stored in a byte is 11111111 in binary,

FF in hexadecimal, and 255 in decimal.

The following table shows how this is so.

128 64 32 16 8 4 2 1

Bit8 7 6 5 4 3 2 1

In the previous table, you see the value of each bit position. Notice that each bit position from the left to right is reduced by half.

Starting at 128, each position halves until it reaches 1. You can convert a binary number to decimal by assigning the decimal value to each bit position and then adding the numbers. The following example shows how a binary number is converted to decimal.

Take the binary number, in this instance: 1 0 1 1 0 1 1 0

Now, place that number in the table:

128 64 32 16 8 4 2 1

1 0 1 1 0 1 1 0

128 32 16 4 2

Every number that corresponds with a one is valid, shown in row three.

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Next, add the decimal numbers in the third row together:

128 + 32 + 16 + 4 + 2 = 182

182 is the correct answer.

This converts a byte from binary to decimal. Learn to do this well.

It is a necessary skill later for subnetting IP addresses.

Decimal Numbers

In the decimal system, the number 249 is familiar to you. You know it is as “two-hundred forty-nine.” What you probably don’t think about is why. Each position of the number represents a number of the units that position represents. In the number 249, the nine represents how many ones are in the number. The number four represents how many tens are in the number, and the number two represents how many hundreds are in the number. Figure 3.1 shows how this breaks down.

In the decimal numbering system, each digit position represents 10 raised to a power (see the table):

10

5

10

4

10

3

100000 10000 1000

10

2

100

10

1

10

10

0

1

Each position is ten, raised to the power of the position-1.

2 4 9 number of hundreds number of tens number of ones

F I G U R E 3 . 1

How decimal numbers represent the values of base 10 numbers.

Binary Numbers

The binary numbering system works exactly the same way, except that each position is two raised to the power of some number (see the following table). Because there are only two digits in the binary numbering system, it takes many more digits to represent the same number as in the decimal system.

2

7

128

2

6

64

2

5

32

2

4

16

2

3

8

2

2

4

2

1

2

2

0

1

Each position is two, raised to the power of the position-1.

In the binary numbering system, the number 11111001 is the same as the previous example. This is 249 (decimal). To determine why, you must first know the value of each position of a binary number.

How to remember the decimal values of each position in the binary system In the previous example, you saw the decimal values for each bit position in a byte of binary data, or an octet. Notice that from right to left each position is double the previous position. Because the table starts at the right with 1, it is easy to reproduce this table when taking the CCNA exam. Simply write the number 1 and then double it to find the decimal value for the next position to the left. Continue doubling the number and moving left until you reach eight values representing the eight binary positions of an octet.

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Converting Numbers From Binary to

Decimal

To convert the previous binary number, 11111001, to decimal, place the number below the chart as shown in the following example.

128 64 32 16 8 4 2 1

1 1 1 1 1 0 0 1

128 64 32 16 8 1

Again, every number that corresponds with a one is valid, as shown in the third row.

Add up the decimal numbers for each position with a binary one.

128 + 64 + 32 + 16 + 8 + 1 = 249

You have just converted a binary octet to decimal.

Converting Numbers from Decimal to

Binary

While working with IP addresses, you will need to convert numbers from decimal to binary. To perform this function, you start by finding the largest binary position that will go into the number you are converting.

For the decimal number 249, the largest binary position that will go into it is 128, the highest bit position of an octet. Place a 1 in that bit position.

128 64 32 16 8 4 2 1

1

Subtract 128 from 249:

249 – 128 = 121

Now perform the same over again. What is the largest bit position that will go into 121? 64 is the largest bit position that will go into

121. Place a 1 in that bit position and continue from there:

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128

1

128

1

128

1

128

1

64 32 16 8 4 2 1

1

121 – 64 = 57

64 32 16 8 4 2 1

1 1

57 – 32 = 25

64 32 16 8 4 2 1

1 1 1

25 – 16 = 9

64 32 16 8 4 2 1

1 1 1 1

9 – 8 = 1

64 32 16 8 4 2 1

1 1 1 1 0 0 1

128

1

Decimal number 249 is binary number 1111001. You have now converted from decimal to binary. This is a skill you will need often when working with IP addresses. Remember the simple method of creating the decimal equivalent chart for converting from one system to the other. Start with the number 1 on the right, and double each number moving left until you reach 128 (8 bits).

IP A

DDRESSES

You will often hear people describe the IP address as a TCP/IP address. Technically this is incorrect. TCP/IP describes an entire suite of protocols. IP is the Layer 3 protocol that TCP uses. The IP address is used by the Layer 3 protocol, Internet Protocol (IP), to uniquely identify the station on the IP network.

An IP address defines two things: the network and the host address on the network. The network portion of the IP address is used for determining routing information. Routers look at the network portion of an IP address to determine the proper path for forwarding packets. After the packet arrives at the proper network, the host address is used to deliver the packet to the proper networking station.

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F I G U R E 3 . 2

.

An IP address in dotted decimal format.

The network mask, sometimes called the subnet mask, determines which part of the IP address represents the network and which part represents the host. The network mask is described in the next section.

IP addresses are currently 32 bits long. (There is a new version of IP being developed that uses 128 bits.) This represents 4 bytes of data called octets. Figure 3.2 shows what an IP address looks like.

8 bits

Network

8 bits

32 bits

8 bits

Host

8 bits

172

172 .

.

16

17 .

.

122

122 .

.

204

204

10.201

44.191

00001010 11001001 00101100 10111111

8 Bits

1 octet

32 Bits

F I G U R E 3 . 3

An IP address in both dotted decimal form and binary.

Computers perform their functions in binary, humans don’t. If an

IP address is to be used by human users, it is represented in decimal numbers, which are easier to understand. Each octet of the IP address is separately converted to decimal. The four octets are shown separated by periods or dots. This is called dotted decimal notation.

Figure 3.3 shows why it is easier to read IP addresses in dotted decimal form instead of binary.

Convert each octet to a decimal number, then separate the numbers by periods. This creates a representation of the IP address that is much easier to remember and to enter without mistakes. Life would not be easy if you had to enter IP addresses in binary format.

As nice as representing IP addresses in dotted decimal makes life, this format does create some headaches.

Network Mask

An IP address is really made up of two parts: the IP address and the network mask. The network mask defines the portion of the 32 bit

IP address that represents the network. The remaining portion represents the host address. The network portion of the address is used for routing packets. Routers build tables of the networks in an internetwork. When packets enter a router, the router uses the subnet mask to determine how much of the address represents the network.

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After this is determined, the router compares the network to the routing tables and determines whether it can forward the packet. If it is able, the router forwards the packet to the next hop along its trip to the destination. After the packet arrives on the destination network, the host portion of the IP address is used to determine which device ultimately takes delivery of the packet.

This may sound complex, but it’s really not.

Consider an IP address that looks like this:

Network = 192.168.2 Computer = 26

That is pretty straightforward. The network the packet belongs on is

192.168.2. The computer on that network the packet should be delivered to is 26.

The network mask is just a shorthand method of representing this same concept. The previous example would look like the following with a network mask:

IP Address = 192.168.2.26 Network Mask = 255.255.255.0

Computing devices take these two numbers in their binary form and perform a binary AND on them. A binary AND says “compare the two binary numbers; if both bits in a position are 1, the result is 1.

If either bit is 0, the result is 0.”

The rules of the binary AND are as follows:

1 + 1 = 1

1 + 0 = 0

0 + 0 = 0

Apply the binary AND to the IP address and the subnet mask:

IP Address

Network Mask

Binary IP Address

Binary Network Mask

Results of AND

Results in Decimal

192.168.2.26

255.255.255.0

11000000.10101000.00000010.00011010

11111111.11111111.11111111.00000000

11000000.10101000.00000010.00000000

192.168.2.0

This is the network.

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The IP address together with the network mask computers and routers can determine how to forward or route packets.

An example of how a TCP/IP computer determines how to forward a packet is outlined in the following numbered list.

1. A user enters a Web site name in his browser.

2. The TCP/IP stack does a Domain Name Server lookup of the name to determine the IP address of the site. (DNS is how

TCP/IP resolves a computer name to an IP address. An explanation of DNS is beyond the scope of this book. (See the

Suggested Readings and Resources section at the end of the chapter.)

3. After the computer has the IP address of the system with which it needs to communicate, it compares whether the IP address is on the same IP network. The computer’s IP address is 192.168.86.58 with a netmask of 255.255.255.0; the computer with which it wants to talk has an IP address of

10.137.88.19.

4. Based on the netmask, the network portion of the originating computer is 192.168.86. This is compared to the same region of the destination IP address. If they match, the packet is placed on the network. The destination system will see it.

However, if the network portions of the IP address do not match, the computer first checks its own routing tables.

(Unless the computer has multiple network interfaces to different networks, this usually doesn’t apply.) The packet is then sent to a router—either one specified in the computer’s routing tables, or (if there is not a specific entry for this network in the routing tables) the packet is sent to the default gateway.

5. The router receives the packet. It compares the network portion of the IP address to its routing tables. If it finds a match, it forwards the packet to the next hop along the way. If the router does not have a specific route to this network, it may have a default gateway of its own. This may be a router connected to the Internet or an internal router with more complete routing tables.

The network mask simply designates what part of the IP address represents the network and what part represents the host.

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A network mask must be a continuous run of binary ones from left to right. This allows for another method of IP address notation.

Because an IP address is really useful only with its netmask, you usually find IP addresses listed as such. An example Class C address would be 192.168.1.32 netmask 255.255.255.0. There is a shorter way to represent the same thing, however. The same IP address can be written as 192.168.1.32/24. This representation lists the number of netmask bits after the IP address. The following are some other IP addresses in both notations to help you grasp the different notations.

10.100.243.76 netmask 255.255.128.0

172.16.99.128 netmask 255.255.0.0

192.168.31.22 netmask 255.255.255.240

10.100.243.76/17

172.16.99.128/16

192.168.31.22/28

Following is a valid netmask:

11111111.11111111.11111111.00000000 255.255.255.0

Following is an invalid netmask:

11111111.11111110.11111111.00000000 255.254.255.0

Because subnet masks are broken into bytes or octets, there are a few numbers you should learn, remember, and learn again. These are the decimal equivalents of an octet being filled from the left to right with bits. When a subnet is split within an octet, these are the decimal equivalents of the binary representations:

10000000

11000000

11100000

11110000

11111000

11111100

11111110

11111111

128

192

224

240

248

252

254

255

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108 Pa r t I E X A M P R E PA R AT I O N

Understanding IP Address Classes

IP addresses are divided into classes. For each class, there is an assumed network mask. The most common IP address classes are shown in the following table:

Class

Range of

First Octet

Maximum

Networks

Maximum

Hosts First Bits

A

B

1–126

128–191

126

16,384

16,777,214

65,534

0

10

C 192–223 2,097,152 254 110

The class of an IP address is actually defined by the first few bits.

á If the first bit of an IP address is 0, it is a Class A address. By definition, a Class A IP address has 8 bits of network address and 24 bits of host address. The subnet mask for a Class A address is 255.0.0.0, or /8.

á

If the first two bits are 10, it is a Class B address. By definition, a Class B IP address has 16 bits of network address and

16 bits of host address. The subnet mask for a Class B address is 255.255.0.0, or /16.

á

If the first three bits are 110, it is a Class C address. By definition, a Class C IP address has 24 bits of network address and

8 bits of host address. The subnet mask for a Class C address is

255.255.255.0, or /24.

It is important that you learn the classes of IP addresses and their default subnet masks. Most people think of subnet bits in terms of the entire subnet. For example, a Class A address 10.201.44.1 that has a subnet mask of 255.255.255.0 is generally thought of as a

Class A address with 24 bits of subnet mask. Cisco has decided to confuse this subject: This is actually a Class A address with 16 bits of subnet mask. The first 8 bits are assumed because it is a Class A IP address. Thinking this way, an IP address of 10.201.44.1 with a subnet mask of 255.0.0.0 is a Class A IP address with 0 bits of subnet mask even though there are 8 bits in the subnet mask. Be sure to remember this distinction when dealing with Cisco equipment and information. The following table shows how IP addresses are broken down by class.

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Class A: N.H.H.H

Class B: N.N.H.H

Class C: N.N.N.H

Class D: multicast

Class E: experimental

N = Network portion of address

H = Host portion of address

Understanding Networks and Broadcasts

IP addresses represent many things. An IP address with all of the host bits set to 0 represents the network itself. Network IP addresses are used by routers to represent the path to a network. The following is part of an IP routing table that shows how the network address is used to identify each network.

Router#sh ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M -

➥mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF

➥inter area

E1 - OSPF external type 1, E2 - OSPF external type

➥2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, *

➥- candidate default

U - per-user static route

Gateway of last resort is 192.250.152.1 to network 0.0.0.0

D 192.168.104.0/24 [90/11151872] via 10.1.1.90,

➥02:06:36, Serial5/0.10

D 192.168.106.0/24 [90/11151872] via 10.1.1.90,

➥02:06:36, Serial5/0.10

D 192.168.107.0/24 [90/11151872] via 10.1.1.90,

➥02:06:36, Serial5/0.10

D 192.168.108.0/24 [90/11151872] via 10.1.1.14,

➥02:06:36, Serial5/0.4

D 192.168.109.0/24 [90/2809856] via 10.1.1.2, 02:06:36,

➥Serial5/0.1

D 192.168.110.0/24 [90/11151872] via 10.1.1.18,

➥02:06:36, Serial5/0.5

D 192.168.111.0/24 [90/11151872] via 10.1.2.122,

➥02:06:36, Serial5/0.133

10.0.0.0/8 is variably subnetted, 270 subnets, 5 masks

D 10.1.3.8/30 [90/7690496] via 10.1.1.26, 02:06:36,

➥Serial5/0.7

D 10.1.2.8/30 [90/11023872] via 10.1.1.10, 02:06:36,

➥Serial5/0.3

C 10.1.1.8/30 is directly connected, Serial5/0.3

Router#

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110 Pa r t I E X A M P R E PA R AT I O N

In the preceding routing table you can see the first entry

192.168.104.0/24 as being accessible via interface Serial5/0.10.

Because 192.168.104.0 has 24 bits of network mask, the host portion of the address consists of the last 8 bits. The representation here has all 8 of these bits set at 0.

The broadcast address is just the opposite of the network address. The broadcast address for a network has all of the host bits set to one. The broadcast address is used to send a packet to every host on a network.

One reason to send broadcasts is to resolve a host name to an IP address. Some name-to-IP address mapping schemes use broadcast addresses to send out a query looking for a host name. Every computer on the network receives the query, and most ignore it. The computer with the name the query is looking for replies with its IP address in the source field of the packet. The name-to-IP address mapping scheme can then add that host name and IP address combination to its cache. For the network 192.168.104.0/24, the broadcast address is 192.168.104.255—the same as the network but with all of the host bits set to one. A broadcast of this type is called a directed broadcast because it specifies the network for the broadcast. Broadcasts that originate on the network for which the broadcast is intended can use the broadcast address 255.255.255.255, which will be translated into the

Layer 2 broadcast address for the network topology in use.

Special IP Address Ranges

There are two other classes of IP addresses that are not used in the same way as Class A, B, and C addresses: multicast and experimental.

Multicast addresses fall in the range of 224–239 in the first octet.

Multicast IP addresses are classified as Class D addresses. A multicast address represents a multicast group. Multicast groups are used to send large amounts of data to a group of computers rather than sending the same information to each one individually. Multicasting requires special configurations in the routers and switches to work properly. However, when used for such things as RealAudio or Video

Streaming, multicasting can reduce network traffic significantly.

Multicasting is beyond the scope of the CCNA exam and is not covered in detail in this book.

The second type of IP address is the experimental class. These IP addresses are rarely used. Experimental IP addresses fall in the range of 240–254 in the first octet. Experimental IP addresses are classified as Class E addresses.

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There are some other special ranges of IP addresses. There are several regions of IP addresses that are reserved for private networks. These

IP addresses are not used for the Internet and will be dropped by

Internet routers if they enter the Internet.

The first reserved IP range offers flexibility in network design to individual companies. The Class A IP network 10.0.0.0 is reserved for private use. This IP range allows companies to build elaborate internal networks. By subnetting the IP range, a company can create networks for a large number of offices without problem. Most companies are moving their internal networks to this range. This reserved IP range is commonly referred to as Ten Net or Ten Net Addresses.

There also is a reserved IP range in the Class B IP range. This range consists of IP networks from 172.16.0.0–172.31.0.0. This Class B range is not routed on the Internet and can be used by companies for use on private networks.

Another area of reserved IP addresses consists of IP networks

192.168.0.0–192.168.255.0. These Class C IP addresses are not routed on the Internet and thus are good for use on private networks. Because the Ten Net range offers more flexibility in network design, most organizations use it for network stations. It is common to use addresses from each range in an enterprise for different purposes.

S

UBNETTING

IP A

DDRESS

R

ANGES

It would be rare for any organization to have sixteen million workstations on one network. In fact, it would make for one very slow network. To use their address space more efficiently, the organization could subnet a class A address.

Subnetting is the process of dividing a network range into smaller

subnetworks. Subnetting allows a network designer more flexibility when allocating IP address ranges. It does not make sense to use an entire range of 254 Class C addresses for an office with only 15 computers. Network designers can create “subnets” of a Class C range and assign parts of the range to different offices. Subnetting allows network designers to make more efficient use of the IP network ranges assigned to them.

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112 Pa r t I E X A M P R E PA R AT I O N

Subnetting a Class A Address

A Class A IP address can support 16,777,212 hosts with the default subnet mask of 255.0.0.0. To make better use of this IP address range, an organization could divide it up into smaller networks. To subnet a larger IP range into more networks with fewer hosts per network, you add subnet mask bits. For this example, use the private network Class A address of 10.0.0.0 netmask 255.0.0.0. This private

Class A network can support one network with 16,777,212 hosts.

Most organizations don’t have quite this many computers on one network, so they subnet the Class A into many networks with fewer computers per network. By adding subnet mask bits to the default subnet of 255.0.0.0 divides the Class A into multiple networks. For example, the Class A IP address 10.110.2.1 could use a subnet mask of

255.255.255.0. Now the IP address scheme will support 65,534 networks with 254 hosts per network. This is much more useful. You can distribute IP addresses to division offices and each office can have 254 hosts. To determine how many networks a subnet creates and how many hosts per subnet there can be, use the equation 2n–2, where n is the number of bits. This works for both networks and hosts per network. In the previous example, 16 bits of subnet were added to a Class

A address. Using the equation 2^16 = 65536 – 2 = 65,534 networks.

In the previous example, 8 bits were used for hosts. Using the equation 2^8 = 256 – 2 = 254 hosts. Table 3.1 lists the Class A subnets.

TA B L E 3 . 1

C

L A S S

A S

U B N E T S

7

8

5

6

3

4

# bits

2

9

10

Subnet Mask

255.192.0.0

255.224.0.0

255.240.0.0

255.248.0.0

255.252.0.0

255.254.0.0

255.255.0.0

255.255.128.0

255.255.192.0

# Subnets

2

6

14

30

62

126

254

510

1022

# Hosts

4194302

2097150

1048574

524286

262142

131070

65534

32766

16382

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19

20

21

15

16

17

18

# bits

11

12

13

14

22

Subnet Mask

255.255.224.0

255.255.240.0

255.255.248.0

255.255.252.0

255.255.254.0

255.255.255.0

255.255.255.128

255.255.255.192

255.255.255.224

255.255.255.240

255.255.255.248

255.255.255.252

C h a p t e r 3 U N D E R S TA N D I N G A N D C O N F I G U R I N G T C P / I P 113

# Subnets

2046

4094

8190

16382

32766

65534

131070

262142

524286

1048574

2097150

4194302

# Hosts

8190

4094

2046

1022

510

254

126

62

30

14

6

2

Subnetting a Class B Address

Class B IP addresses have a default subnet mask of 255.255.0.0, or

/16. Class B address space provides you with a great deal of flexibility in designing your network.

Suppose you have been given the IP address space 130.214.0.0.

With this address space, you must subnet it to accommodate 487 offices with no more than 100 hosts per office. Look to the table of

Class B subnets (see Table 3.2) and find the number of subnet bits required. You find that 9 bits of subnet mask allows for 510 subnets with 126 hosts per subnet. You can now divide the IP address space into the networks and assign them to different offices.

2

3

4

TA B L E 3 . 2

C

L A S S

B S

U B N E T S

# bits Subnet Mask

255.255.192.0

255.255.224.0

255.255.240.0

# Subnets

2

6

14

# Hosts

16382

8190

4094

continues

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114 Pa r t I E X A M P R E PA R AT I O N

8

9

10

11

5

6

7

12

13

14

TA B L E 3 . 2

C

L A S S

B S

U B N E T S

# bits Subnet Mask

255.255.248.0

255.255.252.0

255.255.254.0

255.255.255.0

255.255.255.128

255.255.255.192

255.255.255.224

255.255.255.240

255.255.255.248

255.255.255.252

# Subnets

30

62

126

254

510

1022

2046

4094

8190

16382

# Hosts

2046

1022

510

254

126

62

30

14

6

2

Subnetting a Class C Address

By now, you should be getting the hang of this. This section takes you through the process of subnetting a Class C address.

Suppose your ISP (Internet service provider) has assigned a Class C

IP address (199.250.155.0) for your office. However, you have five offices needing IP addresses reachable from the Internet. Each of the offices has 25 computers that will need IP addresses from this range.

How do you subnet this? See Step by Step 3.1.

S T E P B Y S T E P

3.1 Subnetting a Class C Address

1.

You have a Class C IP address range. You must divide it into five subnets that allow for 25 hosts on each subnet.

Can this be done? Look ahead to Table 3.3 to find a subnet mask that will support at least five networks and at least 25 hosts per network.

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2.

255.255.255.224 allows for six networks with 30 hosts each. That will work. Now what are the six networks this subnet creates? You have subnetted using 3 bits. Write out the chart from converting binary to decimal and place the three bits in beneath the chart.

128 64 32 16 8 4 2 1

1 1 1 0 0 0 0 0

3.

The last bit of subnet mask falls under the 32. Your first valid network will be 199.250.155.32. Because the network is represented by 199.250.155.32, the first valid IP address you can assign to a host is 199.250.155.33. Each network is 32 more. The number of hosts is equal to 32 minus 2.

4.

Add 32 to 199.250.155.32 and you find the next network starts at 199.250.155.64. The following is a list of all the networks created by this subnet and the host addresses for each.

Subnet

32

64

Hosts

33–62

65–94

Broadcast

63

95

96

128

160

97–126

129–158

161–190

127

159

191

192 193–222 223

5.

Now look at this octet of the address in binary. Notice the underlined bits in the following table. These bits are the three bits of a subnet. In the subnet column, notice each subnet is a unique combination of bits. In the hosts column, notice how when the subnet bits are discarded each range goes from 00001-11110. In the broadcast column, notice when the subnet bits are discarded, each broadcast address is 11111. It is much easier to see how subnetting works when you look at it in binary. However, we use IP addresses represented in decimal.

continues

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116 Pa r t I E X A M P R E PA R AT I O N

continued

Subnet

00100000

01000000

01100000

10000000

11000000

Hosts

00100001–00111110

01000001–01011110

01100001–01111110

10000001–10011110

11000001–11011110

Broadcast

00111111

01011111

01111111

10011111

11011111

6.

From the previous table, notice the underlined bits. These bits are the three bits of subnet. In the Subnet column, notice that each subnet is a unique combination of bits. In the Hosts column, notice how when the subnet bits are discarded, each range goes from 00001–11110. In the

Broadcast column, notice that when the subnet bits are discarded, each broadcast address is 11111. It is much easier to see how subnetting works when you look at it in binary.

However, we use IP addresses represented in decimal.

In looking at the two previous tables, you may wonder why

0–31 and 224–254 are not valid address ranges. To understand why, look at these two ranges in the following table.

Subnet Hosts Broadcast

0 1–30 31

00000000

224

00000001–00011110

225–254

00011111

255

11100000 11100001–11111110 11111111

Subnetting on byte boundaries is easy. However, when your subnet mask splits a byte boundary, as the previous example does, there are some rules. When a subnet splits an octet, the subnet bits in the partial octet cannot be all

1s or all 0s. This is why the two previous subnets are not valid.

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4

5

2

3

TA B L E 3 . 3

C

L A S S

C S

U B N E T S

# bits Subnet Mask

255.255.255.192

255.255.255.224

255.255.255.240

255.255.255.248

6 255.255.255.252

# Subnets

14

30

2

6

62

# Hosts

62

30

14

6

2

Subnet masks can be represented in a shorthand method. It is somewhat cumbersome to write out 199.250.155.35 255.255.255.0.

Another way to represent the same subnet mask is

199.250.155.35/24. The /24 means 24 bits of network mask.

Thus, if you added 3 bits of subnet mask to this address, it would be written 199.250.155.35/27. This shorthand method is used when

Cisco routers display routing, as shown in the following example: c3620r1#sh ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, *

➥- candidate default

U - per-user static route, o - ODR

Gateway of last resort is 10.1.99.1 to network 0.0.0.0

D 192.168.104.0/24 [90/41689600] via 10.1.99.1, 1d20h,

➥BRI1/0

D 192.168.105.0/24 [90/41689600] via 10.1.99.1, 1d20h,

➥BRI1/0

D 192.168.106.0/24 [90/41689600] via 10.1.99.1, 1d20h,

➥BRI1/0

D 192.168.107.0/24 [90/41689600] via 10.1.99.1, 1d20h,

➥BRI1/0

D 192.168.108.0/24 [90/41689600] via 10.1.99.1,

➥07:26:06, BRI1/0

continues

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118 Pa r t I E X A M P R E PA R AT I O N

continued

D 192.168.109.0/24 [90/41689600] via 10.1.99.1, 1w0d,

➥BRI1/0

D 192.168.110.0/24 [90/41689600] via 10.1.99.1, 1d11h,

➥BRI1/0

D 192.168.111.0/24 [90/41689600] via ➥10.1.99.1, 1w0d,

➥BRI1/0

Notice how in the preceding example the router shows the network mask in the /bits format.

Good Design Principles When

Assigning IP Addresses

When you create IP subnets and assign them to offices, there are some principles you should follow to make life easier for your routers. When a router learns of a network, it places an entry for that network in its routing table with a pointer to the next hop to reach that network. If your network is hierarchical in design, you can help your router reduce the size of its routing tables.

Variable-Length Subnet Masking

(VLSM)

Sometimes you may have a limited amount of address space and an odd combination of networks to use it on. Suppose that when you subnet a Class C address with a subnet mask of 255.255.255.240, you get 14 networks that can accommodate 14 hosts each. However, one of your locations has 15 hosts.

By creating different subnets using different subnet masks, you can make better use of address space—but at a cost. The price is complexity. Creating a VLSM network requires a great deal of planning and a strong understanding of subnetting and routing. Figure 3.4

shows how VLSM can be used in a network.

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5 HOSTS

192.168.50.224/28

FDDI

15 HOSTS

192.168.50.192/27

C h a p t e r 3 U N D E R S TA N D I N G A N D C O N F I G U R I N G T C P / I P 119

192.168.50.0

2 HOSTS

192.168.50.240/30

F I G U R E 3 . 4

An example of how VLSM can be use to create different size networks from one Class C address space.

50 HOSTS

192.168.50.128/26

2 HOSTS

192.168.50.244/30

Token

Ring

100 HOSTS

192.168.50.0/25

Classless InterDomain Routing (CIDR)

When the Internet began its explosion of growth, IP addresses became very scarce. To help keep from running out of IP addresses, and to make good use of the IP addresses available, Classless

InterDomain Routing (CIDR) was created. CIDR allows multiple

Class C addresses to be combined by supernetting the subnet mask.

Supernetting is accomplished by changing the network mask in the

opposite direction of subnetting. Supernetting removes bits from the default subnet mask to create a larger network containing multiple

Class C addresses.

For an organization that needs more than 254 IP addresses, a CIDR block can be assigned. By removing two subnet mask bits from the normal Class C subnet mask of 255.255.255.0, you get a subnet mask of 255.255.252.0. The list below shows the IP ranges that fall under this scheme.

192.34.128.0

192.34.129.0

192.34.130.0

Class C address

Class C address

Class C address

192.34.131.0

Class C address

———————————————

192.34.128.0

255.255.252.0

192.34.131.255

Supernetted address

Subnet Mask for Supernet

Broadcast address for Supernet

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By making use of Supernetting, Internet routers can be more efficient with routing table entries. Rather than having four entries for four different Class C IP address spaces, the routers can have one entry for the Supernet or CIDR block. More information on CIDR can be found in RFC1519.

Configuring IP Addresses

Now that you understand IP address classes and how to subnet, you need to apply these addresses to router interfaces.

To configure a router interface with an IP address, you use the interface configuration command

IP address

. The following is an example of how you would place an IP address on an interface: c2513r1#config t

Enter configuration commands, one per line. End with

CNTL/Z.

c2513r1(config)#int ethernet 0 c2513r1(config-if)#ip address 192.168.31.1 255.255.255.0

c2513r1(config-if)#exit c2513r1(config)#exit c2513r1#

This assigns the IP address 192.168.31.1 255.255.255.0 to interface

Ethernet0 on the router.

In large enterprises where networks grow quickly, it is sometimes necessary to place more than one IP network on the same physical network. You can configure this by using secondary IP addresses on an interface. When a router interface is configured with multiple IP addresses, the router will forward traffic from one IP network to the other on the interface. Essentially, it will route traffic from one IP network to the other on the same physical network. To configure a secondary IP address on an interface, you add the secondary keyword to the IP address as shown following: c2513r1#config t

Enter configuration commands, one per line. End with

CNTL/Z.

c2513r1(config)#int ethernet 0 c2513r1(config-if)#ip address 192.168.32.1 255.255.255.0

➥ secondary c2513r1(config-if)#exit c2513r1(config)#exit c2513r1#wr

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If you view a router configuration file with multiple IP addresses assigned to an interface, the interface configuration will resemble the following: interface Ethernet0 ip address 192.168.32.1 255.255.255.0 secondary ip address 192.168.31.1 255.255.255.0

Now that you understand IP addresses and how to subnet and verify that the IP subnets are valid, it is time to move on to some of the functions Cisco’s IOS has for TCP/IP services.

TCP/IP O

VERVIEW

Transmission Control Protocol/Internet Protocol (TCP/IP) delivers more data every day than any other protocol. TCP/IP was originally developed for use on the fledgling Internet by the United States

Department of Defense. The design criteria for TCP/IP included the requirement for interoperability between different platforms.

TCP/IP is the most widely supported protocol today. Almost every computer manufacturer, has implemented TCP/IP to some degree on their equipment. TCP/IP is not the fastest protocol. It’s not the most efficient protocol. It’s not even the easiest protocol to use. TCP/IP quite simply is the most widely available protocol for interconnecting different systems. The protocol is simple enough to be implemented in embedded systems like factory control devices. It is also complex enough to be the foundation of the largest network in the world, the

Internet. One of TCP/IP’s greatest assets is that it belongs to no one organization. Other protocols are driven by the needs and desires of companies. The development of SNA is almost exclusively in the hands of IBM. IPX is in the hands of Novell, and DecNET is now in the hands of Compaq since the company’s acquisition of Digital

Equipment Corporation.

TCP/IP is a suite of protocols that supports a vast array of services.

In the suite, there are elements to support Layer 3 network connectivity and IP. TCP and UDP are Transport protocols (Layer 4).

There are even Layer 7 application implementations such as Telnet.

TCP/IP’s greatest strength is its widespread availability and its ability to connect dissimilar computing systems in a manner that they can exchange data effectively.

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F I G U R E 3 . 5

Some of the application layer implementations of TCP/IP.

At the Application layer, TCP/IP supports such services as file transfers (FTP), email delivery (SMTP and POP), remote login (Telnet and rlogin), network management (SNMP), and name resolution

(DNS). These applications define a methodology for working together no matter on what platform they are implemented. This allows an IBM mainframe to exchange email with an Alpha-based

Linux computer using SMTP. Figure 3.5 shows how the Application layer protocols fit into the OSI model.

Application

Transport

Internet

Network

Interface

Hardware

File Transfer

-TFTP

-FTP

-NFS

E-Mail

-SMTP

Remote Login

-Telnet

-rlogin

Network Management

-SNMP

Name Management

-DNS

The Transport protocols (OSI Layer 4) of TCP/IP include TCP and

UDP. Functions supported at the Transport layer include flow control and acknowledgments for data delivery. Figure 3.6 shows how the Transport protocols fit into the OSI model.

F I G U R E 3 . 6

How the transport protocols fit into the OSI model.

Application

Transport

Internet

Network

Interface

Hardware

Transmission Control

Protocol (TCP)

User Datagram

Protocol (UDP)

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The following list describes the different transport protocols of the

TCP/IP suite:

á

TCP is a connection-oriented protocol that guarantees packet delivery using acknowledgments. When a packet is dropped,

TCP is responsible for making sure it is re-sent. TCP is responsible for breaking messages into segments for transmission.

After the segments arrive at the destination, TCP reassembles them. TCP creates a connection between networking stations before transferring data.

á

UDP is a connectionless protocol that makes no guarantees about data delivery. UDP can transmit data faster than TCP because it does use acknowledgments. UDP does not break messages into segments—that function is handled by a different layer. Reliability is also the responsibility of another layer.

Reliability can be handled by an Application layer protocol or by a reliable lower-layer protocol.

TCP connections include several steps. First the TCP protocol creates a connection. Then data is delivered. While data is being delivered, reliability information in the form of acknowledgments is sent.

After all of the data is delivered, the connection is torn down. The process of creating a connection, using acknowledgments, and tearing down the connection makes TCP slower than UPD. However,

TCP provides a quick, efficient, reliable connection.

UDP data transfer does not include the creation of a connection. UDP can start sending data immediately. However, there is no method for

UDP to ensure that data makes it to the destination. UDP sends the data and assumes it arrives at the destination intact. For UDP transfers, another layer of the protocol must handle the function of reliable data delivery whether it be a lower layer or a higher layer.

TCP Segment Format

The TCP segment format supports several functions of the protocol.

Figure 3.7 shows the format of a TCP segment.

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F I G U R E 3 . 7

The TCP segment format.

# Bits 16

Source

Port

16

Dest.

Port

32

Sequence

Number

32

Acknowledgment

Number

4

HLEN

6 6

Reserved

Code

Bits

16 16

Window

Checksum

16

Urgent

0 or 32

Option Data

Descriptions of the TCP segment are as follows:

á Source Port. The source port is the port number of the originating upper-layer protocol.

á Destination Port. The destination port is the port number of the destination upper-layer protocol port.

á

Sequence Number. The sequence number is this segments position in the data stream.

á

Acknowledgment Number. The acknowledgment number is the next expected TCP octet. This is used to guarantee reliable transfer of data.

á

HLEN. The HLEN is the number of 32-bit words in the header of the segment. The HLEN indicates where the data begins.

á Reserved. This position is set to zero. This position is not used currently.

á

Code Bits. The code bits are used for signaling. The code bits are used to indicate the creation of a connection and the termination of a connection.

á Window. The window defines the number of bytes or octets the receiver will accept before acknowledging.

á

Checksum. The checksum is a calculated checksum of the header and data fields. It is used to determine whether the data sent is the data received.

á Urgent Pointer. The urgent pointer indicates the end of urgent data.

á Option. The option field currently has one use: to define the maximum TCP segment size.

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á Data. The data field is filled with the data of the transmission.

This data will be passed up to the layer above TCP at the destination network workstation.

53

69

80

Because TCP services many upper-level protocols, it must have some way to indicate which upper-layer protocol for which a message is intended. TCP uses port numbers for this purpose. Port numbers define which upper-layer protocol and perhaps even which session of an upper-layer protocol for which a message is intended. Some common port numbers are listed in the following table:

Port Number

21

23

25

Function

FTP

Telnet

SMTP

DNS

TFTP

HTTP

161 SNMP

TCP/IP S

ERVICES

TCP/IP is an elegant protocol. After the basics are mastered, it is a very simple protocol to understand and configure. The TCP/IP protocol includes many different implementations of protocols to solve the problem of connecting computers. The following are of some of the protocols and their functions:

á Internet Protocol (IP). The Layer 3 (Network) protocol of the suite. Used for communication at the network layer of the

OSI model.

á Address Resolution Protocol (ARP). Used to map IP addresses to MAC addresses.

á Transmission Control Protocol (TCP). A Layer 4 protocol.

TCP provides guaranteed data delivery.

á

User Datagram Protocol (UDP). A Layer 4 protocol. UDP does not guarantee data delivery.

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á Internet Control Message Protocol (ICMP). Provides diagnostic information about the protocol suite. Most people are familiar with the diagnostic utility ping. Ping provides a means of confirming network connectivity between two stations. Ping is an implementation of ICMP. (Today it is best to specify an

ICMP ping because many other protocols now support ping in their implementations. Novell has added ping to IPX. IBM has even added some ping-like functions to SNA.)

á

Telnet. An Application layer implementation of a program that allows remote terminal access to a device. For internetworking engineers, Telnet is often used to log in to routers.

á

File Transfer Protocol (FTP). Allows stations to transfer files from one to another. FTP used TCP packets to guarantee data delivery.

á Trivial File Transfer Protocol (TFTP). A file transfer implementation. It differs from FTP in that it uses UDP packets to deliver its data; hence, it does not guarantee delivery of data.

á Simple Mail Transfer Protocol (SMTP). An email implementation that delivers mail from network to network or from user to user within one domain. SMTP is used throughout the

Internet for delivery of millions of emails daily.

á Post Office Protocol (POP3). An email implementation that allows users to connect with the system that receives their email and download that email to their computers. POP is used by millions of users everyday to get email from their ISP.

Programs that can use POP include Eudora Pro, Outlook,

Pegasus Mail, and the email portion of most browsers.

á

Network Time Protocol (NTP). Used to synchronize the clocks of networking computers. NTP is typically used to synchronize your system’s clock with one of the very accurate time sources available on the Internet. Cisco routers support NTP and can be configured to synchronize their clocks. This can be very helpful when diagnosing problems.

á HyperText Transfer Protocol (HTTP). The protocol that

Netscape Navigator uses to talk with Web servers. Your browser uses HTTP to download HTML from Web servers and display the information on your screen.

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á Domain Name System (DNS). Used to map domain names like www.cisco.com

to IP addresses. When you type the name of a site you want to visit in your browser, the first thing the computer does is resolve that name to an IP address using

DNS. After the IP address is found, it is passed to the IP stack, and HTTP is used to communicate with the Web server.

Without DNS, you would have to remember IP addresses instead of domain names. DNS makes life much easier.

á

Simple Network Managment Protocol (SNMP). Used to gather statistics from networking devices for monitoring and reporting. Most networking monitoring software, such as

Cisco Works, use SNMP to gather information.

Each of these protocols serves a purpose in the overall data communication between networking stations. As new needs arise, protocols are updated or expanded to accommodate these new needs. Therefore, it is very important to stay abreast of changes in networking technologies.

Host Name-to-IP Address Mapping

Computers are content using IP addresses to communicate. Names really mean very little to a computer. However, human users prefer to address network stations by names that indicate something about the stations. For instance, would you rather remember 192.31.7.130

or www.cisco.com

? www.cisco.com

is much more meaningful to you isn’t it? For networking to function well for both human users and computers, there must be a way to map names like www.cisco.com

to

IP addresses like 192.31.7.130. On the Internet, this is done with

Domain Name Server (DNS).

Domain Name Server (DNS)

The inner workings of DNS are beyond the scope of this book.

However, using DNS is something you will need to do often in internetworking. In most places in which you are required to enter a network station address, either the IP address or the name is acceptable.

For instance, ping 192.31.7.130

is equivalent to ping www.cisco.com

.

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The latter requires the router to resolve the name www.cisco.com

to the

IP address 192.31.7.130 before performing the ping. If everything is configured properly, this resolution of the name happens very quickly.

If you have not configured any name to IP address mapping and you attempt to use a hostname in place of an IP address the following will occur:

ACS-2500#ping www.cisco.com

Translating “www.cisco.com”

% Unrecognized host or address, or protocol not running.

ACS-2500#

Cisco’s IOS has different ways to map names to IP addresses. If your organization makes use of DNS, you can configure your routers to use this DNS to resolve names to IP addresses. To turn this on in the router, you use the ip name-server command to identify the

DNS machine to the router. This is the machine that provides the

DNS service. You must also turn on domain lookup in the router.

This is done with the command ip domain-lookup

.

The following example shows how to configure the DNS server in the router’s configuration:

ACS-2500#config t

Enter configuration commands, one per line. End with

➥CNTL/Z.

ACS-2500(config)#ip name-server 155.229.1.69

ACS-2500(config)#ip domain-lookup

ACS-2500(config)#exit

Now that the name server is configured and domain lookup is enabled, the same command that failed before will work:

ACS-2500#ping www.cisco.com

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echoes to 192.31.7.130, timeout is

➥2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max =

➥92/99/108 ms

ACS-2500#

When the router performs a DNS lookup, it caches the information for future use. You can view the contents of the cache with the show hosts command.

ACS-2500#sh hosts

Default domain is not set

Name/address lookup uses domain service

Name servers are 155.229.1.5, 155.229.1.69

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Host Flags Age Type Address(es) www.kmahler.com (temp, OK) 0 IP 199.250.137.37

cio-sys.cisco.com (temp, OK) 0 IP 192.31.7.130

www.cisco.com

ACS-2500#

This shows each of the domain names cached in the router. Notice that the router reports that the Default domain is not set. The default domain is the domain in which the router resides. The router can provide a shortcut to accessing stations within its domain. If you set the default domain in the router, it will then append this default domain to any name that is not fully entered. If you wanted to ping kestrel.kmahler.com

and the default domain name kmahler.com

was defined in the router with the name server set along with domain name lookup on, the router will be able to identify that station with just kestrel

.

This router has been configured with a name server and IP domain lookup has been turned on.

c3620r1#ping kestrel.kmahler.com

Translating “kestrel.kmahler.com”...domain server

➥(199.250.137.37) [OK]

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 199.250.137.34, timeout

➥is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max =

➥1/4/8 ms c3620r1#

Now try it without the full name. Try pinging just kestrel

”: c3620r1#ping kestrel

Translating “kestrel”...domain server (199.250.137.37)

% Unrecognized host or address, or protocol not running.

c3620r1#

Now turn set the default domain name in the router: c3620r1#config t

Enter configuration commands, one per line. End with

➥CNTL/Z.

c3620r1(config)#ip domain-name kmahler.com

c3620r1(config)#exit c3620r1#

Now try pinging kestrel again: c3620r1#ping kestrel

continues

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130 Pa r t I E X A M P R E PA R AT I O N

continued

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 199.250.137.34, timeout

➥is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max =

➥1/2/4 ms c3620r1#

The router automatically added

.kmahler.com

to the name. This produced kestrel.kmahler.com

, which was then resolved with DNS to the IP address 199.250.137.34 and the ping was successful.

Because the router keeps prior name resolutions cached for some time, it is possible that the IP address of a network station can change and the router will have the wrong IP address cached. You can force the router to clear its name cache with the clear host command.

With this command, you can specify a specific host to clear, or you can use the wildcard (

*

) to clear all name-to-IP address mappings: c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type Address(es) www.kmahler.com (temp, OK) 0 IP 199.250.137.37

linux.kmahler.com (temp, OK) 0 IP 199.250.137.55

kestrel.kmahler.com (temp, OK) 0 IP 199.250.137.34

c3620r1#

To clear an individual host mapping, use the clear host with the host name specified: c3620r1#clear host kestrel.kmahler.com

c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type Address(es) www.kmahler.com (temp, OK) 0 IP 199.250.137.37

linux.kmahler.com (temp, OK) 0 IP 199.250.137.55

c3620r1#

Now if you attempt to ping kestrel.kmahler.com

again, the router will resolve the name instead of using the cache. If the DNS has been updated with the new IP address, the ping will be successful.

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To clear all name to IP address mappings use the wildcard “(

*

)”: c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type Address(es) kestrel.kmahler.com (temp, OK) 0 IP 199.250.137.34

www.kmahler.com (temp, OK) 0 IP 199.250.137.37

linux.kmahler.com (temp, OK) 0 IP 199.250.137.55

c3620r1#clear host * c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type Address(es) c3620r1#

Because there is not always a DNS available, Cisco has provided another method of mapping host names to IP addresses. Static name

mappings can be made part of the router’s configuration.

Static Name Mappings

Not every organization is connected to the Internet or is running

DNS. Cisco has provided a method for mapping your host names to

IP addresses as well. IOS provides a way to enter host names and IP mappings directly in the router’s configuration. Using these static mappings, you can enter commonly used network hosts and reference them by the name you assign. In a router without an IP name server configured and IP domain lookup disabled, try to ping the hostname kestrel

.

c3620r1#ping kestrel

Translating “kestrel”

% Unrecognized host or address, or protocol not running.

c3620r1#

The router has no way to resolve the name to an IP address. To create a static host mapping in the router’s configuration, use the command ip host

. The following is an example to map the name kestrel to the

IP address 199.250.137.34:

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132 Pa r t I E X A M P R E PA R AT I O N c3620r1#config t

Enter configuration commands, one per line. End with

CNTL/Z.

c3620r1(config)#ip host kestrel 199.250.137.34

c3620r1(config)#exit

After assigning this hostname to the IP address, try to ping the host again.

c3620r1#ping kestrel

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 199.250.137.34, timeout

➥ is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max =

4/4/4 ms c3620r1#

The router can now resolve the name kestrel to the IP address

199.250.137.34. Performing the show host command displays the following: c3620r1#sh host

Default domain is not set

Name/address lookup uses static mappings

Host Flags Age Type Address(es) kestrel (perm, OK) 0 IP 199.250.137.34

c3620r1#

R E V I E W B R E A K

Host Name-to-IP Address Mapping

Review

.

To make life easier for human users, computers provide a method of mapping meaningful names to IP addresses.

.

One very common way to do this is with Domain Name

Servers (DNS).

.

Cisco’s IOS supports DNS by configuring the name server’s IP address with the command ip name-server and turning on domain lookup with the command ip domain-lookup

.

.

You can provide a default domain name with the ip domainname command. The router will append this default domain name to any abbreviated name entered. If the entire name is entered, the router does not append the default domain name.

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.

Name resolutions are cached in the router.

.

You can view the cached names with the command show host

.

.

If you have a need to clear a cached name resolution, you can use the command clear host

.

.

In the event that you are not running DNS, you can statically map names to IP addresses in the router’s configuration file. The command ip host is used to create these mappings.

Internet Control Message Protocol

(ICMP)

Internet Control Message Protocol (ICMP) is responsible for reporting

messages and errors at the Network layer of the TCP/IP protocol suite. Many people use ICMP every day when they perform the most basic IP diagnostic tool: Ping.

Using the ping

Command

Ping is actually an acronym for Packet INternet Groper. Ping provides a very basic diagnostic to confirm that one network station can communicate on a Network level with another network station. Ping sends an ICMP echo packet to the destination network station. If it arrives at the destination, the destination echoes the packet back to the sender. In reporting this information, Ping also supplies information about how long it took for a packet to make a round trip from the source station to the destination station.

Cisco routers support Ping from the IOS Command Line Interface

(CLI). Cisco’s implementation of Ping supports some unusual options.

At its basic level, Ping allows you to specify a network station to ping and then it reports its success or failure. Following is an example of a ping from a Cisco router: c3620r1#ping 199.250.136.10

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 199.250.136.10, timeout

➥is 2 seconds:

continues

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134 Pa r t I E X A M P R E PA R AT I O N

continued

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max =

36/70/196 ms c3620r1#

In the previous example, the ping command was entered with an IP address to attempt Layer 3 communication with the destination device. The router then sent five (the default number) pings to the destination station. With each ping, the router reports the success or failure of the ping.

á

An exclamation point (!) indicates a successful ping.

á A period (.) indicates a failure.

á

The letter “U” indicates that the destination is unreachable.

á

The ampersand (&) indicates that the Time To Live (TTL) was exceeded.

The router then reports the success rate in percentage and fraction.

It also reports the minimum, average, and maximum times it took for a ping packet to make the round trip from the source station to the destination and back.

As mentioned before, Cisco’s implementation of Ping supports some options not available in many other implementations. A ping packet contains a source IP address and a destination IP address. Because two stations communicating properly is often a function of the source address, and routers generally have many IP addresses, Cisco’s

Ping allows you to specify the source address.

Cisco calls this the extended ping. The extended ping is available only in the privileged exec mode. Entering the ping command without an address accesses options for the extended ping. The CLI will then prompt you for several options including the target IP address. One of the prompts will ask if you would like Extended Commands.

Answer

Yes to this question to enter other options. One of these options is the source IP address. After you complete the options, the router will attempt to ping the destination IP address using the source IP address you specified.

c3620r1#ping

Protocol [ip]:

Target IP address: 19.250.138.1

Repeat count [5]:

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Datagram size [100]:

Timeout in seconds [2]:

Extended commands [n]: y

Source address or interface: 65.64.176.1

Type of service [0]:

Set DF bit in IP header? [no]:

Validate reply data? [no]:

Data pattern [0xABCD]:

Loose, Strict, Record, Timestamp, Verbose[none]:

Sweep range of sizes [n]:

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 19.250.138.1, timeout is

➥2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max =

➥32/32/36 ms c3620r1#

Again, the ping command reports the success or failure of the ping and the round-trip times for the ping packets.

Using the traceroute

Command

Traceroute shows the path from one end of the connection to the

other, listing each hop along the way. Hop is a term used to describe each router a packet passes thorough to reach its destination. The following example shows a traceroute from a router to Cisco’s Web site. Notice that the actual URL is used rather than the IP address.

This can be done because this router has an IP domain-server defined in the configuration.

Router#traceroute www.cisco.com

Type escape sequence to abort.

Tracing the route to cio-sys.cisco.com (192.31.7.130)

1 cisco_2500_t1.cancer.org (199.250.136.10) 0 msec 0 msec 4 msec

2 s0.atl514.gw.eni.net (155.229.99.229) 8 msec 4 msec 4 msec

3 fe1-0.atl100.gw.eni.net (155.229.0.1) 4 msec 4 msec 4 msec

4 H3-1-0.dca100.gw.eni.net (155.229.120.130) 180 msec 76 msec 16 msec

5 f1-0-0.maeeast.BBNPLANET.net (192.41.177.2) 20 msec 20 msec 24 msec

6 p2-2.vienna1-nbr2.BBNPLANET.net (4.0.1.93) 24 msec 20 msec 20 msec

7 p1-0.vienna1-nbr3.BBNPLANET.net (4.0.5.46) 24 msec 24 msec 20 msec

8 p3-1.paloalto-nbr2.BBNPLANET.net (4.0.3.178) 100 msec 100 msec 100 msec

9 p0-0-0.paloalto-cr18.BBNPLANET.net (4.0.3.86) 104 msec 96 msec 104 msec

10 h1-0.cisco.BBNPLANET.net (131.119.26.10) 100 msec 124 msec 100 msec

11 sty.CISCO.com (192.31.7.39) 100 msec 104 msec 100 msec

12 cio-sys.cisco.com (192.31.7.130) 100 msec * 100 msec

Router#

Ping is limited IP Ping is very useful for determining whether two network stations can successfully communicate at the Layer 3 protocol. Do not attempt to use the information provided by Ping to make other assessments of the network. Trying to use ping times to determine bandwidth or some other aspect of the network is fruitless.

Ping determines if two stations can communicate at the Network layer— that is all.

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Want to find the travel path?

To determine where in the United States your packets are traveling, look at the names returned. Not all Internet backbones support this, but most use the three letter abbreviation for the major airport near the area of the network facility. In the previous example, two are shown:

ATL refers to Atlanta

Hartsfield Airport, and

DCA is National

Airport in Washington, D.C. A list of these abbreviations can be found at gopher://wiretap.spies.com/00/

Library/Article/Aero/airport.lis

.

Each hop along the way is listed with the times required for each of three probes to return. The name for each hop is the second item listed on each line. For example, the name for the 11th hop is sty.CISCO.com

. In the previous example, the router used had an

IP domain-server defined in the configuration. This allows the router to use DNS to resolve names to IP addresses and IP addresses to names. For each hop in this example the name is found.

Because Traceroute shows the path between two network stations, it can be very useful for discovering routing problems. When Traceroute reaches a router and fails to go any further, the connectivity problem is usually just beyond the last router Traceroute was able to successfully reach.

Traceroute provides some other feedback as well. Responses, other than times for probes, can indicate other types of problems:

!H

P

N

U

*

Indicates that the probe was received by a router but not forwarded. This is generally due to an access list filtering the packet.

Indicates that the protocol was unreachable.

Indicates that the network was unreachable.

Indicates that the port was unreachable.

Indicates that a timeout occurred.

Traceroute is a very useful tool when trying to diagnose connectivity problems. Whereas Ping only lets you know whether you have connectivity, Traceroute lets you know what path the packets are taking and where in the path a failure is occurring.

Remote Access Using Telnet

As an internetworking engineer, you will use Telnet on a regular basis. Telnet defines a protocol for remote access. After you have initially configured a router using the serial port, you will most likely access that router using Telnet for future changes and additions.

Telnet is available on most any computer system. Most every version of UNIX has a Telnet client available. Windows 95, 98, and

NT all have Telnet clients. However, the Windows implementations of Telnet are very slow. Get a good Telnet client that is fast and supports large scroll back buffers and macros. You will be surprised how much typing a simple terminal macro can save you.

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A very good implementation of Telnet is included with Datastorm

Technologies’ Procomm Plus 32.

The Telnet protocol is an Application layer implementation of a

TCP/IP protocol. Because Cisco routers support Telnet from both a connection side as well as a client side, Telnet can be used to confirm network connectivity all the way to the Application layer.

Successfully telnetting from one router to another confirms that you have network connectivity on all seven layers of the OSI model. If you are unable to telnet, try to ping the remote router. If that is successful, the problem is most likely above the Network layer of the

OSI model. However, if your ping is unsuccessful, try a traceroute to find where the connection is lost.

From the IOS CLI, you can telnet to a device by entering the command telnet 192.168.1.1

. c3620r1#telnet 199.250.137.50

Trying 199.250.137.50 ... Open

This router was replaced 1/09/99

User Access Verification

Password:

Because Telnet is such a common utility, Cisco has provided a shortcut. By entering an IP address directly on the command line without any command, the router will initiate a Telnet connection to the other device: c3620r1#199.250.137.50

Trying 199.250.137.50 ... Open

This router was replaced 1/09/99

User Access Verification

Password:

If you mistype a command at the command prompt, the route will try to resolve it as a host name and try to telnet to it. This can actually become somewhat of a nuisance because internetworking engineers are not necessarily great typists. The following example shows the command sh run without a space. The router tries to resolve the name “shrun” with the DNS, which is not configured.

After a lengthy delay waiting for the router to timeout looking for the DNS, the router reports that it is unable to resolve the name.

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138 Pa r t I E X A M P R E PA R AT I O N c3620r1#shrun

Translating “shrun”...domain server (255.255.255.255)

% Unknown command or computer name, or unable to find computer address c3620r1#

If you have DNS configured and the server is responsive, the router responds very quickly with the failure. When you are not running

DNS, however, the router will take a long time to report the failure.

To reduce the annoyance, you can disable domain lookup in the router.

To turn off domain name resolution in the router, use the command no ip domain-lookup

. This is a global configuration command: c3620r1#config terminal c3620r1(config)#no ip domain-lookup c3620r1(config)#exit c3620r1#copy running-config startup-config

Now that domain lookup is disabled, it will no longer try to resolve the name to an IP address. The following example shows how this speeds up the router reporting back that you entered an incorrect command: c3620r1#shrun

Translating “shrun”

% Unknown command or computer name, or unable to find computer address c3620r1#

The router did not try to resolve the name to an IP address and thus reported immediately that your typing skills could use some polishing.

C

A S E

S

T U D Y

: I

N T E R N E T

I P A

D D R E S S

E S S E N C E O F T H E C A S E

Here are the essential elements in this case:

.

Your job is to subnet the IP address space assigned to your company in such a way that each office will have enough IP addresses for all of their networking devices and allow for the future addition of another office.

S C E N A R I O

You are the network guru for Emotion

Cybernetics, a robotics company that makes robots with human emotions. Your company has decided to get Internet service for the entire company. Emotion Cybernetics currently has four offices but has plans to soon open another. All of the offices are connected via a Frame Relay WAN.

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C

A S E

S

T U D Y

: I

N T E R N E T

I P A

D D R E S S

.

Your company currently has four offices and expects to open another soon.

.

The largest number of network devices in an office is 25. There is little expected growth of this number.

.

You have been assigned the IP address range 63.250.143.1–63.250.143.254.

Your building (the main office) will be getting a T1 to the Internet. You want to allow the people in your remote offices to access the Internet over the WAN and through your T1. The Internet service provider (ISP) has assigned you one Class A address with a 24-bit subnet mask

(63.250.143.0/24). Your largest office has 25 networked devices installed and does not expect any substantial growth in the number of devices.

A N A LY S I S

You start by writing out the bit positions for the last octet of the IP range.

128 64 32 16 8 4 2 1

Remembering the method of determining the number of workstations in a subnet, you determine that 3 bits of subnet mask will give you 30 workstations per subnet.

128 64 32 16 8 4 2 1

1 1 1

3 subnet bits puts you in the position of the decimal number 32. This will give you 30 usable IP addresses per subnet.

Now you check whether this subnet will provide the necessary number of subnetworks. You begin by writing out the ranges of addresses for each subnet. Because your last subnet bit falls in the position of 32, your first usable network range begins at 32. Each following IP range begins at the last network + 32.

Network

63.250.143.0

First IP Address Last IP Address

63.250.143.1

63.250.143.31 Invalid

63.250.143.32

63.250.143.33

63.250.143.62

continues

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C

A S E

S

T U D Y

: I

N T E R N E T

I P A

D D R E S S

63.250.143.64

63.250.143.96

63.250.143.128

63.250.143.160

63.250.143.192

63.250.143.224

63.250.143.65

63.250.143.97

63.250.143.94

63.250.143.126

63.250.143.129

63.250.143.158

63.250.143.161

63.250.143.190

63.250.143.193

63.250.143.222

63.250.143.225

63.250.143.254 Invalid

Looking at the subnets this creates, you find that you have six valid ranges, each with 30 usable IP addresses. This subnetting scheme will accommodate the needs of Emotion Cybernetics.

C

H A P T E R

S

U M M A R Y

KEY TERMS

• IP address

• Network mask

• subnet

• Class A IP address

• Class B IP address

• Class C IP address

• multicast

• DNS

• MAC address

• secondary IP address

This chapter covers IP addressing and subnetting. Understanding how IP addresses work and how to create subnets is important in any internetworking engineer’s daily tasks. Because IP is quickly becoming the standard protocol for most businesses, a thorough understanding of how this protocol’s addressing works is important.

Along with IP addressing, this chapter covered some of the IP services Cisco’s IOS provides. Configuring DNS on a router allows the router to perform name resolution for IP addresses. This can be helpful when diagnosing problems.

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A

P P L Y

Y

O U R

K

N O W L E D G E

Review Questions

1. What are the classes of IP addresses?

2. What is the equation for determining the number of subnets or the number of hosts based on the number of bits in the subnet mask?

3. What are some of the functions of ICMP?

4. What does the subnet mask define?

5. How does a router use the IP address and subnet mask to determine how to forward a packet?

6. Which class of IP address allows the greatest number of hosts per network?

7. What are the numbers you should remember for subnetting IP networks?

8. What are the rules for a binary AND function?

9. Briefly describe TCP and UDP.

10. What is the function of DNS?

Exam Questions

1. What is the proper equation for finding the number of networks or hosts that a subnet mask allows?

A. 2n – 2

B. 2 - n

×

2

C. 255 – n^2

D. n*2 – 2

2. Which of the following is a valid IP address for a host computer?

A. 192.168.45.32 netmask 255.255.255.224

B. 192.168.45.159 netmask 255.255.255.224

C. 192.168.45.169 netmask 255.255.255.252

D. 192.168.45.167 netmask 255.255.255.252

3. What command will show the host mappings in a router?

A.

Router#show mapping

B.

Router(config)#show host

C.

Router#map host

D.

Router#show host

4. How many networks and hosts will the subnet mask 255.255.255.224 allow on a Class C IP address?

A. 30 networks with six hosts each

B. 32 networks with eight hosts each

C. Six networks with 30 hosts each

D. Eight networks with 32 hosts each

5. What IOS command shows the path IP packets travel to reach a destination?

A.

ping

B.

tracert

C.

pong

D.

traceroute

6. Which of the following is valid?

A.

Router(config-if)#ip address

199.250.137.63 255.255.255.224

B.

Router(config)#ip address 199.250.137.63

255.255.255.224

C.

Router(config-if)#ip address

199.250.137.98 255.255.255.224

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A

P P L Y

Y

O U R

K

N O W L E D G E

D.

Router(config-if)#ip address

199.250.137.63 255.255.255.224

7. Which command will clear the router’s host mapping table without any other input?

A.

Router#clear host

B.

Router#clear host *

C.

Router#clear host mapping table

D.

Router#clear mapping

8. Which of the following is a valid static IP hostname mapping?

A.

Router#ip host computername 192.168.1.1

B.

Router(config)#ip host computername

192.168.1.1

C.

Router(config)#ip hostname computername

192.168.1.1

D.

Router(config)>ip host computername

192.168.1.1

9. Which of the following best describes the following two IP addresses?

192.168.1.0 netmask 255.255.255.0

192.168.1.255 netmask 255.255.255.0

A. Class A addresses, valid IP addresses for workstations

B. Class A addresses, invalid IP addresses for workstations

C. Class C addresses, valid IP addresses for workstations

D. Class C addresses, invalid IP addresses for workstations.

10. Which of the following are private network ranges that are not routed on the Internet? Select three.

A. 10.0.0.0

B. 199.168.0.0

C. 192.168.0.0

D. 172.16.0.0

Answers to Review Questions

1. There are five classes of IP addresses. Class A, B, and C are used for assigning to computers and routers. Class D addresses are used for multicast, and Class E addresses are for experimental use.

For more information, see “IP Addresses.”

2. 2n - 2. For more information, see “IP Addresses.”

3. ICMP provides echo services for the Ping diagnostic. It also sends source quench messages when a device is being overrun. ICMP is also responsible for sending host unreachable packets. For more information, see “TCP/IP Services.”

4. The subnet mask separates the two parts of an IP address: the network and the host address. For more information, see “IP Addresses.”

5. The router ANDs the network mask to the IP address to get the network portion of the IP address. The router then tries to find the network in its routing tables. If the network is found, the router forwards the packet. For more information, see “IP Addresses.”

6. Class A IP addresses allow the greatest number of hosts per network. For more information, see “IP

Addresses.”

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A

P P L Y

Y

O U R

K

N O W L E D G E

7. 128, 192, 224, 240, 248, 252, 254, 255. These numbers represent an octet filled from the right to left with bits. For more information, see

“Subnetting IP Address Ranges.”

8. 1 AND 1 = 1; 1 AND 0 = 0; 0 AND 0 = 0. For more information, see “Binary Math.”

9. TCP and UDP are both Layer 4 Transport protocols. TCP provides a connection-oriented link.

It also provides reliable data transmission and guaranteed delivery of data using acknowledgments. UDP provides a connectionless link and does not provide any guarantee of delivery. For more information, see “TCP/IP Services.”

10. DNS is Domain Name Server. It provides a method of mapping meaningful names to IP addresses, and vice versa. For more information, see “TCP/IP Services.”

Answers to Exam Questions

1. A. The proper equation for finding the number of subnets or the number of hosts a subnet mask will support is 2n – 2. For more information, see

“Subnetting IP Address Ranges.”

2. C. 192.168.45.169 netmask 255.255.255.252 is the only valid IP address. All of the others are either network addresses (all host bits set to zero) or broadcast addresses (all host bits set to one).

For more information, see “IP Addresses.”

3. D. The show host command displays a table of host mappings, both static and dynamic. For more information, see “TCP/IP Services.”

4. C. The subnet mask 255.255.255.224 on a Class

C IP address allows for six networks with 30 hosts each. For more information, see

“Subnetting IP Address Ranges.”

5. D. traceroute is the proper command for finding the path that packets take to reach a destination.

For more information, see “TCP/IP Services.”

6. C. The other answers have invalid IP addresses or the router prompts are incorrect. For more information, see “IP Addresses.”

7. B. clear host * clears the host mapping tables.

For more information, see “Host Name to IP

Address Mapping.”

8. B. ip host computername 192.168.1.1

is the proper syntax for defining static hostname mappings. For more information, see “Static

Name Mappings.”

9. D. The IP addresses fall in the range of Class C

IP addresses. They are also not valid IP addresses for workstations because 192.168.1.0 describes the network and 192.168.1.255 is the broadcast address for the network. For more information, see “IP Addresses.”

10. A, C, D. 10.0.0.0, 192.168.0.0, and 172.16.0.0

are private networks that are not routed on the

Internet. For more information, see “IP

Addresses.”

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration, Cisco Press, 1998.

2. Albitz, Paul, et al. DNS and BIND, O’Reilly,

1998.

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O

B J E C T I V E S

Cisco provides the following objectives for the

IPX/SPX portion of the CCNA exam:

.

Monitor Novell IPX operation on the router.

You must be able to monitor Novell’s IPX protocol on the router to successfully maintain networks using this protocol. Novell network problems can be difficult to diagnose. Making use of the monitoring tools in the router can aid in discovering what is happening and why.

.

Describe the two parts of network addressing, then identify the parts in specific protocol address examples.

To successfully maintain internetworking systems, you must understand the two parts of network addressing so that you can effectively determine how routing of traffic will occur.

List the required IPX address and encapsulation types.

.

Understanding IPX address and encapsulation types is necessary to maintain internetworking systems because many problems occur regarding the encapsulation types.

.

Enable the Novell IPX protocol and configure interfaces.

In order to maintain internetworking systems, you must be able to configure and enable Novell IPX protocol on Cisco routers.

C H A P T E R

4

Understanding and

Configuring IPX/SPX and

Other Network Protocols

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O

B J E C T I V E S

( c o n t i n u e d

)

O

U T L I N E

Configure IPX access lists and SAP filters to control basic Novell traffic.

.

Novell traffic can produce a large amount of network traffic. Understanding how to filter SAPs and control the protocol is essential for internetworking engineers. Novell networks generate a large number of broadcasts to advertise services. By making use of access lists, you can limit which services will be advertised. This can be beneficial for both reducing the bandwidth impact and for securing networks from unauthorized use.

Introduction

NOVELL IPX/SPX Protocol

Internet Packet eXchange (IPX)

Sequenced Packet eXchange (SPX)

NetWare Core Protocol (NCP)

Routing Information Protocol (RIP)

NetWare Link Services Protocol (NLSP)

Service Advertising Protocol (SAP)

Network Addresses

148

148

149

149

149

149

150

150

151

Multiple Encapsulations

A Warning about Bridging

Configuring IPX Routing

Configuring Novell Routing

Novell Access Lists

Standard IPX Access Lists

Extended IPX Access Lists

SAP Filtering Access Lists

Diagnosing IPX Problems

Displaying IPX Addresses

Displaying Servers

Working with IPX Routing Tables

Load Sharing with IPX

Chapter Summary

151

152

161

161

164

166

167

169

153

156

157

157

159

159

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S

T U D Y

S

T R AT E G I E S

.

The network protocols portion of the CCNA test is perhaps the most important for which to study. Network protocols make up the pieces of the whole. The two networking protocols you are required to understand for the CCNA exam are IPX and TCP/IP. These two are the most used in today’s networks. TCP/IP is quickly becoming the standard of many companies, and many would argue it already is a standard.

It scales well. This is proven by the success of the largest WAN to date, the Internet.

Learn Novell’s types of encapsulation and how they relate to Cisco’s encapsulation types.

Learn to recognize Novell network addresses.

Understand how each of these network protocols is configured on the router and how to perform simple diagnostics.

Spend time working with the protocols on real networks. Learn to solve real problems with IPX networks.

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I

NTRODUCTION

Novell’s IPX protocol has been the foundation of many networks for years. IPX is small and fast. It has delivered much data since its introduction. Novell introduced IPX in the 1980s. IPX was based on networking technology developed at Xerox.

Novell has recognized that the world is becoming an IP-based world.

In release 5.0 of Novell’s networking operating system, they have included IP as a native protocol. IPX is still available for compatibility with existing systems.

Although Novell is moving toward IP-based networks, IPX is installed in many organizations and will be around for many more years.

F I G U R E 4 . 1

The layers of the IPX protocol and OSI model.

Novell IPX/SPX P

ROTOCOL

Like other protocols, Novell’s IPX has portions of the protocol that fit into the OSI layers (see Figure 4.1).

3

2

1

5

4

7

6

OSI Reference Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

NetWare

Applications

NetBIOS emulator

NetWare shell

(client)

NetWare

Core

Protocol

(NCP)

RPCbased application

RPC

LU 6.2

support

SPX

IPX

Ethernet/

IEEE

802.3

Token

Ring/

IEEE

802.5

FDDI ARCnet PPP

IPX includes functions at layers three and four of the OSI model.

Thus it is responsible for determining the routing of network traffic.

Novell’s implementation includes two routing protocols: RIP and

NLSP.

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Internet Packet eXchange (IPX)

Internet Packet eXchange (IPX) is the Layer 3 protocol in the Novell

suite. It is a connectionless protocol like TCP/IP’s IP protocol.

There are no acknowledgments and no guarantees of data delivery.

Like TCP’s ports, IPX uses sockets to communicate with upper-layer protocols and applications. An IPX network address consists of two parts: a network number and a node number. The network administrator assigns the IPX network number. The network number is up to eight hexadecimal digits (32 bits, or 4 bytes) long. The node number is usually the MAC address for the network interface and is twelve hexadecimal digits (48 bits, or 6 bytes) in length. When an

IPX network address is displayed in a Cisco router, it is displayed with the network number first, a period, and then the MAC address with periods every two bytes. 3485BA61.0000.0000.0001 is an example of an IPX address. This example address is actually a server.

When the network number has leading zeros, the zeros are not displayed. In the case of network 00000121, a network address for the server would be displayed as 121.0000.0000.0001.

Sequenced Packet eXchange (SPX)

Novell’s implementation includes a connection-oriented protocol,

Sequenced Packet eXchange (SPX). SPX functions similar to TCP/IP’s

TCP protocol. SPX provides a virtual circuit or connection between network nodes and guarantees delivery of data.

NetWare Core Protocol (NCP)

NetWare Core Protocol (NCP) gives client nodes access to server

resources such as printing, file access, and security.

Routing Information Protocol (RIP)

Routing Information Protocol (RIP) is a distance-vectoring routing

protocol used to route IPX traffic in internetworks. IPX RIP uses ticks and hop count to determine the best path for network traffic to travel in an internetwork. It should be noted that IPX RIP functions similarly to IP’s RIP, but there are some differences. (See Chapter 6,

“Routing,” for more information.)

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NetWare Link Services Protocol (NLSP)

NetWare Link Services Protocol (NLSP) provides an advanced link

state routing protocol to the Novell suite. Novell developed it as a replacement for both SAP and RIP. NLSP was included in release

3.12 and 4.x of NetWare.

Novell realized that IPX RIP had limitations and developed NLSP as its replacement. NLSP works similar to IP’s Open Shortest Path First

(OSPF) routing protocol. NLSP offers many advantages over IPX

RIP. NLSP is more efficient, scalable, and offers better routing.

NLSP is more efficient because it only sends updates when a topology change occurs or every two hours, whichever comes first. This is in contrast to RIP sending the entire routing table every 60 seconds whether or not the topology has changed. NLSP also sends serviceinformation updates only when a service status has changed. This is in contrast to SAPs being sent every 60 seconds whether or not the service status has changed. NLSP is scalable, supporting up to 127 hops. This is in contrast to 15 hops for RIP.

NLSP also offers load balancing of traffic over parallel paths. NLSP periodically checks network paths for integrity. If a path is found unusable, NLSP selects an alternative link and updates the network topology.

Service Advertising Protocol (SAP)

Novell has an interesting way of advertising which services are available. It uses Service Advertising Protocol (SAP) to broadcast which services are available from which servers. SAPs present a great challenge to internetworking engineers. Each type of service uses a SAP identifier. File servers use SAP=4 and print servers use SAP=7. By default,

SAP updates are sent every 60 seconds.

Routers listen to the SAP broadcasts and build a table of available services on the network the service is available on. When a client requests a network service, the router responds with the network address the requested service is on. The client then contacts the service directly.

In release 4.x of NetWare, Novell introduced Novell Directory

Services (NDS). NDS is a distributed directory service that reduces

the need for SAPs. SAPs are still used by NetWare 4.x clients when they first boot to discover NDS servers.

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Each of these protocols serves a purpose on the network. As an internetworking engineer, it’s your job to understand how the protocols function so that when there is a connectivity problem, you are able to determine why the problem is occurring and find a resolution.

IPX and SPX generally carry the data around the network for applications. When a client is connected to a server, it can use either protocol during a conversation depending on what transactions are occurring between the server and workstation.

SAPs are broadcasts that file servers, access servers, print servers, and other devices with services to advertise use. Cisco routers configured for IPX routing will listen for these SAPs and build a table of the services being advertised. In an organization with many servers and offices connected through a WAN, this can create a large SAP table in the routers. Most of the time services such as print servers are used only on the local area network and do not need to be broadcast across WAN links where bandwidth is a premium. By using access lists to limit which SAPs are placed in the SAP table, you can reduce the impact of these SAPs on the WAN.

N

ETWORK

A

DDRESSES

The IPX network address consists of two parts: the network number and the node number. When an IPX address is displayed in a router, it takes the form of 4c.03cb.530c.1cd3, where 4c is the network number and 03cb.530c.1cd3 is the node number. Novell’s IPX protocols have an interesting way of mapping the network address to the MAC address. The MAC address is actually used to derive the network address. Given the previous example, the node number 03cb.530c.1cd3

is also the MAC address of the node. Therefore it is simple for the IPX network to determine for which node the packet is destined.

M

ULTIPLE

E

NCAPSULATIONS

Novell’s NetWare protocols support multiple protocol encapsulations. This has been a source of headaches for network administrators for quite some time. Often times, a new networking station would not be able to communicate with the server. After hours of investigation, it would be discovered that the encapsulation setting of the networking station was different than that of the server.

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Remembering the Encapsulation

Types It can be quite challenging to remember the encapsulation types and what their corresponding Cisco types are. The following associations might make it somewhat easier.

SNAP is a “snap” for Ethernet or token ring.

SAP is short, and 802.2 is less than 802.3 and token ring, too.

Novell_Ether is longer, and 802.3 is greater than 802.2.

ARPA doesn’t really fit, but then neither does Ethernet_II.

To add even more confusion, Novell changed the default encapsulation method used starting with NetWare Version 3.12.

Now that you will be connecting Novell networks to Cisco routers,

Cisco has made this source of headaches even worse by coming up with its own names for the encapsulation methods. You not only need to remember the various methods of encapsulation Novell uses, but now, when you connect these networks to a Cisco router, you must also know what Cisco calls the different methods of encapsulation.

á Novell_Ether encapsulation. This is the equivalent of Novell’s

Ethernet_802.3. This frame format includes the IEEE 802.3

length field, but not an IEEE 802.2 Logical Link Control

(LLC) header. This encapsulation was the default through

NetWare 3.11.

á SAP encapsulation. This is the equivalent of Novell’s

Ethernet_802.2. This frame format is the standard IEEE frame format including an 802.2 LLC header. This encapsulation method became the default with the release of NetWare 3.12

and 4.x. For token ring use, SAP also equates to Novell’s

Token_Ring encapsulation.

á

ARPA encapsulation. This is the equivalent of Novell’s

Ethernet_II or Ethernet version 2. It uses the standard

Ethernet 2 header.

á SNAP encapsulation. This is the equivalent of Novell’s

Ethernet_SNAP or SNAP. It extends the IEEE 802.2 header by adding a SubNetwork Access Protocol (SNAP) header. This header provides an encapsulation type code similar to that of

Ethernet version 2. Cisco’s SNAP also equates to Novell’s

Token_Ring_SNAP.

A W

ARNING ABOUT

B

RIDGING

Cisco routers support Concurrent Routing and Bridging (CRB). This is a feature by which the router can route routable protocols like IP and IPX on an interface and, at the same time, bridge non-routable protocols like SNA and NetBEUI. The danger here is that when bridging of any type is configured on an interface, it will bridge all non-routed protocols—including routable protocols that are not currently configured for routing.

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To configure an interface for routing is not complex. Simply assign the interface an address for the routable protocol. Assigning an IP address to an interface configures the interface for IP routing. Assigning an

IPX address to an interface configures it for IPX routing.

When an interface has any bridging configured, whether it is Source

Route Bridging (SRB) or Data Link Switching (DLSw), the interface

will bridge any traffic that is not routable.

The danger occurs when an interface has bridging configured and an

IP address, but no IPX network, configured. All IPX traffic will be bridged with the other bridged networks. You will know this is happening when system administrators call to complain that the Novell servers are beeping constantly and giving the warning message of a misconfigured router.

What is happening is that the IPX traffic from one network is bridged to another network without the network portion of the address being adjusted for the new network. The server sees the network address with the incorrect IPX network number and gives warning messages.

Be careful when configuring bridging on interfaces. Make sure you consider what traffic, other than the intended traffic, might end up on the destination networks.

C

ONFIGURING

IPX R

OUTING

Configuring a Cisco router for IPX routing involves multiple steps.

IPX routing is turned off by default in Cisco’s IOS. This is in contrast to TCP/IP routing, which is on by default.

The first step is to enable IPX routing on the router. This is done with the global configuration command ipx routing

.

Router#config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#ipx routing

Router(config)#exit

Router#sh run

Building configuration...

Current configuration:

!

version 11.2

no service password-encryption service udp-small-servers

continues

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continued

service tcp-small-servers

!

hostname Router

!

!

ipx routing 0010.7b3b.d0a9

!

interface Ethernet0 ip address 199.250.137.59 255.255.255.224

!

interface Serial0 no ip address encapsulation frame-relay

!

no keepalive interface Serial1 no ip address

Notice that after entering the ipx routing statement, a show run was executed on the router. In the configuration, the command ipx routing 0010.7b3b.d0a9

is shown. The MAC address after the ipx routing statement is necessary for the IPX protocol. As you remember, IPX uses the MAC address as part of the network address.

Therefore, the router needs a MAC address to use as its node address on any IPX networks it routes. You can specify a MAC address to use in the IPX routing statement by simply entering the MAC address you wish to use after the ipx routing command. If you do not specify the MAC, the router uses the MAC address of the first

Ethernet, token ring, or FDDI interface. If the router does not have any of these interfaces, then you must specify a MAC address for

IPX routing to use.

The next step in configuring IPX routing is to set the IPX network number on each of the interfaces routing IPX traffic. The following example shows the network 3c being placed on the interface

Ethernet0 and the network 5c being placed on the interface Serial0.

Router#config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#int Serial0

Router(config-if)#ipx network 3c

Router(config-if)#exit

Router(config)#int ethernet1

Router(config-if)#ipx network 5c

Router(config-if)#exit

Router(config)#exit

Router#

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When configuring IPX routing over WAN links, the common interfaces of the WAN routers need to have a unique IPX network number just as LAN interfaces do. The WAN interfaces use the encapsulation type of the interface.

Now that IPX routing has been enabled and the network numbers have been assigned to the interfaces, you must ensure that the encapsulation types are correct. If you do not define an encapsulation type, the router uses the default for the media type. The following list shows the defaults as of IOS version 11.2:

á

Ethernet. The default is Novell_Ether, known as

Ethernet_802.3. This is the default encapsulation type for Novell through NetWare 3.11. Starting with Novell 3.12 the default encapsulation for Novell servers changed to Ethernet_802.2.

á Token Ring. The default is SAP, or Token-Ring to a Novell server.

á FDDI. The default is SNAP or FDDI_SNAP to a Novell server.

To specify the encapsulation type, add the encapsulation keyword to the ipx network configuration command. The following example shows the encapsulation types added to the configuration:

Router#config terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#int ethernet0

Router(config-if)#ipx network 3c encapsulation sap

Router(config-if)#exit

Router(config)#int ethernet1

Router(config-if)#ipx network 5c encapsulation arpa

Router(config-if)#exit

Router(config)#exit

Router#

In the previous example, the encapsulation types for each of the interfaces are different. The router will exchange packets correctly from one network to the next, making the necessary changes to the packet format for each network.

In some environments, multiple Novell networks are present on one

LAN; however, the different logical networks must be using different encapsulation types. To configure for this in a Cisco router requires the use of subinterfaces. For an interface, only one IPX network can be defined.

Check the Version Defaults for certain items change from revision to revision of IOS, so be sure to check the defaults for the version of IOS you are running. This is true of all defaults, not just IPX encapsulation types. Many problems occur when the version of IOS is upgraded in a router and certain settings are not specified in a router’s configuration. A new version of IOS may use different defaults for the setting and something that used to work no longer works.

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R E V I E W B R E A K

However, if multiple encapsulation types are used on one network, the different encapsulation types can be configured using subinterfaces.

You can create subinterfaces on a LAN interface and then assign a different encapsulation type to each subinterface and a different network number to each. Each subinterface must have a different encapsulation type and IPX network number. Once configured properly, the router will route traffic between the two different logical networks. Following is part of an IPX configuration that shows different IPX networks on the subinterfaces of an Ethernet port: hostname Router

!

!

ipx routing 0010.7b3b.d0a9

!

interface Ethernet0 ip address 192.168.33.1 255.255.255.0

!

interface Ethernet0.1

arp timeout 0 ipx network 3C encapsulation SAP

!

interface Ethernet0.2

arp timeout 0 ipx network 54 encapsulation ARPA

!

interface Serial0 no ip address

Router#

Configuring Novell Routing

As you can see, configuring Novell routing is not terribly complicated.

There are several things you need to remember when configuring IPX routing on a Cisco router:

.

IPX routing is not active in a default Cisco router configuration. The first step to setting up IPX routing is to turn on IPX routing with the global command ipx routing [mac-address]

.

The MAC address can be specified or not. If it is not specified, the MAC address of the first Ethernet, token ring, or FDDI interface is used. If the router does not have any of these types of interfaces, you must specify the MAC address to use.

.

The IPX network number must be placed on the interfaces attached to which Novell networks you wish to route data.

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.

Within the Novell protocols, there are multiple encapsulation types. To properly route IPX traffic, the encapsulation type used by the router must match the encapsulation type used by the Novell network. This is an area that can produce problems.

The names of the encapsulation types used by Cisco do not match what Novell servers use.

After these settings are properly configured, the router should begin discovering Novell services by listening for SAPs on the network.

The router will begin building a table of the services it detects.

Because many of these services are important only to the local network and do not need to be advertised or transmitted to remote networks, it is useful to limit through the use of Access Lists which of these services will be added to the table.

N

OVELL

A

CCESS

L

ISTS

In the Novell environment, it is often useful to control which networks can communicate with others.

Novell access lists fall into four categories: standard, extended, SAP filtering, and NLSP route aggregation filtering.

When using IPX access lists, it is sometimes necessary to indicate all

IPX networks. This is represented in an access list as f f f f f f f f.

Another way to represent all networks is by using the number –1.

Standard IPX Access Lists

Standard IPX access lists are numbered 800–899. In a standard IPX

access list, you can permit or deny traffic based on source address and destination address.

A standard access list can be used to limit which networks can communicate with others. Given the following example shown in Figure

4.2, you want to create an access list that will allow only networks 5c and 3b to communicate with network 21d. However, you do not want 3b to be able to communicate with 5c.

NLSP Filters Not on Exam For the

CCNA exam, you will need to understand the standard, extended, and

SAP filtering access lists. NLSP route aggregation filtering is not covered in this book.

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158 Pa r t I E X A M P R E PA R AT I O N

F I G U R E 4 . 2

Novell networks connected via a router.

IPX net 3b

E0

E1

E2 IPX net 21d

Novell

Server

Router

IPX net 5c

Novell

Server

Novell

Server

To create an access list to accomplish this type of traffic control, you must first create an access list for interface Ethernet2 that allows traffic from networks 3b and 5c. Following is an example of how to create a standard IPX access list for this purpose:

Router#config t

Router(config)#access-list 800 permit 3c 21d

Router(config)#access-list 800 permit 5c 21d

Router(config)#exit

Router#

Now that the access list 800 is created, you need to apply it to the interface Ethernet2. The default mode for an access list is to effect outgoing traffic on the interface. Thus this access list will allow IPX traffic from only networks 3c and 5c to exit interface Ethernet2:

Router#config t

Router#(config)#interface Ethernet2

Router(config-if)#ipx access-group 800 out

Router(config-if)#exit

Router(config)#exit

Router#

For interface Ethernet0 and Ethernet1, you want to limit traffic to just IPX network 21d. This is accomplished in much the same way. Following is an example that creates two additional access lists,

801 and 802, that permit traffic only from 21d to the respective networks:

Router#config t

Router(config)#access-list 801 permit 21d 5c

Router(config)#access-list 802 permit 21d 3b

Router(config)#exit

Router#

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Each of these new access lists must be applied to the respective interfaces. Following is an example of how this is accomplished:

Router#config t

Router#(config)#interface Ethernet0

Router(config-if)#ipx access-group 801 out

Router(config-if)#exit

Router(config)interface Ethernet1

Router(config-if)#ipx access-group 802 out

Router(config)#exit

Router#

With these access lists in place, only traffic from network 21d is permitted to exit interfaces Ethernet0 and Ethernet1. Traffic from both networks 5c and 3b are allowed to exit interface Ethernet2. The goal is accomplished.

Extended IPX Access Lists

Extended IPX access lists are numbered 900-999. In an extended

access list, you can permit or deny traffic based on source address, destination address, protocol, source socket, and destination socket.

You can also log when access list violations occur.

SAP Filtering Access Lists

SAP filtering access lists allow you to filter the SAPs exchanged with

other routers. This is very useful because many IPX networks include elements such as network printers that send SAP broadcasts. Generally you do not need these SAPs to be broadcast across WAN links to remote sites. You can filter these SAPs by using a SAP access list. For example, if you wanted to allow only server SAPs from all networks to be broadcast over a WAN link, you could use an access list as follows:

Router(config)#access-list 1000 permit ffffffff 4

Router(config)#

The global command access-list creates the access list. You have assigned this access list 1000, which indicates that it is a SAP access list. The network ffffffff indicates all networks. You could also have used -1 here and had the same result. The number 4 indicates the

SAP type to permit. After the access list is created, it must be applied to the WAN link as follows. SAP access lists can be applied to either outgoing SAPs or incoming SAPs. When you apply a

SAP access list to outgoing SAPs, you are limiting the SAPs sent from the router’s table out of the interface the access list is applied.

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When the access list is applied to incoming SAPs, you are limiting the SAPs the router will add to its table from the interface the access list is applied. The important difference here is that if the SAPs you are not allowing are important only to the local segment, an incoming SAP filter will reduce the size of the router’s SAP table, thus requiring less router memory and processing overhead.

Router#config t

Router(config)#interface serial0

Router(config-if)#ipx output-sap-filter 1000

Router(config-if)#

This access list will allow only server SAPs to be broadcast through the interface Serial0. Table 4.1 lists the more common SAP types used in IPX networks.

4C

7A

98

2D

2E

47

4B

9A

TA B L E 4 . 1

C

O M M O N

S A P

S

U

S E D I N

I P X N

E T W O R K S

9

A

21

5

7

3

4

1

2

Service Type

(Hexadecimal) Description

User

User group

Print server queue

File server

Job server

Print server

Archive server

Queue for job servers

Network Application Support Systems Network Architecture

(NAS SNA) gateway

Time Synchronization value-added process (VAP)

Dynamic SAP

Advertising print server

Btrieve VAP 5.0

SQL VAP

TES—NetWare for Virtual Memory System (VMS)

NetWare access server

Named Pipes server

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Service Type

(Hexadecimal)

9E

107

111

166

26A

Description

Portable NetWare—UNIX

RCONSOLE

Test server

NetWare management (Novell’s Network Management Station

[NMS])

NetWare management (NMS console)

D

IAGNOSING

IPX P

ROBLEMS

When diagnosing an IPX problem, you will find that it is useful to view what SAPs a router is seeing, and from what networks and interfaces they are being seen. There are several IOS commands that help in finding out where in the network the failure is occurring.

Displaying IPX Addresses

One of the first things to do when diagnosing an IPX problem is to check the IPX network numbers assigned to a router’s interfaces.

The command show ipx interface displays IPX-related information about the router’s interfaces. The IPX network number is displayed in full, including the node number. The command can be used without an interface description and will then display information for each interface in the router with an IPX address assigned. Following is an example output from the show ipx interface command:

Router#sh ipx interface

FastEthernet0/0 is up, line protocol is up

IPX address is 11010.00e0.a38d.0800, SAP [up] line-up,

➥RIPPQ: 0, SAPPQ: 0

Delay of this IPX network, in ticks is 1 throughput 0 link

➥delay 0

IPXWAN processing not enabled on this interface.

IPX SAP update interval is 1 minute(s)

IPX type 20 propagation packet forwarding is disabled

Incoming access list is not set

Outgoing access list is not set

IPX helper access list is not set

continues

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continued

SAP GNS processing enabled, delay 0 ms, output filter list

➥is not set

SAP Input filter list is not set

SAP Output filter list is not set

SAP Router filter list is not set

Input filter list is not set

Output filter list is not set

Router filter list is not set

Netbios Input host access list is not set

Netbios Input bytes access list is not set

Netbios Output host access list is not set

Netbios Output bytes access list is not set

Updates each 60 seconds, aging multiples RIP: 3 SAP: 3

SAP interpacket delay is 55 ms, maximum size is 480 bytes

RIP interpacket delay is 55 ms, maximum size is 432 bytes

IPX accounting is disabled

IPX fast switching is configured (enabled)

RIP packets received 9229358, RIP packets sent 846238

SAP packets received 35260817, SAP packets sent 1498927

Token Ring1/0 is administratively down, line protocol is

➥down

IPX address is 88888888.0007.c5b1.1010, SAP [up] line-

➥down, RIPPQ: 0, SAPPQ: 0

Delay of this IPX network, in ticks is 1 throughput 0 link

➥delay 0

IPXWAN processing not enabled on this interface.

IPX SAP update interval is 1 minute(s)

IPX type 20 propagation packet forwarding is disabled

Incoming access list is not set

Outgoing access list is not set

IPX helper access list is not set

SAP GNS processing enabled, delay 0 ms, output filter list

➥is not set

SAP Input filter list is not set

SAP Output filter list is not set

SAP Router filter list is not set

Input filter list is not set

Output filter list is not set

Router filter list is not set

Netbios Input host access list is not set

Netbios Input bytes access list is not set

Netbios Output host access list is not set

Netbios Output bytes access list is not set

Updates each 60 seconds, aging multiples RIP: 3 SAP: 3

SAP interpacket delay is 55 ms, maximum size is 480 bytes

RIP interpacket delay is 55 ms, maximum size is 432 bytes

IPX accounting is disabled

IPX fast switching is configured (enabled)

RIP packets received 0, RIP packets sent 0

SAP packets received 0, SAP packets sent 0

Router#

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If you are diagnosing a problem specific to one interface in the router, you can limit the information shown to just that interface.

The interface about which you are seeking information can also be specified and then only IPX information about that interface will be displayed. Following is an example of how to specify the interface about which you wish to see IPX information.

Router#sh ipx interface token ring1/1

TokenRing1/1 is up, line protocol is up

IPX address is 33333333.0007.c5b1.1090, SAP [up] line-up,

➥RIPPQ: 0, SAPPQ: 0

Delay of this IPX network, in ticks is 1 throughput 0 link

➥delay 0

IPXWAN processing not enabled on this interface.

IPX SAP update interval is 1 minute(s)

IPX type 20 propagation packet forwarding is disabled

Incoming access list is not set

Outgoing access list is not set

IPX helper access list is not set

SAP GNS processing enabled, delay 0 ms, output filter list

➥is not set

SAP Input filter list is not set

SAP Output filter list is not set

SAP Router filter list is not set

Input filter list is not set

Output filter list is not set

Router filter list is not set

Netbios Input host access list is not set

Netbios Input bytes access list is not set

Netbios Output host access list is not set

Netbios Output bytes access list is not set

Updates each 60 seconds, aging multiples RIP: 3 SAP: 3

SAP interpacket delay is 55 ms, maximum size is 480 bytes

RIP interpacket delay is 55 ms, maximum size is 432 bytes

IPX accounting is disabled

IPX fast switching is configured (enabled)

RIP packets received 136, RIP packets sent 246974

SAP packets received 101, SAP packets sent 7493686

Router#

If you want to see a brief description of the IPX information for the interfaces in a router, you can use the variation of the previous command, show ipx interface brief

. This command simply lists each interface in the router, the IPX network number assigned, the encapsulation type, the interface status, and the IPX state. Following is an example of the output from the show ipx interface brief command:

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Router#sh ipx interface brief

Interface IPX Network Encapsulation Status IPX State

FastEthernet0/0 11010 SAP up [up]

TokenRing1/0 88888888 SAP administratively down [up]

TokenRing1/1 33333333 SAP up [up]

TokenRing1/2 11111111 SAP up [up]

TokenRing1/3 unassigned not config’d administratively down n/a

Ethernet2/0 011019 NOVELL_ETHER administratively down [up]

Ethernet2/1 unassigned not config’d up n/a

Ethernet2/2 unassigned not config’d administratively down n/a

Ethernet2/3 unassigned not config’d administratively down n/a

Serial5/0 unassigned not config’d up n/a

Serial5/0.1 unassigned not config’d up n/a

Serial5/0.2 112233 FRAME-RELAY-I up [up]

Serial5/0.3 unassigned not config’d up n/a

Serial5/0.4 unassigned not config’d up n/a

Serial5/0.5 unassigned not config’d up n/a

Serial5/0.6 unassigned not config’d up n/a

Serial5/0.7 unassigned not config’d up n/a

Serial5/0.8 1174 FRAME-RELAY-I up [up]

Serial5/0.9 unassigned not config’d up n/a

Serial5/0.10 unassigned not config’d up n/a

Serial5/0.11 unassigned not config’d up n/a

Serial5/0.12 unassigned not config’d up n/a

Serial5/0.122 unassigned not config’d up n/a

Serial5/0.123 unassigned not config’d up n/a

Serial5/0.133 unassigned not config’d up n/a

Serial5/0.138 unassigned not config’d down n/a

Serial5/0.146 unassigned not config’d up n/a

Serial5/0.147 unassigned not config’d up n/a

Serial5/0.148 unassigned not config’d up n/a

Serial5/0.149 unassigned not config’d up n/a

Serial5/0.150 unassigned not config’d up n/a

Serial5/1 1011088F HDLC up [up]

Serial5/2 unassigned not config’d administratively down n/a

Serial5/3 unassigned not config’d administratively down n/a

Loopback1 unassigned not config’d up n/a

Router#

Displaying Servers

The command show ipx servers is a very useful command when diagnosing an IPX problem. When the router receives SAP broadcasts from servers, it places them in its server table. When two routers are exchanging IPX information, the server tables are exchanged so that each then knows about the servers available through the other router.

For the router to be able to route traffic to a server, it must first see that server’s SAP broadcasts. When the router receives SAP broadcasts for a server, certain information from that SAP is placed in the router’s SAP table. The command show ipx servers allows you to view this table.

The following is an example of the output of this command:

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Router#sh ipx ser

Codes: S - Static, P - Periodic, E - EIGRP, N - NLSP, H -

Holddown, + = detail

221 Total IPX Servers

Table ordering is based on routing and server info

Type Name Net Address Port Route Hops Itf

P 4 RESEARCH A5.0000.0000.0001:0451 2185984/00 1 Fa0/0

P 4 BENEFIT 36B08649.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 EPI A6.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 FINANCE 3634E2C8.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 NACSS01 351F5D27.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 NAPPD01 36B71C16.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 NATIONAL B3.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 NHOFSE01 34197AED.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 NHO_CSS_5 ADF0FCF.0000.0000.0001:0451 2/01 1 Fa0/0

P 4 NOV1 12497.0000.0000.0001:0451 2/01 1 Fa0/0

E 4 APC_NOVELL 3485BA61.0000.0000.0001:0451 269824000/02 2 Se5/0.2

P 4 NAGOV01 3238529B.0000.0000.0001:0451 8/02 2 Se5/0.8

Router#

The table lists the SAP type, server name, network address and port, route to the network, hops to the network, and the interface on which the SAP was seen. Earlier you learned that the node address of an IPX networking station is the MAC address of the node. However, in the preceding table, the node addresses of the different Novell servers shows 0000.0000.0001. This is due to an operating characteristic of

Novell servers. A Novell server using the IPX protocol has two node addresses. The first is the MAC address of the node and the second is

0000.0000.0001, which represents an internal network number in the server.

It is not unlikely that you would need to search for a server name in a large table of servers. The command show ipx servers has several options to make this easier. You can sort the table by server name, network number, or SAP type. However, starting in Release 11.2,

Cisco added the option to search for a specific server using the option regexp

. The following example shows how the regexp option is used to find a server named NOV1:

Router#sh ipx server regexp NOV1

Codes: S - Static, P - Periodic, E - EIGRP, N - NLSP,

H - Holddown, + = detail

208 Total IPX Servers

continues

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continued

Table ordering is based on routing and server info

Type Name Net Address Port Route Hops Itf

P 4 NOV1 12497.0000.0000.0001:0451 8/02 2 Se0/0.1

P 107 NOV1 12497.0000.0000.0001:8104 8/02 3 Se0/0.1

Router#

Working with IPX Routing Tables

When a router is routing IPX traffic, it builds a routing table to determine the proper path to forward traffic. Although this topic is discussed in more detail in Chapter 6, “Routing,” a few commands specific to IPX are covered here.

To view the IPX routing table, use the command show ipx route

.

This will display all of the networks the router has determined paths for and certain information about those paths. Following is an example IPX routing table:

Router#sh ipx route

Codes: C - Connected primary network, c - Connected secondary network

S - Static, F - Floating static, L - Local (internal), W - IPXWAN

R - RIP, E - EIGRP, N - NLSP, X - External, A - Aggregate s - seconds, u - uses

34 Total IPX routes. Up to 1 parallel paths and 16 hops allowed.

No default route known.

C 1174 (FRAME-RELAY-IETF), Se5/0.8

C 11010 (SAP), Fa0/0

C 112233 (FRAME-RELAY-IETF), Se5/0.2

C 1011088F (HDLC), Se5/1

C 11111111 (SAP), To1/2

C 33333333 (SAP), To1/1

R 1 [02/01] via 11010.0060.0803.8c4d, 54s, Fa0/0

R 58 [02/01] via 11010.0080.2968.af0c, 21s, Fa0/0

E A3 [2185984/0] via 112233.4145.0000.0003, age 17:39:14,1267u, Se5/0.2

E A4 [2185984/0] via 112233.4145.0000.0003, age 3d04h,1311u, Se5/0.2

E A5 [2185984/0] via 112233.4145.0000.0003, age 3d04h,25742u, Se5/0.2

R A6 [02/01] via 11010.0060.979b.61a4, 54s, Fa0/0

R B3 [02/01] via 11010.0060.979b.6218, 37s, Fa0/0

E F0 [269824000/2] via 112233.4145.0000.0003, age 3d04h,317u, Se5/0.2

E FD [269824000/2] via 112233.4145.0000.0003, age 3d04h,317u, Se5/0.2

R 100 [07/01] via 1174.00e0.1e89.5841, 29s, Se5/0.8

R 101 [07/01] via 1174.00e0.1e89.5841, 29s, Se5/0.8

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R 444 [02/01] via 11010.0060.b01a.3678, 47s, Fa0/0

R FFFF [02/01] via 11010.0060.b067.b81d, 0s, Fa0/0

R 11011 [02/01] via 11010.0060.979b.6218, 37s, Fa0/0

R 12497 [02/01] via 11010.0080.2968.af0c, 22s, Fa0/0

R ADF0FCF [02/01] via 11010.0010.4bd2.0977, 38s, Fa0/0

R 1011088A [07/01] via 1011088F.00e0.1ea9.ce78, 48s, Se5/1

R 3238529B [08/02] via 1174.00e0.1e89.5841, 29s, Se5/0.8

R 34197AED [02/01] via 11010.0060.08a4.e896, 45s, Fa0/0

E 3485BA61 [269824000/2] via 112233.4145.0000.0003, age 17:38:43,1097u, Se5/0.2

R 351F5D27 [02/01] via 11010.0060.979b.6180, 16s, Fa0/0

R 3634E2C8 [02/01] via 11010.0060.083a.93be, 56s, Fa0/0

R 36B08649 [02/01] via 11010.0060.97d1.9a35, 43s, Fa0/0

R 36B71C16 [02/01] via 11010.0090.2728.3699, 1s, Fa0/0

R 75757657 [02/01] via 11010.00c0.490e.703d, 27s, Fa0/0

R A31B0596 [08/02] via 1011088F.00e0.1ea9.ce78, 50s, Se5/1

R A31B3AF5 [08/02] via 1011088F.00e0.1ea9.ce78, 50s, Se5/1

R B067B81D [21/01] via 11010.0060.b067.b81d, 2s, Fa0/0

Router#

The routing table lists each of the IPX networks in the table. The letter to the left of each network represents how the router learned of the network. Some networks are connected directly to this router, which are represented by a “C.” An “R” represents networks learned through the RIP routing protocol. Networks learned through the routing protocol EIGRP are represented by an “E.”

The routing table can sometimes become unstable because of network topology changes or software bugs. You can force the router to rebuild the routing table by clearing it. The command clear ipx route * will clear all routes in the table:

Router#clear ipx route *

Router#

You can clear a specific network route by specifying the route in the command, such as clear ipx route 3c

. This will cause the router to clear this route and relearn the route if possible.

Router#clear ipx route 3c

Router#

Load Sharing with IPX

Cisco routers routing IPX traffic can perform load sharing across equal cost paths. When two paths of equal cost or delay exist from one network to another, the router can distribute the data across those paths.

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This is called load sharing. Using load sharing can increase the usable bandwidth between the networks. To allow a Cisco router to make use of multiple equal cost paths, you must set the parameter ipx maximum-paths

. The default value is 1. You can allow a Cisco router to use up to 512 equal cost paths for IPX traffic, but a more realistic limit would be two or three paths. Following is an example of how to set the maximum number of paths the router will use for load sharing to 3:

Router(config)#ipx maximum-paths 3

Router(config)#

C

A S E

S

T U D Y

: A

C M E

P

A I N T

C

O M PA N Y

E S S E N C E O F T H E C A S E

.

You must configure the two Cisco routers for IPX networking and create an IPX link across the Wide Area Network.

.

The router at your office is named Acme and is connected to the accounting department on interface Ethernet1/2.

.

The router at the remote office is named

PPlace and is connected to the accounting department on interface Ethernet0/1.

.

The two routers are connected via a leased line. This line is connected to interface

Serial0/0 on each router. You must configure an IPX network on these interfaces also.

.

The routers are currently configured to route IP traffic from one office to the other.

.

The IPX network for the accounting department at Acme is 00001b0c and is using

802.3. The IPX network for the accounting department at Paint’n Place is 00002f43 and is using Ethernet_802.2 encapsulation.

S C E N A R I O

You are the network guru for Acme Paint

Company. Your company just purchased its largest competitor, Paint’n Place. During the merger of the two companies, you are responsible for ensuring that the accounting department of your company can access the financial data of the purchased company. Both your accounting department and the accounting department of

Paint’n Place use Novell Networks. As part of the merger, your company has installed a leased line between the two offices and you are responsible for the routers on each end.

Your office has a Cisco 7206 router named Acme with four Ethernet interfaces installed. Your accounting department has its own LAN and is connected to interface Ethernet1/2 of Acme.

Paint’n Place has a Cisco 3640 router named

PPlace with two Ethernet interfaces installed. The accounting department at Paint’n Place is connected to Ethernet0/1.

The two routers are connected using a point-topoint leased line connected at each end to

Serial0/0.

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A S E

S

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: A

C M E

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A I N T

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O M PA N Y

A N A LY S I S

You log in to the router at Paint’n Place and make the necessary changes:

PPlace(config)#interface Ethnernet0/1

PPlace(config-if)#ipx network 2f43 encapsulation sap

PPlace(conifg-if)#exit

PPlace(config)#interface serial0/0

PPlace(config-if)#ipx network 7700

PPlace(config-if)#exit

PPlace(config)#exit

PPlace#copy running-config startup-config

You then log in to the router at Acme and make the necessary changes:

Acme(config)#interface Ethernet1/2

Acme(config-if)#ipx network 1b0c encapsulation

Novell_Ether

Acme(config-if)#exit

Acme(config)#interface Serial0/0

Acme(config-if)#ipx network 7700

Acme(config-if)#exit

Acme(config)#exit

Acme#copy running-config startup-config

You have successfully configured the routers and find that the accounting department at Acme is now able to access the data stored on the server at Paint’n Place.

C

H A P T E R

S

U M M A R Y

This chapter covered the aspects of Novell Networking. You have learned to configure Novell networking on Cisco routers and have been exposed to some of the techniques for diagnosing problems.

Although Novell networking is relatively simple to implement on

Cisco routers, problems with Novell routing can prove to be elusive.

There is no better teacher than experience.

Unlike IP routing, IPX routing is not turned on by default in Cisco routers. For a Cisco router to route IPX traffic, IPX routing must first be enabled. This is done with the ipx routing command at the global configuration. Follow that by configuring the IPX network numbers for each segment on the interfaces connected to the segments. If an encapsulation type other than Cisco’s default is being used, make sure you configure the encapsulation also.

IPX access lists offer a means of enforcing security policies as well as reducing the overhead of excessive SAPs traveling over slow WAN links.

KEY TERMS

• IPX dncapsulation

• IPX network

• Internet Packet eXchange (IPX)

• Sequenced Packet eXchange (SPX)

• Service Advertising Protocol (SAP)

• Novell Directory Service (NDS)

• NetWare Core Protocol (NCP)

• NetWare Link Services Protocol

(NLSP)

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A

P P L Y

Y

O U R

K

N O W L E D G E

Review Questions

1. What is the purpose of the SAP protocol within an IPX network?

2. What must you do prior to configuring an IPX network on an interface of a router?

3. What is significant about the IPX address

5c.0000.0000.0001?

4. How can you configure multiple IPX encapsulation types on one interface of a router?

5. What does network f f f f f f f f represent?

6. How could you determine if a router is receiving

SAPs for a particular server?

7. What does IPX load sharing do?

8. List some of the useful information displayed with the command show ipx interface

.

9. What are some of the advantages of NLSP over

RIP?

10. On which protocol was IPX based?

Exam Questions

1. Describe the parts of this IPX address:

76c.03cd.feed.beef

A. 76c is the network and 03cd.feed.beef is the node address.

B. 76c.03cd is the network and feed.beef is the node address.

C. You need a subnet mask to determine the network portion of the address.

D. 76c is the node address and 03cd.feed.beef is the network.

2. Which of the following encapsulation types are properly matched? Select two.

A. Ethernet_802.3 - SAP

B. Ethernet_802.2 - Novell_Ether

C. Ethernet_II - ARPA

D. TokenRing_SNAP - SNAP

3. What is the affect of the following partial configuration?

Ethernet0

Ipx network 7d

Ipx access-group 800

Access-list 800 permit 55fd

A. Any network including network 55fd will be allowed to talk to network 7d.

B. Only network 55fd will be allowed to talk to network 7d.

C. Only network 800 will be allowed to talk to network 7d.

D. No network will be able to talk to 7d because the access list is improper.

4. What is the proper configuration for IPX network 1fd using Ethernet_II encapsulation?

A.

Router(config-if)#ipx network 1fd encapsulation Ethernet_II

B.

Router(config-if)#ipx network 1fd encapsulation ARPA

C.

Router(config)#ipx network 1fd encapsulation Ethernet_II

D.

Router(config)#ipx network 1fd encapsulation ARPA

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P P L Y

Y

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5. What command limits how many equal-cost parallel paths will be used to load share?

A.

ipx load-sharing-paths

B.

ipx maximum-paths

C.

ipx equal-costs

D.

ipx parallel-paths

6. Which of the following commands will display the IPX address of an interface? Select two.

A.

show interface

B.

show ipx interface

C.

show ipx interface brief

D.

show interface ipx

7. Which command will fully clear the router’s IPX routing table?

A.

Router#clear ipx route

B.

Router#clear ipx route *

C.

Router#clear ipx route ffffffff

D.

Router#clear ipx routing table

8. Which of the following statements are true of the IPX protocol? Select two.

A. IPX is a connection-oriented protocol.

B. IPX uses subnet masks to differentiate the network from the node.

C. IPX uses the MAC address of the network station as the node address in the network address.

D. IPX is a connectionless protocol.

Answers to Review Questions

1. SAP broadcasts announce the availability of a service on the network. For more information, see

“Service Advertising Protocol (SAP).”

2. By default IPX routing is not on. It must be turned on with the global configuration command ipx routing

. For more information, see

“Configuring IPX Routing.”

3. The IPX address 5c.0000.0000.0001 is a server’s internal network address. For more information, see “Network Addresses.”

4. By using subinterfaces, you can configure an interface with multiple IPX encapsulation types.

For more information, see “Configuring IPX

Routing.”

5. f f f f f f f f is a wildcard address representing all IPX networks. For more information, see

“Novell Access Lists.”

6. The command show ipx servers shows a list of services including servers for which the router has received SAPs. For more information, see

“Diagnosing IPX Problems.”

7. IPX load sharing allows a router to use equal-cost parallel paths to increase the bandwidth between networks. For more information, see

“Configuring IPX Routing.”

8. The command show ipx interface displays a great deal of useful information, including IPX address, IPX SAP update interval, input and output access list status, IPX accounting status,

RIP packets received and sent, and SAP packets received and sent. For more information, see

“Diagnosing IPX Problems.”

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A

P P L Y

Y

O U R

K

N O W L E D G E

9. NLSP is a link state routing protocol that is scalable. It reduces bandwidth overhead by sending only topology changes rather than entire routing tables. For more information, see “NetWare Link

Services Protocol (NLSP).”

10. IPX is based on XNS. XNS was developed by

Xerox. For more information, see “Novell

IPX/SPX Protocol.”

Answers to Exam Questions

1. A. The network portion of an IPX address is the first group of hexadecimal digits. The rest of the address is the node address. For more information, see “Network Addresses.”

2. C, D. The first two answers are reversed.

For more information, see “Multiple

Encapsulations.”

3. B. Only network 55fd will be allowed to talk to network 7d. For more information, see

“Novell Access Lists.”

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration, Cisco Press, 1998.

4. B. B represents the proper configuration statement. For more information, see “Configuring

IPX Routing.”

5. B. The command ipx maximum-paths limits how many equal-cost parallel paths will be used for load sharing of IPX traffic. For more information, see “Configuring IPX Routing.”

6. B, C. show ipx interface and show ipx interface brief will display the IPX address of interfaces in the router. For more information, see “Diagnosing IPX Problems.”

7. B. clear ipx route * is the proper syntax to clear the entire IPX routing table. The

* indicates all routes. For more information, see “Working with IPX Routing Tables.”

8. C, D. IPX does use the MAC address as the node address and IPX is a connectionless protocol. For more information, see “Novell IPX/SPX

Protocol.”

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O

B J E C T I V E S

Cisco provides the following objectives for the WAN

Protocols portion of the CCNA exam:

.

Differentiate between the following WAN services: Frame Relay, ISDN/LAPD, HDLC, and

PPP.

To be an internetworking professional, you must be able to differentiate between these services.

.

Recognize key Frame Relay terms and features.

Because Frame Relay is a very popular WAN technology, you must be well-versed in the terms and features of this very flexible technology.

List commands to configure Frame Relay LMIs, maps, and subinterfaces.

.

To be able to maintain Frame Relay systems, you must be familiar with the commands used to configure its features.

.

List commands to monitor Frame Relay operation of the router.

To diagnose problems with Frame Relay systems, you must be familiar with the monitoring commands.

.

Identify PPP operations to encapsulate WAN data on Cisco routers.

As an internetworking professional, you must be familiar with PPP operations and technologies.

C H A P T E R

5

WAN Protocols

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O

B J E C T I V E S

( c o n t i n u e d

)

O

U T L I N E

State a relevant use and context for ISDN networking.

.

As an internetworking professional, you must know when to use such technologies as ISDN.

Identify ISDN protocols, function groups, reference points, and channels.

.

ISDN is another popular WAN technology. You must be able to identify its protocols, function groups, reference points, and channels.

.

Describe Cisco’s implementation of ISDN BRI.

You must be able to relate ISDN technologies to

Cisco’s implementation.

Introduction 176

WAN Terms

Central Office

Plain Old Telephone Service (POTS)

Local Loop

Demarcation Point

Customer Premises Equipment (CPE)

T1 Service

Channel Service Unit/Data Service Unit

(CSU/DSU)

Integrated Services Digital Network

(ISDN)

ISDN Layer 2 Protocols (LAPD)

ISDN Layer 3 Protocols

Network Termination

ISDN Reference Points

Basic Rate Interface (BRI)

Understanding SPIDs

Configuring BRI ISDN

Monitoring ISDN

Primary Rate Interface (PRI)

ISDN Review

Remote Access Server (RAS)

176

176

177

177

177

178

178

179

180

180

180

180

181

181

182

182

184

185

185

186

How Routers Communicate via

Serial Links

Synchronous Data-Link Control (SDLC)

High-Level Data-Link Control (HDLC)

Point-to-Point Protocol (PPP)

186

186

187

188

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O

U T L I N E

( c o n t i n u e d

)

S

T U D Y

S

T R AT E G I E S

Frame Relay

Frame Relay Cloud

Committed Information Rate (CIR)

Data-Link Connection Identifier (DLCI)

Permanent Virtual Circuit (PVC)

Switched Virtual Circuit (SVC)

Local Management Interface (LMI)

Configuring Frame Relay

Configuring Multipoint Frame Relay

Configuring Point-to-Point Frame Relay

Separating Logical Connections with

Subinterfaces

Monitoring Frame Relay

Monitoring the Serial Port of a

Frame Relay Connection

Seeing Active PVCs

Viewing the Frame Relay Map

Frame Relay Review

189

195

196

197

197

198

198

199

201

203

190

191

191

194

194

195

WAN Technologies—When to Use What 204

.

Wide Area Networking protocols, configuration, troubleshooting, and design are the areas in which most internetworking professionals spend the majority of their time. Most companies have some sort of connection between offices or a connection to the Internet. As an internetworking professional, you will be expected to be knowledgeable about these technologies. You must know not only how to configure connections and troubleshoot problems, but also when each type of connection should be used.

Become proficient and knowledgeable about the products offered by your communications company. Not all products are available everywhere in the United States. If your company is international, you will need to become proficient with the offerings of companies in other countries.

Knowing what is available and how cost effective your options are will make you more valuable to your company. You will be able to make informed recommendations and decisions about the best technologies to utilize when connecting your company. Spend time with a representative of your communication company to learn what they offer.

Chapter Summary 208

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I

NTRODUCTION

Wide Area Networks (WANs) require their own set of protocols for communication. Wide Area Networking generally defines connecting geographically separated locations together for network communication. The methods used to do this vary from normal dialup phone lines to high-speed fiber connects, even satellite communication can be used to build a WAN.

WANs are implemented in a variety of ways. Not too long ago, most companies connected remote offices using leased point-topoint lines. This was expensive and not an effective use of bandwidth. From this grew technologies for sharing bandwidth such as

Frame Relay. This chapter explains different WAN technologies and how they are implemented using Cisco routers.

WAN T

ERMS

When building or maintaining WANs, you must understand a few new terms and technologies. You will be interfacing with equipment and services from telephone companies. Following are some terms with descriptions.

Central Office

A Central Office (CO) is the local telephone office serving your location. The CO houses the switching equipment for Voice and Data circuits. Figure 5.1 shows how your office and remote offices are connected to COs and then to each other.

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CO

Remote office

Local loop CO

Your office

CO

Local loop

Remote office

Plain Old Telephone Service (POTS)

It’s amazing how something that never had a name needs a name when a new technology comes along. Plain Old Telephone Service

(POTS) is one of those things. POTS describes the normal tele-

phone service most people have installed in their homes. It’s a normal telephone line capable of connecting to a modem and providing connections to other telephone numbers. POTS never really had a name until newer technologies such as ISDN and ATM came about.

Local Loop

The local loop is the part of the telephone network that runs from your local CO to your office. It connects you to the telephone network where your data can then be placed on shared services like

Frame Relay. The local loop is commonly referred to as the “last mile.” Figure 5.1 shows where the local loop falls in the network.

Demarcation Point

Before the telephone companies were split from AT&T, the telephone company was responsible for everything connected to its network, including your telephone equipment. In fact, it was illegal to connect any of your own equipment to the telephone network. When AT&T was broken apart from the local Bell companies, this changed. Now you can purchase your own equipment and connect it to the network.

C h a p t e r 5 WA N P R OTO C O L S 177

F I G U R E 5 . 1

This figure illustrates the relationship between your office, the CO, and remote offices.

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With this change came the need for a point of demarcation that defined where the telephone company’s responsibility ended and the customer’s began. This means if there is a problem with wiring on the customer side of the demarcation point, the customer is responsible for repairing it. The customer can contract the local Bell company to perform this work, but at the customer’s expense. If a problem develops on the Bell side of the demarcation point, it is the Bell company’s responsibility to repair the problem at their expense.

The demarcation point is not always well defined and has become more vague than telephone companies would like. Generally speaking, the demarcation point is where the telephone company terminates its lines in your building. However, when ordering data lines, it is common to request an extended demarcation point. When requesting an extended demarcation point, you tell the telephone company where to terminate the line and they will extend your circuit to that point, generally at an additional cost.

Customer Premises Equipment (CPE)

Customer Premises Equipment (CPE) is equipment that resides at the

customer site. This equipment interfaces with the telephone company’s network. A CSU/DSU is a good example of Customer

Premise Equipment.

T1 Service

A T1 is a digital telephone line that provides a bandwidth of 1.54

million bits per second. This bandwidth is delivered as 24 channels of 64Kbps data channels. Thus 24

×

64 = 1536Kbps, or a rounded

1.54 million bits per second. T1s are often used for data connections between offices, local loop connections to Frame Relay networks, or connections to the Internet.

In many areas of the country, T1s can be ordered as fractional circuits.

If you need 512Kbps of bandwidth, you can order a fractional T1 with eight channels. These eight 64Kbps channels make up the 512Kbps bandwidth you needed. When a T1 is connected to a CSU/DSU, one parameter that must be set is the number of active channels.

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Typically this is all 24 channels; however, in the case of a fractional

T1 this must be set per circuit. When diagnosing installation problems in which fractional T1s are involved, always confirm that the

CSU/DSUs are configured properly for the number of channels used.

Channel Service Unit/Data Service

Unit (CSU/DSU)

A Channel Service Unit/Data Service Unit (CSU/DSU) (also often called just a CSU) interfaces with digital telephone lines. A CSU provides an interface for the router to connect to the digital line delivered by the telephone company. There are generally two types of CSUs: 56Kbps CSUs connect to 56Kbps lines and provide one channel of data over the line. T1 CSUs connect to fractional T1s and full T1s. A T1 CSU can usually be configured to use any number of channels on the line. This allows the use of fractional T1s.

A special type of CSU, called an add/drop CSU, can actually split the use of the channels between devices or even types of equipment. This feature can make better use of resources. Suppose a company has two offices with sophisticated phone switches. These two phone systems are connected together with a PRI line. (A PRI is an ISDN line packaged in a T1 span. This provides 23 channels for voice or data and one channel for signaling. The PRI line is described in more detail in the later section “Primary Rate Interface (PRI).”) This PRI allows calls to be transferred and connected throughout the company as if it were all in one building. This company has purchased a full PRI line with 23 channels. One channel is used for signaling. Of these 23 channels, the company never uses more than 19. The four channels that are never used can be used for data. An add/drop CSU can be installed at either end of the PRI. The CSUs are configured to split the channels, giving the first 19 channels to the telephone system.

The remaining four channels are given to the router. This gives the router a bandwidth of 256Kbps. Now the company not only has voice connectivity through the PRI, but it also has data connectivity without any additional monthly cost.

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Integrated Services Digital Network

(ISDN)

Integrated Services Digital Network (ISDN) is a technology that has

been around for many years. However, only recently has it become a commonly used technology. ISDN defines a digital communication link for voice and data communication. ISDN can be ordered in two different ways: Basic Rate Interface (BRI) and Primary Rate Interface

(PRI). With each type of ISDN, there are data channels and a signal-

ing channel. The data channels are used for the delivery of data. This data can consist of network packets of data or digitized voice data for normal telephone conversations. Data channels usually have a bandwidth of 64Kbps each. However, there are installations of ISDN that have 56Kbps data channels. The signaling channel is used for communication with the telephone network. This signaling includes dialing and special features such as initiating conference calls or call waiting.

ISDN Layer 2 Protocols (LAPD)

The ISDN specification includes implementations of layer 2 and layer 3 protocols. The layer 2 signaling protocol is Link Access

Procedure, D channel (LAPD). LAPD is used across D channels

to provide control and signaling information. LAPD is formally specified in ITU-T Q.920 and ITU-T Q.921.

ISDN Layer 3 Protocols

There are two layer 3 protocols defined within the ISDN framework for signaling: ITU-T Q.930 and ITU-T Q931. These two protocols work together to provide user-to-user, circuit-switched, and packetswitched connections. These protocols specify how call establishment, call termination information, and miscellaneous messages are communicated.

Network Termination

In the United States, when an ISDN BRI line is installed, it requires equipment to provide power to the line from the customer location.

This is unlike a normal telephone line, where operating power is supplied from the Central Office. The equipment at the customer location that provides this power is called a Network Termination 1 (NT1).

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The device that interfaces with the line and provides data connectivity to the ISDN line is not a modem. A modem modulates and demodulates digital data to analog signals. In ISDN, the digital data is placed on the ISDN line in digital fashion. Therefore, no modulation or demodulation is required. The device that interfaces with the ISDN line is called a Terminal Adapter (TA). Some implementations of terminal adapters include an NT1. These devices can be plugged directly into the ISDN line. Other implementations do not include the NT1 and require this device for operation.

In the United States, when an ISDN line is installed, the NT1 is not provided by the telephone company. In many parts of Europe, when an ISDN line is installed, the telephone company supplies the NT1.

For this reason, ISDN interfaces can commonly be purchased with or without an integrated NT1. Because Cisco 2500 routers do not have removable modules, Cisco decided not to include NT1s in the

ISDN implementation of these routers. This makes the router usable in both the U.S. and Europe. It does create the need for an external

NT1 for U.S. installations, however.

ISDN Reference Points

ISDN specifies several reference points that define the logical interface between functional devices such as TAs and NT1s. The ISDN reference points are as follows:

á R. This is the reference point between non-ISDN equipment and a TA.

á

S. This is the reference point between user terminals and the

NT2.

á

T. This is the reference point between NT1 and NT2 devices.

á U. This is the reference point between NT1 devices and linetermination equipment. The U reference point is only relevant in

North America where the carrier does not provide the NT1.

Basic Rate Interface (BRI)

Basic Rate Interface (BRI) ISDN consist of two communication channels, called bearer channels and one signaling channel called the data channel.

BRI ISDN is often referred to as 2B+D. This refers to the two bearer channels and one data channel. The two bearer channels are normally

64Kbps each; however, there are installations with 56Kbps channels.

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182 Pa r t I E X A M P R E PA R AT I O N

F I G U R E 5 . 2

ISDN line makeup.

Check with your local telephone company about the speed of your bearer channels. The installation information from the telephone company should tell you. Along with the two bearer channels, there is a 16Kbps data channel. This channel is used for communicating with the CO equipment. BRI bearer channels can be used together to get a data throughput of 128Kbps data transfer. Often this is mistakenly called bonding the channels. (Bonding is an actual protocol that is not often used.) Typically when the channels are used together it is done through a protocol called multilink. Multilink is an extension of point-to-point protocol (PPP). Figure 5.2 shows a representation of an ISDN BRI line.

64Kbps bearer channel

64Kbps bearer channel

16Kbps data channel

2B+D ISDN BRI

Understanding SPIDs

A Service Profile ID (SPID) is used between your ISDN equipment and the telephone companies switch. A SPID is usually some variation on the telephone number for your ISDN line. BRI lines normally have two telephone numbers assigned—one to each bearer channel.

There are implementations of ISDN in which one SPID is used for both B channels.

Configuring BRI ISDN

There are several pieces of information you will need before attempting to configure ISDN BRI on a Cisco router. When your ISDN line is installed, the telephone company should supply you with this information. You will need to know the SPIDs of the bearer channels and the switch type that is supplying your ISDN service. There is a move toward ISDN devices autodetecting SPIDs and other necessary information. However, it is generally still necessary to configure this information manually.

The following example shows a configuration of a router with a

BRI line. version 11.2

service timestamps log datetime msec localtime show-

➥timezone

08 051-6 CH 05 8/10/99 9:21 AM Page 183 service password-encryption service udp-small-servers service tcp-small-servers

!

hostname Router1

!

boot system flash 2: enable secret 5 $1$v5wf$t4849SbzncSTyNbE1qWOi/

!

username Router password 7 49494938292 partition flash 2 4 4

!

isdn switch-type basic-ni1 clock timezone EST -5 clock summer-time EDT recurring

!

interface Ethernet0 ip address 10.250.1.99 255.255.255.0

media-type 10BaseT

!

interface Serial0 no ip address shutdown

!

interface BRI0 ip address 10.1.99.1 255.255.255.0

encapsulation ppp dialer map ip 10.1.99.2 broadcast 7705557435 dialer map ip 10.1.99.2 broadcast 7705557443 dialer load-threshold 1 either dialer-group 1 isdn spid1 40455512120100 5551212 isdn spid2 40455512130100 5551213 no fair-queue ppp authentication chap ppp chap password 7 030B594831C79 ppp multilink

!

router eigrp 100 network 10.0.0.0

no auto-summary

!

ip classless logging buffered 4096 debugging dialer-list 1 protocol ip permit

!

line con 0 password 7 848473837211 login line vty 0 4 password 7 382483249238 login

!

end

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184 Pa r t I E X A M P R E PA R AT I O N

The previous example shows a BRI configuration. The first line in the configuration having to do with ISDN is isdn switch-type basic-ni1

.

ISDN is supposed to be standardized; however, there are several different implementations. The switch type tells the router how to communicate with the CO’s ISDN equipment. Basic-NI1 is the most common switch type. Many ISDN switches emulate NI1 signaling.

In one installation, the telephone company supplied the ISDN information listing the switch type as 5ESS which is one of the switch types a Bell Company might use. After hours of diagnosing, the switch type in the router was changed to Basic-NI1 and the circuit came up. This is an example of why you shouldn’t blindly trust the information you are given by the telephone company. This

BRI interface has an IP address assigned and encapsulation is set to

PPP. The dialup configuration is using CHAP as the authentication method.

Monitoring ISDN

Cisco’s IOS has several commands for monitoring ISDN. One of these commands is show isdn status

. Its output is shown in the following example.

Router#show isdn status

The current ISDN Switchtype = basic-ni1

ISDN BRI1/0 interface

Layer 1 Status:

ACTIVE

Layer 2 Status:

TEI = 72, State = MULTIPLE_FRAME_ESTABLISHED

TEI = 81, State = MULTIPLE_FRAME_ESTABLISHED

Spid Status:

TEI 72, ces = 1, state = 5(init) spid1 configured, spid1 sent, spid1 valid

Endpoint ID Info: epsf = 0, usid = 0, tid = 1

TEI 81, ces = 2, state = 5(init) spid2 configured, spid2 sent, spid2 valid

Endpoint ID Info: epsf = 0, usid = 1, tid = 1

Layer 3 Status:

2 Active Layer 3 Call(s)

Activated dsl 0 CCBs = 3

CCB: callid=0x0, sapi=0, ces=1, B-chan=0

CCB: callid=0x19, sapi=0, ces=2, B-chan=1

CCB: callid=0x1A, sapi=0, ces=1, B-chan=2

Total Allocated ISDN CCBs = 3

Router#

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Primary Rate Interface (PRI)

Primary Rate Interface (PRI) ISDN is implemented over the same

type line as a T1. This gives the PRI line 24 channels of 64Kbps data.

Of these 24 channels, one must be used for signaling. This gives a

PRI line 23 bearer channels of 64Kbps, or in some cases 56Kbps, data. These channels can be used independently or used together in a multilink configuration. In some installations where multiple PRI lines are used together, one signaling channel can be used for multiple lines. This gives one PRI line use of all 24 channels and the second

PRI use of 23 channels. The signaling channel of the second PRI provides signaling for both lines. Special configuration from the CO is required to do this. Work with your local telephone company when you order PRIs if you would like to use multiple PRIs in this way.

A PRI is a versatile type of data circuit. Many modern telephone systems use PRI lines to connect with the COs. Telephone conversations

(voice traffic) is digitized by the telephone system and transmitted over the digital telephone line. The digital data can be transmitted through the telephone network until it reaches its destination where it is converted back into analog signals.

ISDN Review

.

ISDN comes in two packages: BRI and PRI.

.

BRI ISDN has two bearer channels of 64Kbps (or 56Kbps) bandwidth and one signaling channel of 16Kbps.

.

PRI ISDN is delivered on the same type circuit as a T1 and has 23 bearer channels and one signaling channel.

.

A SPID is a Service Profile ID and is usually a variation of the telephone number for the bearer channels in your ISDN line.

.

To configure a BRI line, you set the switch type and set up the

SPIDS and the encapsulation type.

.

ISDN lines do not provide power like normal POTS lines.

ISDN lines require an NT1 at the customer site. Some ISDN devices have an NT1 built into them and some do not. Cisco

2500s with BRI require an external NT1 to work with ISDN.

.

The command show isdn status provides information about an ISDN line and the status of the channels.

R E V I E W B R E A K

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186 Pa r t I E X A M P R E PA R AT I O N

Remote Access Server (RAS)

Remote Access Servers (RAS) can be as simple as a few modems con-

nected to a computer running Windows NT or as complex as a Cisco

AS5300. However it’s implemented, a RAS provides remote network connectivity through dialup services. RAS can be used to dial into the company network to retrieve email and access network resources such as shared storage. RAS is also used for dialup access to the Internet.

Most ISPs use RAS to provide their dialup access. More sophisticated

RAS systems can make use of PRI lines to provide both analog mode access and digital ISDN access. It is the use of digital PRI lines for

RAS systems that made 56Kbps modems possible. 56Kbps modems take advantage of the digital connection between the RAS and the

CO to increase the potential bandwidth for analog modems.

H

OW

R

OUTERS

C

OMMUNICATE VIA

S

ERIAL

L

INKS

Routers communicate over serial lines when connected to dedicated circuits, Frame Relay, dialup lines, and other types of WAN links.

When two routers communicate over the serial ports, they must use a protocol to encapsulate the data they are transferring. These protocols include Synchronous Data-Link Control (SDLC), High-Level

Data Link Control (HDLC) and Point-to-Point Protocol (PPP).

Synchronous Data-Link Control (SDLC)

Synchronous Data-Link Control (SDLC) was developed by IBM for

use with its SNA protocol. Cisco routers support SDLC on their serial ports. A common use of SDLC is when an automated teller machine (ATM) is attached to a router. The router can communicate with the ATM using SDLC protocol. The traffic can then be encapsulated in TCP/IP and sent to a bank’s mainframe. The following example shows the output from a show interfaces on a serial port using SDLC encapsulation.

Router#show interfaces serial0

Serial0 is up, line protocol is up

Hardware is HD64570

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MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 1/255

Encapsulation SDLC, loopback not set

Router link station role: NONE (DTE)

Router link station metrics: group poll not enabled poll-wait 40000 seconds

N1 (max frame size) 12016 bits modulo 8 sdlc vmac: 0000.0000.00—

Last input 00:00:31, output 00:00:30, output hang never

Last clearing of “show interface” counters 00:06:41

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

137 packets input, 8110 bytes, 0 no buffer

Received 90 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

63 packets output, 3302 bytes, 0 underruns

0 output errors, 0 collisions, 5 interface resets

0 output buffer failures, 0 output buffers swapped out

8 carrier transitions

DCD=up DSR=up DTR=up RTS=up CTS=up

Router#

High-Level Data-Link Control (HDLC)

The High-Level Data-Link Control (HDLC) protocol grew from

SDLC. HDLC is a Data Link layer protocol. It encapsulates data for transmission on synchronous serial connections. Although most vendors support HDLC, many implementations are slightly different.

This creates some confusion for those new to internetworking. If you are connecting a Cisco router to a Bay Networks router, for example, and try to use HDLC as the protocol on the serial link, you will find that they will not communicate. This is due to the differences in the implementations. HDLC is the default encapsulation type for serial ports on Cisco routers and is often used when connecting Cisco to Cisco routers. The following example shows the output from a show interfaces command on a serial port using

HDLC encapsulation.

Router#show interfaces seriial0

Serial0 is up, line protocol is up

Hardware is HD64570

Internet address is 192.168.220.2/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 1/255

continues

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188 Pa r t I E X A M P R E PA R AT I O N

continued

Encapsulation HDLC, loopback not set, keepalive set

➥(10 ➥sec)

Last input 00:00:00, output 00:00:07, output hang never

Last clearing of “show interface” counters 00:02:58

Input queue: 0/75/0 (size/max/drops); Total output drops:

0

Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/

➥drops)

Conversations 0/1/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

70 packets input, 3582 bytes, 0 no buffer

Received 64 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

40 packets output, 1998 bytes, 0 underruns

0 output errors, 0 collisions, 4 interface resets

0 output buffer failures, 0 output buffers swapped out

4 carrier transitions

DCD=up DSR=up DTR=up RTS=up CTS=up

Router#

Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) is a Data Link layer protocol that will

work over synchronous or asynchronous serial connections. PPP uses

Link Control Protocol (LCP) to build and monitor connections. PPP

provides a non-vendor-specific standard for serial link communication.

PPP is often used when routers from two different manufactures must communicate with each other. PPP is the protocol used by most all dial-up Internet connections.

Because PPP is often used over dial-up lines, the LCP portion supports authentication using two protocols: Password Authentication Protocol

(PAP) and Challenge-Handshake Authentication Protocol (CHAP). PAP

provides a simple authentication technique. Its biggest disadvantage is that passwords are transmitted over the link in clear text. CHAP provides a more secure authentication technique using a “magic” number.

CHAP authentication never transmits the password over the link.

PPP supports many different network protocols, including TCP/IP,

IPX/SPX, AppleTalk, Transparent Bridging, DECnet, and

OSI/CLNS.

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To configure PPP on a serial port, use the command encapsulation ppp

, as shown in the following example.

Router#configure terminal

Enter configuration commands, one per line. End with

➥CNTL/Z.

Router(config)#interface serial1

Router(config-if)#encapsulation ppp

Router(config-if)#

The following example shows the output from a show interfaces for a serial port using PPP encapsulation.

Router#

Router#show interfaces serial0

Serial0 is up, line protocol is up

Hardware is HD64570

Internet address is 192.168.220.2/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 1/255

Encapsulation PPP, loopback not set, keepalive set (10

➥sec)

LCP Open

Open: IPCP, CDPCP

Last input 00:00:01, output 00:00:01, output hang never

Last clearing of “show interface” counters 00:00:02

➥0

Input queue: 0/75/0 (size/max/drops); Total output drops:

Queueing strategy: weighted fair

Output queue: 0/1000/64/0 (size/max total/threshold/

➥drops)

Conversations 0/1/256 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

3 packets input, 96 bytes, 0 no buffer

Received 3 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

2 packets output, 32 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions

DCD=up DSR=up DTR=up RTS=up CTS=up

Router#

F

RAME

R

ELAY

Frame Relay is a packet-switching network. This type of network

shares expensive resources among subscribers. By sharing the expensive resources such as long distance digital connections, the

Frame Relay provider can offer a more cost-effective solution than dedicated lines. Dedicated lines are charged on a per-mile basis.

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190 Pa r t I E X A M P R E PA R AT I O N

When dedicated lines cross state lines, costs and complexity of problem solving are increased. Because different areas are handled by different Bell Operating Companies (BOCs) it is difficult to get one BOC to take responsibility for a line crossing several states. This creates problems when a line is having problems. Many times, vendors will point fingers at one another rather than try to resolve the problem.

Frame Relay circuits save the customer money because the only mileage charge is from the customer site to the Frame Relay provider’s local Point Of Presence (POP). Once in the provider’s

POP, the traffic can travel throughout the Frame Relay network without regard to distance. A business can save a tremendous amount of money by switching from dedicated lines to Frame Relay.

Frame Relay allows the telephone companies to make more efficient use of their networks, therefore costing the customer less. Frame

Relay is an enhancement of the X.25 packet-switching network.

In a Frame Relay network, customer data is fed into the Frame Relay network by a local loop. After the data enters the Frame Relay network, it can travel over circuits with data from other companies. The data travels within the Frame Relay network until it reaches the local

Frame Relay switch for the destination. It then exits the network on the destination’s local loop and arrives at the destination network.

During the time the data is in the Frame Relay network, it can be sent over different paths based on network load and availability of circuits. In a Frame Relay network, if a circuit is cut, data can be rerouted automatically to other circuits. This gives Frame Relay very high reliability.

Frame Relay Cloud

Frame Relay cloud, or just cloud, is used to describe the multitude of

Frame Relay switches and circuits that make up the Frame Relay network. It describes most everything after the local loop of the source network to the local loop of the destination network.

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Committed Information Rate (CIR)

When you order a Frame Relay circuit, you request a Committed

Information Rate (CIR). The CIR is how much bandwidth the Frame

Relay vendor guarantees you. Because Frame Relay networks are shared, this number can be very important. What you are charged for a Frame Relay circuit is directly related to the CIR. If you order a circuit with 384Kbps of bandwidth and a CIR of 256Kbps, then your Frame Relay vendor guarantees delivery of 256Kbps of data.

Some Frame Relay providers allow you to burst up to the full speed of your circuit, thus exceeding your CIR. Some vendors do not. This is something you should consider when choosing a Frame Relay provider. If your provider allows you to burst above the CIR, data in excess of the CIR is marked Discard Eligible (DE), which is explained later in this chapter.

Data-Link Connection Identifier (DLCI)

In a Frame Relay network, one physical circuit can support multiple virtual circuits The router needs some way to identify for which virtual circuit a packet of data is destined. The Data-Link Connection

Identifier (DLCI, pronounced “del-see”) is used to identify this vir-

tual circuit. After the packet of data is received by the Frame Relay switch, the router is attached, and the Frame Relay switch makes a determination as to how to forward the packet. The Frame Relay switch has been programmed by the Frame Relay vendor to know where to forward packets containing particular DLCI numbers. The programming is specific to the local Frame Relay switch with which the router is connected. After the packet of data exits the local Frame

Relay switch and starts, it travels through the Frame Relay cloud, the local DLCI number is no longer significant. The local DLCI number is used only between the router and the Frame Relay switch with which the router is connected.

To review, the DLCI represents a virtual circuit to the Frame Relay switch attached to the router. Beyond that point in the Frame Relay cloud, the DLCI has no significance. This is what people mean when they state that a DLCI has “local significance” only.

This topic can be very confusing. A DLCI does not identify the connection between two routers, it identifies the virtual circuit only between the Frame Relay switch and the locally connected router.

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192 Pa r t I E X A M P R E PA R AT I O N

F I G U R E 5 . 3

A method of assigning DLCIs to simplify the network.

Figure 5.3 shows a typical Frame Relay network. Tampa, Fl. is connected to a Frame Relay switch through its local loop. This Frame

Relay switch identifies this connection as DLCI 100. The Frame Relay provider created a connection between this Frame Relay switch and the Frame Relay switch connected to Atlanta, Ga.. This connection is called a Permanent Virtual Circuit (PVC). (PVCs are explained in more detail later in this chapter.) The Frame Relay switch connected to Atlanta, Ga. communicates with the destination router through its local loop. This connection is identified as DLCI 101. To the router in

Tampa, Fl., it communicates with the Frame Relay switch using DLCI

100. The router in Atlanta, Ga. communicates with it’s Frame Relay switch using DLCI 101. Each of the DLCIs has local significance because the router on the other end of the connection does not know of the use at the other Frame Relay switch.

Frame Relay providers have begun to configure the DLCIs in such a way that they seem more logical. The providers will assign the same

DLCI for each connection back to one location to make it appear as if the DLCI refers to a location rather than only being locally significant.

Figure 5.3 shows how this might look.

DLCI 100

Refers to the PVC to Atlanta

Tampa, FL

DLCI 101 to Tampa

Atlanta, GA

DLCI 102 to Boston

DLCI 103 to Newport News

PVC

DLCI 100

Refers to the PVC to Atlanta

Newport News, VA

Boston, MA

DLCI 100

Refers to the PVC to Atlanta

Configurations from the routers in Figure 5.3 would look like the ones shown in the following four examples.

08 051-6 CH 05 8/10/99 9:21 AM Page 193

Atlanta, GA interface Serial0 no ip address no ip directed-broadcast encapsulation frame-relay IETF

!

interface Serial0.101 point-to-point description Tampa, FL ip address 10.1.1.5 255.255.255.252

!

frame-relay interface-dlci 101 interface Serial0.102 point-to-point description Boston, MA ip address 10.1.1.9 255.255.255.252

frame-relay interface-dlci 102

!

interface Serial0.103 point-to-point description Newport News, VA

!

ip address 10.1.1.13 255.255.255.252

frame-relay interface-dlci 103

Tampa, FL interface Serial0 no ip address no ip directed-broadcast encapsulation frame-relay IETF

!

interface Serial0.100 point-to-point description Atlanta, GA ip address 10.1.1.6 255.255.255.252

frame-relay interface-dlci 100

Boston, MA interface Serial0 no ip address no ip directed-broadcast

!

encapsulation frame-relay IETF interface Serial0.100 point-to-point description Atlanta, GA ip address 10.1.1.10 255.255.255.252

frame-relay interface-dlci 100

Newport News, VA interface Serial0 no ip address no ip directed-broadcast encapsulation frame-relay IETF

!

interface Serial0.100 point-to-point description Atlanta, GA ip address 10.1.1.14 255.255.255.252

frame-relay interface-dlci 100

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In the previous four examples, it appears that the DLCI for Atlanta is

100, and each of the other routers reference its DLCI. Actually, the

DLCI 100 is unknown to the Atlanta router. DLCI 100 is defined in each of the other router’s local Frame Relay switches and it refers to the PVC that connects to Atlanta. This means the DLCI numbers are locally significant to the Frame Relay switch at which they are defined.

When you are bringing up a new Frame Relay site and it does not appear to be working properly, try reversing the DLCI numbers at the two ends. It is not uncommon for a Frame Relay provider to accidentally program the Frame Relay switches with the DLCI numbers opposite of what your documentation says. Trying this first can save you days of headaches.

Permanent Virtual Circuit (PVC)

A Permanent Virtual Circuit (PVC) is a connection created within the

Frame Relay cloud by the Frame Relay provider. The PVC is created as a virtual connection between two locations. A PVC can seem like a dedicated connection between two different locations.

The Frame Relay provider programs PVCs into the Frame Relay switches. When data enters the Frame Relay cloud, it is identified by a DLCI. This DLCI relates to a PVC within the network. The data is switched through the network to the destination for that PVC.

It is important to realize that one Frame Relay circuit can support multiple PVCs. This is one of the cost-saving advantages of Frame

Relay. By supporting multiple connections on one local loop, a company can save money on the recurring circuit costs and on the hardware required to support another circuit.

Switched Virtual Circuit (SVC)

A Switched Virtual Circuit (SVC) works much like a Permanent

Virtual Circuit. However, an SVC is created and deleted by the customer’s equipment. This allows greater flexibility in the network. An

SVC is much like making a telephone call: You pick up the telephone and dial a phone number. The telephone switch translates this number and switches the lines required to connect your telephone to the telephone you are calling. When your call is complete, you hang up the telephone and the switch terminates all of the connections it created for your telephone call.

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An SVC works much like this. When your router needs to communicate with another router, it sends the signals to the Frame Relay switch to connect it to the other router. The Frame Relay switch dynamically creates a path between the two routers. The routers communicate and when they are finished the router tells the Frame

Relay switch to terminate the connection. The Frame Relay switch then removes the dynamic connection it created. SVCs are not used very much but do serve a useful purpose.

Local Management Interface (LMI)

Local Management Interface (LMI) is a protocol used between the

Frame Relay switch and your router. In 1990 Cisco Systems,

StrataCom, Northern Telecom, and Digital Equipment worked together to create what is known as “Gang-of-Four LMI,” or “Cisco

LMI.” They took the original Frame Relay protocol and added extensions that make supporting larger internetworks easier.

The extensions that were added include the following:

á Virtual Circuit Status Messages. This extension provides synchronization between the network and the user device. It periodically reports the existence of net PVCs and the deletion of already existing ones. It provides basic information about the integrity of PVCs.

á Multicasting. This extension allows a sender to transmit a single frame and have it delivered to multiple recipients.

á Global Addressesing. This extension gives connection identifiers global rather than local significance.

á

Simple Flow Control. This extension offers a flow control mechanism that applies to the entire Frame Relay interface.

C h a p t e r 5 WA N P R OTO C O L S 195

C

ONFIGURING

F

RAME

R

ELAY

Configuring Frame Relay on a Cisco router involves configuring the interface for each of the protocols that will be used such as IP or

IPX, and relating the interface to the DLCI for the connection.

There are two different ways to configure Frame Relay on a router supporting multiple connections: multipoint and point-to-point.

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196 Pa r t I E X A M P R E PA R AT I O N

F I G U R E 5 . 4

Frame Relay network using multipoint interfaces.

Configuring Multipoint Frame Relay

In Figure 5.4, you see a Frame Relay network with multiple sites connecting to the Atlanta office. This Frame Relay network is set up in a multipoint configuration. In a multipoint configuration, all of the routers are on one network.

Atlanta, GA

IP Address

10.100.1.1 Mask 255.255.255.0

DLCI 101

PVC

Tampa, FL

IP Address

10.100.1.2 Mask 255.255.255.0

DLCI 102

Newport News, VA

IP Address

10.100.1.4 Mask 255.255.255.0

DLCI 104

Boston, MA

IP Address

10.100.1.3 Mask 255.255.255.0

DLCI 103

In a multipoint Frame Relay configuration, each of the DLCIs are listed under one subinterface. The following example shows how the serial port would be configured in the Atlanta router in Figure 5.4.

interface Serial0/0.1 multipoint ip address 10.100.1.1 255.255.255.0

frame-relay interface-dlci 102 frame-relay interface-dlci 103 frame-relay interface-dlci 104

Multipoint Frame Relay networks present a challenge for network designers. Note that in Figure 5.3 each of the routers has a direct connection back to Atlanta. However, none of the routers have connections to the other branch routers. This means that if the router in

Tampa needs to communicate with the router in Newport News, it must send the data to Atlanta first. The way the IP protocol works is different. Because Tampa and Newport News are in the same subnet, the routers would try to communicate directly. This is why most

Frame Relay networks are configured in a point-to-point configuration.

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Configuring Point-to-Point Frame Relay

The opposite of multipoint is point-to-point. In a point-to-point

Frame Relay network, each PVC is represented by a separate interface connection within the router. Since Frame Relay can support multiple connections on one physical circuit, there needs to be a way of separating these connections within the router. Cisco supports this logical separation of a physical connection through the use of subinterfaces. To configure Frame Relay on a serial port, use the command encapsulation frame-relay

, as shown in the following example.

Router#configure terminal

Enter configuration commands, one per line. End with

➥CNTL/Z.

Router(config)#interface serial0

Router(config-if)#encapsulation frame-relay

Router(config-if)#

Separating Logical Connections with

Subinterfaces

Subinterfaces provide a method of separating one physical network

connection into multiple logical connections. This is useful for

Frame Relay because one physical local loop can support many

PVCs. This creates the appearance of many different physical connections to other sites on one interface. The router must have some means of separating these logical connections so it can properly deliver the traffic destined to them. Subinterfaces provide this means. Subinterfaces have uses beyond just Frame Relay, but this section focuses on how subinterfaces are used with Frame Relay.

To create a subinterface, use the interface command in the configure mode of the router. Enter the interface description to which you want to add a subinterface, with an added period and a number following to identify the subinterface. When configuring Frame Relay, it is a good idea to use the DLCI number for the subinterface number. This makes it easier to identify items in the config later. The following example shows how DLCI 199 is configured as a subinterface of interface Serial0.

Router#config terminal

Router(config)#interface Serial0.199

Router(config-if)#ip address 10.199.2.1 255.255.255.0

Router(config-if)#frame-relay interface DLCI 199

Router(config-if)#exit

Router(config)#

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198 Pa r t I E X A M P R E PA R AT I O N

F I G U R E 5 . 5

An example of how subinterfaces can be used in a Frame Relay network.

Figure 5.5 shows how subinterfaces connect with the router. Serial0 is connected to the local loop that connects the office with the

Frame Relay cloud. Serial0 is subinterfaced to three interfaces.

Serial0.1 has an IP address of 10.100.2.1 and connects to the PVC for Boston. Serial0.2 has an IP address of 10.200.2.1 and connects to the PVC for Newport News. Serial0.3 has an IP address of

10.1.2.1 and connects to Atlanta. Each of these interfaces works logically as a separate network interface although they all exist on one physical interface. Each subinterface can be shut down independently of the others and can support different protocols. For example, Serial0.2 might support IPX traffic as well as TCP/IP, although the other two subinterfaces only support TCP/IP.

Serial0

Boston, MA

Serial0.1

Ser ial0.2

Newport News, VA Atlanta, GA

Monitoring Frame Relay

There are several commands specific to Frame Relay monitoring. These commands allow you to see specific data on Frame Relay networks.

Monitoring the Serial Port of a Frame Relay

Connection

The show interfaces command lists some Frame Relay specific information for interfaces connected to Frame Relay networks. The following example shows the output from show interfaces for a

Frame Relay network.

Router#sh int s5/0

Serial5/0 is up, line protocol is up

Hardware is M4T

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 3/255

Encapsulation FRAME-RELAY IETF, loopback not set,

➥keepalive set (10 sec)

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LMI enq sent 18350, LMI stat recvd 18350, LMI upd recvd

➥6, DTE LMI up

LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0

LMI DLCI 1023 LMI type is CISCO frame relay DTE

Broadcast queue 0/64, broadcasts sent/dropped 0/0,

➥interface broadcasts 102718

0

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of “show interface” counters 2d02h

Input queue: 0/75/0 (size/max/drops); Total output drops:

0

Queueing strategy: weighted fair

Output queue: 0/64/0 (size/threshold/drops)

Conversations 0/163 (active/max active)

Reserved Conversations 0/0 (allocated/max allocated)

5 minute input rate 24000 bits/sec, 24 packets/sec

5 minute output rate 23000 bits/sec, 21 packets/sec

5397749 packets input, 1050654351 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored,

➥0 abort

5045209 packets output, 1227209722 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 output buffer failures, 0 output buffers swapped out

0 carrier transitions DCD=up DSR=up DTR=up

➥RTS=up CTS=up

Router#

The show interfaces command shows a subset of this information for each of the subinterfaces. Because a subinterface is a logical separation of networks, it is not necessary for it to display all of the physical information about the interface. The following example shows the output from show interfaces for a Frame Relay subinterface.

Router#show interfaces serial5/0.1

Serial5/0.1 is up, line protocol is up

Hardware is M4T

Description: Boston

Internet address is 10.110.17.1/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely

➥255/255, load 4/255

Encapsulation FRAME-RELAY IETF

Router#

Seeing Active PVCs

To view the PVCs your router can see, use the show frame-relay pvc

. The following example shows the output from this command.

Tampa_FL#show frame-relay pvc

PVC Statistics for interface Serial0/0 (Frame Relay DTE)

continues

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200 Pa r t I E X A M P R E PA R AT I O N

continued

DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE,

➥INTERFACE = Serial0/0.100

input pkts 26318755 output pkts 33295916 in

➥bytes 2677624387 out bytes 3218704665 dropped pkts 46 in

➥FECN pkts 312 in BECN pkts 0 out FECN pkts 0 out

➥BECN pkts 0 in DE pkts 170015 out DE pkts 0 out bcast pkts 5016413 out bcast bytes 402198896 pvc create time 35w4d, last time pvc status changed 2d18h

DLCI = 101, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE,

➥INTERFACE = Serial0/0.101

input pkts 13540113 output pkts 12640482 in

➥bytes 476157705 out bytes 1523146301 dropped pkts 42 in FECN

➥pkts 30858 in BECN pkts 0 out FECN pkts 0 out

➥BECN pkts 0 in DE pkts 1998143 out DE pkts 0 out bcast pkts 4981728 out bcast bytes 399422790 pvc create time 35w4d, last time pvc status changed 1d03h

DLCI = 161, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE,

➥INTERFACE = Serial0/0.161

input pkts 5229620 output pkts 5285702 in

➥bytes 525271058 out bytes 1180706730 dropped pkts 63 in FECN

➥pkts 7 in BECN pkts 22057 out FECN pkts 0 out

➥BECN pkts 0 in DE pkts 85329 out DE pkts 0 out bcast pkts 3266531 out bcast bytes 261916791 pvc create time 26w1d, last time pvc status changed 2d18h

Tampa_FL#

In the preceding example you see the statistics shown with the command show frame-relay pvc

. Each of the DLCIs assigned is listed with statistics about each. The input/output packets and bytes are self-explanatory. They represent the amount of data that has been passed using this DLCI. The number of dropped packets represents the number of packets that were dropped at the Frame Relay level because an active outbound DLCI was not found. The next two statistics are FECN and BECN. These are methods of informing the network devices of congestion in the Frame Relay network.

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á Forward Explicit Congestion Notification (FECN). A method used in Frame Relay to inform the routers of congestion on the network. When a Frame Relay switch detects congestion, it can set a bit in the frame to notify the receiving network device that the packet experienced congestion in the network. This is called

FECN (pronounced feck-in).

á

Backward Explicit Congestion Notification (BECN). Often used in conjunction with FECN. When a packet is passing back to the sending station, the Frame Relay switch can set the

BECN bit to tell the sending station its traffic is experiencing congestion. The sending station can then reduce the speed at which it is sending data. This is called BECN (pronounced beck-in).

Because Frame Relay is a shared network, there are times when your traffic may not be delivered due to network congestion. When a

Frame Relay switch experiences congestion, it will first start discarding traffic. One way it decides what traffic to discard is to look at the

Discard Eligible (DE) bit. Your router can set this bit when the traffic enters the Frame Relay network, indicating that this packet can be dropped if necessary. A common use of the DE bit is when bandwidth utilization exceeds the Committed Information Rate (CIR).

Traffic in excess of the CIR can be set as Discard Eligible (DE).

Because many networking protocols such as TCP guarantee delivery of packets, if the Frame Relay network drops a packet, the TCP protocol will resend the data.

Viewing the Frame Relay Map

Cisco routers create an internal map of frame relay DLCIs and the ports they are mapped with. This map can be viewed using the command show frame-relay map

, as shown in the following example.

Router#sh frame-relay map

Serial5/0.123 (up): point-to-point dlci, dlci

➥123(0x7B,0x1CB0), broadcast, BW =

64000 status defined, active

Serial5/0.149 (up): point-to-point dlci, dlci

➥141(0x8D,0x20D0), broadcast, BW =

16000

continues

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202 Pa r t I E X A M P R E PA R AT I O N

continued

status defined, active

Serial5/0.133 (up): point-to-point dlci, dlci

➥168(0xA8,0x2880), broadcast, BW =

16000 status defined, active

Serial5/0.148 (up): point-to-point dlci, dlci

➥164(0xA4,0x2840), broadcast, BW =

16000 status defined, active

Serial5/0.7 (up): point-to-point dlci, dlci

➥102(0x66,0x1860), broadcast, BW = 16

000 status defined, active

Serial5/0.9 (up): point-to-point dlci, dlci 16(0x10,0x400),

➥broadcast, BW = 1600

0 status defined, active

Serial5/0.5 (up): point-to-point dlci, dlci

➥106(0x6A,0x18A0), broadcast, BW = 16

000 status defined, active

Serial5/0.8 (up): point-to-point dlci, dlci

➥111(0x6F,0x18F0), broadcast, BW = 16

000 status defined, active

Serial5/0.122 (up): point-to-point dlci, dlci

➥122(0x7A,0x1CA0), broadcast, BW =

64000 status defined, active

Serial5/0.2 (up): point-to-point dlci, dlci

➥100(0x64,0x1840), broadcast, BW = 16

000 status defined, active

Serial5/0.146 (up): point-to-point dlci, dlci

➥146(0x92,0x2420), broadcast, BW =

16000 status defined, active

Serial5/0.10 (up): point-to-point dlci, dlci

➥113(0x71,0x1C10), broadcast, BW = 1

6000 status defined, active

Serial5/0.11 (up): point-to-point dlci, dlci

➥134(0x86,0x2060), broadcast, BW = 1

6000 status defined, active

Serial5/0.6 (up): point-to-point dlci, dlci

➥105(0x69,0x1890), broadcast, BW = 16

000 status defined, active

Serial5/0.4 (up): point-to-point dlci, dlci

➥108(0x6C,0x18C0), broadcast, BW = 16

000

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C h a p t e r 5 WA N P R OTO C O L S 203 status defined, active

Serial5/0.138 (up): point-to-point dlci, dlci

➥138(0x8A,0x20A0), broadcast, BW =

16000 status defined, active

Serial5/0.147 (up): point-to-point dlci, dlci

➥157(0x9D,0x24D0), broadcast, BW =

16000 status defined, active

Serial5/0.3 (up): point-to-point dlci, dlci

➥107(0x6B,0x18B0), broadcast, BW = 16

000 status defined, active

Serial5/0.150 (up): point-to-point dlci, dlci

➥206(0xCE,0x30E0), broadcast, BW =

16000 status defined, active

Serial5/0.12 (up): point-to-point dlci, dlci

➥114(0x72,0x1C20), broadcast, BW = 1

6000 status defined, active

Serial5/0.1 (up): point-to-point dlci, dlci

➥109(0x6D,0x18D0), broadcast, BW = 16

000 status defined, active

Router#

In the previous example you see that each interface that has been set for frame relay encapsulation is listed along with other information about the DLCI. Listed as BW is the bandwidth of the circuit’s CIR.

Frame Relay Review

.

Frame Relay is a packet-switching network that provides low cost connections due to the sharing of resources.

.

The entire Frame Relay network is called the cloud.

.

The Committed Information Rate (CIR) is how much bandwidth your Frame Relay provider guarantees you will have available.

.

A Data-Link Connection Identifier (DLCI) is a number assigned to map an IP address to a PVC. DLCIs have local significance. This means they can be different on either end of a

PVC and can be reused within the network.

.

A Permanent Virtual Circuit (PVC) is a path created in the

Frame Relay network between two points. The Frame Relay provider creates PVCs.

R E V I E W B R E A K

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204 Pa r t I E X A M P R E PA R AT I O N

.

A Switched Virtual Circuit (SVC) is a path created dynamically in the Frame Relay network in much the same way a telephone call is placed. When one router needs to communicate with another, it requests that the Frame Relay switch create an

SVC. The switch creates the SVC dynamically. When the routers no longer need to communicate, the SVC is deleted.

.

Local Management Interface (LMI) is the protocol used between the router and the Frame Relay switch to add management functions for large internetworks.

.

To configure Frame Relay on an interface, use the command encapsulation frame-relay

.

.

To separate a physical interface into subinterfaces, use the configuration command interface serial0.100

, where

.100

is the subinterface you want to create. Add the frame-relay interface DLCI

100 command, where 100 is the DLCI number for the PVC.

.

To diagnose Frame Relay problems, the commands show interfaces and show frame-relay pvc offer much information.

WAN T

ECHNOLOGIES

—W

HEN TO

U

SE

W

HAT

Wide Area Network (WAN) technologies are tricky. One of the variables that can greatly affect the decision of one technology over another is cost. Cost varies greatly across the United States and ever more so in other countries. This creates quite a challenge when trying to determine which WAN technology to use.

There are several factors to consider when choosing a WAN technology for a remote site. What facilities do you have at the site to which you are connecting? If you are connecting an office in Boston to an office in Atlanta, and the office in Atlanta can only support Frame

Relay, that pretty much makes the decision for you.

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The most wisely designed WANs use a variety of technologies, each used where it is strong in both cost and versatility. It wouldn’t make sense to use ISDN to connect two offices when the long distance charges between the offices would exceed the cost of a point-topoint fractional T1. It also wouldn’t make sense to use a full T1 point-to-point connection to connect an office of three light email users to the company’s network, unless the cost of a full T1 in that part of the country was cheaper than an ISDN line.

With every decision of how to connect offices, weigh all the factors.

Consider the options and make a wise decision. Too many companies set a standard and live by that standard no matter what. In short, use common sense.

There are parts of the country where you can get a dedicated 56Kbps line for under $100 a month. This is an excellent deal. There are also parts of the country where ISDN lines are very inexpensive. In these parts of the country it might make sense to connect smaller offices to a larger one via ISDN and then connect the larger office to the company’s Frame Relay network with a T1 or fractional T1.

Use your company’s money wisely. In the case of the American

Cancer Society, for example, the money you save could save lives. In the case of an automotive manufacturer, on the other hand, the money you save may mean a bigger bonus.

It is difficult to set standards for how large a circuit is required for what size office. It depends greatly on the network traffic that office will generate. An office of salespeople who mainly use email and browse the Web will generate far less traffic than a data center that constantly queries database servers distributed around your WAN.

Find a way to monitor the traffic on your circuits and adjust the bandwidths based on this information. An excellent product for monitoring circuit utilization is Multi Router Traffic Graphic

(MRTG). MRTG is a freeware product that will run on

Linux/UNIX or Windows NT. (It runs much better on

Linux/UNIX than it does on Windows NT. It tends to bog down a

Windows NT server.) It is available at no cost from its creator at http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/mrtg.html

.

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C

A S E

S

T U D Y

: R

A P T O R

R

E S C U E

O

R G A N I Z AT I O N

E S S E N C E O F T H E C A S E

.

You need to install a T1 line with very little information. The sales representative who sold the circuit no longer works for the ISP so you cannot question him about what he told the Raptor Rescue. The person from the Raptor Rescue who ordered the circuit is out of the country for a month, so you are unable to find out any information from him. You must use your background in WAN technologies to help you figure out how things work together.

S C E N A R I O

You have been hired to install and connect a T1 line for the Raptor Rescue organization. This organization takes in birds of prey that have been injured. Birds that can recover are rehabilitated and released back into the wild. Birds that have sustained more serious injuries are kept well and used for educational programs to teach people about the benefits of Raptors. The Raptor Rescue has ordered a T1 connection for the Internet and the ISP was supposed to provide installation.

Unfortunately, at the last minute the Raptor

Rescue found out they would have to hire someone for the installation. You are that lucky person.

The Raptor Rescue knows very little about what they have ordered. They simply listened to the sales representative that sold them the T1. They purchased a Cisco 2501 router and a Kentrox

D-Serv 56/64 DSU.

A N A LY S I S

You have been told that the Raptor Rescue ordered a T1 line from the ISP. You take the equipment to the telephone room and evaluate the situation. You notice the Kentrox D-Serv

56/64 DSU and think to yourself, “If they truly get a T1, this DSU won’t work because it’s only for 56/64Kbps lines.” A CSU/DSU capable of handling 24 channels would be required for a full

T1 line. The Kentrox 56/64Kbps DSU handles only one channel of either 56Kbps or 64Kbps.

In the telephone room you find a work order on the floor. The work order is from the telephone company. It says they were to install a 56Kbps line to

Acme Internet. Finally, some clue as to what is happening here! They are not getting a T1 as was described to you; they are getting a 56Kbps line.

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C

A S E

S

T U D Y

: R

A P T O R

R

E S C U E

O

R G A N I Z AT I O N

You have found that it is not uncommon to find people not familiar with WAN technologies referring to any dedicated circuit as a T1.

You contact the ISP and get the IP address that has been assigned to the Raptor Rescue. You configure the router with that IP address and set the bandwidth on the interface for 56Kbps. You remove the DSU from its box and refer to the configuration guide.

The DSU has two connectors on the back: one is an RS-232 and the other a V.35. You have a Cisco V.35 cable in the box with the router.

In the manual, you find how to set the DSU to use the V.35 connector. You also find how to set the data rate to 56Kbps. After completing this configuration, you plug the DSU into the jack marked by the telephone company. You connect the V.35 cable from the DSU to the router and turn on all of the equipment. The DSU starts to light up and the router is now up and running.

You plug the router into the local network and configure your laptop with the IP address of the router as your default gateway. After you have connected your laptop to the network, you try and connect to the Cisco Web site. Success!

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208 Pa r t I E X A M P R E PA R AT I O N

C

H A P T E R

S

U M M A R Y

KEY TERMS

• Integrated Services Digital Network

(ISDN)

• Basic Rate Interface (BRI)

• Primary Rate Interface (PRI)

• Service Profile ID (SPID)

• Central Office (CO)

• Plain Old Telephone Service (POTS)

• Local loop

• Demarcation point

• Customer Premises Equipment (CPE)

• T1

• Channel Service Unit/Data Service

Unit (CSU/DSU)

• Synchronous Data-Link Control

(SDLC)

• High-Level Data-Link Control

(HDLC)

• Point-to-Point Protocol (PPP)

• Frame Relay

• Committed Information Rate (CIR)

• Data-Link Connection Identifier

(DLCI)

• Permanent Virtual Circuit (PVC)

• Switched Virtual Circuit (SVC)

• Local Management Interface (LMI)

This chapter covered WAN protocols, terms, and technologies. In any internetwork, you will find connections to the outside world in one or more of these fashions. T1s are commonly used for connections to the Internet. Frame Relay is often used for connecting branch offices of a company together. ISDN is a good solution for backup circuits and intermittent connections for branch offices.

ISDN is also a good solution for small office Internet connectivity.

Knowing and understanding these technologies will make your life as an internetworking engineer much easier. There are many nuances in dealing with local telephone companies that you will simply have to learn through experience.

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C h a p t e r 5 WA N P R OTO C O L S 209

A

P P L Y

Y

O U R

K

N O W L E D G E

Review Questions

1. Describe the channel makeup of a T1 circuit.

2. Describe a CO and some of the services it can provide.

3. Describe an ISDN BRI line and PRI line.

4. Describe HDLC as a protocol and how it interoperates with different vendors’ equipment.

5. How do you know how much bandwidth will be guaranteed by your Frame Relay provider?

6. What are two ways that Frame Relay can be configured within a router?

7. How are DLCIs assigned?

8. What techniques do Frame Relay switches use to inform routers of congestion in the network?

9. How is a subinterface created? What is its purpose?

10. What is an NT1? What is its function?

Exam Questions

1. Which of the following are related to Frame

Relay? Select two.

A. SPID

B. LMI

C. CIR

D. 2B+D

2. An ISDN BRI line can be described as which of the following?

A. 23B+D

B. 2B+D

C. 23D+B

D. 2D+B

3. What does CIR represent?

A. A CIR is the number used to represent the connection between a router and the Frame

Relay switch.

B. A CIR connects two Frame Relay sites through Frame Relay switch programming.

C. The CIR is the amount of bandwidth your

Frame Relay provider guarantees you will have available.

D. CIR is the protocol used between a router and a Frame Relay switch.

4. To configure ISDN, you must set which of the following?

A. ISDN switch-type; SPIDs; encapsulation

B. SPIDs; encapsulation; NT1

C. cir; ISDN switch-type; encapsulation

D. ISDN switch-type; LMI; SPIDs

5. Why is Frame Relay more cost effective than leased lines?

A. Smaller connections are cheaper than leased lines.

B. Frame Relay connections are cheaper because of competition between providers.

C. Because Frame Relay providers can share resources between many companies, they can charge less for the service.

D. Frame Relay uses bearer channels to increase the bandwidth available.

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A

P P L Y

Y

O U R

K

N O W L E D G E

6. Which answer best describes DLCIs?

A. DLCIs are global throughout the network.

B. DLCIs are locally significant.

C. DLCIs set the amount of bandwidth allocated to your circuit.

D. DLCIs prioritize traffic so that more important traffic is delivered first.

7. Which of the following commands shows the status of PVCs?

A.

Router#show frame-relay pvc

B.

Router#list frame-relay pvc

C.

Router#show frame-relay dlci

D.

Router#list frame-relay dlci

8. Which of the following commands shows the status of an ISDN line?

A.

Router#show isdn status

B.

Router(config)#show isdn status

C.

Router#show spid status

D.

Router(config)#show spid status

9. How do you create a subinterface?

A.

Router(config)#interface serial0 subinterface 100

B.

Router(config)#subinterface serial0.100

C.

Router(config)#create subinterface 100

D.

Router(config)#interface serial0.100

10. What method do Frame Relay switches use to inform routers of congestion within the Frame

Relay network?

A. Forward Error Congestion Notification

(FECN) and Backward Error Congestion

Notification (BECN)

B. Congestion Control Protocol (CCP)

C. Data-Link Connection Identifier (DLCI)

D. Latency Algorithm, Traffic Errors (LATE)

Answers to Review Questions

1. A T1 circuit contains 24 channels, each having a bandwidth of 64Kbps, giving the T1 a bandwidth of 1.54Mbps. For more information, see “T1

Service.”

2. A CO is a Central Office and can provide POTS,

ISDN, Drame Relay, and local loop connections.

For more information, see “Central Office.”

3. An ISDN BRI line has two bearer channels with a bandwidth of 64Kbps or 56Kbps each, and a signaling channel with a bandwidth of 16Kbps. A

PRI line is delivered on the same type of circuit as a T1 and contains 23 64Kbps bearer channels with one channel used for signaling. For more information, see “Basic Rate Interface (BRI)” and

“Primary Rate Interface (PRI).”

4. HDLC is used for serial communication and is generally vendor specific. Being vendor specific, routers from different manufacturers cannot use

HDLC for communication with one another. For more information, see “High-Level Data-Link

Control (HDLC).”

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C h a p t e r 5 WA N P R OTO C O L S 211

A

P P L Y

Y

O U R

K

N O W L E D G E

5. When you order a circuit, you specify a

Committed Information Rate (CIR). This CIR is how much bandwidth the provider guarantees.

The higher the CIR, the higher the price. For more information, see “Committed Information

Rate (CIR).”

6. Frame Relay can be configured as either multipoint or point-to-point. In multipoint, all of the

DLCIs are listed under one subinterface. In point-to-point, each DLCI is separated under its own subinterface with a separate network. For more information, see “Configuring Frame

Relay.”

7. DLCIs are assigned by the Frame Relay vendor and are only significant in the local connection between the Frame Relay switch and the router.

Although in some implementations they may appear to be global, they are not. For more information, see “Data-Link Connection Identifier

(DLCI).”

8. Frame Relay switches use FECN and BECN to let routers know there is congestion within the frame relay network. For more information, see

“Seeing Active PVCs.”

9. A subinterface is created in the config mode by typing the command interface serial 0.100

, where the number

100 is the subinterface you want to create. It is generally a good idea to match the subinterface number with the DLCI number to make the config easier to read. For more information, see “Separating Logical

Connections with Subinterfaces.”

10. An NT1 provides termination of an ISDN line and provides power to the line. In the U.S., an NT1 is not provided with an ISDN line.

In some parts of Europe, the telephone company supplies an NT1 with the ISDN line. This creates the need for ISDN interfaces with and without integrated NT1. Cisco 2500 routers, not being modular, do not have an integrated NT1. When ordering equipment for other routers, be sure to order the correct module. For more information, see “Integrated Services Digital Network

(ISDN).”

Answers to Exam Questions

1. B, C. LMI and CIR are related to Frame Relay.

SPID and 2B+D relate to ISDN. For more information, see “Frame Relay.”

2. B. An ISDN BRI line has two bearer channels and one data channel for signaling. It is sometimes called a 2B+D. For more information, see

“Basic Rate Interface (BRI).”

3. C. The CIR is the Committed Information Rate.

This is the amount of bandwidth a Frame Relay provider guarantees will be available for you. For more information, see “Committed Information

Rate (CIR).”

4. A. Configuring ISDN requires you set the switch type, SPIDs, and encapsulation. For more information, see “Configuring BRI ISDN.”

5. C. By sharing resources, Frame Relay providers can reduce the cost to customers. For more information, see “Frame Relay.”

6. B. DLCIs are locally significant, even if they seem otherwise. For more information, see “Data-Link

Connection Identifier (DLCI).”

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A

P P L Y

Y

O U R

K

N O W L E D G E

7. A. show frame-relay pvc gives information and statistics for PVCs. For more information, see

“Monitoring Frame Relay.”

8. A. show isdn status from the exec prompt, not the config, gives information about an ISDN line and its status. For more information, see

“Monitoring ISDN.”

9. D. Simply use the interface serial0.100

command from the config prompt, where

100 is the subinterface you wish to create. Remember that it’s a good idea to match the subinterface with the

DLCI number for readability of the configuration.

For more information, see “Separating Logical

Connections with Subinterfaces.”

10. A. FECN and BECN are used to inform routers of congestion in the Frame Relay network. For more information, see “Monitoring Frame Relay.”

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration, Cisco Press, 1998

2. Ford, Merilee; Lew, H. Kim; Spanier, Steve;

Stevenson, Tim. Internetworking Technologies

Handbook, Cisco Press, 1997

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O

B J E C T I V E S

Cisco provides the following objectives for the Routing

Protocols portion of the CCNA exam:

.

Add the RIP routing protocol to your configuration.

RIP is a routing protocol that is used in many legacy systems. It is often used to communicate between different manufactures’ routers. Being able to configure a router for RIP is necessary because it is often the common routing protocol between different brands of routers. RIP is also implemented in many operating systems such as Windows NT and UNIX.

.

Add the IGRP routing protocol to your configuration.

IGRP is a popular routing protocol. Being able to configure a router for IGRP is important because many organizations are using more advanced routing protocols like IGRP.

Explain the services of separate and integrated multiprotocol routing.

.

Because many organizations have a variety of equipment and often interface with outside organizations, it is likely that you will need to understand how different routing protocols can be integrated together. There are two ways to have multiple protocols on a router: separate or integrated. Cisco uses a combination. Each protocol has its own routing table, but different routing protocols in the same stack can share the same table.

C H A P T E R

6

Routing

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O

B J E C T I V E S

( c o n t i n u e d

)

O

U T L I N E

List problems that each routing type encounters when dealing with topology changes and describe techniques to reduce the number of these problems.

.

Understanding what happens when a routing protocol experiences a change in network topology is important when designing and maintaining networks. Different routing protocols handle network changes differently. One routing protocol that reconverges quickly for a small simple network may take an unreasonably long time to reconverge a larger network.

Introduction

Routing Protocol Basics

Path Selection

Static and Dynamic Routing

Multiprotocol Routing

Administrative Distance

Autonomous Systems (AS)

One more objective The objective

“Describe the benefits of network segmentation with routers” is covered in detail in Chapter 8, “LAN Switching.”

Routing Problems

Split Horizon

Poison Reverse Update

Routing Recap

Distance Vector Protocols

Routing Information Protocol (RIP)

RIP Routing Tables

Regular Updates and Triggered

Updates

Hop Count

Routing Update Timer

Route Invalid Timer

Route Flush Timer

Hold-Down Timer

Configuring RIP

Working with RIP

The Limitations of RIP

216

223

224

224

224

226

226

227

228

228

228

229

229

229

230

230

232

218

219

219

220

221

222

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O

U T L I N E

( c o n t i n u e d

)

S

T U D Y

S

T R AT E G I E S

Internet Gateway Routing Protocol (IGRP) 233

IGRP Metrics

Similarities Between IGRP and RIP

233

233

Differences Between IGRP and RIP

IGRP Timers

234

234

Configuring IGRP

Working with IGRP

234

235

Link-State Protocols 237

.

Studying for the routing protocols section of the

CCNA exam can be challenging. Routing protocols exchange network information between routers. Understanding how these routing protocols perform this operation and the problems that can occur will help you diagnose problems when they occur.

Try configuring RIP and IGRP on Cisco routers.

Experiment with how different settings can affect the operation of the routing protocol.

Experience is the best teacher.

Selecting a Routing Protocol 238

Chapter Summary 240

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I

NTRODUCTION

Routing traffic is the process of getting a packet of data from the

originating station to the destination station. This could be as simple as putting the packet on the network where the local destination station can receive the packet. It can also be as complex as sending the packet to a default gateway (a router) where the packet is compared to a routing table and then forwarded to the next router that can help the packet along its path. The next router then compares the packet to its routing tables and forwards it to the next hop along its journey. This continues until the packet reaches a router directly attached to the network of the destination station. The packet is then delivered to the destination station.

TCP/IP traffic is routed based on the Network portion of the address. If the network node generating the packet is on the same network as the destination node, then the packet is simply placed on the network where the destination node will see it. If however, the destination node is on a remote network, the source node must make a decision. In most cases, network nodes are attached to only one network. A desktop PC in an office for instance is usually connected to one network—the office network. In the case of a network node that is attached to only one network, there is usually a “default gateway” IP address defined in the routing table. This default gateway IP address is usually a router.

When the network node generates an IP packet for a destination node that is on a remote network, the source node compares the network portion of the IP address to its own network. The two will not match and thus the source node sends the packet to the default gateway IP address for forwarding. The default gateway can be a router attached to many networks like a router connected to the Internet. After the packet is received at the default gateway, it is compared to the routing tables to determine how to forward it. In some cases, the router itself may be attached to the destination network, in which case the packet is simply placed on the destination network for the destination node to receive. However, if the router is not attached to the destination network, the routing tables are searched for the destination network.

If a match is found in the routing tables, the packet is sent to the next hop defined in the table. If a match is not found and there is not a default gateway, the packet is dropped and a “network unreachable” message is sent to the originator of the packet.

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What this chapter covers is how the routing tables of the router are built and maintained. Routing protocols are used to exchange information about the networks to which they are directly connected.

Routing exchanges also include information about routes they have learned from other routers. This allows a router to build a table of paths to each network in an internetworking system. There are several challenges that must be overcome for a routing protocol to be successful.

People often confuse routing protocols with routable protocols. There is a big difference. A routable protocol is a network protocol that transports data across a network and includes some sort of structure that allows the traffic to be routed to the proper network. A routing

protocol is a method by which routers exchange information about

the networks they can reach. This exchange of information allows the routers to build tables of how to reach each network. This is relatively simple until changes start happening in the network. These changes could include a WAN link failing because a construction crew cut a fiber, or a power outage at an office location. Just about anything that causes a portion of the network to become unreachable can be called a network outage. Basically, any change in the network that would affect a router’s ability to reach a network would create a topology change. When a topology change occurs, routers update each other with new information. This information may simply say that the network is no longer reachable. In the case where there are multiple paths to a network, the information may provide an alternate path to the network. This process of updating routing tables is called conver-

gence. When a network change occurs the routers reconverge.

Routing protocols are responsible for determining the path by which to send data between networks. When there is only one path from one network to another, the decision is rather easy. However, when multiple paths exist, the task becomes more complex. A routing protocol uses information it has gathered to determine which is the best path for the transport of data. This information is commonly referred to as metrics, or distance. Metrics may include information such as bandwidth of the path, delay of the path, number of hops to the destination, reliability of a path, and current load on an interface.

There are two main types of routing protocols: distance vector and link state.

Although there are many routing protocols, this chapter covers the routing protocols you are required to know for the CCNA exam.

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The dynamic routing protocols include RIP and IGRP. In addition to these you need to understand static routes. Static routes are simply routes that you pre-select. You can specify in a router that to reach a certain network, take this path. Static routes can be used for many things. The following sections will elaborate on these topics for you.

R

OUTING

P

ROTOCOL

B

ASICS

A routing protocol is responsible for exchanging valid routing information between routers so that routers can find the best path for traffic between two networks. There are several elements that might make one routing protocol a better choice than another. Some routing protocols are available on most vendors’ equipment. In cases where multiple types of routers will be exchanging routing information, you must choose a routing protocol common to the routers involved. In the case of Bay Router talking to Cisco routers, for example, you might choose RIP or OSPF as a routing protocol because both of these routing protocols are supported by both Bay and Cisco. In other cases, you may be forced to use a certain routing protocol because your network is interfacing with another that is already using a particular routing protocol. Whatever the variables, choosing a routing protocol can be an enlightening experience. Each has its advantages and disadvantages. Each also has a following with a faith stronger than some religions.

There are some terms relating to routing protocols with which you must be familiar:

á

Topology change. Any change in the layout of a network that will affect routing of traffic. This could include an interface being shut down by an administrator or a link failing due to a line being cut. It could also include the addition of a new circuit to a network. Any occurrence that causes a path to be unusable or unreliable can cause a topology change.

á Convergence. The processes of all routers on a network updating their routing tables when a topology change occurs.

á Metric. Any piece of data that a routing protocol uses to make a routing decision. Metrics can include hop count, bandwidth, reliability, ticks, cost, delay, load, and MTU.

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C h a p t e r 6 R O U T I N G 219

Path Selection

The elements used to determine the best path vary with the routing protocol. Some simple protocols use the number of hops between networks. This is not necessarily the most accurate method of deciding the best path. Consider the network shown in Figure 6.1.

Router 2

1.544 Mb

1.544 Mb

Network B

Network A

56 KB

Router 3

Router 1

F I G U R E 6 . 1

A network where number of hops is not the best method of choosing a path.

If Network A has traffic for Network B, the fastest path would be through Router 2 because the bandwidth is much higher than the direct route from Router 1 to Router 3. However, a routing protocol that only considers the number of hops between two points when choosing a path would take the direct route between Router 1 and

Router 3. A more “intelligent” routing protocol would consider the bandwidth of the links and then would choose the faster link. An even more intelligent routing protocol would consider the reliability of the link and the load already on the link when making a routing decision. This is an example of how different routing protocols choose a path. An important consideration when designing a network is how the routing protocol you choose will work within the design. If the network you are designing does not include redundant paths, then a routing protocol that considers link speed and link load is not as important as it would be in a network with redundant paths.

Static and Dynamic Routing

Routers can obtain routing information multiple ways. When a network is configured, you can place static routes in the router’s configuration. A static route is a route statement that you place in the router’s configuration. You, the network designer, determine the path to a network and use a configuration command to tell the router how to reach that network. These routes tell the router where to forward traffic destined for a remote network. Static routes are useful in many cases.

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However, the biggest disadvantage to static routes is they cannot adapt to a changing network. Not too many networks fall in the category of “never changing.” Most networks are in a state of constant growth and change. Static routes do have uses, however; they are generally used in conjunction with a dynamic routing protocol.

Static routes are routes that you enter in the configuration of the router.

They do not change dynamically with the changes in network topology. Static routes have specific uses within a network. Most Internet connections for organizations use a static route to the ISP. It would be useless for an organization with one connection to the Internet to maintain routing tables for the Internet. By using a static route to the default gateway, the router can be configured to send any traffic for which it doesn’t specifically have a routing entry to the Internet.

Dynamic routing is when the router uses some sort of routing protocol

to learn about the network and create routing tables based on this information. In a network using dynamic routing, when changes occur in the network, the changes are reflected in the routing tables soon after.

A common way that static and dynamic routing are combined is by creating a static default gateway entry and letting everything else be learned dynamically. This way, the router will reference its routing tables for a path to a remote network. If the remote network is not found in the routing tables, the packet is then forwarded to the default gateway, which would normally be a router with more complete routing tables.

In most cases in which a company has a connection to the Internet, the default gateway approach is used to forward Internet traffic to the ISP. This is helpful because maintaining complete routing tables for the Internet is a large task for a router and is also not necessary when there is only one path to the Internet.

M

ULTIPROTOCOL

R

OUTING

Cisco routers can run and use multiple routing protocols simultaneously. This makes it important to understand what they do when they receive routes for the same network from two different routing protocols.

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The router assigns an administrative distance to each routing protocol that it runs. This distance is used to weigh routes to the same network that were learned on two different routing protocols.

Administrative Distance

When a Cisco router receives routing information, this information is evaluated and valid routes are placed in the routing tables of the router. It is possible for a router to be running multiple routing protocols and to receive valid routing information for the same route via two different routing protocols. How does the router decide which route to use? Just as routes within a routing protocol are weighted with metrics, different routing protocols are weighted using a metric called administrative distance. To determine which routing protocol to select over the other, the router uses the administrative distance.

The default values for administrative distance are as follows:

Network directly connected to the router

Static route

EIGRP summary route

External BGP

Internal EIGRP

IGRP

OSPF

IS-IS

RIP

EGP

Internal BGP

Unknown

100

110

115

120

140

5

20

0

1

90

200

255

When a route is received from two different routing protocols, the route from the protocol with the lowest administrative distance is the one placed in the routing tables.

For instance, a route to 10.110.33.0 is received via the RIP routing protocol. A route to 10.110.33.0 is also received via the IGRP routing protocol. The router compares the administrative distances for the two routing protocols: RIP is 120 and IGRP is 100. Therefore, the route from the IGRP routing protocol is placed in the routing tables.

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Why are some routing protocols preferred over others? IGRP uses more meaningful metrics than RIP. RIP simply uses hop count to select the most efficient path between two networks. IGRP uses bandwidth, reliability, and load in calculating the most efficient path. These metrics are far more meaningful than those used by RIP. Therefore a route selected by IGRP is preferred over a route selected by RIP.

F I G U R E 6 . 2

Two autonomous networks connected together.

Autonomous Systems (AS)

An autonomous system (AS) is a group of routers under one administrative control that shares a routing strategy. The routers in an autonomous system can freely exchange routing information.

Routing protocols used within an AS are commonly called interior

routing protocols.

When networks that have different administrators are connected, there is more concern about the routing information that is being exchanged. A common example of this type of environment is the

Internet. The Internet consists of many networks administered by different groups that must exchange routing information with one another. By using a border gateway routing protocol, these networks can exchange routing information without concern that routing errors in one AS will adversely affect another AS. Figure 6.2 shows an example of this.

AS 100

AS 200

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C h a p t e r 6 R O U T I N G 223

R

OUTING

P

ROBLEMS

The biggest challenge for a routing protocol is a topology change. A topology change could be a new network path or the loss of an existing one. When a topology change occurs, one of the problems that can occur is a routing loop. A routing loop is a situation that occurs when two routers have routing entries that point to the same network where each route sends the data to the other router. Figure 6.3 shows an example network.

10.192.68.0

Router 1

F I G U R E 6 . 3

Example network with routing tables.

Router 1 Routing Table

10.110.1.0 Router 2

Routing

Loop

Link 1

Router 2

Router 2 Routing Table

10.192.68.0 Router 1

10.110.1.0 Router 3

Link 2

Router 3

Router 3 Routing Table

10.192.68.0 Router2

In Figure 6.3, you see that Router 2 has routes to two networks. If

Link2 goes down, Router 2 loses its path to network 10.110.1.0. If this happens, Router 1 sends a routing update to Router 2, indicating that it has a path to network 10.110.1.0. Now Router 2 thinks it can reach network 10.110.1.0 through Router 1. In reality this is not true. However, if Router 1 forwards a packet for network

10.110.1.0 to Router 2, Router 2 will forward the packet back to

Router 1, which will forward the packet back to Router 2. This will continue until the packet’s Time To Live (TTL) expires.

TTL is a field in the packet header of an IP packet that is set to a number when the packet is generated. Every time the packet passes through a router, the TTL is decremented. If this number reaches zero, the packet is discarded. This keeps packets from endlessly being caught in a routing loop.

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R E V I E W B R E A K

Different routing protocols have different ways of trying to avoid routing loops. Two such techniques are the split horizon rule and the poison reverse update.

Split Horizon

Split horizon is a rule that states “do not send routing updates back

out of the interface they were learned on.” This keeps a router from sending routing updates back to the router from which they were learned. If you refer to Figure 6.3, you will see that if Router 1 did not advertise the route for network 10.110.1.0 back out of the interface from which it learned the path, Router 2 would not have been able to create a routing loop. Another method to avoid routing updates is the poison reverse update.

Poison Reverse Update

A poison reverse update is a routing update that contains a route that is no longer reachable. The hop count for the route is set to infinity (as defined by the routing protocol) so the route will be marked as invalid.

A poison reverse update allows a router that knows a path is no longer reachable to update other routers in the network with this information.

Routing Recap

.

Routed protocols are protocols that carry data for applications.

These protocols include an addressing scheme that contains some hierarchical information that allows the packets to be routed. Examples of routed protocols include IP and IPX.

.

Routing protocols are the protocols used by routers to exchange

information about connected networks. Routing protocols are used by routers to make decisions about which path to use when forwarding packets from a routed protocol. Examples of routing protocols include RIP and IGRP.

.

A metric is any piece of information a routing protocol uses to decide which network path to use when forwarding packets of data. Some items that can be considered metrics are hop count, bandwidth, reliability, ticks, cost, delay, load, and MTU.

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.

Convergence is the process a routing protocol goes through

when a topology change occurs in a network. During convergence, the routing tables of the routers in the network are updated with new information about the network.

.

Static routes are determined by the network designer and are

added to the configuration of the router. Static routes are static. They do not change unless you physically change them in the configuration.

.

Dynamic routes are determined by a routing protocol. These

routes are added to the routing table by the routing protocol after the routing protocol considers all of the metrics and determines the best path to forward data to a particular network.

.

A topology change is anything that changes the connections in a network. This could be the addition of a connection, a lost connection, or even a congested connection.

.

When a router is using multiple routing protocols, and multiple routing protocols report paths to a particular network, the router uses the administrative distance to determine which routing protocol’s path to use. The routing protocol with the lowest administrative distance is the one used to forward packets.

.

An autonomous number creates a group of routers that freely exchange routing information.

.

A routing loop is a situation in which two routers each think the other is a path to a particular network. Router A will forward a packet to Router B. Router B will reference its routing table for a path to the network and find Router A as the path. Router B then forwards the packet to Router A. Router A again forwards the packet to router B. This continues until the Time To Live

(TTL) of the packet expires and the packet is dropped.

.

One of two methods used to avoid routing loops is the split horizon rule. Split horizon is a rule that states “do not advertise routes on interfaces they were learned from.” Basically, this rule states “do not tell the router you learned a route from that you are a path to that network.”

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.

The second method used to avoid routing loops is a poison

reverse update, which is a routing update that marks a network

as unreachable. When a router attached to a network determines the network is no longer reachable, the router sends out a poison reverse update to tell other routers that the network is no longer reachable.

Beyond understanding the basic workings of routing, you should be familiar with the inner workings of the different protocols at your disposal. The remainder of the chapter will go into detail regarding the two different types of routing protocols (link state and distance vector) and the different options and configurations for each, including examples.

D

ISTANCE

V

ECTOR

P

ROTOCOLS

A distance vector routing protocol uses the number of hops between networks. Any time a packet must be read off a network, a routing decision is made and the packet is forwarded to another network.

This trip through a router is called a hop. A distance vector routing protocol exchanges two pieces of information in a routing exchange: distance and vector. Distance is defined as a metric and the vector is defined as next hop router.

When distance vector protocols send updates to neighboring routers, the default is to send the entire routing table. This can use up valuable bandwidth on a network with many routers. This also leads to routing loops as described previously.

Distance vector routing protocols include Routing Information

Protocol (RIP) and Internetwork Gateway Routing Protocol (IGRP), which are discussed in the following sections.

Routing Information Protocol (RIP)

Routing Information Protocol (RIP) is a distance-vector routing

protocol that uses hop count as the metric to determine the best path. RIP is used for routing IPX and IP traffic. For IP traffic, there are two versions of RIP currently in use: RIPv1 and RIPv2.

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The biggest difference between the two versions has to do with subnet masks. RIPv1 does not include the subnet mask in the routing update. Because RIPv1 does not include subnet mask information with the update, all networks connected via RIP must use the same subnet mask. If you are designing a network where different subnet masks will be used on different networks, you must use RIPv2 if you are using RIP as a routing protocol.

RIP Routing Tables

Different routing protocols maintain different information in the routing tables. RIP maintains the following information in its routing table:

Destination Next Hop Distance Timers Flags

10.110.20.0

10.44.17.0

10.202.14.0

10.65.32.1

10.65.32.1

10.44.23.1

2

3

13 t1, t2, t3 t1, t2, t3 t1, t2, t3 x, y x, y x, y

The destination network is listed with the IP address of the next hop along the path to reach that network. The distance to the network in hops is the metric used to determine the best path. The timers are used to determine when a route is no longer valid and to avoid routing loops. The timers will be discussed in more detail later in this section. The flags indicate whether the route is valid and whether it is currently being held down.

The router builds its routing tables by listening to its neighbors. The neighbor routers send out updates with the networks reachable through that router and the number of hops to that network. The receiving router takes that information and builds a table like the previous one. If the router receives an update from another router with a path to a network it already has in its routing table, the hop counts are compared. If the new route is shorter in terms of hops than the one in the routing table, the routing table entry is replaced with the new path. This is an important point. RIP maintains only one path to any destination network although there may be multiple paths to the network. Some other routing protocols keep multiple paths in the routing tables and select the one of lower cost. This helps other routing protocols make quicker decisions about routing traffic when a path becomes unusable.

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Regular Updates and Triggered Updates

RIP has two types of updates: regular updates that occur periodically based on the routing update timer, and triggered updates that occur anytime there is a network topology change. Regular updates are used to update routing tables and indicate a route is still available. Triggered updates make the network converge faster since routers do not wait until the regular update to report the changes to the network.

Hop Count

RIP uses the number of hops to a network as the metric for determining the best path. This is not necessarily the best way to determine the most efficient path for network traffic. There is a limit of 15 hops in a RIP network. Any network 16 or more hops away is considered unreachable. This is significant because this is the method RIP uses to report a down path. When a route becomes unreachable, the router connected to the path sends out a triggered update reporting the path as 16 hops away. Because 16 hops is considered unreachable, routers receiving this update will mark the path unreachable in their routing tables.

Routing Update Timer

RIP uses timers as methods to many of its functions. The first timer to discuss is the routing update timer. The routing update timer is normally set to 30 seconds. This ensures that every router sends a complete copy of its routing table every time the routing update timer counts down to zero, or 30 seconds if the default is kept.

Each time the routing update timer counts down to zero, the router sends its entire routing table out to all active interfaces. After the complete table is sent, the timer is reset.

One of the disadvantages of RIP is that it sends the entire routing table periodically even if no topology changes have occurred. This is very inefficient compared to some other routing protocols. This is one of the things that limits RIP’s scalability for large networks. Also, if the routing tables are large, the routing updates can consume a large amount of bandwidth from low bandwidth WAN connections.

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Route Invalid Timer

The route invalid timer determines the amount of time that must pass without hearing a routing update before the router considers a route invalid. The default setting for the route invalid timer is 90 seconds. This is three times the default for the routing update timer.

There is a route invalid timer for each entry in the routing table.

Each time an update for that entry is received, the route invalid timer is reset. If the timer counts down to zero, the route is then marked as invalid. After a route is marked invalid, neighbor routers are sent this information.

Route Flush Timer

The route flush timer determines the amount of time that must pass without receiving an update for a routing table entry before the entry is removed from the table. There is a route flush timer attached to each entry in the routing table. When an update is received for that entry, this timer, along with the route invalid timer, is reset. If the route flush timer counts down to zero, the route entry is completely removed from the routing table. The default for the route flush timer is 270 seconds, or nine times the update timer.

Hold-Down Timer

Hold-down timers are used to prevent a regular routing update from

reinstating a route that is down. When a route goes down, the neighboring routers detect it and calculate new routes. These new routes are sent out in routing updates to inform their neighbors of the change.

This change ripples through the network. Because this update does not instantly arrive at every router in the network, it is conceivable that a router that has not yet received the update could send out a regular update indicating the route that has just gone down is still good.

A router that has just received the update that the route has gone bad could receive this update indicating the route is good.

A hold-down timer tells the router to hold down any changes that might have recently gone down for a period of time. This prevents a stale routing update from changing the route to an up status during this time. This allows time for the update indicating that a route has gone down to converge throughout the network.

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Configuring RIP

To configure RIP in a router, you must first have your interfaces configured with the IP addresses. Configuring an IP address on an interface is covered in Chapter 3, “Understanding and Configuring

TCP/IP.” Then you configure the router rip as follows: c2513r2#config t

Enter configuration commands, one per line. End with

➥CNTL/Z.

c2513r2(config)#router rip c2513r2(config-router)#network 10.0.0.0

c2513r2(config-router)#exit c2513r2(config)#exit c2513r2#

This configures RIP on all of the networks that fall under the Class A range of 10.0.0.0. It is conceivable that you may not want RIP routing updates to be broadcast out all of the interfaces on a router. Many times it is not desirable for routing updates to be broadcast on LAN interfaces if there are no routers on that segment to receive the updates.

In this case, you can set the interface to passive in the router configuration. When an interface is set to passive, it will not broadcast routing updates but it will still listen for them. Following is an example of how to set the Ethernet segment as passive for this configuration: c2513r2#config t

Enter configuration commands, one per line. End with

➥CNTL/Z.

c2513r2(config)#router rip c2513r2(config-router)#network 10.0.0.0

c2513r2(config-router)#passive-interface ethernet 0 c2513r2(config-router)#exit c2513r2(config)#exit c2513r2#

After RIP is configured, it will begin exchanging routing updates with other RIP enabled networking devices. It is helpful to view the routing tables and statistics about what RIP has learned.

Working with RIP

As with everything in life, routing protocols are not perfect. There will be times you think you have configured everything correctly but the routers just can’t seem to find a path to the network you just added. In this case, it is necessary to check how the routing protocol is reacting. The first place to start looking is the routing table.

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The following example shows how to check the routing tables using the show ip route command:

Router#show ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, *

➥- candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets

C 10.245.34.0 is directly connected, TokenRing0

R 192.168.99.0/24 [120/1] via 10.245.34.2, 00:00:12,

➥TokenRing0

199.250.137.0/27 is subnetted, 1 subnets

C 199.250.137.32 is directly connected, Ethernet0

Router#

The routing table includes a great deal of useful information for diagnosing a routing problem. In the routing table above, notice the route to 10.245.34.0 is listed as directly connected to TokenRing0.

This is information that RIP will broadcast to other routers. Network

10.245.34.0 is reachable via this router. To the left of the route is a letter indicating how the route was learned. In the case of

10.245.34.0 it is

C

, which stands for connected network. The table shown at the top of the routing table lists each letter and its meaning.

The next route in the table is to 192.168.99.0 and it was learned via the RIP routing protocol. The numbers [

120/1

] indicate that it has an administrative distance of 120 and a RIP distance of 1 hop. The first number is the administrative distance assigned to the routing protocol. The second number is the calculated distance for the routing protocol and thus will be greatly different for different routing protocols.

The next piece of information about this route is that it was learned via 10.245.34.2. This indicates the router that advertised this route.

The next entry is the time since the route was last updated. If this time reaches the invalid timer, then the route will be considered invalid and will not be used. The last piece of information about the route indicates the interface from which the routing update was received. In this case, it was learned on interface TokenRing0.

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The following example shows how to view the settings of the majority of RIP’s timers and other important information using the command show ip protocol

: c2513r2#sh ip protocol

Routing Protocol is “rip”

Sending updates every 30 seconds, next due in 24 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 Key-chain

Loopback0 1 1 2

TokenRing0 1 1 2

Routing for Networks:

192.168.99.0

192.168.42.0

10.0.0.0

Passive Interface(s):

Ethernet0

Routing Information Sources:

Gateway Distance Last Update

10.245.34.1 120 00:00:15

Distance: (default is 120) c2513r2#

The command show ip protocol will list each of the routing protocols the router is running and statistics on each. For RIP, the information shown includes the settings for each of the timers, which interfaces are being used to broadcast the RIP information, for which networks information is being advertised, any passive interfaces, and the sources from which routing information has been learned.

The Limitations of RIP

RIP is a useful routing protocol in small networks where compatibility is necessary with other equipment that only supports RIP. However,

RIP has some serious limitations that make it unsuitable for large complicated networks.

The first limitation is that of the hop count. RIP supports no more than 15 hops from one end of a network to the other. This limits the physical size of the network on which RIP can be used.

Another limitation of RIP is the fact that RIP sends its entire routing table every 30 seconds by default. By sending the entire routing table periodically, RIP generates a fair amount of network traffic. In a large network this could become very significant.

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RIP also is slow to converge, which can cause problems in large networks when topology changes occur.

The single metric of hop count creates another limitation for RIP.

Because RIP considers only the number of hops when determining the best path for traffic, it will not always select the most efficient route for the traffic. More useful routing protocols consider many other factors when selecting a best path for traffic, such as bandwidth and current load.

Given the option, it is best to use a routing protocol more sophisticated than RIP. IGRP is a more robust routing protocol and is also easy to configure.

Internet Gateway Routing Protocol

(IGRP)

Internet Gateway Routing Protocol (IGRP) was developed by Cisco as

a routing protocol more scalable than RIP. IGRP offers many other advantages, as well. Its main disadvantage is that it is Cisco proprietary and only works with Cisco routers. Therefore, in a network where multiple vendors’ routers are used, IGRP is not an option.

IGRP uses more meaningful metrics, however, when selecting a path for network traffic.

IGRP Metrics

When IGRP selects the best path for network traffic, it uses many metrics. IGRP considers bandwidth, delay, load, and reliability when deciding which path is the best to a network. Each metric in

IGRP is weighted and a distance is equated. This distance is used to determine the best path to a network.

Similarities Between IGRP and RIP

In many ways, IGRP is not much different than RIP. They are both distance vector routing protocols. Both protocols periodically transmit their entire routing table, with the exception of routes suppressed by split horizon. They both use split horizon with poison reverse, triggered updates, and hold-down timers to prevent routing loops.

Exam-related protocols Most people use EIGRP or OSPF (BGP for

ISPs). But these routing protocols are not covered on the CCNA exam.

OSPF and EIGRP are on the ACRC test. BGP is touched on the ACRC test, but is mainly CCIE material.

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Differences Between IGRP and RIP

IGRP has several significant differences from RIP. IGRP uses autonomous systems to group routers in a routing domain.

IGRP broadcasts its full routing table every 90 seconds by default.

RIP does this every 30 seconds.

IGRP Timers

IGRP uses timers much like RIP does. These timers ensure that certain events occur periodically—such as routing updates. They can also indicate when a route is no longer valid or when a route should be purged from the routing tables.

IGRP uses the following timers to operate properly:

á Update timer. This timer indicates when a new routing update should be sent out. The default value for this timer is

90 seconds.

á

Invalid timer. An invalid timer is attached to every route in the routing tables. When an update is received for this route, the timer is reset. If the timer ever reaches zero, the route is then considered invalid and is no longer used. The default is three times the update timer, or 270 seconds.

á

Hold-down timer. When a route is marked invalid, it is kept marked invalid for the duration of the hold-down timer. The default is three times the update timer plus 10 seconds.

á

Flush timer. A flush timer is attached to every route in the routing table. The flush timer is reset every time a route update is received for the route. If the route flush timer counts down to zero, the route entry is completely removed from the routing table. The default time is 270 seconds.

Configuring IGRP

Configuring IGRP is very much like configuring RIP—with one caveat: IGRP requires an autonomous number. This autonomous number indicates a group of routers participating in a routing scheme.

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Routers using the same autonomous number will freely exchange routing information. Some networks have multiple autonomous numbers and one or two routers will be used to exchange the routing information between the two systems. These routers will control what routing information is exchanged between the systems. The concept of exchanging information between different autonomous systems is beyond the scope of the CCNA exam and thus is not covered in this book.

IGRP, like RIP, does require that the interfaces be configured with IP addresses because it is the IP protocol that is being routed. To configure IGRP on a router, start with the routing igrp asnumber command where asnumber is the number representing the autonomous system in which the router will participate. It is common to use 100 as a starting number in private networks; however, you are free to use any number you prefer between 1 and 65,535 if you are in sole control of the network. If your network is participating with other networks outside of your control, you will need to coordinate with the other network administrators. After the routing command is configured, list the networks under the control of this routing protocol. Following is an example of how IGRP is configured on a router.

Router#config t

Enter configuration commands, one per line. End with

➥CNTL/Z.

Router(config)#router igrp 100

Router(config-router)#network 199.250.137.0

Router(config-router)#network 10.0.0.0

Router(config-router)#exit

Router(config)#exit

Router#

Working with IGRP

Just as with RIP, after you have configured IGRP on a router, it is helpful to check the routing tables to see what the router has learned and placed in the routing tables. Because the routing information from all routing protocols is consolidated in one routing table, the same command is used to view this information in IGRP as it was for RIP.

The following example shows how to check the routing tables using the show ip route command:

Router#sh ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M -

➥mobile, B - BGP

continues

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236 Pa r t I E X A M P R E PA R AT I O N

continued

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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, *

➥- candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets

C 10.245.34.0 is directly connected, TokenRing0

I 192.168.99.0/24 [100/1188] via 10.245.34.2, 00:00:03,

➥TokenRing0

I 192.168.33.0/24 [100/1188] via 10.245.34.2, 00:00:03,

➥TokenRing0

199.250.137.0/27 is subnetted, 1 subnets

C 199.250.137.32 is directly connected, Ethernet0

Router#

The routing table information looks much the same as RIP with one noticeable difference. Look at the route for 192.168.99.0. The distance to this route is [

100/1188

]. The 100 is the administrative distance for the routing protocol IGRP. The 1188 is the metric IGRP calculated for this path. The metric is based on all of the factors that

IGRP uses to calculate the most efficient path.

There are times at which you may want to force the router to rebuild its routing table. When diagnosing problems, you might find that a router has an improper route for a network. If you think the router may have missed an update for some reason, you can clear the router’s routing table. This will force the router to rebuild this table from current information. To clear the routing table, use the command clear ip route followed by the network for which you want to clear the route. If you want to clear the entire routing table, use the command clear ip route *

. This clears the entire routing table and rebuilds it. Depending on the routing protocol being used, rebuilding the routing table may take a few minutes.

Just like RIP, the show ip protocol command will display information about the IGRP protocol.

Router#sh ip protocol

Routing Protocol is “igrp 100”

Sending updates every 90 seconds, next due in 43 seconds

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Invalid after 270 seconds, hold down 280, flushed after

➥630

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

IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

IGRP maximum hopcount 100

IGRP maximum metric variance 1

Redistributing: igrp 100

Routing for Networks:

199.250.137.0

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.245.34.2 100 00:00:10

Distance: (default is 100)

Router#

The information displayed is similar to that of RIP. There are a few exceptions, however. There are metric weights listed, which are parameters that allow you to change the behavior of the IGRP routing protocol. You can adjust how much weight is given to each of the metrics considered by IGRP. You could adjust bandwidth to have less weight than reliability when making a routing decision. Other information different from RIP includes the maximum hop count that can be adjusted.

L

INK

S

TATE

P

ROTOCOLS

Link state routing protocols were designed to make up for some of

the shortcomings of distance vector methods. Link state protocols do not forward their entire route table, instead sending only information about networks to which the router has direct connections. This saves bandwidth and helps avoid routing loops caused by ‘secondhand’ information. These updates, called Link State Packets (LSPs) are forwarded around the network so that all routers get a consistent view of the network as a whole.

Link state protocols are also designed to handle much larger networks. Many distance vector protocols are limited in size to 15 hops, whereas link state protocols can go much higher. This adds a scalability that is not possible with distance vector routing.

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The primary disadvantages of link state routing are expense and implementation. Link state routing requires significantly more processing than distance vector protocols do, which makes it more expensive to implement in a router.

S

ELECTING A

R

OUTING

P

ROTOCOL

When you are faced with making a decision on which routing protocol to use, there are many factors you need to consider. First, you need to ask yourself what routing protocols are supported by all of the routers that will participate in the routing domain. If there is only one common routing protocol, your choice is fairly simple. If multiple routing protocols are support by all of the routers that will participate, then you must consider the ease of configuration. The two routing protocols you are required to know for the CCNA exam are RIP and IGRP, which are both simple to configure. However, there are other routing protocols that take a great deal more time and effort to configure. This should be considered when selecting a routing protocol. For instance, the size of the network is also a consideration when selecting a routing protocol. If you are designing the next Internet, RIP is not an option because its limitation of 15 hops would not work for a worldwide network like the Internet.

C

A S E

S

T U D Y

: T

H E

P

A S S I V E

I

N T E R FA C E

E S S E N C E O F T H E C A S E

.

A newly installed router has been configured to use the RIP routing protocol in an attempt to have it and the existing router exchange routing information. However, after installing the new router, workstations on the new network segment cannot communicate with the existing network segment.

.

You must use the diagnostic commands to determine why you are unable to communicate between the networks.

S C E N A R I O

You are a network engineer for a computer training facility. Recently your company added a new network for one of the classes. The network is segmented from the main office network by a router named Classrm. In the classroom, there are 10 workstations and one server. Each of the workstations is assigned an IP address in the range

10.200.1.10–19. The server is assigned the IP address 10.200.1.5 and the router’s Ethernet0 port is connected to the network in the classroom.

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C h a p t e r 6 R O U T I N G 239

C

A S E

S

T U D Y

: T

H E

P

A S S I V E

I

N T E R FA C E

Ethernet0 has been assigned the IP address

10.200.1.1. This network is using a subnet mask of 255.255.255.0.

The main router for the office is named Denver.

Denver is connected to the main office LAN using

Ethernet0. The IP address assigned to Ethernet0 of Denver is 10.100.1.1 and subnet mask

255.255.255.0. The router Classrm is connected to the main LAN using interface Ethernet1, which has been assigned the IP address 10.100.1.2

and subnet mask 255.255.255.0. After installing the router Classrm, you find that the workstations in the classroom can communicate among themselves and the server in the classroom but none of the computers can communicate with the computers on the main office network. You need to fix this so the computers can access the testing software that is on a server on the main network.

Router Denver has already been configured to use the RIP routing protocol, so the person who installed the new router configured Classrm to use it as well.

A N A LY S I S

You begin by checking the routing tables of the router Classrm. You find that the route to network 10.100.1.0 is in the routing table as a connected network. You also find a route to network

10.200.1.0 as a connected network. From the router, you ping the server on the main network that houses the testing software. The ping is successful.

You then log in to the router Denver and check the routing tables. You find that the network

10.100.1.0 is listed in the routing table as a connected network. However, you do not see the network 10.200.1.0. You check the routing statements in the router and they appear as follows:

Router rip

Network 10.0.0.0

You then log back into the router Classrm and check the routing statements in that router. They appear as follows:

Router rip

Network 10.0.0.0

Passive-interface Ethernet1

This does not appear correct to you. You check your reference material and find that the passiveinterface command is telling the router not to advertise RIP on the network attached to

Ethernet1. This could be the problem. You remove the passive-interface command from the configuration and save the changes.

You wait a couple of minutes to allow the routing broadcasts to update the routing tables.

Again you check the routing tables in the router

Denver and find that now network 10.200.1.0 is appearing in the table and is listed as being learned through RIP.

You go to the workstations in the classroom and ping the server on the main network where the testing software is housed. The ping is successful. You then attempt to run the testing software and it works. Congratulations!

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C

H A P T E R

S

U M M A R Y

KEY TERMS

• routing protocol

• RIP

• IGRP

• metric

• hop

• update timer

• hold-down timer

• invalid timer

• flush timer

• routing loop

• split horizon

• poison reverse update

• autonomous system

• administrative distance

Routing protocols are used by routers to exchange information about

the networks with which they are connected. By exchanging this information, other routers in the network can learn what path to take to reach a destination network. Routing protocols use metrics to determine the most efficient path between to routers. These metrics can include such items as the number of hops between the networks, the bandwidth of the links between the networks, the reliability of the link, and the load on the link. The routing protocol being used determines how the metrics are used in calculations to determine the best path. Know how to configure both RIP and

IGRP, as well as the strengths and weaknesses of both.

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C h a p t e r 6 R O U T I N G 241

A

P P L Y

Y

O U R

K

N O W L E D G E

Review Questions

1. What metrics are considered by the RIP routing protocol?

2. What is considered unreachable by the RIP routing protocol?

3. What situation could be true that would make

RIP select a path that is not the most efficient?

4. What are some of the metrics considered by the

IGRP routing protocol?

5. What timers are used in RIP and IGRP to maintain the routing tables and prevent routing loops?

6. Describe split horizon.

7. Describe a poison reverse update.

8. What is an autonomous system?

9. What is the function of the flush timer?

10. Describe how a router uses administrative distance when making a routing decision.

Exam Questions

1. What is the proper way to configure RIP in a router?

A.

Router(config)#router rip

B.

Router(config)#router rip 100

C.

Router(config)#routing rip

D.

Router(config)#routing rip 100

2. After RIP is configured, how do you specify which networks to distribute information about?

A.

Router(config-router)#distribute 10.0.0.0

B.

Router(config)#network 10.0.0.0

C.

Router(config-router)#network 10.0.0.0

D.

Router(config)#distribute 10.0.0.0

3. What is special about a hop count of 16 in the

RIP protocol?

A. It indicates that the network is directly connected to the router.

B. It indicates a network 16 hops away.

C. It indicates that the route was received 16 seconds ago.

D. It indicates that the network is unreachable.

4. Which timer limits how long a route that has been marked invalid will be kept in the routing tables?

A. Update timer

B. Invalid timer

C. Flush timer

D. Hold-down timer

5. What does the statement “Routing information is not sent back out of the interface from which it was learned” describe?

A. The split horizon rule

B. Poison reverse updates

C. A routing loop

D. The divided parallel rule

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A

P P L Y

Y

O U R

K

N O W L E D G E

6. Suppose a router is running two routing protocols and a valid route is received from both protocols to one remote network. How does the router choose which route to place in its routing table?

A. The first routing protocol to report the route is used.

B. The first routing protocol configured is preferred over the next.

C. The administrative distance of the routing protocols are used to select which route to add.

D. The route with the lowest metric is used.

7. What command will display statistics about the routing protocols running and their timers?

A.

Router#show ip interface

B.

Router#show routing protocols

C.

Router#show ip protocol

D.

Router#sh ip stats

8. What command will clear the entire routing table and force the router to rebuild them?

A.

Router#clear routing tabl e

B.

Router#clear ip route

C.

Router#clear ip route *

D.

Router#rebuild routing table

9. Given the following line from a routing table, what does the [

120/12

] indicate?

R 192.168.99.0/24 [120/12] via

➥10.245.34.2, 00:00:06, TokenRing0

A. The administrative distance of RIP is 120 and the network is 12 hops away.

B. The administrative distance of RIP is 12 and the network is 120 hops away.

C. The route was received 120 seconds ago and is 12 hops away.

D. The route was received 12 seconds ago and is

120 hops away.

Answers to Review Questions

1. RIP considers only the number of hops between networks when making routing decisions. For more information, see “Routing Information

Protocol (RIP).”

2. RIP considers any network more than 15 hops away unreachable. For more information, see

“Routing Information Protocol (RIP).”

3. Because RIP considers only the number of hops, it is possible that RIP could select a slow link of only one hop rather than a much faster link that is two hops away. This would cause RIP to select a less efficient path for network traffic. For more information, see “Routing Information Protocol

(RIP).”

4. IGRP considers bandwidth, delay, reliability, and load on the link when making a routing decision.

For more information, see “Internet Gateway

Routing Protocol (IGRP).”

5. RIP and IGRP use the update timer, invalid timer, hold-down timer, and flush timer. For more information, see “Routing Information

Protocol (RIP)” and “Internet Gateway Routing

Protocol (IGRP).”

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C h a p t e r 6 R O U T I N G 243

A

P P L Y

Y

O U R

K

N O W L E D G E

6. Split horizon is a rule that states “routes are not sent back out of the interface from which they were learned.” For more information, see “Split

Horizon.”

7. A poison reverse update is a routing update with a metric that makes the network unreachable. This is a method for a router to force other routers to update their routing tables with information about a down network. For more information, see

“Poison Reverse Update.”

8. An autonomous system is a group of routers freely exchanging routing information. For more information, see “Autonomous Systems (AS).”

9. The flush timer specifies how long a route is kept in the routing table after it has been marked invalid. For more information, see “Route Flush

Timer.”

10. A router uses the administrative distance to select the route to add to the routing table when a path to a network is learned from two different routing protocols. For more information, see

“Routing Information From Multiple Protocols.”

Answers to Exam Questions

1. A. The proper way to configure RIP on a router is router rip

. For more information, see

“Routing Information Protocol (RIP).”

2. C. The proper way to list a network for RIP to distribute is

Router(config-router)#network

10.0.0.0

. For more information, see “Routing

Information Protocol (RIP).”

3. D. In RIP, a hop count of 16 is considered unreachable. For more information, see “Routing

Information Protocol (RIP).”

4. C. The flush timer limits how long a route that has been marked invalid will be kept in the routing table. For more information, see “Flush

Timer.”

5. A. The statement describes the split horizon rule.

For more information, see “Split Horizon.”

6. C. The administrative distance is used to select which routing protocol to use. For more information, see “Administrative Distance.”

7. C. show ip protocol lists a great deal of information about the routing protocols running on a router and the timers associated with those routing protocols. For more information, see

“Working with IGRP.”

8. C. clear ip route * will clear the routing tables and force the router to rebuild them. For more information, see “Working with IGRP.”

9. A. The [

120/12

] indicates that the administrative distance of RIP is 120 and the network is 12 hops away. For more information, see “Working with RIP.”

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration, Cisco Press, 1998.

2. Doyle, Jeff. Routing TCP/IP Volume I, Cisco

Press 1998.

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O

B J E C T I V E S

Cisco provides the following objectives for the Network

Security portion of the CCNA exam:

Configure standard and extended access lists to filter IP traffic.

.

In order to secure complex networks, you must be able to configure standard and extended access lists for filtering IP traffic.

.

Monitor and verify selected access list operations on the router.

To maintain the security of complex networks where access lists are implemented, you must be able to monitor and verify these access lists.

C H A P T E R

7

Network Security

10 051-6 CH 07 8/10/99 9:23 AM Page 246

O

U T L I N E

S

T U D Y

S

T R AT E G I E S

Introduction

Understanding Access Lists

Uses of Access Lists

The Parts of an Access List

How an Access List Works

How Access Lists Affect Performance

Types of Access Lists

Standard IP Access Lists

Extended IP Access Lists

Implementing Standard IP Access Lists 251

Restricting Traffic to One Network

Restricting Traffic from One Network

Restricting Traffic from One Network

Station

Allow Only Traffic from IP Addresses

Ending in .242

Wildcards in the IP Address

Implementing Access Lists

252

252

253

254

255

256

Implementing Extended IP Access Lists 256

Deny Telnet Requests to a Host on

Your Network 258

Managing Access Lists

Editing Access Lists

Deleting Access Lists

Verifying Access Lists

Viewing Access Lists

247

247

248

249

250

250

251

251

258

259

259

260

260

Named Access Lists 261

247

.

The key to studying for the security objectives is to practice using access lists. Create access lists and test them to be sure they are working as you expect. Experiment with different ways of creating access lists to find the most efficient method of arranging the tests. Access lists can be tricky. When you create a new access list, check to be sure it does what you want it to do.

Chapter Summary 265

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I

NTRODUCTION

When designing networks that will interface with other networks beyond your control, you need methods to secure your network. With the rush for companies to connect their networks to the Internet, there is a real need to secure local networks from distrusted network attacks, configuration errors, and failures. One way to secure a local network is through the use of access lists within the router’s configuration.

An access list can filter the type of network traffic that will be allowed to pass through the router. Cisco’s IOS provides a means to identify traffic in many of the protocols it supports. This chapter focuses on how to create and apply TCP/IP access lists.

C h a p t e r 7 N E T WO R K S E C U R I T Y 247

U

NDERSTANDING

A

CCESS

L

ISTS

Access lists are basically a set of tests that packets must pass before being allowed to enter. In an outgoing access list, the packet must be allowed by the access list before being forwarded out of an interface.

An incoming access list tests each packet entering the router from an interface. If the packet is allowed by the access list, it may enter. If it is not allowed, the packet is dropped.

When a packet is being evaluated with an access list, the router checks to see if a packet matches the test in each line of the access list and then either allows or denies the packet. If the packet doesn’t match any of the lines in an access list, it is discarded. This is called the implicit deny all. If a packet is not explicitly allowed by an access list, it will be dropped.

An access list is created in the router configuration with parameters identifying the traffic for special handling. This access list is identified by a number or, starting with release 11.2 of Cisco’s IOS, a name. This access list is then applied to an interface of the router or to some other part of the configuration for special handling.

Uses of Access Lists

An access list can be used to identify network traffic for priority handling. Traffic matching the access list can be given higher priority when the network is congested and traffic is queued for an interface.

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Access lists identify interesting traffic for dial-up links. Interesting traffic is network traffic you have determined can initiate dialing on a dial-up link. This can be very important when you’re facing the high cost of some ISDN links.

An access list can be used to limit routing updates or to limit certain

SAPs for bridging. Access lists have many uses. As you climb the

Cisco Certification ladder, the uses you are required to understand become more complex.

The most common use of an access list is as a filter. In the access list, you identify certain types of traffic that will not be allowed to exit an interface. This creates a filter for network traffic and is often used to secure networks from other networks, such as the Internet. If you have a router that attaches a private network to the Internet, it is common to create access lists to limit the types of traffic that can come from the Internet to your private network.

The Parts of an Access List

An access list has two elements: the list itself, and the statement that assigns a router interface to the access list. The statement assigning an interface to an access list is called the access group statement.

You can assign only one access list per protocol to an interface. You can have one IP access list and one IPX access list.

Access lists are identified by number. Starting with IOS 11.2, named access lists are supported for some protocols. By carefully selecting a name, you can provide a clue as to what the access list is doing. This can be very helpful when you’re looking at a config that hasn’t been touched for a year and you’re trying to determine the access list’s goal.

Here is a list of the types of access lists available under Cisco’s IOS 11.2:

Protocol Range

IP

Extended IP

1-99

100-199

Ethernet type code

DECnet

XNS

200-299

300-399

400-499

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C h a p t e r 7 N E T WO R K S E C U R I T Y 249

Protocol

Extended XNS

AppleTalk

Ethernet address

Novell

Extended Novell

Novell SAP

Range

500-599

600-699

700-799

800-899

900-999

1000-1099

How an Access List Works

An access list can be applied to traffic entering or exiting an interface. These two ways of implementing an access list are called

inbound and outbound. Inbound access lists create more of a load on

the router’s processor, and thus, are not recommended unless absolutely necessary. In most cases, you should be able to enforce network traffic or routing polices using outbound access lists. The discussions in this chapter focus on outbound access lists.

Figure 7.1 shows how the router evaluates a packet for forwarding with an access list.

F I G U R E 7 . 1

The flow of an access list.

Packet

No

Can I route this packet?

Yes

Select interface

Bit bucket

No

Test packet against access list permit?

Yes

Yes

Access list?

No

Send packet

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Network Traffic Must Be Explicitly

Allowed When an access list is applied to an interface, all traffic is disallowed except that which is explicitly allowed in the access list.

When you’re creating an access list, remember that the router adds the deny all command to the end of the access list.

Each statement in the access list is evaluated. The packet is either permitted to pass, denied passage and tossed in the bit bucket, or falls through to the next statement in the access list because it doesn’t match that statement. If a packet reaches the end of the access list and it hasn’t matched any statements, it is discarded. This is because access lists have an implicit “deny all” added to the end. This is an important concept to understand. If you don’t explicitly permit some traffic in your access list, all traffic will be denied.

When the router receives a packet for routing, it first checks the routing tables to determine how to forward the packet. If it determines that it doesn’t have a way to forward the packet, the packet is dropped.

However, if the router determines it can forward the packet and knows which interface the packet should be sent to, it then checks to see if the interface is grouped to an access list. If not, the packet is copied to the interface’s output buffer, and the packet is forwarded.

If the router determines that the exiting interface is grouped to an access list, the packet is subjected to the test of the access list before it is forwarded. Only if the packet is specifically allowed in the access list is it forwarded.

How Access Lists Affect Performance

An access list adds much overhead to the forwarding process. The router must perform a great deal of work each time it forwards a packet. How you design an access list and apply it to an interface can help reduce some of this overhead.

When the router is evaluating a packet through an access list, it works through the list from top to bottom. Remember this when creating an access list. The router will stop processing the access list on the first match. Whenever possible, design your access lists so that the majority of traffic matches the first few statements. This will keep the processor load created by the access list to a minimum.

Types of Access Lists

You will deal with two types of access lists for the CCNA exam: the standard IP access list and the extended IP access list.

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C h a p t e r 7 N E T WO R K S E C U R I T Y 251

Standard IP Access Lists

A standard IP access list is a simple list of permit and deny statements based on a source IP address. The standard IP access list permits or denies the entire IP protocol suite based on the source IP address. The extended IP access list offers more granular control over network traffic.

Extended IP Access Lists

An extended IP access list gives you more granular control over traffic identification. It allows decisions based on source and destination IP addresses, particular protocols within the IP protocol suite, and port numbers. The extended IP access list offers much more flexibility in describing the type of network traffic to allow or deny.

I

MPLEMENTING

S

TANDARD

IP

A

CCESS

L

ISTS

The access list itself is created in the router’s global configuration.

Then, the access-group command is used in the interface configuration of the interface you want the access list applied to. Here is the syntax for creating a standard IP access list: access-list 1-99 {permit|deny} address mask

The access-list command identifies this as an access list.

1-99 identifies it as a standard access list entry. The permit or deny command tells the router what to do with traffic that matches this statement. address is the source IP address. mask is used to identify wildcard bits, which identify parts of the IP address to ignore when doing the test.

Here is the syntax for the access-group command that is applied to the interface:

Router(config)#ip access-group 1-99 {out|in}

The ip access-group command identifies this interface as belonging to an IP access list group.

1-99 identifies which standard access list to apply to this interface. The out or in command tells the router to apply this access list to either outgoing traffic or incoming traffic.

The default is outgoing traffic.

Incoming Traffic Versus Outgoing

Applying an access list to incoming traffic creates more of a load on the router’s processor than applying it to outgoing traffic.

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Restricting Traffic to One Network

For security reasons, you might want to limit the traffic allowed to pass through a router to certain networks. In the first example, you will limit traffic to only 10.0.0.0. This network might represent your company’s entire network.

The following example shows a standard access list that allows only traffic from the network 10.0.0.0. You can’t assign more than one access list to an interface for a particular protocol type.

.

.

.

interface Serial1 ip address 192.168.42.1 255.255.255.0

ip access-group 1 out

.

.

.

router rip network 10.0.0.0

network 192.168.42.0

.

.

!

no ip classless

!

access-list 1 permit 10.0.0.0 0.255.255.255

.

This code shows that the access-list statement permits traffic from the source address 10.0.0.0 0.255.255.255, which includes any network station that has an IP address beginning with 10. Since access lists include an implicit deny all at the end, any network traffic originating from a network other than 10.0.0.0 would be denied. The access list is applied to interface Serial1. Notice that the access-group statement is stated as out

, meaning that the access list applies to traffic exiting this interface. This access list will have no effect on traffic entering the router through Serial1.

Restricting Traffic from One Network

You just created an access list that allows only traffic from the network

10.0.0.0. Now, you will restrict traffic from only one network. You will allow traffic from any network other than 10.0.0.0 to pass. Any traffic originating from network 10.0.0.0 will be dropped into the bit bucket.

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C h a p t e r 7 N E T WO R K S E C U R I T Y 253

This can be useful for securing an internal network. If your company connects to another company’s network, you might want to restrict which parts of your network that company’s traffic travels.

.

.

.

interface Serial1 ip address 192.168.42.1 255.255.255.0

ip access-group 1 out

.

.

.

router rip network 10.0.0.0

network 192.168.42.0

!

no ip classless

.

.

!

access-list 1 deny 10.0.0.0 0.255.255.255

access-list 1 permit any

.

The first test for traffic exiting interface Serial1 is whether the IP address falls within the network 10.0.0.0. If it does, the packet is denied and sent off to the bit bucket. Otherwise, the packet is evaluated by the second line of the access list, which permits any traffic.

Since any packet making it past the first statement would be permitted by the second, no traffic would fall to the implicit deny all statement at the end of the access list. The command permit any is shorthand for

0.0.0.0 255.255.255.255. Either of these notations will work, but permit any is more readable and makes it easier to see what the access list is doing. Access lists can become very complex. Anything you can do to make them more readable will make your life easier.

Restricting Traffic from One Network

Station

In this example, you will restrict traffic from one specific network station. In the following code, the access list specifies the IP address of a station that will be denied access. Any other station on the network will be allowed to pass. You might use this to keep a machine from being accessed outside your local network. By not allowing traffic from that machine to exit the local network through the router, you ensure some security over who can access it.

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254 Pa r t I E X A M P R E PA R AT I O N

.

.

interface Serial1 ip address 192.168.42.1 255.255.255.0

.

.

.

ip access-group 1 out router rip network 10.0.0.0

network 192.168.42.0

!

no ip classless

.

.

!

access-list 1 deny 10.99.68.102 0.0.0.0

access-list 1 permit any

.

The wildcard bits are all set to zero. This tells the router to evaluate all the bits of the IP address when comparing traffic to the access list. This example allows all traffic to pass except traffic originating from IP address 10.99.68.102.

Allow Only Traffic from IP Addresses

Ending in .242

In this example, you will allow only traffic from IP addresses ending in .242. All other traffic will be denied. The wildcard bits are set to ignore the first three octets when comparing network traffic. If the last octet is not equal to .242, traffic is denied.

To secure data within your company, you place systems in each office that transfers customer information to the home office. In order to secure this data from possible access by other machines, you assign each of the database machines an IP address with a final octet of .242.

You then set up access lists to allow only traffic from IP addresses ending in .242 to access the network where the consolidated customer database is stored.

.

.

interface Serial1 ip address 192.168.42.1 255.255.255.0

.

.

.

ip access-group 1 out

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C h a p t e r 7 N E T WO R K S E C U R I T Y 255 router rip network 10.0.0.0

network 192.168.42.0

!

no ip classless

!

access-list 1 permit 0.0.0.242 255.255.255.0

.

.

access-list 1 deny any

.

Here, the wildcard bits in the first three octets are set to one. This tells the router to ignore these bits when evaluating network traffic. If the last octet of the IP address is not equal to .242, the traffic is denied.

Note that even though the access list would perform the same function without the second line of the access list, it is easier to see how this access list will perform when deny any is shown in the configuration.

Wildcards in the IP Address

When defining an IP address in an access list, it is often necessary to identify entire networks instead of a specific network station. In an access list, you can use wildcard bits to identify parts of the network address to not check. Wildcard bits work much like subnet mask bits but identify the station portion of the address instead of the network portion. Earlier, you saw how the entire network 10.0.0.0 could be denied. You also saw how these wildcard bits are applied to the IP address to identify what traffic matches this test.

The following are some examples of wildcard bits and how they might be used in access lists.

In the following example, any station on network 192.168.200.0

will be permitted. Since the wildcard bits in the last octet are all ones, any address in the last octet will be permitted: permit 192.168.200.0 0.0.0.255

The next example denies all traffic from one IP address: deny 192.168.42.109 0.0.0.0

The following example permits any IP address. A shortcut for this is to use the keyword any in the access list, as shown in the second line.

These two lines are synonymous and can be used interchangeably.

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R E V I E W B R E A K

However, using the keyword any can make it easier for someone reading the router’s configuration to quickly identify what the access list is doing.

permit 0.0.0.0 255.255.255.255

permit any

Implementing Access Lists

.

Standard IP access lists allow filtering on the source address only.

.

When a wildcard bit is set to one, the router doesn’t care what value is in that bit position.

.

By default, access lists are applied to outgoing traffic. An access list can be applied to incoming traffic, but at the cost of performance to the router.

.

Access lists are evaluated from the top down. Once a match is found, the router either forwards or drops the packet.

I

MPLEMENTING

E

XTENDED

IP

A

CCESS

L

ISTS

An extended access list offers more detailed control over traffic. With an extended access list, you can specify the source IP address, destination IP address, protocol, and port used. You use an extended access list when you need more granular control over what traffic to filter. If you simply need to filter traffic based on the source IP address, the standard access list will work and should be used.

Here is the syntax for an extended access list:

Router(config)#access-list 100-199 {permit|denyip|tcp|udp|icmp}

➥source source-mask dest dest-mask [lt|gt|eq|neq dest-port]

The access-list command is the same as for the standard IP access list. The number of the access list,

100-199

, identifies it as an extended IP access list. The commands permit and deny work as they do in standard access lists. However, in an extended access list, you can add the protocol to permit or deny. The source address and source mask work as they do in a standard access list. In the extended access list, you can also identify the destination IP address and mask.

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C h a p t e r 7 N E T WO R K S E C U R I T Y 257

Extended access lists also let you specify a port number. You can specify whether the port number should be less than, greater than, equal to, or not equal to the specified port.

Putting a question mark at the end of your extended access list entry will list the well-known TCP/IP port numbers and an abbreviation that can be used in place of the actual number. These abbreviations make access lists more readable and make it easier to determine their purpose. The following is an example of the router’s output. The first column shows the abbreviation you can use in the router configuration. The second column describes the service.

The number in parentheses is the actual port number used by that service.

c2513r1(config)#access-list 100 permit tcp any any eq ?

bgp Border Gateway Protocol (179) chargen Character generator (19) cmd Remote commands (rcmd, 514) daytime Daytime (13) discard Discard (9) domain Domain Name Service (53) echo Echo (7) exec Exec (rsh, 512) finger Finger (79) ftp File Transfer Protocol (21) ftp-data FTP data connections (used infrequently, 20) gopher Gopher (70) hostname NIC hostname server (101) ident Ident Protocol (113) irc Internet Relay Chat (194) klogin Kerberos login (543) kshell Kerberos shell (544) login Login (rlogin, 513) lpd Printer service (515) nntp Network News Transport Protocol (119) pim-auto-rp PIM Auto-RP (496) pop2 Post Office Protocol v2 (109) pop3 Post Office Protocol v3 (110) smtp Simple Mail Transport Protocol (25) sunrpc Sun Remote Procedure Call (111) syslog Syslog (514) tacacs TAC Access Control System (49) talk Talk (517) telnet Telnet (23) time Time (37) uucp Unix-to-Unix Copy Program (540) whois Nicname (43) www World Wide Web (HTTP, 80)

The ability to specify the source and destination IP address along with protocol and port number offers you greater control over your network.

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The Evolution of Access Lists One area of Cisco’s IOS that changes rapidly with new versions is access lists. New releases of IOS often add capabilities to traffic control with access lists. When working with access lists, you should consult the documentation to find out which version of IOS you are using.

Deny Telnet Requests to a Host on

Your Network

This example uses an extended IP access list to deny requests to telnet to a host on your network. This could be useful if your network is attached to the Internet and you have a UNIX computer you need to protect from telnet access over the Internet. By filtering telnet requests to the UNIX computer with an access list, you ensure that the system is safe even if the administrator of the UNIX system fails to secure it from the operating system.

.

.

.

.

.

interface Ethernet0 ip address 192.168.42.1 255.255.255.0

.

ip access-group 101 out router rip network 10.0.0.0

!

network 192.168.42.0

no ip classless

!

.

.

access-list 101 deny tcp 0.0.0.0 255.255.255.255

[ic:ccc]192.168.42.231 0.0.0.0 eq telnet access-list 101 permit ip any any

.

Here, the access list will deny telnet traffic from any computer to the host 192.168.42.231. Any other network traffic for 192.168.42.231

will be allowed. A common use of this type of access list is to limit network addresses that can telnet to your router. If you have a router that borders the Internet, it’s a good idea to deny telnet requests from the Internet to keep hackers at bay.

M

ANAGING

A

CCESS

L

ISTS

Once you have created your access lists, you will no doubt need to edit them. After they are edited, you will want to verify that they are operating as expected. Another topic pertaining to managing access lists is viewing access lists. As an internetworking professional,

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C h a p t e r 7 N E T WO R K S E C U R I T Y 259 you will often be introduced to a new network and will need to figure out the configuration objectives.

Editing Access Lists

Cisco’s IOS does not provide a way to insert a statement in an access list above current lines. If you need to make changes to an access list other than adding statements to the end of the access list, you must remove the access list from the configuration and reenter it in the correct order. One method of editing an access list is to copy the configuration from the router to a TFTP server. Edit the configuration file with a text editor and then copy the modified configuration back to the router from the TFTP server.

Deleting Access Lists

To remove an access list from a configuration, use the no access list command. The following example shows how to remove an access list:

.

.

c2513r1#sh run

.

router eigrp 100 network 192.168.220.0

!

router rip network 10.0.0.0

network 192.168.42.0

!

no ip classless

!

access-list 1 permit 192.168.42.1

access-list 1 permit 10.0.0.0 0.255.255.255

.

.

access-list 1 permit 192.168.200.0 0.0.0.255

access-list 1 deny any

.

c2513r1#config t

Enter configuration commands, one per line. End with CNTL/Z.

c2513r1(config)#no access-list 1 c2513r1(config)#exit

This code shows how the access list looks in the configuration before its removal. There are four tests in the access list. The command no access-list 1 removes all four lines from the router’s configuration.

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Verifying Access Lists

To determine whether an interface is grouped to an access list, you can use the command show ip interface

:

.

.

c2513r2#sh ip interface

.

Serial0 is up, line protocol is up

Internet address is 192.168.42.2/24

Broadcast address is 255.255.255.255

Address determined by non-volatile memory

MTU is 1500 bytes

Helper address is not set

Directed broadcast forwarding is enabled

Multicast reserved groups joined: 224.0.0.9

Outgoing access list is 1

Inbound access list is not set

Proxy ARP is enabled

Security level is default

Split horizon is enabled

ICMP redirects are always sent

ICMP unreachables are always sent

ICMP mask replies are never sent

IP fast switching is enabled

IP fast switching on the same interface is enabled

IP multicast fast switching is enabled

Router Discovery is disabled

IP output packet accounting is disabled

IP access violation accounting is disabled

TCP/IP header compression is disabled

Probe proxy name replies are disabled

Gateway Discovery is disabled

Policy routing is disabled

Network address translation is disabled

.

.

.

c2513r2#

Viewing Access Lists

To view access lists created in a router, you can use the show access-lists command: c2513r2#sh access-lists

Standard IP access list 1 permit 192.168.42.1

permit 192.168.200.0, wildcard bits 0.0.0.255

permit 10.0.0.0, wildcard bits 0.255.255.255

deny any

Extended IP access list 101

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C h a p t e r 7 N E T WO R K S E C U R I T Y 261 permit ip 192.168.201.0 0.0.0.255 any deny tcp any host 10.100.2.101 eq ftp c2513r2#

You see each access list in the configuration, whether or not it is applied to an interface. Each permit or deny statement of the access list is shown.

N

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Release 11.2 of Cisco’s IOS introduced named access lists. This feature allows you to give a standard IP or extended IP access list a meaningful name to make reading the configuration easier.

Named access lists have the added bonus of allowing you to edit the access list and remove lines without having to delete and reenter the access list.

Here is how a named access list is created: c2513r2#config t

Enter configuration commands, one per line. End with CNTL/Z.

c2513r2(config)#ip access-list extended warez_protect c2513r2(config-ext-nacl)#deny tcp any host 192.168.41.23 eq

➥ftp c2513r2(config-ext-nacl)#deny tcp any host 192.168.41.23 eq

➥ftp-data c2513r2(config-ext-nacl)#permit ip any any c2513r2(config-ext-nacl)#exit c2513r2(config)#exit c2513r2#

The preceding code shows you how the named access list is created.

In the global configuration, enter the command ip access-list extended access-list-name

. In this example, you will filter FTP requests to a server in your building that has been compromised in the past by pirates creating Warez FTP sites. This begins the named access list configuration. The router will then enter the named access list configuration mode. This is shown by the change in the router’s prompt to c2513r2(config-ext-nacl)#

. This indicates that you’re configuring an extended named access control list. Next, you enter each of the permit or deny statements for the access list. Here is what the access list you just created will look like in the configuration: c2513r2#sh run

.

.

continues

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262 Pa r t I E X A M P R E PA R AT I O N

continued

.

no ip classless

!

ip access-list extended warez_protect deny tcp any host 192.168.41.23 eq ftp deny tcp any host 192.168.41.23 eq ftp-data permit ip any any logging buffered 1024000 debugging access-list 1 permit 192.168.42.1

access-list 1 permit 192.168.200.0 0.0.0.255

access-list 1 permit 10.0.0.0 0.255.255.255

access-list 1 deny any access-list 101 permit ip 192.168.201.0 0.0.0.255 any access-list 101 deny tcp any host 10.100.2.101 eq ftp

!

!

line con 0 exec-timeout 0 0 password 7 094F471A1A0A login line aux 0 password 7 094F471A1A0A login c2513r2#

You now need to apply the access list to the interface that traffic for this host will exit. The following code shows how this is done: c2513r2#config t

Enter configuration commands, one per line. End with CNTL/Z.

c2513r2(config)#int e0 c2513r2(config-if)#ip access-group warez_protect c2513r2(config-if)#exit c2513r2(config)#exit c2513r2#

When a named access list is applied to an interface, the name of the access list shows in the show ip interface command’s output: c2513r2#sh ip interface

Ethernet0 is up, line protocol is down

Internet address is 192.168.41.1/24

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 enabled

Outgoing access list is warez_protect

Inbound access list is not set

Proxy ARP is enabled

Security level is default

Split horizon is enabled

ICMP redirects are always sent

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C h a p t e r 7 N E T WO R K S E C U R I T Y 263

ICMP unreachables are always sent

ICMP mask replies are never sent

IP fast switching is enabled

IP fast switching on the same interface is disabled

IP multicast fast switching is enabled

Router Discovery is disabled

IP output packet accounting is disabled

IP access violation accounting is disabled

TCP/IP header compression is disabled

Probe proxy name replies are disabled

Gateway Discovery is disabled

Policy routing is disabled

Network address translation is disabled

The named access list also shows in the output of the show access-lists command: c2513r2#sh access-lists

Standard IP access list 1 permit 192.168.42.1

permit 192.168.200.0, wildcard bits 0.0.0.255

permit 10.0.0.0, wildcard bits 0.255.255.255

deny any

Extended IP access list 101 permit ip 192.168.201.0 0.0.0.255 any deny tcp any host 10.100.2.101 eq ftp

Extended IP access list warez_protect deny tcp any host 192.168.41.23 eq ftp deny tcp any host 192.168.41.23 eq ftp-data permit ip any any c2513r2#

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The facts of the case are as follows:

.

The LAN has been compromised by hackers on the Internet.

.

You must provide a method of securing the network while still allowing use of the Web server.

.

You must secure the router so that it isn’t susceptible to login attempts from the Internet.

S C E N A R I O

The Schutzhund club whose Internet connection you set-up calls and says it is experiencing significant delays in Internet traffic. Users of its Web site are complaining that it is slow and that sometimes requests fail.

You arrive at its office and place your laptop with sniffer software on the network. You notice a large amount of traffic from Internet IP addresses to the

NT server running the Web site. Further investigation shows that most of this traffic is FTP data.

continues

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You investigate the server and find that the FTP portion of the server was left available for anyone to use and has been compromised. You find a directory on the server called Warez with several subdirectories underneath containing commercial software. You ask if the Schutzhund Club uses the

FTP server to upload updates to its Web site. It does use the FTP server for this purpose but always loads updates from computers in its office.

It never uses the FTP server from the Internet.

You shut down the FTP service on the server and check the sniffer again. The traffic has stopped.

You try accessing Internet sites and find that the speed has increased significantly. It obviously was unknowingly hosting a pirate Warez site. It’s your job to secure the network so that this can’t happen again.

The Schutzhund Club needs its Web server to be accessible so that handlers can register their dogs in events. Pirates are using the club’s network for illegal activities, which is keeping the network from being available for its intended purpose. You must secure the network in such a way as to allow the desired network traffic but not allow the unwanted use of the network by pirates.

A N A LY S I S

You explain that the router can be used to keep this from happening again by filtering FTP requests that come to the network from the Internet. The

FTP server will still be accessible from the local network, but no one coming across the router from the Internet will be able to access the FTP server. The president of the Schutzhund Club likes the idea of securing his network and asks if there are any other safeguards you should put in place.

You explain that you can secure the router to not accept telnet requests from the Internet either.

By limiting telnet access from the local network, no one on the Internet will be able to make changes to the router to reverse the filters you will put in place for the FTP server.

You begin by copying the current configuration from the router to the TFTP server on your laptop.

You do this because it is always a good idea to have a copy of the configuration from where you started.

You create an access list to deny any host talking with any other host using the TCP and the FTP data ports. You also add the access list to the router’s Ethernet port since this is the interface you don’t want the traffic to exit. Here is the access list you create: c2513r2(config)#access-list 101 deny tcp any

➥any eq ftp c2513r2(config)#access-list 101 deny tcp any

➥any eq ftp-data c2513r2(config)#interface e0 c2513r2(config-if)#ip access-group 101 c2513r2(config-if)#exit c2513r2(config)#exit c2513r2#

You create another access list that denies any address except one on the local network: c2513r2(config)#access-list 102 permit tcp

➥192.168.41.0 0.0.0.255 eq telnet

You then apply this access list to incoming traffic to the VTY lines of the router: c2513r2#config t

Enter configuration commands, one per line.

➥End with CNTL/Z.

c2513r2(config)#line vty 0 4 c2513r2(config-line)#access-class 102 in c2513r2(config-line)#exit c2513r2(config)#exit c2513r2#

You connect to the Internet with your dial-up account and attempt to connect to the FTP server.

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C h a p t e r 7 N E T WO R K S E C U R I T Y 265

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The connection is refused. You then try to telnet to the router, and that connection is refused also.

The Schutzhund Club president thanks you for helping. He invites you to a Schutzhund trial the following weekend, where you can see the famous

Keyser Soze vom Wardlaw try for his Schutzhund

III title. He assures you it will be a fascinating show. Keyser’s grandsire is Quando von Arminius, who was Weltsieger in 1986 and 1987. This brave dog will surely be the star of the show.

C

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This chapter showed you how to use access lists to restrict network traffic that passes through a router. By using access lists wisely, you can enforce policies and secure your network from attacks. Because many companies are connecting their internal networks to the

Internet, private networks can be open to numerous attacks. By making use of access lists, you can restrict the type of attack.

Standard access lists allow you to restrict data based on the source IP address only. They permit or deny the entire IP protocol suite based solely on the source IP address. By using the wildcard bits, you can identify entire networks from which to restrict network traffic.

Extended access lists provide greater flexibility in identifying the types of traffic to restrict. An extended IP access list lets you specify the source and destination IP addresses. It also lets you identify the protocol and port number.

Access lists can be applied to traffic either entering or exiting an interface. Access lists that are applied to traffic entering an interface are called inbound access lists. Inbound access lists create a great deal of load on the router’s processor. Outbound access lists create less of a load on the processor, and thus, are the preferred method. Either way, access lists do create a load on the router’s processor, and thus, should be well-designed. The router processes the access list from the top down. When a match is found, the router either forwards or drops the packet. Design your access lists so that, for the majority of traffic, the router processes the fewest lines of the access list possible while still enforcing the traffic policy.

How access lists are implemented in the IOS changes often and rapidly. Always check the documentation for the current version of the IOS you are running for the correct way to create an access list.

KEY TERMS

• access list

• destination IP address

• destination IP access list

• octet

• packet

• port number

• wildcard bits

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Review Questions

1. How many types of IP access lists are there, and what are their capabilities?

2. What is added to the end of every access list?

3. What performance considerations should you take into account when creating an access list?

4. What are wildcard bits?

5. What are some uses for access lists?

6. What version of IOS introduced named access lists?

7. When you apply an access list to an interface, does it filter incoming or outgoing traffic or both?

8. How can you edit an access list?

9. Why is it important to pay attention to the order of statements in an access list?

10. What is the purpose of named access lists and the use of built-in abbreviations for well-known TCP port numbers?

Exam Questions

1. An access list applied to an interface affects traffic going in which direction?

A. Network traffic coming into the interface is filtered.

B. Network traffic exiting the interface is filtered.

C. Both incoming and outgoing traffic is filtered.

D. The access list filters traffic in the direction specified when the access-group command is applied to the interface.

2. Which range of numbers indicates an extended

IP access list?

A. 0-99

B. 100-199

C. 1-99

D. 101-199

3. What is the proper syntax for applying a standard

IP access list to an interface?

A.

Router(config-if)#access-list 101

B.

Router(config-if)#access-group 22 in

C.

Router(config-if)#access-group 101 out

D.

Router(config-if)#access-list 22 out

4. Which command will show which IP access lists are applied to which interfaces?

A.

Router#show ip interface

B.

Router#show interface

C.

Router#show access-lists

D.

Router#show traffic-filters

5. What is the proper syntax for an access list to allow all traffic from network 192.168.42.0?

A.

Router(config)#access-list 1 permit

192.168.42.0 255.255.255.0

B.

Router(config-if)#access-list 1 permit

192.168.42.0 255.255.255.0

C.

Router(config)#access-list 1 permit

192.168.42.0 0.0.0.255

D.

Router(config)#access-list 1 permit

192.168.42.0 0.0.0.255 out

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6. How many access lists can be applied to an interface for one protocol?

A. Two

B. One

C. Up to 99

D. As many as the router’s processor can adequately service

7. What command will display the access lists currently defined?

A.

Router#show access-lists

B.

Router#show access lists

C.

Router#show traffic-filters

D.

Router#list access-lists

8. Which of the following is a properly formatted extended IP access list statement?

A.

Router(config)#access-list 99 permit tcp

192.168.41.0 0.0.0.255 any eq telnet

B.

Router(config)#access-list 102 permit tcp

192.168.41.0 0.0.0.255 any eq telnet

C.

Router(config)#access-list 102 permit

192.168.41.0 255.255.255.0 any tcp eq telnet

D.

Router(config)#access-list 99 permit tcp

192.168.41.0 255.255.255.0 eq telnet

9. What implied command is added to the end of every IP access list?

A.

deny any

B.

permit any

C.

deny ip traffic

D.

permit ip traffic

10. Access list statements are executed in what order?

A. From the top down, executing every statement, even after a match is found

B. Randomly until a match is found

C. In order of importance

D. From the top down until a match is found; then, the packet is either forwarded or dropped

Answers to Review Questions

1. There are two types of IP access lists—standard and extended. Standard access lists let you identify traffic based on source IP address. Extended

IP access lists let you specify source and destination IP address, protocol, and port number. For more information, see “Implementing Standard

IP Access Lists” and “Implementing Extended IP

Access Lists.”

2. An implicit deny any

. You must have at least one permit statement in your access list to allow any traffic to pass the access list. For more information, see “Implementing Standard IP Access Lists” and “Implementing Extended IP Access Lists”.

3. Access lists cause the router to look at each packet entering or exiting an interface. When the router evaluates an access list, it does so from the top down. When it finds an applicable command, it either forwards the permitted packet or drops the denied packet. Well-designed access lists deal with most of the traffic in the first few lines, so the router doesn’t have to process the entire access list for most traffic. For more information, see “How

Access Lists Affect Performance.”

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4. Wildcard bits are similar to a subnet mask, but they identify bit positions that the access list doesn’t care about. When the wildcard bit is set to one, the access list algorithm doesn’t evaluate that bit position. For more information, see

“Wildcards in the IP Address.”

5. Access lists can be used to filter traffic, identify interesting traffic to bring up a dial-up connection, filter routing updates, and limit SAPs. For more information, see “Uses of Access Lists.”

6. Cisco’s IOS version 11.2 introduced named access lists. For more information, see “Named

Access Lists.”

7. An access list filters traffic in the direction specified—either incoming or outgoing traffic. The default is outgoing traffic, since this creates the least load on the router’s processor. For more information, see “Implementing Standard IP

Access Lists.”

8. A named access list can be edited line-by-line like most configuration commands. Numbered access lists must be deleted and reentered, or the configuration can be copied to a TFTP server and then edited with a text editor and copied back. For more information, see “Editing Access Lists.”

9. Access lists are processed from the top down.

Once a match is found, the router either forwards the packet or drops it. When most of the network traffic matches the first line of the access list, the load on the router’s processor is lessened. For more information, see “How Access Lists Affect

Performance.”

10. Named access lists and abbreviations for wellknown TCP ports make access lists easier to read and figure out. It’s common to have to look at a configuration of a router you have never seen before and figure out what the purpose of each configuration line is. By giving access lists meaningful names and using the built-in abbreviations, you can make it easier for yourself later when you must evaluate the router’s configuration. For more information, see “Implementing Extended

IP Access Lists” and “Named Access Lists.”

Answers to Exam Questions

1. D. By default, an access list applies to traffic exiting an interface. However, it applies to traffic in the direction specified when the access list is applied to the interface. For more information, see “Implementing Standard IP Access Lists” and

“Implementing Extended Access Lists.”

2. B. Extended access lists are numbered 100-199.

For more information, see “The Parts of an

Access List.”

3. B. When applying an access list to an interface, the access-group command is used. Standard IP access lists fall in the range of 1-99. For more information, see “Implementing Standard IP

Access Lists.”

4. A. show ip interface lists many parameters that deal with the interface, including the IP access lists applied to it. For more information, see

“Verifying Access Lists.”

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5. C. Wildcard bits are different from subnet masks. For more information, see “Wildcards in the IP Address.”

6. B. Only one access list per protocol per interface is allowed. For more information, see “The Parts of an Access List.”

7. A. show access-lists will display all the access lists in the configuration, even if they aren’t applied to an interface. For more information, see “Viewing Access Lists.”

8. B. The proper format for an extended IP access list is access-list 100-199 {permit|denyip|tcp|udp|

➥icmp} source source-mask dest dest-mask

➥[lt|gt|eq|neq dest-port]

For more information, see “Implementing

Extended IP Access Lists.”

9. A. Every access list has an implicit deny any command added. If you don’t implicitly allow some IP traffic to pass, none will. For more information, see “Understanding Access Lists.”

10. D. Access list statements are executed from the top down until a match is found. Once a match is found, the packet is either forwarded or dropped. For more information, see “How Access

Lists Affect Performance.”

Suggested Readings and Resources

1. Chappell, Laura. Introduction to Cisco Router

Configuration. Cisco Press, 1998.

2. Cisco Press. Cisco IOS Solutions for Network

Protocols, Volume I. Cisco Press, 1998.

3. Cisco Press. Cisco IOS Solutions for Network

Protocols, Volume II. Cisco Press, 1998.

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O

B J E C T I V E S

Cisco provides the following objectives for the LAN

Switching portion of the CCNA exam.

.

Describe LAN segmentation using bridges.

To be an internetworking professional, you will need to understand the benefits of network segmentation using bridges. As LANs grow in size, network segmentation becomes more important.

.

Describe LAN segmentation using routers.

To be an internetworking professional, you must understand the benefits of network segmentation using routers. As LANs grow in size, network segmentation becomes more important.

.

Describe LAN segmentation using switches.

To be an internetworking professional, you will need to understand the advantages of network segmentation using switches. As LANs grow in size, network segmentation becomes more important.

.

Name and describe two switching methods.

To be an internetworking professional, you must understand the advantages and disadvantages of different methods of LAN switching.

.

Describe full- and half-duplex Ethernet operation.

As an internetworking professional, you must be familiar with full- and half-duplex Ethernet operations.

C H A P T E R

8

LAN Switching

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O

B J E C T I V E S

( c o n t i n u e d

)

Describe network congestion problems in

Ethernet networks.

.

As an internetworking professional, you must understand the effects of network congestion on

Ethernet networks and how to overcome these problems with LAN segmentation.

Describe the benefits of network segmentation with bridges.

.

As an internetworking professional, you must understand the benefits of network segmentation with bridges.

Describe the benefits of network segmentation with switches.

.

As an internetworking professional, you must understand the benefits of network segmentation with switches.

Describe the features and benefits of Fast

Ethernet.

.

As an internetworking professional, you must understand the features and benefits of Fast

Ethernet.

Describe the guidelines and distance limitations of Fast Ethernet.

.

As an internetworking professional, you must be able to design networks using Fast Ethernet. To do this effectively, you must understand its limitations.

Distinguish between cut-through and storeand-forward LAN switching.

.

As an internetworking professional, you must be able to distinguish between cut-through and storeand-forward LAN switching.

Describe the operation of Spanning Tree

Protocol and its benefits.

.

As an internetworking professional, you must understand the operation of Spanning Tree

Protocol and how it works to guarantee a loop-free network.

.

Describe the benefits of virtual LANs.

As an internetworking professional, you must understand the benefits of VLANs and where they should be used.

Define and describe the function of a MAC address.

.

As an internetworking professional, you must understand the function of a MAC address.

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U T L I N E

Introduction

The Problems with Large Networks

Fundamentals of Ethernet

Collision Domains

Network Segmentation

LAN Segmentation Using Bridges

LAN Segmentation Using Routers

LAN Segmentation Using Switches

Cut-Through Switching

Fragment-Free Switching

Store and Forward Switching

Bridges, Routers, and Switches in a Nutshell

Full-Duplex and Half-Duplex

The Speakerphone

The Bridge Over a River

Ethernet Full and Half Duplex

277

277

279

279

280

280

280

280

275

275

276

277

Fast Ethernet

The Different Media Types of Fast

Ethernet

Distance Limitations

Spanning Tree Protocol

Election of a Root Switch

Spanning Tree Protocol Port States

Blocking State

Listening State

Learning State

Forwarding State

Disabled State

Virtual LANs (VLANs)

282

282

282

283

Chapter Summary

284

284

285

286

287

287

287

288

288

288

289

289

292

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S

T U D Y

S

T R AT E G I E S

.

Begin studying for the LAN switching section of the CCNA exam by making sure you understand the effects of collisions on a large LAN. Then, make sure you understand each of the methods covered in this chapter for segmenting a LAN to avoid the problems caused by excessive collisions.

Finally, be sure you can differentiate between store-and-forward and cut-through LAN switching. If you have a Cisco LAN switch available, spend some time learning how it is configured and review the statistics in the switch to see how it affects LAN traffic. This chapter begins with a review of how Ethernet functions and how segmenting a LAN can improve the performance of a LAN.

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I

NTRODUCTION

When LANs were first created, it was inconceivable that there would be a network that connected the world like the Internet does today.

In the 1980s, a large network consisted of 20 or 30 PCs. Today, many networks consist of 500 or more PCs. The sheer number of computers on the network creates new challenges to overcome.

This chapter discusses some of the techniques used to overcome the challenges of larger networks.

T

HE

P

ROBLEMS WITH

L

ARGE

N

ETWORKS

Large networks present several problems that must be overcome. By understanding the challenges first, you will better grasp how the technology addresses these problems.

Some of the challenges large networks create are as follows:

á Because a large number of network stations are contending for the LAN media, a large number of collisions usually occur.

These collisions seriously degrade the performance of the

LAN. Each area of the LAN that competes for media time is called a collision domain.

á

Most networks have groups of computers that typically communicate with each other. The computers in human resources, for example, usually communicate with the HR server. The computers in finance, on the other hand, usually communicate with the Finance server. When all of these computers share a common network, they are competing for LAN time with all the other departments. By segmenting the LAN into logical groups this can be overcome.

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Fundamentals of Ethernet

Ethernet is a Carrier Sense Multiple Access/Collision Detect

(CSMA/CD) network—but what does that really mean? Carrier

Sense refers to the ability of Ethernet nodes to detect the Ethernet

carrier. A carrier is a signal that is transmitted constantly. When data is transmitted, the carrier is modulated or slightly modified in a way that represents the data to be transmitted.

An example of a carrier signal is with your car radio. FM radio stations transmit a constant carrier signal. Your car stereo detects this signal.

When your stereo is tuned to a frequency where there is no radio station carrier, you hear static. When you tune to a station that is currently not playing any music, you no longer hear the static. The radio is detecting the carrier and is decoding the information on the carrier, which, when there is no sound, is nothing. Ethernet uses this same technique to detect the Ethernet carrier. Most Ethernet cards have some sort of link light that shows when the card is detecting the carrier.

Multiple Access refers to the ability of multiple nodes to access the

media simultaneously.

Collision Detection refers to the ability of Ethernet nodes to detect

when two stations send data simultaneously and create a collision of data. When a collision occurs, the data sent is corrupt. When a collision occurs, all of the nodes involved in the collision wait a random amount of time before trying to send their data again.

Before an Ethernet node starts to send data, it listens for network traffic on the network. If there is no traffic, the Ethernet node starts to send its frame. After completion, it listens again to check if another station collided with the sending of its frame.

Because the number of network stations on a network greatly affects how many collisions will occur, this is a factor on network performance. At some point, the amount of traffic will increase to the point that very little traffic will be sent without experiencing a collision. When this occurs, network congestion becomes a significant problem. Network congestion occurs when a large number of collisions are occurring due to the shear number of nodes contending for the shared media. When a network reaches this point, the performance of the network degrades quickly.

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Collision Domains

A collision domain is a group of network nodes on Ethernet that share the network media. By sharing the media, they can experience or cause collisions with other stations in the collision domain. In large networks you can divide the network into multiple collision domains to avoid the performance degradation associated with large collision domains. This dividing of the network is called segmentation, which is discussed in the following section.

N

ETWORK

S

EGMENTATION

To keep collision domains small on large networks, you can make use of network segmentation. By segmenting the network into small collision domain segments that can communicate with each other, you avoid the performance degradation that occurs with a large number of nodes in one collision domain. There are several different ways to segment a network:

á Bridges

á Routers

á

Switches

The following sections detail each of these techniques.

The advantage of LAN segmentation is that it increases network performance by reducing the number of nodes participating in a collision domain.

LAN Segmentation Using Bridges

A bridge is a network device that is connected to multiple network segments. A bridge is a Layer 2 device. It operates at the Data Link layer, using MAC addresses to determine when to forward traffic.

Years ago bridges were physical devices that connected to the network. Today when the functions of a bridge are needed, a router is typically used. Cisco routers can function as bridges as well as routers. Figure 8.1 shows a network segmented with a bridge.

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F I G U R E 8 . 1

Network segmented by a bridge.

Workstation

MAC

0060.9726.5000

Forwarding Table

0060.9276.5000 0060.9726.517F

Workstation

MAC

0060.9726.517F

Bridge

In Figure 8.1, you see a network segmented by a bridge. The bridge listens to each segment to which it is connected. When it hears network traffic, it reads the MAC address of the sending station and places that MAC address in a table. This table determines how the bridge forwards traffic. Suppose the bridge detects

0060.9726.5000 trying to reach 0060.9726.517F on segment A.

Because the frame originated from 0060.9726.5000 on segment A, the bridge places this MAC address in the table for segment A. It has learned where MAC address 0060.9726.5000 resides. Because at this point the bridge does not know where 0060.9726.517F

resides, the bridge reads the frame from segment A and floods it to segment B.

Node 0060.9726.517F reads the frame destined for it directly from segment A, and thus the frame forwarded to segment B goes unanswered. Node 0060.9726.517F responds to the frame. The bridge sees a frame from 0060.9726.517F destined for 0060.9726.5000 on segment A. The bridge consults its bridging table and finds that node 0060.9726.5000 is on segment A, the same segment the frame was detected on, so the bridge does nothing. The conversation between 0060.9726.517F and 0060.9726.5000 continues without causing any traffic on segment B.

Node 0060.9726.517F on segment A now sends a frame to

0060.9726.5003 on segment B.

Bridges learn MAC addresses (Layer 2) and forward traffic to other segments based on tables they build of which MAC addresses reside on which segments. Bridges have no knowledge of upper-layer protocols. All forwarding is based on MAC address.

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LAN Segmentation Using Routers

A router can be used to segment a network into different collision domains (see Figure 8.2). Assuming that the router is routing between the networks and not bridging, this actually divides the networks into separate Layer 3 networks. Each different Layer 3 network is a separate collision domain. One of the disadvantages of separating the networks with a router is it can take a very powerful router to handle many high-speed network connections with large amounts of data.

Workstation

192.168.42.12

255.255.255.0

Interface Address

192.168.55.1

255.255.255.0

F I G U R E 8 . 2

A network segmented using a router.

Router

Interface Address

192.168.42.1

255.255.255.0

Workstation

192.168.55.42

255.255.255.0

LAN Segmentation Using Switches

LAN switches work much like bridges with many ports. A LAN switch learns what MAC addresses are connected to which ports.

After the switch learns this information, it then forwards frames only to the ports on which the destination MAC address resides.

LAN switches learn on which port network nodes reside by listening to network traffic. If a LAN switch receives a frame with a destination MAC address that is not in its forwarding tables, it floods all ports with the frame. After the destination station replies to the frame, the LAN switch will learn on which port it resides and won’t need to flood its traffic again.

LAN switches reduce collisions by increasing the number of collision domains. By increasing the number of collision domains, you reduce the number of network nodes contending for the media.

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R E V I E W B R E A K

A LAN switch can operate in three different modes:

á Cut-through

á

Fragment-free

á

Store-and-forward

Cut-Through Switching

A switch operating in cut-through mode will start forwarding a frame as soon as it reads the destination address. At that point the switch looks up the MAC address in its forwarding table and starts forwarding the frame out of the destination port. Cut-through switching produces the lowest amount of latency through the switch.

Fragment-Free Switching

A switch operating in fragment-free mode will start forwarding a frame as soon as the first 64 bytes of the frame are received. Because most collisions occur during the first 64 bytes, this helps keep the switch from forwarding corrupt frames. It also allows the switch to read the destination MAC address and make a forwarding decision.

Fragment-free mode creates the second least amount of latency or delay through the switch.

Store-and-Forward Switching

A switch operating in store-and-forward mode receives the entire frame before forwarding it. This ensures that corrupt frames are not forwarded. Store-and-forward mode creates more latency or delay through the switch because it must receive the entire frame before it begins to forward it. This means larger packets will have a greater latency through the switch than smaller packets.

Bridges, Routers, and Switches in a

Nutshell

.

Bridges. Bridges are useful for segmenting a LAN. Cisco routers can function as bridges as well as routers. Bridges operate at Layer 2 (the MAC layer). A bridge listens to the

MAC addresses on each LAN segment to which it is connected.

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It builds a table of these MAC addresses along with which segment it detected the station on. When the bridge detects traffic for a MAC address that is on a remote segment, it copies the frames and forwards them to the segment on which the destination MAC address was detected. If a bridge sees traffic for a

MAC address the bridge has not yet detected, it copies the frame to all LAN segments to which it is attached. It then listens for the response to the frame and then adds that station to the table. Bridges are older technology and are not used much anymore. Many of the functions of bridges have been replaced by LAN switches.

.

Routers. Routers are useful for segmenting a LAN at Layer 3.

A router uses the Network layer protocol to determine when to forward traffic to another network. The disadvantage to using a router to segment a high speed LAN is that the router must make routing decisions and generally will slow down the connection between two high-speed LAN segments.

.

LAN Switches. LAN switches are an evolution of bridges.

Basically, a LAN switch can be thought of as a bridge with many segments. LAN switches operate at Layer 2, as do bridges. It is not uncommon today to have desktops on a large

LAN connected directly to switch ports.

A switch works much like a bridge in that it listens for MAC addresses and builds a table of the port on which the MAC address was detected. When traffic for that MAC address is detected on another port the frame is copied to the port the destination MAC address was detected on. This keeps the entire network from having to contend with that traffic.

LAN switches can operate in three modes. Store-and-forward mode receives the entire frame, stores it, looks up the destination address in the table and then forwards the frame out of the destination port. Cut-through mode starts forwarding the frame as soon as the destination address is read. Fragment-free mode begins forwarding as soon as 64 bytes are received, which is when most collisions would occur. Store-and-forward mode creates greater latency through the switch than the cutthrough and fragment-free modes. Store-and-forward also has more latency for larger packets than smaller ones because the entire packet must be received before forwarding.

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F

ULL

-D

UPLEX AND

H

ALF

-D

UPLEX

To understand the differences between full- and half-duplex

Ethernet, you must first have a strong grasp on the concept of fullversus half-duplex. Full-duplex operation allows both sides of the connection to send as well as receive data simultaneously. With half-

duplex operation, only one side can send data at a time, and while

that side is sending, it is unable to receive, or listen.

The Speakerphone

Most everyone has experienced half-duplex when talking on a speakerphone. Very few speakerphones support full-duplex. On a half-duplex speakerphone, only one person can talk at a time. In other words, you must take turns using the network media (the telephone line).

When you pick up the receiver and use the handset as opposed to the speakerphone, you are using full-duplex. Both parties can speak without blanking out the other.

The Bridge over a River

Another example that might help you fully understand the difference between full- and half-duplex is a bridge. A bridge with one lane each way allows traffic from both directions to cross a river at the same time. This is an example of full-duplex.

If a construction crew were working on the bridge and had one lane blocked, traffic would have to alternate across the bridge. A flagman at each end of the bridge would stop the traffic from one direction to allow the traffic from the other direction to pass. After some time, the traffic from that direction would have to stop to allow some of the traffic from the other direction to pass. This is an example of half-duplex.

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Ethernet Full- and Half-Duplex

Ethernet can work similarly. In a shared network with no switches, the network operates at half-duplex. Only one network station can talk at a time. When you employ switches on a network, you can support full-duplex Ethernet. Full-duplex Ethernet is supported when there are only two nodes in a collision domain, or on the shared media. This could be a workstation plugged directly into a network switch. The workstation is one node and the network switch is the other node. Both devices must support full-duplex operation.

A common problem when using full-duplex-capable devices is they often do not properly auto-negotiate. Often, one device will enter full-duplex mode and the other will not. It would seem that if this occurs, traffic would not pass. However, in the experience of the author, traffic will pass, but will generate a large number of errors and cause a substantial slowdown of the network. It is best to not let devices auto-negotiate the full-duplex mode. If you want two devices to operate at full-duplex, specifically set them to do so.

Most Ethernet ports on Cisco routers do not support full-duplex mode. Before trying to interface a router with a switch in full-duplex mode, be sure that the interface on the router supports it.

Full-duplex Ethernet doubles the effective bandwidth of the link. A

10Mb link run in full-duplex mode becomes 20Mb. A 100Mb link run in full-duplex mode becomes 200Mb. Obviously, full-duplex

Ethernet can help alleviate bottlenecks in networks. A common place to fully utilize full-duplex Ethernet is on network servers. These devices typically receive and send more data than any other network device. Another common use of full-duplex Ethernet is high-speed links that collapse a network backbone to one switch. Figure 8.3

shows a fully switched network with a collapsed backbone.

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F I G U R E 8 . 3

A network with a collapsed backbone to a highspeed switch.

Fast

Ethernet

Catalyst 5500 Switch

1900 switch

1900 switch

1900 switch

WS WS WS

F

AST

E

THERNET

Over the past few years, networks have grown quickly. More demands are being placed on LANs with new technologies. Audio and video needs have created a great deal of demand for faster networks. To address these needs, manufacturers came up with Fast

Ethernet. Fast Ethernet increases the bandwidth of Ethernet from

10Mbps to 100Mbps.

To make the transition easier, most Fast Ethernet devices can also operate at 10Mbps. However, many organizations have large numbers of 10Mbps devices installed. These organizations can benefit from Fast Ethernet by installing a Fast Ethernet switch for their servers and connect the 10Mbps devices through the switch. This way the servers have access to high-speed connections and can service more 10Mbps workstations than before. This upgrade is generally inexpensive compared to upgrading an entire network to Fast

Ethernet all at once.

The Different Media Types of Fast

Ethernet

Fast Ethernet supports different media types. 100BaseTX can be used on two pairs of Category 5 copper wire like most 10BaseT installations.

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This is convenient because you will not have to install new wiring in most cases.

100BaseFX is an implementation of Fast Ethernet on Fiber Optic

strands. Because 100BaseFX has longer maximum distance limitations, it is commonly used to connect outlying switches with a backbone switch.

100BaseT4 is an implementation of Fast Ethernet using four pairs of

Category 3, 4, or 5 wire. This implementation is good for organizations that already have a large investment of Category 3 or 4 wire in place and cannot justify upgrading the cabling to Category 5.

Distance Limitations

With Fast Ethernet advances in speed come some new limitations in distance. Because most of the Ethernet specification remained unchanged while the speed of transmission increased, there are some resulting limitations. Part of Ethernet’s specification calls for devices to be able to detect a collision within the first 64 bytes. Because Fast

Ethernet is now 10 times faster than 10BaseT, the time to detect a collision in 64 bytes is now 10 times shorter. Although electricity travels very quickly, it is not instantaneous. Copper Fast Ethernet has a distance limitation of 100 meters per segment. The maximum network diameter (the distance from one end of the network to the other) is 200 meters. These limitations keep the network small enough so that if nodes at each of the far ends of the network are involved in a collision, they can detect it during the first 64 bytes.

Table 8.1 shows the different limitations of Fast Ethernet.

TA B L E 8 . 1

E

T H E R N E T

D

I S TA N C E

L

I M I TAT I O N S

Characteristics

Cable

Number of pairs or strands

100BaseTX

Category 5 62.5/125

UTP,or Type 1 micron multiand 2 STP

100BaseFX 100BaseT4

Category 3, 4, or 5 UTP mode fiber

2 pairs 2 strands 4 pairs

continues

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TA B L E 8 . 1

continued

E

T H E R N E T

D

I S TA N C E

L

I M I TAT I O N S

Characteristics

Connector

Maximum segment length

100BaseTX 100BaseFX 100BaseT4

ISO 8877 Duplex

(RJ-45) SCmediaconnector

ISO 8877

(RJ-45) interface connector connector

(MIC) ST

100 meters 400 meters 100 meters

Maximum network diameter

200 meters 400 meters 200 meters

Because network switches are active devices that regenerate the frame, their use can extend the limitations of a Fast Ethernet network.

S

PANNING

T

REE

P

ROTOCOL

The Spanning Tree Protocol (STP) is a link management protocol. It manages redundant links in a network while at the same time preventing undesirable loops. To operate properly, Ethernet requires only one active path between two nodes.

Cisco switches use the Spanning Tree Protocol to maintain a loopfree network.

The Spanning Tree Protocol calculates the most efficient loop-free path through the network. To maintain this loop-free environment, the Spanning Tree Protocol defines a tree that spans all switches in the network. When a network loop exists, switches can see a node’s

MAC address on both sides of a port. This causes confusion for the forwarding algorithm and allows duplicate frames to be forwarded.

The Spanning Tree Protocol determines where multiple paths exist and forces certain ones into standby (blocked) state. If a network segment in the Spanning Tree becomes unreachable or if the Spanning

Tree Protocol calculates that the cost of a path has changed, the

Spanning Tree algorithm reconfigures the Spanning Tree topology by activating the path that was placed in standby state.

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STP operation is transparent to end stations, which do not detect whether they are connected to a single LAN segment or a switched

LAN of multiple segments.

Election of a Root Switch

For a Spanning Tree to operate properly a root switch must be elected. This root switch becomes the logical center of the Spanning

Tree topology.

When switches in a network are powered on, they begin exchanging data messages called Bridge Protocol Data Units (BPDUs). By using the BPDUs, the Spanning Tree Protocol elects a root switch and designates a switch for every switched LAN segment. The Spanning

Tree Protocol also detects loops in the network and places redundant switch ports in standby state.

Spanning Tree Protocol Port States

The Spanning Tree Protocol maintains a loop-free network by setting the state of switched ports. The Spanning Tree can set one of five states:

á Blocking

á Listening

á

Learning

á Forwarding

á

Disabled

Blocking State

A port placed in blocking state does not forward frames. When a port is placed in blocking state, it discards frames received from the attached segment. It also discards frames switched from other ports.

The blocked port does not place MAC addresses in the forwarding tables.

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Listening State

Listening state is the first state a port enters after the blocking state.

When the Spanning Tree Protocol has determined that a port should participate in frame forwarding, it is placed in listening state.

Learning is disabled while in listening state. While in listening state, a port does the following:

á

Discards frames received from the attached segment

á Discards frames switched from other ports for forwarding

á

Does not learn MAC addresses

á

Receives BPDUs

á Processes BPDUs

á Receives and responds to network management messages

Learning State

After a port is placed in learning state it prepares to participate in frame forwarding. The port enters learning state from the listening state. While in learning state, the port does the following:

á

Discards frames received from the attached segment

á Discards frames switched from another ports for forwarding

á

Learns MAC address and places them in the address database

á

Receives, processes, and transmits BPDUs

á Receives and responds to network management messages

Forwarding State

A port enters forwarding state from the learning state. While in forwarding state, the port performs the following functions:

á

Forward frames received from the attached segment

á

Forwards frames switched from other ports

á Learns MAC addresses and adds them to the forwarding database

á

Receives BPDUs

á Processes BPDUs

á Receives and responds to network management messages

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Disabled State

A port placed in disabled state does not participate in frame forwarding or the Spanning Tree Protocol. A port placed in disabled state is not functioning.

V

IRTUAL

LAN

S

(VLAN

S

)

Now that technology has given you Fast Ethernet and LAN switching technology, you can build very large networks. But, should you? Does it not make sense to group computers together by function? However, you don’t want to have to worry about a large number of small networks. There is a happy medium: Virtual LANs (VLANs). On a switched network, VLANs allow you to define logical groups. Why would you want to do this? Although switches reduce the problems with collisions occurring, broadcasts still propagate throughout a switched network. These broadcasts might be Novell SAPs or ARP requests. Either way, they are forwarded to every workstation and server on your large switched network every time a broadcast is generated. Not only do broadcasts generate network overhead, but every workstation on the network must process the broadcast. This creates processing overhead for your computers as well.

By dividing the network into logical pieces with computers grouped by function, you can reduce the overhead of broadcasts on the network and workstations. Think of switches as reducing collision domains and VLANs as reducing broadcast domains.

VLANs were designed to solve two main problems:

á

Scalability of flat networks

á Simplification of network management

VLANs solve these problems by segmenting large flat networks into logically grouped VLANs. These VLANs are easier to manage because they are grouped logically. They also increase scalablity of the network because a large number of nodes can be supported as a number of smaller groups.

By adding a router in the network, you can route data from one

VLAN to another just as you can route data from physically separate networks.

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C

A S E

S

T U D Y

: A

C M E

I

N T E R A C T I V E I S

G

R O W I N G

T

O O

Q

U I C K LY

!

E S S E N C E O F T H E C A S E

The facts of the case are as follows:

.

Acme Interactive has too much traffic for a shared media network.

.

The company’s expected growth will only compound the problem.

.

To effectively make a recommendation, you must understand the benefits of LAN segmentation and the methods of implementing it.

.

You need to make sure your recommendation will be able to grow with the company as it expands.

S C E N A R I O

You have been hired by Acme Interactive to help improve the performance of its LAN. Chuck, the technical manager, explains that the company has recently purchased another company and added about 100 workstations to its LAN. This makes the count of workstations on the LAN now 220.

Since the expansion, the LAN users have been complaining about how slow access has become.

Acme Interactive is expected to continue growing at a rapid pace and needs a solution that can grow with it.

The company is currently running 10BaseT

Ethernet with Category 5 cable throughout the building. All of the servers and most of the workstations are equipped with network cards capable of running at 100Mbps as well.

The server room contains three servers. A Linux machine runs the intranet Web site and the

Internet Web site for the company. A Novell server is used for file and print sharing. A

Windows NT computer runs Microsoft Exchange for email and scheduling. All of the users on the network use all of these servers. There is a router installed that connects the network to the

Internet.

A N A LY S I S

First you need to evaluate how the company’s network traffic is distributed. Do small groups of computers normally communicate among themselves, or is the traffic more distributed? With this company, it is more evenly distributed because all users make use of the three servers housed centrally in the server room.

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C

A S E

S

T U D Y

: A

C M E

I

N T E R A C T I V E I S

G

R O W I N G

T

O O

Q

U I C K LY

!

Because you are certain that the degradation in network speed has to do with a drastic increase in collisions, you know some sort of LAN segmentation is necessary. You place a sniffer on the network to confirm your suspicions and find that you are correct. The network is experiencing an excessive amount of collisions. To segment the LAN, there are several options.

A router could be used to segment the LAN into separate networks. This would reduce the effects of collisions on each of the network segments.

However, installing a router would require the network addressing scheme to change. This means the technical staff at Acme Interactive would have to make changes on each computer at the office. The router also would not be able to pass traffic from one segment to another as quickly as a LAN switch.

A bridge could be used to segment the LAN.

Because a bridge operates at the Data Link layer, no changes in networking addresses would be required. The segments would be separated into different collision domains, which would reduce the effects of collisions on network traffic.

However, a bridge is not necessarily the best solution. A bridge does not offer the growth potential that LAN switches would, and LAN switches could provide more segments, thus reducing the effects of collisions even more.

LAN switches are the best solution in this scenario. LAN switches would provide the best means of reducing the collision domains on the network as well as provide for future growth.

Because all of the workstations on the network access the same servers, you recommend placing the servers on a Cisco 5500 switch using full-duplex Fast Ethernet. Many of the workstations can also be connected directly to the 5500 switch. Another switch is required for workstations at the far end of the building. For these workstations, you recommend installing a Cisco

5505 switch connected to the 5500 via a fiber

Fast Ethernet connection. This option provides the best performance because each workstation will be placed in its own collision domain. The growth potential is very good; adding more switches allows expansion of the LAN with little to no effect from collisions. Also, because switches operate at the Data Link layer, there is no need to reconfigure the network addresses.

Chuck, the technical manager, calls you into his office and explains he is pleased with your recommendation. He contracts you to oversee the implementation project.

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C

H A P T E R

S

U M M A R Y

KEY TERMS

• Collision domain

• Broadcast domain

• Full-duplex

• Half-duplex

• Network segmentation

• Bridge

• Network switch

• Cut-through

• Store-and-foward

• Full-duplex Ethernet

• 100BaseTX

• 100BaseFX

• 100BaseT4

• Spanning Tree Protocol

• Root switch

• Virtual LAN

This chapter covered different methods of segmenting a LAN to reduce the effects of large amounts of network traffic combined with the exponential increase in collisions large amounts of traffic create.

Three devices are discussed for use in segmenting a LAN: bridges, routers, and LAN switches.

Bridges operate at Layer 2, the Data Link layer. A bridge listens to

network traffic on each segment to which it is attached and builds a table of MAC addresses listing which segment they are on. When the bridge sees traffic on a segment that has a destination MAC address of a station on a different segment, the bridge copies the frame from the source segment and places it on the segment the destination address is on. The bridge determines this by referring to the MAC addresses and segments in the table it has built. When a bridge sees a frame for a MAC address that is not in its table yet, it copies the frame to all segments except the one the frame originated on. Then the bridge listens for a reply to the frame and places the MAC address with the segment it heard the reply on in its table.

Routers separate LAN segments at Layer 3, the Network layer.

Routers are typically slower than LAN switches. Routers are most useful when separating a LAN at the Network layer.

LAN switches operate at Layer 2, the Data Link layer. LAN switches

operate much like bridges. Switches listen to MAC addresses on each port and build a table of these MAC addresses.

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A

P P L Y

Y

O U R

K

N O W L E D G E

Review Questions

1. Describe the advantages of network segmentation.

2. Describe the three methods of LAN switching.

3. Describe the purpose of the Spanning Tree

Protocol.

4. Describe the difference between segmenting a

LAN with a bridge and with a router.

5. Describe the features and benefits of Fast

Ethernet.

6. What is an example of half-duplex?

7. What is an example of full-duplex?

8. Describe the benefits of VLANs.

9. How can different VLANs communicate with each other?

Exam Questions

1. A bridge segments a LAN at which layer?

A. Layer 1

B. Layer 2

C. Layer 3

D. Layer 5

2. A router segments a LAN at which layer?

A. Layer 1

B. Layer 2

C. Layer 3

D. Layer 5

3. A LAN switch segments a LAN at which layer?

A. Layer 1

B. Layer 2

C. Layer 3

D. Layer 5

4. A VLAN separates a network into different what?

A. Collision domains

B. Broadcast domains

C. LAN segments

D. Logical groups

5. Which of the following are characteristics of cutthrough LAN switching?

A. Lower latency than store-and-forward

B. Higher latency than store-and-forward

C. The same latency as store-and-forward

D. None of the above

6. When does store-and-forward switching begin forwarding the frame?

A. After the first 64 bytes have been received error free

B. After the entire frame is received error free

C. As soon as the frame is detected

D. After the frame is received and the frame forward timer has elapsed

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294 Pa r t I E X A M P R E PA R AT I O N

A

P P L Y

Y

O U R

K

N O W L E D G E

7. What function does the Spanning Tree Protocol serve?

A. It maintains a loop-free network.

B. It spans the entire network to determine if you have exceeded the maximum length for

Fast Ethernet.

C. It sets switches in store-and-forward or cutthrough mode.

D. It determines where routers are on the network.

8. Full-duplex Ethernet increases effective bandwidth by how much?

A. 1.5 times.

B. 1.75 times.

C. 2 times.

D. It doesn’t increase bandwidth; it simply reduces collisions.

9. What is the maximum segment length for Fast

Ethernet 100BaseTX?

A. 100 feet

B. 200 feet

C. 100 meters

D. 200 meters

10. What is the maximum diameter for a Fast

Ethernet network?

A. 100 feet

B. 200 feet

C. 100 meters

D. 200 meters

Answers to Review Questions

1. Network segmentation allows larger networks because it reduces network congestion caused by a large number of network nodes contending for the shared media. For more information, see

“The Problems with Large Networks.”

2. The three methods of LAN switching are cutthrough, fragment-free, and store-and-forward.

Cut-through starts forwarding as soon as the destination address is read. Fragment-free starts forwarding the frame after the first 64 bytes are received error free. Store-and-forward receives the entire frame before beginning to forward it.

Store-and-forward creates the greatest latency through the switch. For more information, see

“LAN Segmentation Using Switches.”

3. The Spanning Tree Protocol ensures a loop-free path on bridged and switched networks. It does this by electing a root switch and determining loops in the network. Redundant ports are placed in standby state so they do not forward frames. For more information, see “Spanning Tree Protocol.”

4. Segmenting a network with a bridge segments the network at Layer 2. Using a router to segment a network segments the network at Layer 3. For more information, see “Network Segmentation.”

5. Fast Ethernet is 10 times faster than 10BaseT.

This increase in speed provides more bandwidth for high-demand applications such as audio and video. Fast Ethernet is good for servers and highuse workstations. For more information, see “Fast

Ethernet.”

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A

P P L Y

Y

O U R

K

N O W L E D G E

6. A speakerphone is typically half-duplex, allowing only one person to speak at a time. For more information, see “Fast Ethernet.”

7. A two-way bridge over a river is an example of full-duplex. Cars can cross in both directions simultaneously. For more information, see “Fast

Ethernet.”

8. VLANs allow you to separate a network into logical groups. This provides benefits of security and increased scalability. For more information, see

“Virtual LANs (VLANs).”

9. A router must route data between the different

VLANs. For more information, see “Virtual

LANs (VLANs).”

Answers to Exam Questions

1. B. Bridges segment a LAN at Layer 2. For more information, see “LAN Segmentation Using

Bridges.”

2. C. Routers segment a LAN at Layer 3. For more information, see “LAN Segmentation Using

Routers.”

3. B. LAN switches segment the network at Layer 2.

For more information, see “LAN Segmentation

Using Switches.”

4. B. VLANs separate a network into different broadcast domains. For more information, see

“LAN Segmentation Using Switches.”

5. A. Cut-through LAN switching has a lower latency than store-and-forward. For more information, see “LAN Segmentation Using Switches.”

6. B. Store-and-forward LAN switching receives the entire frame error free before forwarding it. For more information, see “LAN Segmentation Using

Switches.”

7. A. The Spanning Tree Protocol maintains a loopfree network. For more information, see

“Spanning Tree Protocol.”

8. C. Full-duplex Ethernet effectively doubles the bandwidth. For more information, see “Fast

Ethernet.”

9. C. The maximum segment length for Fast

Ethernet 100BaseTX is 100 meters. For more information, see “Fast Ethernet.”

10. D. The maximum diameter for a Fast Ethernet network using copper is 200 meters. For more information, see “Fast Ethernet.”

Suggested Readings and Resources

1. Ford, Merilee; Lew, H. Kim; Spanier, Steve;

Stevenson, Tim. Internetworking Technologies

Handbook, Cisco Press, 1997.

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F

INAL

R

EVIEW

Study and Exam Prep Tips

Fast Facts

Practice Exam

P A R T

II

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This chapter provides you with some general guidelines for preparing for a certification exam. It addresses your exam preparation activities, covering general study tips.

To better understand the nature of preparation for the test, it is important to understand learning as a process.

You probably are aware of how you best learn new material. Maybe outlining works best for you, or perhaps you are a visual learner who needs to “see” things.

Whatever your learning style, test preparation takes time. Although it is obvious that you can’t start studying the night before you take these exams and expect to do well, it is very important to understand that learning is a developmental process. Understanding the process helps you focus on what you know and what you need to study further.

Thinking about how you learn should help you recognize that learning takes place when we are able to match new information to old. You have some previous experience with computers and networking, and now you are preparing for this certification exam. Using this book, software, and supplementary materials will not just add incrementally to what you know. As you study, you actually change the organization of your knowledge to integrate this new information into your existing knowledge base. This leads you to a more comprehensive understanding of the tasks and concepts outlined in the objectives and related to computing in general.

Again, this happens as an iterative process rather than a singular event. Keep this model of learning in mind as you prepare for the exam, and you will make better decisions about what to study and how much to study.

Study and Exam

Prep Tips

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S

TUDY

T

IPS

There are many ways to approach studying, just as there are many different types of material to study.

However, the tips that follow should work well for the type of material covered on the certification exams.

Study Strategies

Although individuals vary in the ways they best learn information, some basic principles of learning apply to everyone. You should adopt some study strategies that take advantage of these principles. One of these principles is that learning can be broken into various depths.

Recognition (of terms, for example) exemplifies a surface

level of learning: You rely on a prompt of some sort to elicit recall. Comprehension or understanding (of the concepts behind the terms, for instance) represents a deeper level of learning. The ability to analyze a concept and

apply your understanding of it in a new way or to address

a unique setting represents further depth of learning.

Your learning strategy should enable you to know the material a level or two deeper than mere recognition.

This will help you to do well on any exam. You will know the material so thoroughly that you can easily handle the recognition-level types of questions used in multiple-choice testing. You will also be able to apply your knowledge to solve novel problems.

This outline provides two approaches to studying. First, you can study the outline by focusing on the organization of the material. Work your way through the points and subpoints of your outline with the goal of learning how they relate to one another. For example, be sure you understand how each of the main objective areas is similar to and different from one another. Then do the same thing with the subobjectives. Also, be sure you know which subobjectives pertain to each objective area and how they relate to one another.

Next, you can work through the outline and focus on learning the details. Memorize and understand terms and their definitions, facts, rules and strategies, advantages and disadvantages, and so on. In this pass through the outline, attempt to learn detail as opposed to the big picture (the organizational information that you worked on in the first pass through the outline).

Research shows that attempting to assimilate both types of information at the same time seems to interfere with the overall learning process. If you separate your studying into these two approaches, you will perform better on the exam than if you attempt to study the material in a more conventional manner.

Macro and Micro Study Strategies

One strategy that can lead to this deeper learning involves preparing an outline that covers all the objectives and subobjectives for the particular exam you are working on. You should then delve a bit further into the material and include a level or two of detail beyond the stated objectives and subobjectives for the exam. Finally, flesh out the outline by coming up with a statement of definition or a summary for each point in the outline.

Active Study Strategies

The process of writing down and defining the objectives, subobjectives, terms, facts, and definitions promotes a more active learning strategy than merely reading the material does. In human information processing terms, writing forces you to engage in more active encoding of the information. Simply reading over the material constitutes passive processing.

Next, determine whether you can apply the information you have learned by attempting to create examples and scenarios of your own. Think about how or where you could apply the concepts you are learning. Again, write down this information to process the facts and concepts in a more active fashion.

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The hands-on nature of the Step by Step tutorials and the exercises at the end of the chapters provide further active learning opportunities that will reinforce concepts.

Common Sense Strategies

Finally, you should also follow common sense practices in studying: Study when you are alert, reduce or eliminate distractions, take breaks when you become fatigued, and so on.

You must come up with the answers in your own words. Flash Cards also enables you to test your knowledge of particular objective areas.

You should set a goal for your pre-testing. A reasonable goal would be to score consistently in the 90-percent range (or better). See Appendix D, “How to Use the Top Score Software Suite,” for a more detailed explanation of this test engine.

Pre-Testing Yourself

Pre-testing allows you to assess how well you are learning. One of the most important aspects of learning is what has been called meta-learning. Meta-learning has to do with realizing when you know something well or when you need to study some more. In other words, you recognize how well or how poorly you have learned the material you are studying. For most people, this can be difficult to assess objectively on their own.

Therefore, practice tests are useful because they reveal more objectively what you have and have not learned.

You should use this information to guide review and further study. Developmental learning takes place as you cycle through studying, assessing how well you have learned, reviewing, and assessing again, until you feel you are ready to take the exam.

You may have noticed the practice exam included in this book. Use it as part of this process. In addition to the practice exam element of this book, the Top Score software on the CD-ROM also provides a variety of ways to test yourself before you take the actual exam.

By using the Top Score Practice Exams, you can take an entire practice test. By using the Top Score Study

Cards, you can take an entire practice exam or you can focus on a particular objective area, such as Planning,

Troubleshooting, or Monitoring and Optimization.

By using the Top Score Flash Cards, you can test your knowledge at a level beyond that of recognition:

More Exam Preparation Tips

Generic exam preparation advice is always useful.

Follow these general guidelines:

á

Become familiar with the product. Hands-on experience is one of the keys to success on any certification exam. Review the exercises and the

Step by Step tutorials in the book.

á

Review the current exam preparation guide on

the Cisco Certification Web site. The documentation Cisco makes publicly available over the Web identifies the skills every exam is intended to test.

á

Memorize foundational technical detail as

appropriate. But remember: Most certification exams are generally heavier on problem solving and application of knowledge than they are on questions that require only rote memorization.

á

Take any of the available practice tests. We recommend the one included in this book and those you can create using the Top Score software on the CD-ROM. Although these are fixed-format exams, they provide preparation that is also valuable for taking an adaptive exam. Because of the nature of adaptive testing, it is not possible for these practice exams to be offered in the adaptive format. However, fixed-format exams provide the same types of questions as adaptive exams and are the most effective way to prepare for either type of exam.

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One-of-a-kind product New Riders

Publishing has just released a new product in their certification line:

ExamGear Test Simulation Software.

CCNA ExamGear will be available in stores in August, 1999. This product includes 500 test questions delivered to you in random order, as well as tracking and scoring abilities that no other test simulation product has. If you want to ensure that you are prepared for the CCNA Exam, check out

CCNA ExamGear.

á

Accept the Non-Disclosure Agreement and preliminary survey as part of the examination process. Complete them accurately and quickly move on.

á

Read the exam questions carefully. Reread each question to identify all relevant detail.

á

Tackle the questions in the order they are presented. Skipping around won’t build your confidence; the clock is always counting down.

á

Don’t rush, but at the same time, don’t linger on difficult questions. The questions vary in degree of difficulty. Don’t let yourself be flustered by a particularly difficult or verbose question.

á

Look on the Cisco Web site for samples and

demonstration items. These tend to be particularly valuable, if only for one significant reason:

They allow you to become familiar with any new testing technologies before you encounter them on a Cisco exam.

During the Exam Session

The generic exam-taking advice you’ve heard for years applies when taking a Cisco exam:

á

Take a deep breath and try to relax when you first sit down for your exam. It is very important to control the pressure you may (naturally) feel when taking exams.

á

You will be provided scratch paper. Take a moment to write down any factual information and technical detail that you committed to shortterm memory.

á

Carefully read all information and instruction screens. These displays have been put together to give you information relevant to the exam you are taking.

Fixed-Form Exams

Building from this basic preparation and test-taking advice, you also need to consider the challenges presented by the different exam designs. Because a fixed-form exam is composed of a fixed, finite set of questions, add these tips to your strategy for taking a fixed-form exam:

á

Note the time allotted and the number of questions appearing on the exam you are taking.

Make a rough calculation of how many minutes you can spend on each question, and use that number to pace yourself through the exam.

á

Take advantage of the fact that you can return to and review skipped or previously answered questions. Mark the questions you can’t answer confidently, noting the relative difficulty of each question on the scratch paper provided. When you reach the end of the exam, return to the more difficult questions.

á

If there is session time remaining when you have completed all questions (and you aren’t too fatigued!), review your answers. Pay particular attention to questions that seem to have a lot of detail or that required graphics.

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á

As for changing your answers, the rule of thumb here is don’t ! If you read the question carefully and completely and you felt like you knew the right answer, you probably did. Don’t secondguess yourself. If, as you check your answers, one stands out as clearly incorrect, however, of course you should change it. But if you are at all unsure, go with your first impression.

All that said, the most important thing to remember is that if you know your stuff, you should do fine. Use the self-test elements in this book, such as the practice exams and the review questions. Study where and when you are able to focus on the information, and practice with hands-on experience as much as possible. Good luck!

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14 051-6 Fast Facts 8/10/99 9:28 AM Page 305

The “fast facts” listed in this section are designed to act as a refresher of key points and topics that are required to be successful on the Cisco CCNA exam. By using these summaries of key points, you can spend an hour prior to your exam to refresh key topics, and ensure that you have a solid understanding of the objectives and information required for you to be successful in each major area of the exam.

What follows are the main categories Cisco uses to arrange the objectives:

.

OSI Reference

.

WAN Protocols

.

IOS

.

Network Protocols

.

Routing

.

Network Security

.

LAN Switching

For each of these main sections, or categories, the assigned objectives are reviewed, and following each objective, review material is offered. The information presented is intended as a review only.

If you do not understand the summary information presented, that is a cue for you to return to the main study material in Part 1, “Exam Preparation.”

Fast Facts

CCNA

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306 FA S T FAC T S

OSI R

EFERENCE

Objective OSI Reference: The understanding of the OSI reference model. Considerations include the following:

á

Identify and describe the functions of each of the seven layers of the OSI reference model.

á

Describe connection-oriented network service and connectionless network service and identify the key differences between them.

á

Describe data link addresses and network addresses, and identify the key differences between them.

á

Identify at least three reasons why the industry uses a layered model.

á

Define and explain the five conversion steps of data encapsulation.

á

Define flow control and describe the three basic methods used in networking.

á

List the key internetworking functions of the OSI

Network layer and how they are performed in a router.

Identify and describe the functions of each of the seven layers of the OSI reference model.

á

Application

á

Presentation

á

Session

á

Transport

á

Network

á

Data Link

á

Physical

OSI Layers

The OSI model was developed to divide networking functions into layers that are based on functionality.

The reasons for creating the OSI model are as follows:

á

It divides the complex network operations into less complex pieces.

á

It keeps changes in one area from adversely affecting other layers. This facilitates the evolution of networking components because, for example, companies can focus on improving one area without worrying about the functions handled at other layers.

á

It allows different vendors to create products that will work together.

á

It simplifies understanding the complex concepts of networking by dividing the concepts into smaller, less complex pieces.

Memorizing the sentence “All People Seem To Need

Data Processing” is a good way to remember the first letters of the seven layers of the OSI model.

Each of the seven layers of the OSI model performs a function for networking systems.

Application Layer (Layer 7)

The Application layer provides the interface from the

OSI model to the end user’s application. Some examples of Application layer implementations include the following:

á

Telnet

á

FTP

á

SMTP

á

POP

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Presentation Layer (Layer 6)

The Presentation layer defines how the data will be presented to the application layer. Some examples of

Presentation layer implementations include the following:

á

MIDI

á

JPEG

á

EBCDIC

á

ASCII

á

MPEG

Session Layer (Layer 5)

Communication sessions between network stations are established, managed, and terminated at the Session layer. Sessions handle the requests and responses between applications on the network stations. Examples of Session layer implementations include the following:

á

SQL

á

NFS

Transport Layer (Layer 4)

The Transport layer accepts data from the Session layer and puts it in packets to pass down to the Network layer. It is responsible for making sure packets are delivered without error, in sequence, and without any losses or duplications. Acknowledgments for packets received are generally sent from the Transport layer. Examples of

Transport layer implementations include the following:

á

TCP

á

UDP

á

SPX

Network Layer (Layer 3)

The Network layer defines the network address, which is different than the media access control (MAC) address.

An Internet Protocol (IP) address is an example of a

Network layer address.

The Network layer makes routing decisions and forwards packets that are destined for remote networks.

The Network layer evaluates the network address of the destination system and determines whether it is on the same network as the source system. If the address is identical, the Network layer obtains the destination system’s MAC address, either from a table or through a resolution technique such as IP’s Address Resolution Protocol

(ARP). After the Network layer has addressed the packet

with the source and destination addresses, it passes the packet down to the Data Link layer for delivery.

If the Network layer determines that the destination system is not on the same network as the source system, it addresses the packet with the MAC address of the router that can deliver the packet. It determines this by referencing routing tables that can be built either statically or dynamically. After the Networking layer addresses the packet with the source address and the address of the router that can deliver the packet, it passes the packet down to the Data Link layer for delivery to the router.

Examples of Network layer implementations include the following:

á

IP

á

IPX

Data Link Layer (Layer 2)

The Data Link layer provides an error-free link between two network devices. Data passed down from the Network layer is packaged into frames that will be passed down to the Physical layer. Error checking and correction information is added to the frame.

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One error correction technique used is cyclic redun-

dancy check (CRC). The media access control (MAC)

address is part of the Data Link layer. Logical link con-

trol (LLC) is also a function of the Data Link layer.

Examples of Data Link layer implementations include the following:

á

DLC

á

LLC

á

Build the data.

á

Package the data for end-to-end transport.

á

Append network address in the header.

á

Append local address in the data-link header.

á

Convert to bits for transmission.

Physical Layer (Layer 1)

The Physical layer is responsible for placing the data on the network media. The Physical layer defines such things as voltage levels, wire specifications, maximum distances, and connector types. Examples of Physical layer implementations include the following:

á

10BaseT Ethernet

á

100BaseTX Ethernet

á

Token-Ring

Encapsulation

Each layer uses control information to communicate with the corresponding layer at the other end of the link. This control data is added to the packet as it moves down the stack. This process is called encapsulation.

As a packet travels down through the seven layers, each layer adds control data and then passes the data down to the next layer until the data reaches the network.

After the packet is delivered to the receiving station, the process is reversed. Each layer strips off its control data and passes the packet up to the next layer until the packet reaches the application.

Encapsulation consists of five steps beginning at the

Transport layer:

Connection-Oriented and

Connectionless Network

Service

Connection-oriented protocols establish a connection

with the other end before they begin sending data. The protocol makes a request to the station that it wishes to exchange data with. This request must be accepted before the exchange can occur.

During the exchange of data, the two stations verify that the data is received error free. After the data exchange is complete, the connection between the two stations is terminated.

In a connectionless exchange, the two stations don’t need to establish a connection. One station simply sends data to the other. Connectionless exchanges do not verify that the data is received and make no attempt to ensure that the data is received error free. Generally in a connectionless exchange, layers above the Transport layer are responsible for making sure all the data is exchanged completely and error free.

MAC Address

The Media Access Control (MAC) address uniquely identifies each station on the network media. MAC addresses are 48 bits long and are usually represented as three groups of four hexadecimal digits. An example of a MAC address follows:

00e0.a38d.0800

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FA S T FAC T S 309

Network Address

The network address identifies the station at the Network level (Layer 3) of the OSI model. In TCP/IP, the network address is the IP address of the device. Under

Novell’s IPX, the network address is dynamically assigned as a combination of the IPX network number and the MAC address of the station. Network addresses are usually hierarchical, just as is the mailing address of a business or home.

Windowing

The sending and receiving station agree on a window size. This window defines the number of packets the stations will exchange before an acknowledgment is required. If the window size is five packets, for example, the sending station will send five packets and then wait for an acknowledgment from the receiver.

Address Resolution Protocol

(ARP)

Address Resolution Protocol (ARP) is responsible for

mapping IP addresses to MAC addresses. ARP builds a table of IP addresses and their corresponding MAC addresses.

Ethernet

Ethernet is by far the most popular networking topol-

ogy in use today. Its ease of use, simplicity, and reliability have established it as the mainstream networking topology. Ethernet is a Carrier Sense Multiple

Access/Collision Detect (CSMA/CD) network.

Ethernet can be implemented several different ways:

10Base5, 10Base2, 10BaseT, and 100BaseTX, and

100BaseFX are the most common.

Source-Quench Message

Source-quench messages are used to signal the sending sta-

tion to slow down. One problem with source-quench messages is that if the network is over burdened, there is no guarantee that the source-quench packet will arrive at the sending station.

Buffering

Buffering allows the receiving station to temporarily

store packets received until it can process them. As long as the bursts are short and do not overflow the buffer, this technique works well.

Token Ring

The IEEE 802.5 specification is almost identical to

IBM’s Token Ring. Logically, Token Ring is arranged in a ring in which each station is a node on the ring. These stations pass traffic around the ring. A network station accepts a packet from its upstream neighbor and sends it to its downstream neighbor. Token Ring supports multiple speeds: 4Mbps and 16Mbps are the most common. Newer Token Ring equipment is starting to support 100Mbps.

Serial Ports

Cisco routers use serial ports for connection to many devices including Channel Service Unit/Data Service

Unit (CSU/DSU). CSU/DSUs are similar to modems

but are used for digital circuits such as 64Kbps leased lines and T1s.

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These serial ports are very versatile. They support several different standards from one port. The port configures itself for the different standards based on the cable that is plugged in.

Some of the types of interfaces supported by Cisco serial ports include the following:

á

V.35

á

EIA-232 (RS-232)

á

EIA-449

á

X.21

á

EIA-530

WAN P

ROTOCOLS

Objective WAN Protocols: Understand the technologies behind WAN protcols and how to configure them within a router. The WAN protocols objective includes the following:

á

Differentiate between the following WAN services:

Frame Relay, ISDN/LAPD, HDLC, and PPP.

á

Recognize key Frame Relay terms and features.

á

List commands to configure Frame Relay LMIs, maps, and subinterfaces.

á

List commands to monitor Frame Relay operation in the router.

á

Identify PPP operations to encapsulate WAN data on Cisco routers.

á

State a relevant use and context for ISDN networking.

á

Identify ISDN protocols, function groups, reference points, and channels.

á

Describe Cisco’s implementation of ISDN BRI.

Following is a list of basic concepts and terms you need to be familiar with for this portion of the test.

á

Plain Old Telephone Service (POTS). POTS describes the normal telephone service most people have installed in their homes.

á

Local Loop. The local loop is the part of the telephone network that runs from your local CO to your office.

á

Demarcation Point. The demarcation point is where the telephone company terminates its lines in your building.

á

Customer Premise Equipment (CPE). Customer

Premise Equipment is equipment that resides at

the customer site. This equipment interfaces with the telephone company’s network. A CSU/DSU is a good example of Customer Premise

Equipment

á

T1 Service. A T1 is a digital telephone line that provides a bandwidth of 1.54 million bits per second. This bandwidth is delivered as 24 channels of 64Kbps data channels.

á

Channel Service Unit/Data Service Unit

(CSU/DSU). A Channel Service Unit/Data

Service Unit (CSU/DSU) (also often called just a

CSU) interfaces with digital telephone lines. A

CSU provides an interface for the router to connect to the digital line delivered by the telephone company.

á

Integrated Services Digital Network (ISDN).

ISDN defines a digital communication link for

voice and data communication. ISDN can be ordered in two different ways: Basic Rate Interface

(BRI) and Primary Rate Interface (PRI). A BRI

line has two B channels and one D channel

(2B+D). A PRI line has 23 B channels and one D channel (23B+D).

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FA S T FAC T S 311

á

ISDN Layer 2 Protocols (LAPD). This ISDN specification includes implementations of Layer 2 and Layer 3 protocols. The Layer 2 signaling protocol is Link Access Procedure, D channel (LAPD).

LAPD is used across D channels to provide control and signaling information. LAPD is formally specified in ITU-T Q.920 and ITU-T Q.921.

á

ISDN Layer 3 Protocols. There are two Layer 3 protocols defined within the ISDN framework for signaling: ITU-T Q.930 and ITU-T Q931.

These two protocols work together to provide

user-to-user, circuit-switched, and packet-switched

connections. These protocols specify how call establishment, call termination, information, and miscellaneous messages are communicated.

ISDN Reference Points

á

R. This is the reference point between non-ISDN equipment and a TA.

á

S. This is the reference point between user terminals and the NT2.

á

T. This is the reference point between NT1 and

NT2 devices.

á

U. This is the reference point between NT1 devices and line-termination equipment. The U reference point is only relevant in the North

America where the carrier does not provide the

NT1.

Understanding SPIDs

A Service Profile ID (SPID) is used between your ISDN equipment and the telephone companies switch. A

SPID is usually some variation on the telephone number for your ISDN line.

Monitoring ISDN

The most useful command to monitor ISDN is show isdn status

.

ISDN in a Nutshell

ISDN comes in two packages: BRI and PRI.

BRI ISDN has two bearer channels of 64Kbps (or

56Kbps) bandwidth and one signaling channel of

16Kbps.

PRI ISDN is delivered on the same type circuit as a T1 and has 23 bearer channels and one signaling channel.

A SPID is a Service Profile ID and is usually a variation of the telephone number for the bearer channels in your ISDN line.

To configure a BRI line, you set the switch type, the

SPIDs, and the encapsulation type.

ISDN lines do not provide power like normal POTS lines. ISDN lines require an NT1 at the customer site.

Some ISDN devices have an NT1 built into them and some do not. Cisco 2500s with BRI required an external NT1 to work with ISDN.

The command show isdn status provides information about an ISDN line and the status of the channels.

High-Level Data-Link Control

(HDLC)

HDLC is a link-layer protocol. It encapsulates data for

transmission on synchronous serial connections.

Although most vendors support HDLC, most implementations are slightly different. HDLC is the default encapsulation type for serial ports on Cisco routers and is often used when connecting Cisco to Cisco routers.

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Point-to-Point Protocol (PPP)

Point-to-Point Protocol (PPP) is a Data Link layer proto-

col that will work over synchronous or asynchronous serial connections. PPP uses Link Control Protocol

(LCP) to build and monitor connections.

Because PPP is often used over dial-up lines, the LCP portion supports authentication using two protocols:

Password Authentication Protocol (PAP) and Challenge-

Handshake Authentication Protocol (CHAP). PAP pro-

vides a simple authentication technique. Its biggest disadvantage is that passwords are transmitted over the link in clear text. CHAP provides a more secure authentication technique using a magic number.

CHAP authentication never transmits the password over the link.

PPP supports many different network protocols, including TCP/IP, IPX/SPX, AppleTalk, Transparent

Bridging, DECnet, and OSI/CLNS.

To configure PPP on a serial port, use the command encapsulation ppp

.

Data-Link Connection Identifier

(DLCI)

In a Frame Relay network, one physical circuit can support multiple virtual circuits. The router needs some way to identify which virtual circuit a packet of data is destine for. The DLCI (pronounced “del-see”) is used to identify this virtual circuit. A DLCI is used to identify to which of the multiple virtual circuits a packet of data is destined. After the packet of data is received by the Frame Relay switch, the router is attached, and the

Frame Relay switch makes a determination as to how to forward the packet. The Frame Relay switch has been programmed by the Frame Relay vendor for how to forward packets containing particular DLCI numbers. The programming is specific to the local Frame

Relay switch with which the router is connected. After the packet of data exits the local Frame Relay switch and starts its travel through the Frame Relay cloud, the local DLCI number is no longer significant. The local

DLCI number is only used between the router and the

Frame Relay switch with which the router is connected.

To review, the DLCI represents a virtual circuit to the

Frame Relay switch attached to the router. Beyond that point in the Frame Relay cloud, the DLCI had no significance. This is what people mean when they state that a DLCI has “local significance” only.

Frame Relay

Frame Relay is a packet-switching network. This type of

network shares expensive resources among subscribers.

By sharing the expensive resources such as long distance digital connections, the Frame Relay provider can offer a more cost-effective solution than dedicated lines.

Committed Information Rate (CIR)

When you order a Frame Relay circuit, you request a

Committed Information Rate (CIR). The CIR is how

much bandwidth the Frame Relay vendor guarantees you.

Permanent Virtual Circuit (PVC)

A Permanent Virtual Circuit (PVC) is a connection created within the Frame Relay cloud by the Frame Relay provider. The PVC is created as a virtual connection between two locations. A PVC can seem like a dedicated connection between two different locations.

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Local Management Interface (LMI)

Local Management Interface (LMI) is a protocol used

between the Frame Relay switch and your router. In

1990, Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment worked together to create what is known as “Gang-of-Four LMI” or “Cisco LMI.”

They took the original Frame Relay protocol and added extensions that make supporting larger internetworks easier.

To create a subinterface, use the interface command in the configuration mode of the router. Enter the description of the interface to which you want to add a subinterface, with an added period and a number following it to identify the subinterface. When configuring Frame Relay, it is a good idea to use the DLCI number for the subinterface number. This makes it easier to identify items in the config later. An example for

DLCI 199 would be the following: interface serial0/0.199

Configuring Multipoint Frame Relay

In a multipoint configuration, all of the routers are on one network with DLCIs listed for each connection. All connections fall under one logical port on the router.

Monitoring the Serial Port of a

Frame Relay Connection

The show interfaces command lists some Frame

Relay-specific information for interfaces connected to

Frame Relay networks.

Configuring Point-to-Point Frame

Relay

Because Frame Relay can support multiple connections on one physical circuit, there must be a way of separating these connections within the router. Cisco supports this logical separation of a physical connection through the use of subinterfaces. Using subinterfaces, you can separate the multiple logical connections within the router.

Seeing Active PVCs

To view the PVCs your router can see, use the show frame-relay pvc command.

Separating Logical Connections with Subinterfaces

Subinterfaces provide a method of separating one physi-

cal network connection into multiple logical connections. This is useful for Frame Relay because one physical local loop can support many PVCs. This creates the appearance of many different physical connections to other sites on one interface. The router must have some means of separating these logical connections so it can properly deliver the traffic destined to them.

Subinterfaces provide this means. Subinterfaces have uses beyond just Frame Relay, but this discussion will focus on how subinterfaces are used with Frame Relay.

Congestion Notification

á

Forward Explicit Congestion Notification

(FECN). A method used in Frame Relay to inform the routers of congestion on the network.

When a Frame Relay switch detects congestion, it can set a bit in the frame to notify the receiving network device that the packet experienced congestion in the network. This is called FECN

(pronounced “feck-in”).

á

Backward Explicit Congestion Notification

(BECN). Often used in conjunction with FECN.

When a packet is passing back to the sending station, the Frame Relay switch can set the BECN bit to tell the sending station its traffic is experiencing congestion. The sending station can then reduce the speed at which it is sending data. This is called BECN (pronounced “beck-in”).

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Viewing the Frame Relay Map

Cisco routers create an internal map of frame relay

DLCIs and the ports they are mapped with. This map can be viewed using the command show frame-relay map

.

Frame Relay in a Nutshell

Frame Relay is a packet-switching network that provides

low-cost connections due to the sharing of resources.

The entire Frame Relay network is called the cloud.

Committed Information Rate (CIR) is how much band-

width your Frame Relay provider guarantees you will have available.

The Data-Link Connection Identifier (DLCI) is a number assigned to map an IP address to a PVC. DLCIs have local significance. This means they can be different on either end of a PVC and can be reused within the network.

The Permanent Virtual Circuit (PVC) is a path created in the Frame Relay network between two points. The

Frame Relay provider creates PVCs.

The Switched Virtual Circuit (SVC) is a path created dynamically in the Frame Relay network in much the same way in which a telephone call is placed. When a router needs to communicate with another, it requests that the Frame Relay switch create an SVC. The switch creates the SVC dynamically. When the routers no longer need to communicate, the SVC is deleted.

Local Management Interface (LMI) is the protocol used

between the router and the Frame Relay switch to add management functions for large internetworks.

To configure Frame Relay on an interface, use the command encapsulation frame-relay

.

To separate a physical interface into subinterfaces, use the configuration command interface serial0.100

, in which

.100

is the subinterface you want to create. Add the frame-relay interface DLCI 100 command, in which

100 is the DLCI number for the PVC.

To diagnose Frame Relay problems, the commands show interfaces and show frame-relay pvc offer much information.

IOS

Objective IOS: Understand how to interface with a router using Cisco’s Internetworking Operating System.

Considerations include the following:

á

Log in to a router in both user and privileged modes.

á

Examine router elements (RAM, ROM, CDP, and show).

á

Control router passwords, identification, and banner.

á

Identify the main Cisco IOS commands for router startup.

á

Enter an initial configuration using the setup command.

á

Copy and manipulate configuration files.

á

List the commands to load Cisco IOS software from flash memory, a TFTP server, or ROM.

á

Prepare to back up, upgrade, and load a backup

Cisco IOS software image.

á

Prepare the initial configuration of your router and enable IP.

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Normal Password

Authentication

This method of authentication simply uses a password stored in the configuration file of the router. When you attempt to access a router either through the console port or telnet, the initial login will look like this:

User Access Verification

Password:

Router>

The prompt character is

>

. The

> character indicates that you are now in the user exec mode. In this mode you have access to commands for monitoring the router and its interfaces. What can be performed at this level is restricted. To configure the router or perform advanced diagnostics, you must access the privileged exec mode. To access the privileged exec mode, enter the command enable

. Then enter the enable password, as follows:

User Access Verification

Password:

Router>enable

Password:

Router#

Context-Sensitive Help

Cisco’s IOS provides help for command syntax and options. This help system is available from the user exec or privileged exec mode command prompts.

To see a list of commands available at any time, simply type a question mark (

?

) at the prompt. To see the options for a command, simply type the command followed by a question mark.

Configuring the Router

To configure the router, you must enter the configuration mode. The configuration can be changed from several different sources: terminal, memory, or network.

The command configure followed by the source of the configuration change will put the router into configuration mode. To configure from the terminal, type the command configure terminal

.

Hostname

The hostname of a router is set using the global config command: c2513r1(config)#hostname Keyser

Keyser(config)#

Message of the Day (MOTD)

The message of the day (MOTD) is displayed when someone accesses the router.

To enter a MOTD, use the following command from the global config mode.

c2511r1(config)#banner motd #This router was replaced 1/09/99# c2511r1(config)#

The

# before and after the message is simply a delimiter character and can be any character not included in the message. It simply identifies the beginning and end of the message. The preceding example will appear onscreen the next time someone connects to the router.

This router was replaced 1/09/99

User Access Verification

Password:

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Configuring Interfaces

When configuring interface parameters such as bandwidth, IP address, media type, and most anything else specific to an interface, you use the interface configuration mode. To enter the interface configuration mode, type the command interface followed by the interface identifier. An example would be interface ethernet0/0

.

Saving Changes

To save changes made to the config, use the command copy running-config startup-config

. This copies the running configuration to NVRAM, where it is stored when the router is powered off. This command is commonly abbreviated copy run start

.

To find out which commands begin with te type the following: c2511r1#te?

telnet terminal test c2511r1#te

Managing Passwords

Cisco routers store passwords for login access in the configuration file. There are two types of passwords used in this manner for accessing a router: the login password and the enable password. The login password is used to gain access to the router’s user exec mode.

The enable password is used to enter the privileged exec mode, where configuration changes can be made.

Viewing the Configuration

To view the running configuration of a router, use the command show running-config or write terminal

.

Both commands display the running configuration on the display terminal.

Command History

The command history buffer allows you to recall a previously typed command and execute it again. Use the up arrow to scroll through the last commands executed.

Abbreviated Commands

Any router command can be abbreviated to just enough letters for the router to determine which command you are intending.

If you abbreviate a command too much, the CLI will not be able to uniquely identify which command you are trying to execute. Instead, it will respond with the following: c2511r1#te

% Ambiguous command: “te” c2511r1#

Viewing the Contents of the

Command History Buffer

The contents of the command history buffer can be viewed by typing the command show history

.

Random Access Memory

(RAM)

The router uses RAM to store data, execute programs, and buffer data. Packets of data can be buffered in

RAM until the router has time to process them. In some models of Cisco routers (3600, 7200, 7500, and so on), the IOS runs in RAM. Some routers, like the

2500 series, run the IOS from flash memory.

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Read-Only Memory (ROM)

The instructions for performing a self test of the device are stored in ROM. The ROM also contains a subset

(some routers contain a full version) of the IOS.

Flash Memory

Flash memory is similar to ROM except that flash mem-

ory can be erased and rewritten electronically. Cisco routers use flash memory much like personal computers use disk storage.

In routers in which the IOS is copied to RAM during the boot process, the IOS can be upgraded by copying a new version of IOS to flash while the router runs the copy stored in RAM. After the new version of IOS is stored in flash memory, the router can be rebooted and the new version will be loaded and executed.

This does not work when the router is running the IOS from flash.

Examining the Components of a Router

The router can provide much information about the components within it. Information on the amount of memory, version of IOS, and interfaces installed can be viewed with the show version command.

Interfaces

The interfaces of a router connect it to the networks for which it will be handling traffic. These interfaces provide a great deal of information about the network and how the interface is performing.

To view information about an interface, type the command show interfaces

.

You can also view this information for a specific interface by specifying the interface. An example of how to do this is as follows: show interfaces ethernet0

Cisco Discovery Protocol

(CDP)

The Cisco discovery protocol (CDP) runs on Cisco-manufactured devices including routers, access servers, and workgroup switches. CDP starts automatically when the device is booted. Because CDP operates at the Data Link layer, it is not necessary for the devices to be in common network address domains to exchange CDP data.

CDP provides a method for a Cisco device to discover its neighboring Cisco devices and exchange information about the devices.

Processor

The processor does most of the work in the router. It keeps routing tables updated by calculating paths, moving packets, and performing many other tasks. When you are diagnosing problems, it is often important to check on the load of the processor. To view statistics on the processor, type the command show processes

.

CDP Enable and Disable

CDP is enabled by default; it can be disabled on a router by typing the following global command: c2511r1(config)#no cdp run

You can also disable CDP on a particular interface by typing the following command: c2511r1(config-if)#no cdp enable

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Seeing CDP Neighbors

You can view a router’s CDP neighbors by typing the show cdp neighbors command.

More detailed information about the neighbors can be viewed with the show cdp neighbors detail command.

This information lists each neighbor and the version of

IOS it is running. This is very helpful information when you are trying to diagnose why a feature is not working.

If the neighboring router is running an older version of

IOS, it may not support some newer features. In an environment in which different groups support different routers, this can be helpful in finding information about the routers you may need to communicate with.

CDP Timers

CDP has two timers.

á

CDP timer. Dictates how often a router sends out CDP packets. This timer is adjusted with the command cdp timer xx

, where xx is the number of seconds between CDP packets.

á

CDP holdtime. Sets the time a router will wait for a CDP packet from a neighbor before removing that neighbor from the routers neighbor table.

This is changed with the command cdp holdtime xxx

, where xxx is the number of seconds to hold information in the table before removing it.

c2511r1#copy running-config tftp

Remote host []? 192.169.169.140

Name of configuration file to write [c2511r1confg]?

Write file c2511r1-confg on host

192.168.169.140? [confirm]y

Building configuration...

Writing c2511r1-confg !! [OK] c2511r1#

Each exclamation point (

!

) indicates one packet that has been successfully transferred.

The running-config file is copied via the TFTP protocol to the TFTP server at IP address 129.168.169.140.

The filename used on the TFTP server is c2511r1-config. Finally, the file is transferred to the server. The startup-config can be copied using the same technique.

To reverse the process, simply reverse the order of the devices. Copy from the TFTP server to the filename running-config, as follows: c2511r1#copy tftp running-config

Host or network configuration file [host]?

Address of remote host [255.255.255.255]?

192.168.169.140

Name of configuration file [c2511r1-confg]?

Configure using c2511r1-confg from

192.168.169.140? [confirm]y

Loading c2511r1-confg from 192.168.169.140 (via

Ethernet0): !

[OK - 1260/32723 bytes] c2511r1#

Managing Configuration Files from the Privileged Exec Mode

The configuration file defines how the router will interact with the connected networks and the data on those networks.

To copy the running-config to a TFTP server, use the command copy running-config tftp

, as follows:

Back Up and Upgrade IOS

IOS resides in the router as a file stored in flash memory. This file can become corrupt because of, for example, a power problem or hardware failure. This file may also need to be upgraded to a newer version to support new features or correct problems because of software bugs. The TFTP server is used for this process.

To copy the IOS stored in flash to the TFTP server, use the copy command with the source and destination reversed.

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N

ETWORK

P

ROTOCOLS

Objective Network Protocols: Understand the functions of different network protocols. Understand how to create subnets and configure network protocols on

Cisco routers. Network protocols include the following:

á

Monitor Novell IPX operation on the router.

á

Describe the two parts of network addressing, then identify the parts in specific protocol address examples.

á

Create the different classes of IP addresses (and subnetting).

á

Configure IP addresses.

á

Verify IP addresses.

á

List the required IPX address and encapsulation type.

á

Enable the Novell IPX protocol and configure interfaces.

á

Identify the functions of the TCP/IP Transport layer protocols.

á

Identify the functions of the TCP/IP Network layer protocols.

á

Identify the functions performed by ICMP.

á

Configure IPX access lists and SAP filters to control basic Novell traffic.

á

A Novell IPX address looks like

A31B3AF5.0000.0000.0001

á

An IP address looks like 192.168.19.114

MAC Address

The MAC address identifies a device on the local network. It is mapped in some way to the network address.

For Novell’s IPX protocol, the MAC address is used to create the network address. Because it is made part of the network address, there is no need to create a map between the two addresses. TCP/IP, however, requires a method of mapping IP address to a MAC address.

Address Resolution Protocol (ARP) performs this mapping.

Converting Numbers From Binary to

Decimal

To convert the binary number 11111001 to its equivalent decimal number, place the number below the chart as shown following:

128 64 32 16 8 4

1 1 1 1 1 0

2

0

1

1

Add up the decimal numbers for each position with an equivalent binary 1.

128 + 64 + 32 + 16 + 8 + 1 = 249

Network Addresses

Network addresses are the addresses used at Layer 3 of

the OSI reference model. The network address differs from the MAC address in that a network address is generally routable because it includes some type of hierarchical information. A MAC address is local to the network on which it is connected. Some network addresses use the MAC address to create the network address; Novell’s IPX does this. Two examples of network addresses are IPX and IP addresses.

Converting Numbers from Decimal to Binary

While working with IP addresses, you will need to convert numbers from decimal to binary. To perform this function, you start by finding the largest binary position that will go into the number you are converting.

For the decimal number 249, for example, the largest binary position that will go into it is 128, which is the highest bit position of an octet. Place a 1 in that bit position.

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128 64 32 16 8 4 2 1

1

Subtract 128 from 249: 249 – 128 = 121. Now perform the same over again. The largest bit position that will go into 121 is 64. Place a 1 in that bit position and continue from there.

2 1 128 64 32 16 8 4

1 1

121 – 64 = 57

128 64 32 16 8 4

1 1 1

57 – 32 = 25

128 64 32 16 8 4

1 1 1 1

25 – 16 = 9

128 64 32 16 8 4

1 1 1 1 1

9 – 8 = 1

128 64 32 16 8 4

1 1 1 1 1 0

2

2

2

2

0

1

1

1

1

1

TCP/IP Overview

The key thing to understand for this objective is how the transport protocols fit into the OSI model.

á

TCP is a connection-oriented protocol that guarantees packet delivery using acknowledg-

ments. When a packet is dropped, TCP is responsible for making sure that it is resent. TCP is responsible for breaking messages into segments for transmission. After the segments arrive at the destination, TCP reassembles them. TCP creates a connection between networking stations before transferring data.

á

UDP is a connectionless protocol that makes

no guarantees about data delivery. UDP can transmit data faster than TCP because it does not use acknowledgments. UDP does not break messages into segments; that must be handled by a different layer. Reliability is also the responsibility of another layer. Reliability can be handled by an

Application layer protocol or by a reliable lowerlayer protocol.

IP Addresses

An IP address defines two things: the network and the host address on the network. The network mask, sometimes called the subnet mask, determines which part of the IP address represents the network and which part represents the host.

Network Mask

Consider an IP address that looks like this:

Network = 192.168.2

Computer = 26

The network mask is just a short-hand method of representing this same concept. The previous example would look like this with a network mask:

IP Address = 192.168.2.26

Network Mask = 255.255.255.0

Computing devices take these two numbers in their binary form and perform a binary AND on them. A binary AND says, in effect, “compare the two binary numbers; if both bits in a position are 1, the result is 1.

If either bit is 0, the result is 0.”

TCP/IP Services

Following is a list of some of the protocols and their functions.

á

Internet Protocol (IP). The Layer 3 (Network) protocol of the suite.

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á

Address Resolution Protocol (ARP). Used to map IP addresses to MAC addresses.

á

Transmission Control Protocol (TCP). A Layer

4 protocol. TCP provides guaranteed data delivery.

á

User Datagram Protocol (UDP). A Layer 4 protocol. UDP does not guarantee data delivery.

á

Internet Control Message Protocol (ICMP).

Provides diagnostic information about the protocol suite. Most people are familiar with the diagnostic utility ping. Ping provides a means of confirming network connectivity between two stations. Ping is an implementation of ICMP. Today it is best to specify an ICMP ping because many other protocols now support ping in their implementations. Novell has added ping to IPX. IBM has even added some ping-like functions to SNA.

á

Telnet. An Application layer implementation of a program that allows remote terminal access to a device. For internetworking engineers, Telnet is often used to log in to routers.

á

File Transfer Protocol (FTP). Allows stations to transfer files from one to another. FTP uses TCP packets to guarantee data delivery.

á

Trivial File Transfer Protocol (TFTP). A file transfer implementation. It differs from FTP in that it uses UDP packets to deliver its data; hence, it does not guarantee delivery of data.

á

Simple Mail Transfer Protocol (SMTP). An email implementation that delivers mail from network to network or from user to user within one domain. SMTP is used throughout the

Internet for delivery of millions of emails daily.

FA S T FAC T S 321

á

Post Office Protocol (POP). An email implementation that allows users to connect with the system that receives their email and download that email to their computers. POP is used by millions of users everyday to get email from their ISPs.

Programs that can use POP include Eudora Pro,

Outlook, Pegasus Mail, and the email portion of most browsers.

á

Network Time Protocol (NTP). Used to synchronize the clocks of networking computers.

NTP is typically used to synchronize your system’s clock with one of the very accurate time sources available on the Internet. Cisco routers support NTP and can be configured to synchronize their clocks. This can be very helpful when diagnosing problems.

á

HyperText Transfer Protocol (HTTP). The protocol Netscape Navigator uses to talk with Web servers. Your browser uses HTTP to download

HTML from Web servers and display the information on your screen.

á

Domain Name System (DNS). Used to map domain names such as www.cisco.com

to IP addresses. When you type the name of a site you want to visit in your browser, the computer first resolves that name to an IP address using DNS.

After the IP address is found, it is passed to the

IP stack and HTTP is used to communicate with the Web server. Without DNS, you would have to remember IP addresses rather than domain names. DNS makes life much easier.

á

Simple Network Monitor Protocol (SNMP).

Used to gather statistics from networking devices for monitoring and reporting. Most networking monitoring software, such as Cisco Works, uses

SNMP to gather information.

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Domain Name Server (DNS)

Cisco’s IOS has different ways to map names to IP addresses. If your organization makes use of DNS, you can configure your routers to use DNS to resolve names to IP addresses. To turn this on in the router, you use the ip name-server command to identify the

DNS machine to the router. This is the machine that provides the DNS service. You must also turn on domain lookup in the router, which is done with the command ip domain-lookup

.

Use the following, for example, to configure the DNS server in the router’s configuration:

ACS-2500#config t

Enter configuration commands, one per line. End with CNTL/Z.

ACS-2500(config)#ip name-server 155.229.1.69

ACS-2500(config)#ip domain-lookup

ACS-2500(config)#exit

When the router performs a DNS lookup, it caches the information for future use. You can view the contents of the cache with the show hosts command, as follows:

ACS-2500#sh hosts

Default domain is not set

Name/address lookup uses domain service

Name servers are 155.229.1.5, 155.229.1.69

Host Flags Age Type

Address(es) www.kmahler.com (temp, OK) 0 IP

199.250.137.37

cio-sys.cisco.com (temp, OK) 0 IP

192.31.7.130

www.cisco.com

ACS-2500#

The default domain is the domain the router resides in.

The router can provide a shortcut to accessing stations within its domain. If you set the default domain in the router, it will then append this default domain to any name that is not fully entered. If you wanted to ping kestrel.kmahler.com

and the default domain name kmahler.com

was defined in the router with the name server set, along with domain name lookup on, the router will be able to identify that station with just kestrel

.

To clear an individual host mapping, use the clear host with the host name specified, as follows: c3620r1#clear host linux.kmahler.com

c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type

Address(es) www.kmahler.com (temp, OK) 0 IP

199.250.137.37

kestrel.kmahler.com (temp, OK) 0 IP

199.250.137.34

c3620r1#

To clear all name to IP address mappings, use the wildcard

*

, as follows: c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type

Address(es) kestrel.kmahler.com (temp, OK) 0 IP

199.250.137.34

www.kmahler.com (temp, OK) 0 IP

199.250.137.37

linux.kmahler.com (temp, OK) 0 IP

199.250.137.55

c3620r1#clear host * c3620r1#sh host

Default domain is kmahler.com

Name/address lookup uses domain service

Name servers are 199.250.137.37

Host Flags Age Type

Address(es) c3620r1#

Because there is not always a DNS available, Cisco has provided another method of mapping host names to IP addresses. Static name mappings can be made part of the router’s configuration.

To create a static name mapping, use the command ip host with the hostname and IP address. An example entry would be ip host kestrel 199.250.137.34

.

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Understanding IP Address

Classes

A

B

IP addresses are divided into classes. For each class, there is an assumed network mask. The most common

IP address classes are the following:

Class

Range of

(1 st octet)

Maximum

Networks Maximum Hosts

1–126

128–191

126

16,384

16,777,214

65,534

C 192–223 2,097,152 254

The class of an IP address is actually defined by the first few bits.

á

If the first bit of an IP address is 0, it is a Class A address. By definition, a Class A IP address has eight bits of network address and 24 bits of host address. The subnet mask for a Class A address is

255.0.0.0 or /8.

á

If the first two bits are 10, it is a Class B address.

By definition, a Class B IP address has 16 bits of network address and 16 bits of host address. The subnet mask for a Class B address is 255.255.0.0

or /16.

á

If the first three bits are 110, it is a Class C address. By definition, a Class C IP address has

24 bits of network address and eight bits of host address. The subnet mask for a Class C address is

255.255.255.0 or /24.

The IPX network address consists of two parts: the

network number and the node number. When an IPX

address is displayed in a router, it takes the form of

4c.03cb.530c.1cd3

, where

4c is the network number and

03cb.530c.1cd3

is the node number. Novell’s IPX protocol has an interesting way of mapping the network address to the MAC address. The MAC address is actually used to derive the network address.

Given the previous example, the node number

03cb.530c.1cd3

is also the MAC address of the node.

Therefore, it is simple for the IPX network to determine for which node the packet is destined.

Cisco Novell encapsulation types and their Novell equivalents appear in the following list:

á

Cisco’s Novell-Ether encapsulation is Novell’s

Ethernet_802.3. This frame format includes the

IEEE 802.3 length field, but not an IEEE 802.2

Logical Link Control (LLC) header. This encapsulation was the default for NetWare 2.x and 3.x.

á

Cisco’s SAP encapsulation is Novell’s

Ethernet_802.2. This frame format is the standard IEEE frame format, including an 802.2

LLC header. This encapsulation method became the default with the release of NetWare 3.12 and

4.x. For Token Ring use, SAP also equates to

Novell’s Token_Ring encapsulation.

á

Cisco’s ARPA encapsulation is Novell’s

Ethernet_II or Ethernet version 2. It uses the standard Ethernet 2 header.

á

Cisco’s SNAP encapsulation is Novell’s

Ethernet_SNAP or SNAP. It extends the IEEE

802.2 header by adding a SubNetwork Access

Point (SNAP) header. This header provides an

encapsulation type code similar to that of

Ethernet version 2. Cisco’s SNAP also equates to

Novell’s Token_Ring_SNAP.

Configuring IPX Routing

Configuring a Cisco router for IPX routing involves multiple steps. IPX routing is turned off by default in

Cisco’s IOS. This is in contrast to TCP/IP routing, which is on by default.

The first step is to enable IPX routing on the router.

This is done with the global configuration command ipx routing

.

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324 FA S T FAC T S

The next step in configuring IPX routing is to set the

IPX network number on each of the interfaces routing

IPX traffic.

To specify the encapsulation type, add the encapsulation keyword to the ipx network configuration command.

In some environments, multiple Novell networks are present on one LAN although the different logical networks are using different encapsulation types. To configure for this in a Cisco router requires the use of subinterfaces.

For an interface, only one IPX network can be defined.

However, if multiple encapsulation types are used on one network, the different encapsulation types can be configured using subinterfaces. You can create subinterfaces on a LAN interface and then assign a different encapsulation type to each subinterface, and a different network number to each. Each subinterface must have a different encapsulation type and IPX network number. After it is configured properly, the router will route traffic between the two different logical networks.

Novell Access Lists

In the Novell environment there may be times when you need to control which networks can communicate with others. Using a Novell access list can accomplish this. A Novell access list can be used to limit this access.

Novell access lists fall into four categories: standard, extended, SAP filtering, and NLSP route aggregation filtering. For the CCNA exam, you will need to understand the standard, extended, and SAP filtering access lists.

When using IPX access lists, it is sometimes necessary to indicate all IPX networks. This is represented in an access list as ffffffff or

–1

(minus 1). This is the wildcard to indicate all IPX network numbers.

Standard IPX Access Lists

Standard IPX access lists are numbered 800–899. In a

standard IPX access list, you can permit or deny traffic based on source address and destination address.

Displaying IPX Addresses

One of the first things to do when diagnosing an IPX problem is to check the IPX network numbers assigned to a router’s interfaces.

The command show ipx interface displays IPX-related information about the router’s interfaces. The IPX network number is displayed in full, including the node number. The command can be used without an interface description and will then display information for each interface in the router with an IPX address assigned.

If you are diagnosing a problem specific to one interface in the router, you can limit the information shown to just that interface.

If you wish to see a brief description of the IPX information for the interfaces in a router, you can use the variation of the previous command: show ipx interface brief

.

Displaying IPX Servers

The command show ipx servers is very useful when diagnosing an IPX problem. When the router receives

SAP broadcast from servers, it places them in its server table. The command show ipx servers allows you to view this table. Following is an example of the output of this command:

Router#sh ipx ser

Codes: S - Static, P - Periodic, E - EIGRP, N -

NLSP, H - Holddown, + = detail

221 Total IPX Servers

Table ordering is based on routing and server info

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FA S T FAC T S 325

Type Name Net

Address Port Route Hops Itf

P 4 RESEARCH

A5.0000.0000.0001:0451 2185984/00 1 Fa0/0

P 4 BENEFIT

36B08649.0000.0000.0001:0451 2/01 1

Fa0/0

Working with IPX Routing Tables

When a router is routing IPX traffic, it builds a routing table to determine the proper path to forward traffic.

To view the IPX routing table, use the command show ipx route

. This will display all of the networks the router has determined paths for and certain information about those paths.

The routing table can sometimes become unstable because of network topology changes or software bugs.

You can force the router to rebuild the routing table by clearing it. The command clear ipx route * will clear all routes in the table:

Router#clear ipx route *

Router#

You can clear a specific network route by specifying the route in the command, such as clear ipx route 3c

.

This will cause the router to clear this route and relearn the route if possible:

Router#clear ipx route 58

Router#

Load Sharing with IPX

Cisco routers routing IPX traffic can perform load sharing across equal cost paths. When two paths of equal cost or delay exist from one network to another, the router can distribute the data across those paths. This is called load sharing. Using load sharing can increase the usable bandwidth between the networks. To allow a

Cisco router to make use of multiple equal cost paths, you must set the parameter ipx maximum-paths

. The default value is

1

, which means that load sharing is disabled. You can allow a Cisco router to use up to 512 equal cost paths for IPX traffic, but a more realistic number would be two or three paths.

SAP Access Lists

SAP access lists allow you to filter the SAPs exchanged

with other routers. This is very useful because many

IPX networks include elements such as network printers that send SAP broadcasts. Generally, you do not need these SAPs to be broadcast across WAN links to remote sites. You can filter these SAPs by using a SAP access list. For example, if you wanted to allow only server SAPs from all networks to be broadcast over a

WAN link, you could use an access list.

R

OUTING

Objective Routing: Understand the operations of routing protocols and how to configure them within a Cisco router. The routing objectives include the following:

The routing section of the CCNA exam includes the following objectives:

á

Add the RIP routing protocol to your configuration.

á

Add the IGRP routing protocol to your configuration.

á

Explain the services of separate and integrated multiprotocol routing.

á

List problems that each routing type encounters when dealing with topology changes and describe techniques to reduce the number of these problems.

á

Describe the benefits of network segmentation with routers.

Routing Protocol Basics

A routing protocol is responsible for exchanging valid routing information between routers so that routers can find the best path for traffic between two networks.

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326 FA S T FAC T S

There are some terms relating to routing protocols that you must be familiar with:

á

Topology Change. Any change in the layout of a network that will affect routing of traffic. This could include an interface being shut down by an administrator or a link failing due to a line being cut. It could also include the addition of a new circuit to a network. Any occurrence that causes a path to be unusable or unreliable can cause a topology change.

á

Convergence. The processes of all routers on a network updating the routing tables when a topology change occurs.

á

Metric. Any piece of data that a routing protocol uses to make a routing decision. Metrics can include hop count, bandwidth, reliability, and others.

What to Do with Routing

Information from Multiple Protocols

Cisco routers can run and use multiple routing protocols simultaneously. This makes it important to understand what they do when they receive routes for the same network from two different routing protocols.

The router assigns an administrative distance to each routing protocol that it runs. This distance is used to weigh routes to the same network that were learned on two different routing protocols.

Administrative Distance

When a Cisco router receives routing information, this information is evaluated and valid routes are placed in the routing tables of the router. To determine which routing protocol to select over another, the router uses the administrative distance. Make sure you are familiar with the default values for administrative distance.

Path Selection

The elements used to determine the best path vary with the routing protocol are called metrics. Some simple protocols simply use the number of hops between networks. This is not necessarily the most accurate method of deciding the best path.

Dynamic routing is when the router uses some sort of

routing protocol to learn about the network and create routing tables based on this information. In a network using dynamic routing, when changes occur in the network, the changes are reflected in the routing tables soon after.

It is common for static and dynamic routing to be combined. A usual configuration includes a static default gateway entry, whereas everything else is learned dynamically. In this way, the router will reference its routing tables for a path to a remote network. If the remote network is not found in the routing tables, the packet is then forwarded to the default gateway, which would normally be a router with more complete routing tables.

Autonomous Systems (AS)

An autonomous system (AS) is a group of routers under one administrative control that share a routing strategy.

The routers in an autonomous system can freely exchange routing information. Routing protocols used within an AS are commonly called interior routing

protocols.

Split Horizon

Split horizon is a rule that says, in effect, “do not send

routing updates back out of the interface they were learned on.” This keeps a router from sending routing updates back to the router they were learned from.

Another method to avoid routing updates is the poison reverse update, described next.

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FA S T FAC T S 327

Poison Reverse Update

A poison reverse update is a routing update that contains a route that is no longer reachable. The hop count for the route is set to infinity (as defined by the routing protocol), so the route will be marked as invalid. A poison reverse update allows a router that knows a path is no longer reachable to go through that path to update neighbor routers with that information.

RIP Routing Tables

Different routing protocols maintain different information in the routing tables. RIP maintains the following in its routing table (see Table 1):

TA B L E 1

R I P I

N F O R M AT I O N

I

N C L U D E D

Destination Next Hop

10.110.20.0

10.65.32.1

Distance Timers

2 t1, t2, t3

10.44.17.0

10.65.32.1

10.202.14.0

10.44.23.1

3

13 t1, t2, t3 t1, t2, t3

Flags

x, y x, y x, y

The destination network is listed with the IP address of the next hop along the path to reach that network. The

distance to the network in hops is the metric used to

determine the best path. The timers are used to determine when a route is no longer valid and to avoid routing loops. The timers will be discussed in more detail later. The flags indicate if the route is valid or not and whether it is currently being held down.

Regular Updates and Triggered

Updates

RIP has two types of updates: regular updates that occur periodically based on the routing update timer and

triggered updates that occur anytime there is a network

topology change. Regular updates are used to update routing tables and indicate that a route is still available.

Triggered updates make the network converge more quickly because routers do not wait until the regular update to report the changes to the network.

Routing Update Timer

RIP uses timers as methods to many of its functions.

The first timer to discuss is the routing update timer.

The routing update timer is normally set to 30 seconds.

This ensures that every router sends a complete copy of its routing table every time the routing update timer counts down to 0, or 30 seconds if the default is kept.

Each time the routing update timer counts down to 0, the router sends its entire routing table out all active interfaces. After the complete table is sent, the timer is reset.

One of the disadvantages of RIP is that it sends the entire routing table periodically even if no topology changes have occurred. This is very inefficient compared to some other routing protocols. This is one of the things that limits RIP’s scalability for large networks. Also, if the routing tables are large, the routing updates can consume a large amount of bandwidth from low bandwidth WAN connections.

Route Invalid Timer

The route invalid timer determines the amount of time that must pass without hearing a routing update before the router considers a route invalid. The default setting for the route invalid timer is 90 seconds. This is three times the default for the routing update timer. There is a route invalid timer for each entry in the routing table.

Each time an update for that entry is received, the route invalid timer is reset. If the timer counts down to 0, the route is then marked as invalid. After a route is marked invalid, neighboring routers are sent this information.

Route Flush Timer

The route flush timer determines the amount of time that must pass without receiving an update for a routing table entry before the entry is removed from the table.

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There is a route flush timer attached to each entry in the routing table. When an update is received for that entry, this timer along with the route invalid timer is reset. If the route flush timer counts down to 0, the route entry is completely removed from the routing table. The default for the route flush timer is 270 seconds, or nine times the update timer.

Hold-Down Timer

Hold-down timers are used to prevent a regular routing

update from reinstating a route that is down. When a route goes down, the neighboring routers detect it and calculate new routes. These new routes are sent out in routing updates to inform their neighbors of the change. This change ripples through the network.

Because this update does not instantly arrive at every router in the network, it is conceivable that a router that has not yet received the update could send out a regular update indicating that the route that has just gone down is still good. A router that has just received the update that the route has gone bad could receive this update indicating the route is good.

A hold-down timer tells the router to hold down any changes that might have recently gone down for a period of time. This prevents a stale routing update from changing the route to an up status during this time. This allows time for the update indicating that a route has gone down to converge throughout the network.

Configuring RIP

To configure RIP in a router, you must first have your interfaces configured with the IP addresses. Then you configure the router rip as follows: c2513r2#config t

Enter configuration commands, one per line. End with CNTL/Z.

c2513r2(config)#router rip c2513r2(config-router)#network 10.0.0.0

c2513r2(config-router)#exit c2513r2(config)#exit c2513r2#

When an interface is set to passive

, it will not broadcast routing updates but it will still listen for them.

Following is an example of how to set the Ethernet segment as passive for this configuration: c2513r2#config t

Enter configuration commands, one per line. End with CNTL/Z.

c2513r2(config)#router rip c2513r2(config-router)#network 10.0.0.0

c2513r2(config-router)#passive-interface ethernet 0 c2513r2(config-router)#exit c2513r2(config)#exit c2513r2#

Working with RIP

After RIP is configured on a router, it is helpful to check the routing tables and statistics about the RIP protocol. The following example shows how to check the routing tables using the show ip route command:

Router#show ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-

IS level-2, * - candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets

C 10.245.34.0 is directly connected,

TokenRing0

R 192.168.99.0/24 [120/1] via 10.245.34.2,

00:00:12, TokenRing0

199.250.137.0/27 is subnetted, 1 subnets

C 199.250.137.32 is directly connected,

Ethernet0

Router#

The following example shows how to view the settings of the majority of RIP’s timers and other important information using the command show ip protocol

:

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FA S T FAC T S 329 c2513r2#sh ip protocol

Routing Protocol is “rip”

Sending updates every 30 seconds, next due in

24 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 Key-chain

Loopback0 1 1 2

TokenRing0 1 1 2

Routing for Networks:

192.168.99.0

192.168.42.0

10.0.0.0

Passive Interface(s):

Ethernet0

Routing Information Sources:

Gateway Distance Last Update

10.245.34.1 120 00:00:15

Distance: (default is 120) c2513r2#

Internet Gateway Routing

Protocol (IGRP)

IGRP is more scalable than RIP and offers many other advantages. Its main disadvantage is that it is Cisco proprietary and only works with Cisco routers. IGRP uses more meaningful metrics when selecting a path for network traffic.

IGRP Metrics

When IGRP selects the best path for network traffic, it uses many metrics. IGRP considers bandwidth, MTU, delay, load, and reliability when deciding which path is the best to a network. Each metric in IGRP is weighted and a distance is equated. This distance is used to determine the best path to a network.

The Limitations of RIP

The first limitation of RIP is that of the hop count.

RIP supports no more than 15 hops from one end of a network to the other. This limits the physical size of the network on which RIP can be used.

Another limitation of RIP is the fact that RIP sends its entire routing table every 30 seconds by default. By sending the entire routing table periodically, RIP generates a fair amount of network traffic. In a large network, this could become very significant.

RIP also is slow to converge, which can cause problems in large networks when topology changes occur.

The single metric of hop count creates another limitation for RIP. Because RIP considers the only number of hops when determining the best path for traffic, it will not always select the most efficient route for the traffic. More useful routing protocols consider many other factors when selecting a best path for traffic.

IGRP Similarities with RIP

In many ways, IGRP is not much different than RIP.

They are both distance vector routing protocols. Both protocols periodically transmit their entire routing table with the exception of routes suppressed by split horizon. They both use split horizon with poison reverse, triggered updates, and hold-down timers to prevent routing loops.

IGRP Differences from RIP

IGRP has several significant differences from RIP, however. IGRP uses autonomous systems to group routers in a routing domain.

IGRP broadcasts its full routing table every 90 seconds by default. IP RIP does this every 30 seconds.

IGRP Timers

IGRP uses timers much like RIP does. These timers ensure that certain events occur periodically, such as routing updates. These timers can also indicate when a route is no longer valid or when a route should be purged from the routing tables.

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IGRP uses the following timers to operate properly:

á

Update timer. This timer indicates when a new routing update should be sent out. The default value for this timer is 90 seconds.

á

Invalid timer. An invalid timer is attached to every route in the routing tables. When an update is received for this route, the timer is reset. If the timer ever reaches 0, the route is then considered invalid and no longer used. The default is 270 seconds.

á

Hold-down timer. When a route is marked invalid, it is kept marked invalid for the duration of the hold-down timer. The default is 280 seconds.

á

Flush timer. A flush timer is attached to every route in the routing table. If a route is marked invalid and stays invalid for the duration of the flush timer, the route is completely removed from the routing table. The default is 630 seconds.

Configuring IGRP

Configuring IGRP is very much like configuring RIP, with one caveat. IGRP requires an autonomous number.

This autonomous number indicates a group of routers participating in a routing scheme. Routers using the same autonomous number will freely exchange routing information. To configure IGRP on a router, start with the routing igrp asnumber command, where asnumber is the number representing the autonomous system the router will participate in. Following is an example of how IGRP is configured on a router:

Router#config t

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)#router igrp 100

Router(config-router)#network 199.250.137.0

Router(config-router)#network 10.0.0.0

Router(config-router)#exit

Router(config)#exit

Router#

Working with IGRP

Just like RIP, after you have configured IGRP on a router, it is helpful to check the routing tables to see what the router has learned and placed in the routing tables. Because the routing information from all routing protocols is consolidated in one routing table, the same command is used to view this information in

IGRP as it was for RIP.

The following example shows how to check the routing tables using the show ip route command.

Router#sh ip route

Codes: C - connected, S - static, I - IGRP, 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, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-

IS level-2, * - candidate default

U - per-user static route, o - ODR

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 1 subnets

C 10.245.34.0 is directly connected,

TokenRing0

I 192.168.99.0/24 [100/1188] via

10.245.34.2, 00:00:03, TokenRing0

I 192.168.33.0/24 [100/1188] via

10.245.34.2, 00:00:03, TokenRing0

199.250.137.0/27 is subnetted, 1 subnets

C 199.250.137.32 is directly connected,

Ethernet0

Router#

The routing table information looks much the same as that of RIP, with one noticeable difference. Look at the route for 192.168.99.0. The distance to this route is

[

100/1188

]. The 100 is the administrative distance for the routing protocol IGRP. The 1188 is the metric

IGRP calculated for this path. The metric is based on all of the factors that IGRP uses to calculate the most efficient path.

Just like RIP, the show ip protocol command will display information about the IGRP protocol:

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FA S T FAC T S 331

Router#sh ip protocol

Routing Protocol is “igrp 100”

Sending updates every 90 seconds, next due in

43 seconds

Invalid after 270 seconds, hold down 280, flushed after 630

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

IGRP metric weight K1=1, K2=0, K3=1, K4=0,

K5=0

IGRP maximum hopcount 100

IGRP maximum metric variance 1

Redistributing: igrp 100

Routing for Networks:

199.250.137.0

10.0.0.0

Routing Information Sources:

Gateway Distance Last Update

10.245.34.2 100 00:00:10

Distance: (default is 100)

Router#

An access list can be applied to traffic entering or exiting an interface. These two ways of implementing an access list are called inbound and outbound. Inbound access lists create more of a load on the router’s processor and are thus not recommended unless absolutely necessary. In most cases you should be able to enforce network traffic or routing polices using outbound access lists. The following discussion focuses on outbound access lists.

Extended IP Access List

An extended IP access list gives more granular control to traffic identification. It allows decisions based on source and destination IP addresses, particular protocols within the IP protocol suite, and port numbers. The extended IP access list offers much more flexibility in describing the type of network traffic to allow or deny.

N

ETWORK

S

ECURITY

Objective Network security: Understand how to use access lists to secure a network. Network security objectives include the following:

á

Configure standard and extended access lists to filter IP traffic.

á

Monitor and verify selected access list operations on the router.

Access Lists

An access list is created in the router configuration with parameters identifying the traffic for special handling.

This access list is identified by a number, or, starting with release 11.2 of Cisco’s IOS, a name. This access list is then applied to an interface of the router or to some other part of the configuration for special handling.

How an Access List Works

When the router receives a packet for routing, the router first checks the routing tables to determine how to forward the packet. If it determines that it does not have a way of forwarding the packet, the packet is dropped.

However, if the router determines that it can forward the packet and to which interface the packet should be sent, it then checks to see if the interface is grouped to an access list. If not, the packet is copied to the interface’s output buffer and the packet is forwarded.

If the router determines that the exiting interface is grouped to an access list, the packet is first subjected to the test of the access list before forwarding. Only if the packet is specifically allowed in the access list is it forwarded.

When defining an IP address in an access list, it is often necessary to identify entire networks instead of a specific network station. In an access list, you can use wildcard bits to identify parts of the network address to not check.

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Implementing Extended Access

Lists

An extended access list offers more detailed control over traffic. With an extended access list, you can specify the source IP address, destination IP address, the protocol, and the port used.

Following is the syntax for an extended access list: access-list 100-199 {permit|deny ip|tcp|udp|icmp} source source-mask dest destmask [lt|gt|eq|neq dest-port]

The access-list command is the same as for the simple

IP access list. The number of the access list 100–199 identifies it as an extended IP access list. The commands permit and deny work just as they do in simple access lists. However, in an extended access list, you can add the protocol to permit or deny. Extended access lists also allow you to specify a port number. With the port number, you can specify whether it should be less than, greater than, equal to, or not equal to the specified port.

Entering a question mark (

?

) at the end of your entry of the extended access list will list the well-known

TCP/IP port numbers and abbreviations that can be used in place of the actual numbers.

Viewing Access Lists

To view access lists created in a router, you can use the show access-lists command.

LAN S

WITCHING

Objective LAN switching: Understand how to access lists to secure a network. LAN switching objectives include the following:

á

Describe the advantages of LAN segmentation.

á

Describe LAN segmentation using bridges.

á

Describe LAN segmentation using routers.

á

Describe LAN segmentation using switches.

á

Name and describe two switching methods.

á

Describe full- and half-duplex Ethernet operation.

á

Describe the network congestion problem in

Ethernet networks.

á

Describe the benefits of network segmentation with bridges.

á

Describe the benefits of network segmentation with switches.

á

Describe the features and benefits of Fast

Ethernet.

á

Describe the guidelines and distance limitations of Fast Ethernet.

á

Distinguish between cut-through and store-andforward LAN switching.

á

Describe the operation of the Spanning Tree

Protocol and its benefits.

á

Describe the benefits of virtual LANs.

á

Define and describe the function of a MAC address.

Collision Domains

A collision domain is a group of network nodes on

Ethernet that share the network media. By sharing the media, they can experience or cause collisions with other stations in the collision domain. In large networks, you can divide the network into multiple collision domains to avoid the performance degradation associated with large collision domains. This dividing of the network is called segmentation.

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FA S T FAC T S 333

Network Segmentation

To keep collision domains small on large networks, you can make use of network segmentation. By segmenting the network into small collision domain segments that can communicate with each other, you avoid the performance degradation that occurs with a large number of nodes in one collision domain. There are several different devices you can use to segment a network:

á

Bridges

á

Routers

á

Switches

The following sections detail each of these techniques.

LAN Segmentation Using Bridges

A bridge is a network device that is connected to multiple network segments. A bridge is a Layer 2 device. It operates at the Data Link layer using MAC addresses to determine when to forward traffic. Years ago, bridges were physical devices that connected to the network.

Today, when the functions of a bridge are needed, a router is typically used. Cisco routers can function as bridges as well as routers.

Bridges learn MAC addresses (Layer 2) and forward traffic to other segments based on tables it builds of which MAC addresses reside on which segments.

Bridges have no knowledge of upper-layer protocols.

All forwarding is based on MAC address.

The advantage of LAN segmentation is that it increases network performance by reducing the number of nodes participating in a collision domain.

LAN Segmentation Using Switches

LAN switches work much like bridges with many ports.

A LAN switch learns what MAC addresses are connected to which ports. After the switch learns this information, it then forwards frames only to the ports on which the destination MAC address resides. LAN switches learn which port network nodes reside on by listening to network traffic. If a LAN switch receives a frame with a destination MAC address that is not in its forwarding tables, it floods all ports with the frame.

After the destination station replies to the frame, the

LAN switch will learn on which port it resides and won’t need to flood its traffic again.

LAN switches reduce collisions by increasing the number of collision domains. By increasing the number of collision domains, you reduce the number of network nodes contending for the media.

A LAN switch can operate in two different modes:

á

Cut-through

á

Store-and-Forward

LAN Segmentation Using Routers

A router can be used to segment a network into different collision domains. Assuming the router is routing between the segments and not bridging, this actually divides the segments into separate Layer 3 networks. Each different

Layer 3 network is a separate collision domain. One of the disadvantages of doing this with a router is that it can take a very powerful router to handle many high-speed network connections with large amounts of data.

Full-Duplex Ethernet

Full-Duplex Ethernet doubles the effective bandwidth of

the link. A 10Mbit link run in Full-Duplex mode becomes 20Mbits. A 100Mbit link run in Full-Duplex mode becomes 200Mbits. Obviously, Full-Duplex

Ethernet can help alleviate bottlenecks in networks. A common place to fully utilize Full-Duplex Ethernet is to network servers. These devices typically receive and send more data than any other network device. Another common use of Full-Duplex Ethernet is high-speed links that collapse a network backbone to one switch.

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334 FA S T FAC T S

Fast Ethernet

Fast Ethernet increases the bandwidth of Ethernet from

10Mbits per second to 100Mbits per second—10 times faster.

Fast Ethernet supports different media types:

á

100BaseTX. Can be used on two pairs of

Category 5 copper wire, like most 10BaseT installations. This is convenient because you will not have to install new wiring in most cases.

á

100BaseFX. An implementation of Fast Ethernet on fiber optic strands. Because 100BaseFX has longer maximum distance limitations, it is commonly used to connect outlying switches with a backbone switch.

á

100BaseT4. An implementation of Fast Ethernet, using four pairs of Category 3, 4, or 5 wire. This implementation is good for organizations that already have a large investment of Category 3 or 4 wire in place and cannot justify upgrading the cabling to Category 5.

Because network switches are active devices that regenerate the frame, their use can extend the limitations of a Fast Ethernet network.

Spanning Tree Protocol

The Spanning Tree Protocol (STP) calculates the most efficient loop-free path through the network. To maintain this loop-free environment, the Spanning Tree

Protocol defines a tree that spans all switches in the network. When a network loop exists, switches can see a node’s MAC address on both sides of a port. This causes confusion for the forwarding algorithm and allows duplicate frames to be forwarded. The Spanning

Tree Protocol determines where multiple paths exist and forces certain ones into standby (blocked) state.

If a network segment in the Spanning Tree becomes unreachable, or if the Spanning Tree Protocol calculates that the cost of a path has changed, the Spanning Tree algorithm reconfigures the Spanning Tree topology by activating the path that was placed in standby state.

STP operation is transparent to end stations, which do not detect whether they are connected to a single LAN segment or a switched LAN of multiple segments.

Spanning Tree Protocol Port States

Spanning Tree Protocol maintains a loop-free network by setting the state of switched ports. Spanning Tree can set any one of five states:

á

Blocking

á

Listening

á

Learning

á

Forwarding

á

Disabled

Virtual LANs (VLANs)

On a switched network, VLANs allow you to define logical groups. Although switches reduce the problems with collisions occurring, broadcasts still propagate throughout a switched network. These broadcasts might be Novell SAPs or ARP requests. Either way, they are forwarded to every workstation and server on your large switched network every time a broadcast is generated. Not only do broadcasts generate network overhead, but every workstation on the network must process the broadcast. This creates processing overhead for your computers as well.

By dividing the network into logical pieces with computers grouped by function, you can reduce the overhead of broadcasts on the network and workstations.

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Think of switches as reducing collision domains and

VLANs as reducing broadcast domains.

VLANs were designed to solve two main problems:

á

Scalability of flat networks

á

Simplification of network management

FA S T FAC T S 335

VLANs solve these problems by segmenting large flat networks into logically grouped VLANs. These VLANs are easier to manage because they are grouped logically.

They also increase scalability of the network; a large number of nodes can be supported as a number of smaller groups.

By adding a router in the network, you can route data from one VLAN to another just as you can route data from physically separate networks.

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15 051-6 Practice Exam 8/10/99 9:29 AM Page 337

This is an example test that is designed to resemble what you might see on your CCNA exam. You have

90 minutes to complete the test of 70 questions. Time yourself, and take your time. The test is not difficult but does require careful reading of the questions and answers. Pay attention to details in the answers such as the router prompt. You may use scratch paper and a pencil during the test. However, you will not be able to use a calculator on the real exam so don’t use one here.

Cisco requires a score of 68% to pass. This would be 48 questions correct. There has been talk of Cisco raising the required score to 70%. Do not worry if this happens before you take your test. This only means you have to get one more question correct. You must correctly answer 49 of the 70 questions to score a 70%.

After completing the exam, check your answers and review any areas you missed.

Practice Exam

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338 P R AC T I C E E X A M

E

XAM

Q

UESTIONS

1. Which terms are properly matched with their definitions? Select two.

A. CPE—Equipment at the customer site (such as CSU/DSU)

B. Demarc—The location where the telco switch connects to the Frame Relay network

C. DTE—Equipment used for communication

(such as a modem)

D. CO—The telephone company’s central office, where switching equipment is housed.

2. Changing the cdp timer to 90 performs what function?

A. Sets the cdp timer to 90 seconds

B. Sets the cdp packet size to 90 bytes

C. Sets the priority of cdp traffic

D. Places “

90

” in the header field of the cdp packet

3. Which of the following are Presentation layer implementations? Select three.

A. PICT and JPEG

B. ASCII and EBCDIC

C. MIDI and MPEG

D. NFS and SQL

4. When entering a command at the CLI, what key sequence moves the cursor to the beginning of the line?

A. CTRL+M

B. CTRL+A

C. CTRL+R

D. CTRL+T

5. Which of the following are ports of a router that will allow you to log in to the router? Select three.

A. Console

B. Virtual Terminal

C. Serial0

D. Auxiliary

6. Which of the following are characteristics of

Frame Relay? Select two.

A. PVC

B. 2B+D

C. DLCI

D. SPID

7. Which of the following are Session layer implementations? Select two.

A. MIDI—MPEG

B. NFS—SQL

C. ASCII—EBCIDIC

D. RCP—NetBIOS

8. The following is from a 7500 router; which of the answers best describes its parts?

FastEthernet2/0/1

A. Type FastEthernet, Slot 2, Adapter 0,

Interface 1

B. Type FastEthernet, Adapter 2, Interface 0,

Slot 1

C. Type FastEthernet, Adapter 2, Slot 0,

Interface 1

D. Type FastEthernet, Slot 2, Interface 0,

Adapter 1

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P R AC T I C E E X A M 339

9. Which IP address class allows the fewest number of host addresses?

A. Class A

B. Class B

C. Class C

D. Class F

10. Given the following network address, what is the protocol and what are the parts of the address?

30020a6.0004.ac35.7e5a

A. IP address, network 30020a6, node

0004.ac35.7e5a

B. IP address, network 0004.ac35.7e5a, node

30020a6

C. IPX address, network 30020a6, node

0004.ac35.7e5a

D. IPX address, network 0004.ac35.7e5a, node

30020a6

11. What command allows a Cisco router to use multiple equal cost paths for load sharing on IPX networks?

A.

router(config)#ipx maximum-paths

B.

router(config)#ipx load-sharing

C.

router(config)#ipx multiple-paths

D.

router(config)#ipx path-sharing

12. Which serial protocol has PVCs and DLCIs?

A. HDLC

B. SDLC

C. Frame Relay

D. PLEE

13. Given the following, what is true about this interface?

Serial5/3 is administratively down, and the line protocol is down.

A. The port has shut down due to loss of the line protocol.

B. The port has been shut down with the shutdown command.

C. The port has experienced errors and is down due to excessive errors.

D. The port is functionally operational but is currently not receiving keepalive packets.

14. Which of the following is a function of the

Network layer of the OSI model?

A. Converts data to 1s and 0s and places the data on the network media

B. Uniquely identifies the networking station with a MAC address

C. Uniquely identifies the networking station and is responsible for determining routing of traffic

D. Determines how the data will be presented to the Application layer

15. What is the function of poison reverse updates?

A. Prevents a router from exchanging LMI with a Frame Relay switch

B. Allows a router to delete itself from another router’s CDP update table

C. Prevents routing loops by sending a routing update with a metric marking the route unreachable

D. Prevents a router from accessing a TFTP server for its configuration file

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16. Given the following network address, what is the protocol and what are the parts of the address?

192.168.105.33 netmask 255.255.255.0

A. IPX address, network 192.168.105.0, node 33

B. IP address, network 192.168.105.0, node 33

C. IPX address, network 33, node 192.168.105.0

D. IP address, network 33, node 192.168.105.0

17. Which type of memory is used to store the startup configuration file?

A. RAM

B. Flash memory

C. NVRAM

D. SDRAM

18. ISDN supports which of the following?

A. Voice and data traffic

B. Data traffic only

C. Voice traffic only

D. PVCs and DLCIs

19. Which of the following is a valid range of IP addresses for the address 137.65.29.19 with

12 bits of subnet?

A. 137.65.29.16 to 137.65.29.31

B. 137.65.29.17 to 137.65.29.31

C. 137.65.29.17 to 137.65.29.30

D. 137.65.29.16 to 137.65.29.30

20. What is the function of the ICMP echo?

A. To determine network connectivity

B. To determine the noise level of a Frame Relay line

C. To exchange routing capabilities between routers

D. To determine the type of digital line being used to carry data

21. Which statements are true of store-and-forward switching? Select two.

A. The entire packet is received before forwarding.

B. Latency varies with packet length.

C. The packet is forwarded after the first 48 bytes are received error free.

D. Packets containing errors are forwarded.

22. Which address uniquely identifies each networking station on the network media and is usually burned into ROM?

A. Network address

B. DLCI

C. PVC

D. MAC Address

23. What is the proper method to obtain help about the show command?

A.

show help

B.

show ?

C.

show?

D.

help show

24. You have been assigned the IP network

209.168.54.0. You need to subnet it for the maximum number of networks that will support 40 hosts per network. What is the proper subnet mask?

A. 255.255.255.192

B. 255.255.255.224

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P R AC T I C E E X A M 341

C. 255.255.255.240

D. 255.255.255.248

25. What are the layers of the OSI reference model in order from layer 1 to layer 7?

A. Physical, Network, Data Link, Transport,

Session, Presentation, Application

B. Physical, Data Link, Network, Transport,

Presentation, Session, Application

C. Physical, Data Link, Network, Transport,

Session, Presentation, Application

D. Physical, Data Link, Transport, Network,

Session, Presentation, Application

26. What OSI layer is responsible for determining routing of data?

A. Data Link

B. Network

C. Transport

D. Application

27. Which protocol creates a connection between network stations and ensures reliable delivery of data?

A. IP

B. UDP

C. TCP

D. IPX

28. What is the proper command to back up the router’s IOS to a TFTP server?

A.

router#copy tftp flash

B.

router(config)#copy flash tftp

C.

router#backup tftp flash

D.

router#copy flash tftp

29. Which command properly configures the interface for IP address 65.167.89.1 with 16 bits of subnet mask?

A.

router(config)#ip address 65.167.89.1

255.255.255.0

B.

router(config-if)#ip address 65.167.89.1

255.255.255.0

C.

router(config-if)#ip address 65.167.89.1

255.255.0.0

D.

router(config)#ip address 65.167.81.1

255.255.0.0

30. Given the IP address 126.101.29.67

255.255.255.192, what is the class, subnet, and broadcast?

A. Class A, Subnet 126.101.29.64, Broadcast

126.101.29.127

B. Class A, Subnet 126.101.29.0, Broadcast

126.101.29.255

C. Class B, Subnet 126.101.29.63, Broadcast

126.101.29.128

D. Class B, Subnet 126.101.29.64, Broadcast

126.101.29.127

31. Given the following configuration command, what are the different parts of the command?

Router(config)#ip route 199.250.138.0

255.255.255.0 199.250.137.1 distance 120

A. 199.250.138.0 is the destination network,

255.255.255.0 is the next hop subnet mask,

199.250.137.1 is the next hop, and 120 is the administrative distance you assign to this route.

B. 199.250.138.0 is the next hop,

255.255.255.0 is the next hop subnet mask,

199.250.137.1 is the destination network, and 120 is the administrative distance you assign to this route.

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C. 199.250.138.0 is the destination network,

255.255.255.0 is the destination network subnet mask, 199.250.137.1 is the next hop, and 120 is the administrative distance you assign to this route.

D. 199.250.138.0 is the destination network,

255.255.255.0 is the destination network subnet mask, 199.250.137.1 is the next hop, and

120 is the distance in hops to this network.

32. Which of the following encapsulation types are properly matched? Select two.

A. sap—Ethernet_802.2

B. arpa—Ethernet_802.3

C. snap—Ethernet_II

D. novell_ether—Ethernet_802.3

33. Which OSI layer performs the code conversion and formatting functions?

A. Application

B. Session

C. Presentation

D. Network

34. What information is displayed when you execute a show interfaces command? Select three.

A. IP address

B. Bandwidth

C. IPX address

D. Input/Output statistics

35. Where is the running configuration stored in the router?

A. RAM

B. Flash memory

C. NVRAM

D. ROM

36. Which command will show the IPX address of an interface?

A.

show interface

B.

show ipx networks

C.

show ipx interface

D.

show interface ipx

37. Which terms apply to ISDN? Select two.

A. PVC

B. SPID

C. DLCI

D. 2B+D

38. Which statement is true of MOTD?

A. The MOTD protocol exchanges information with other Cisco devices.

B. MOTD allows a router to discover faster paths through the use of bandwidth calculations.

C. MOTD copies the configuration of the router to a backup.

D. The MOTD is displayed prior to the login prompt.

39. What is true about access lists? Select two.

A. Access lists can be applied to incoming traffic.

B. Access lists have an explicit allow added to the end.

C. Access lists can be applied to outgoing traffic.

D. Access lists can be applied only to IP traffic.

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40. Which are examples of Application layer implementations? Select the best answer.

A. PICT and JPEG

B. NFS and SQL

C. ASCII and EBCIDIC

D. Telnet and SMTP

41. How do you instruct the router to load the IOS from ROM?

A.

router(config)#load IOS ROM

B.

router(config)#boot system ROM

C.

router(config)#boot IOS ROM

D.

router(config)#load system ROM

42. Which OSI layer uses port numbers to communicate with higher layers?

A. Network

B. Transport

C. Data Link

D. Session

43. How can you deny telnet access from IP address

192.168.59.45 entering the router on serial0 ip address 192.168.99.1?

A.

router(config)#access-list 1 deny

192.168.59.45

B.

router(config)#access-list 1 deny

192.168.99.1

C.

router(config)#access-list 1 deny

192.168.59.45 eq 23

D.

router(config)#access-list 1 deny

192.168.99.1 eq 23

P R AC T I C E E X A M 343

44. Which of the following commands will verify that you have properly configured an IP address?

Select two.

A.

testip

B.

verify

C.

traceroute

D.

ping

45. What statement is true of HDLC?

A. HDLC is a dialup protocol that can be used in place of ISDN to combine the two channels of an ISDN line for more bandwidth.

B. HDLC is a serial protocol that varies between vendors. To use HDLC on a Cisco router, the router on the other end of the connection must also be a Cisco.

C. HDLC is a serial protocol to connect SNA devices.

D. HDLC provides handshaking between Cisco routers and Frame Relay switches.

46. Which of the following statements are properly matched? Select two.

A. BRI—two B channels and one D channel

B. BRI—23 B channels and one D channel

C. PRI—two B channels and one D channel

D. PRI—23 B channels and one D channel

47. What is the proper method to enter the privileged

EXEC?

A. Enter the password at the login prompt.

B. Enter the enable password at the login prompt.

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C. Enter the password at the login prompt, then enter the command enable

, then enter the enable password.

D. Enter the password at the login prompt, then enter the command privileged

, then enter the enable password.

48. How do you create a subinterface?

A.

router(config)#interface serial0 sub 1

B.

router(config)#subinterface serial0.1

C.

router(config)#interface serial0.1

D.

router(config-if)#interface serial0.1

49. Which OSI layer uniquely identifies the networking station on the network with an address that is usually burned into ROM?

A. Physical

B. Network

C. MAC

D. Data Link

50. What is true of the IP address 224.11.23.1?

A. It is a Class A address.

B. It is a Class B address.

C. It is a multicast address.

D. It is a loopback address.

51. Which type of routing update includes a path with a routing metric that makes the path unreachable?

A. Poison Reverse

B. Hold Down

C. Unreachable Update

D. Infinity Update

52. What is the purpose of the Novell SAP protocol?

A. It is the core protocol used for server communication.

B. It advertises services available.

C. It is a tick count routing protocol.

D. It is a link state routing protocol.

53. What statements are true of Novell’s IPX protocol?

Select two.

A. IPX provides reliable transport.

B. IPX creates a connection between two networking stations.

C. IPX is a connectionless unreliable protocol.

D. IPX uses the MAC address of the networking station in the network address.

54. The term local loop refers to what?

A. The connection between two frame relay switches

B. The connection from the customer’s location to the local central office

C. Creating a loop in the CSU/DSU for testing purposes

D. A SONET ring that loops around a city

55. You entered te and the router responded ambiguous command

; what should you type next to see a list of commands beginning with te

?

A.

router#te?

B.

router#te ?

C.

router#te[tab]

D.

router#te help

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56. What is the proper way to configure IPX on an interface using the Novell encapsulation

Ethernet_II?

A.

router(config-if)#ipx network 5c encapsulation sap

B.

router(config-if)#ipx encapsulation sap network 5c

C.

router(config-if)#ipx encapsulation arpa network 5c

D.

router(config-if)#ipx network 5c encapsulation arpa

57. What command displays the contents of the configuration register?

A.

show config

B.

show version

C.

show register

D.

show config-reg

58. What commands will display the DLCI numbers and their status? Select two.

A.

show frame map

B.

show interface

C.

show frame lmi

D.

show frame pvc

59. What does the DE bit indicate?

A. The destination address

B. The sequence of the packet

C. How many switches the packet has traversed

D. That the packet can be discarded

60. What are FECN and BECN?

A. Routing protocols for Frame Relay

B. Methods for switches to prioritize traffic

C. Congestion notification methods

D. Frame Relay encapsulations

61. Given the following, which statements are true?

Select three.

Access-list 10 permit 10.100.10.0 0.0.0.255

Ethernet0

Access-group 10 in

A. Traffic from IP address 10.100.10.153 would be allowed to enter Ethernet0.

B. Traffic from IP address 10.100.11.45 would be allowed to enter Ethernet0.

C. Traffic from IP address 192.168.123.56

would be allowed to exit Ethernet0.

D. Traffic destined for 10.100.10.33 would be allowed to exit Ethernet0.

62. Which command will display a list of Novell servers?

A.

show ipx servers

B.

show novell servers

C.

show ipx saps

D.

show novell saps

63. When do configuration changes take effect?

A. After the router is reloaded

B. After the configuration changes are saved

C. After the router completes all routing updates

D. Immediately

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64. What does the command copy running-config startup-config do?

A. Creates an offsite backup of the router’s configuration

B. Saves configuration changes so if the router reboots, the running config will be the same

C. Copies the running-config to flash memory

D. Copies the startup-config to the runningconfig

65. Given the following address, what is the protocol and what are the portions of the address?

192.168.33.56 netmask 255.255.0.0

A. Protocol IP, Network 192.168.33.0, node 56

B. Protocol IPX, Network 192.168.0.0, node 33.56

C. Protocol IP, Network 192.168.0.0, node 33.56

D. Protocol IPX, Network 192.168.33.0, node 56

66. A RIP routing update contains a distance of 16; which of the following statements is true?

A. It will take 16 clock ticks to reach the destination network.

B. The destination is 16 hops away but still reachable.

C. The link between this router and the destination network has a bandwidth of

16 Kbits.

D. The destination network is unreachable.

67. What is a routing loop?

A. A ring of routers exchanging routing information

B. A route that contains two valid paths to a destination network

C. A situation that occurs when multiple routers think they can reach a network through each other when in reality neither has a path to the destination

D. The loop traffic takes to get to the destination network

68. Given the following configuration command, which statement is true?

Router(config)#ip route 0.0.0.0 0.0.0.0

62.123.3.1

A. Traffic for network 62.123.3.1 has no valid path.

B. Packets for network 62.123.3.1 will be forwarded to 0.0.0.0

C. Packets destined for networks not explicitly listed in the routing tables will be forwarded to 62.123.3.1.

D. Packets from network 0.0.0.0 will be forwarded to 62.123.3.1.

69. You want to know what IP address will be assigned to interface Ethernet0 when the router reboots; what is the correct command to investigate this?

A.

sh run

B.

sh start

C.

sh int e0

D.

sh int e 0

70. Given the following configuration commands, which statement is true?

Router eigrp 100

Network 192.168.100.0

Passive-interface ethernet0

A. Routing updates for all networks will be sent out interface Ethernet0.

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B. Only routing updates for 192.168.100.0 will be sent out interface Ethernet0.

C. No routing updates will be sent out

Ethernet0.

D. Routing updates for network 100 are sent out interface Ethernet0.

A

NSWERS AND

E

XPLANATIONS

1. A, D. CPE is customer premise equipment; this is equipment at the customer site, such as CSU/

DSUs. CO is the central office, where the telephone company houses switching equipment.

The Demarc is the location at the customer site where the telephone company’s responsibility ends and the customer’s begins. A modem is data communication equipment (DCE) not data terminal equipment (DTE). For more information, see the section in Chapter 5 titled “WAN Terms.”

2. A. The CDP timer sets how long between CDP neighbor packets are sent. For more information, see the section in Chapter 2 titled “Cisco

Discovery Protocol (CDP).”

3. A, B, C. PICT and JPEG, ASCII and

EBCIDIC, and MIDI and MPEG are all examples of Presentation layer implementations.

The Presentation layer defines how data will be presented to the Application layer. PICT and

JPEG are methods of graphics representation,

ASCII and EBCDIC are methods of text representation, and MIDI and MPEG are methods of multimedia representations. NFS and SQL are examples of Session layer implementations. For more information, see the section in Chapter 1 titled “Presentation Layer.”

4. B. Ctrl+A is the correct keystroke to return the cursor to the beginning of the line. For more information, see the section in Chapter 2 titled

“Editing Features.”

5. A, B, D. Console, Virtual Terminal, and auxiliary ports are correct because they all allow you to access the router and log in. Although you can access a router over Serial0 and log in, you actually are accessing a Virtual Terminal session, which presents you with the login prompt. For more information, see the section in Chapter 2 titled “Logging in to a

Router in Both User and Privileged Modes.”

6. A, C. PVC and DLCI are associated with Frame

Relay networks. PVCs are Permanent Virtual

Circuits and DLCIs are Data Link Connection

Identifiers. 2B+D and SPID are examples of

ISDN implementations. For more information, see the section in Chapter 5 titled “Frame Relay.”

7. B, D. NFS and SQL, and RCP and NetBIOS are examples of Session layer implementations.

MIDI and MPEG, and ASCII and EBCIDIC are examples of Presentation layer implementations.

For more information, see the section in Chapter 1 titled “Session Layer.”

8. A. In a 7000 or 7500 router with VIP cards, the interfaces are referenced as Type, Slot,

Adapter, and Interface. For more information, see the section in Chapter 2 titled “Interfaces.”

9. C. Class C addresses allow for only 254 host addresses. Class A and B addresses allow for more hosts, and there is no class F IP address. For more information, see the section in Chapter 3 titled

“Understanding IP Address Classes.”

10. C. This is a Novell IPX address. The first number is the network number. The following portion represents the MAC address of the station and is the node address. For more information, see the section in Chapter 4 titled “Network Addresses.”

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11. A. The command ipx maximum-paths allows a

Cisco router to use multiple paths to a network in a load sharing capacity. For more information, see the section in Chapter 4 titled “Configuring

IPX Routing.”

12. C. Frame Relay has PVCs and DLCIs. For more information, see the section in Chapter 5 titled “Frame Relay.”

13. B. An interface that shows administratively down has been shut down in the configuration with the shutdown command. For more information, see the section in Chapter 2 titled “Interfaces.”

14. C. The Network layer uniquely identifies the station on the network and is responsible for determining routing of traffic. For more information, see the section in Chapter 1 titled “Network Layer.”

15. C. A poison reverse update is a routing update including a metric that indicates that the network is unreachable. For more information, see the section in Chapter 6 titled “Routing Information

Protocol.”

16. B. IP addresses are represented as four numbers separated by periods. The subnet mask

255.255.255.0 indicates that the last octet is the node. For more information, see the section in

Chapter 3 titled “IP Addresses.”

17. C. The startup configuration is stored in

NVRAM. Flash memory stores the IOS image, the contents of RAM is lost when the power is lost.

For more information, see the section in Chapter 2 titled “Understanding Router Elements.”

18. A. ISDN can be used for voice and data traffic. For more information, see the section in Chapter 5 titled

“Integrated Services Digital Network (ISDN).”

19. C. 137.65.29.17 to 137.65.29.30 are valid IP addresses for 137.65.29.19 with 12 bits of subnetting. Remember that Cisco counts subnet bits from the end of the default subnet mask.

Therefore, the default subnet mask of 8 bits is added to the 12 bits of subnetting, for a total of

20 bits of network mask. For more information, see the section in Chapter 3 titled “IP Addresses.”

20. A. The ICMP echo confirms network connectivity between two stations. For more information, see the section in Chapter 3 titled “Internet

Control Message Protocol (ICMP).”

21. A, B. Store-and-forward switches receive the entire packet before forwarding. This means the latency through the switch varies with the length of the packet. For more information, see the section in

Chapter 8 titled “LAN Segmentation Using

Switches.”

22. D. The MAC address uniquely identifies a station on the network media and is usually burned into ROM. For more information, see the section in Chapter 1 titled “Data Link Layer.”

23. B. show ?

will display a list of options for the show command. show?

will list all commands that begin with show

. For more information, see the section in Chapter 2 titled “Context-Sensitive Help.”

24. A. Two bits of subnetting will allow for 62 hosts per network. Three bits allow for only 30 hosts. For more information, see the section in

Chapter 3 titled “IP Addresses.”

25. C. Remember the sentence “All People Seem

To Need Data Processing”: Application,

Presentation, Session, Transport, Network, Data

Link, Physical. For more information, see the section in Chapter 1 titled “The Seven Layers.”

26. B. The Network layer determines how packets are routed. For more information, see the section in Chapter 1 titled “Network Layer.”

27. C. Transmission Control Protocol (TCP) is a connection-oriented transport protocol that includes error checking to ensure reliable delivery of data.

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UDP is a connectionless protocol that includes no guarantee of data delivery. IP is connectionless with no guarantee of packet delivery. IPX is much like IP in this respect. For more information, see the section in Chapter 1 titled “Connection-

Oriented and Connectionless Services.”

28. D. copy flash tftp at the Privileged EXEC copies the flash memory to a tftp server. For more information, see the section in Chapter 2 titled

“Back Up and Upgrade IOS.”

29. B. Remember that when Cisco talks of subnet bits they are referring to bits after the default netmask for the class of address. The IP address is assigned at the Interface level of the configuration mode. This is indicated by the prompt router(config-if)#

. For more information, see the section in Chapter 3 titled “IP Addresses.”

30. A. This is a Class A IP address. The subnet is

64 and the broadcast is 127. For more information, see the section in Chapter 3 titled “IP

Addresses.”

31. C. A static route statement includes the destination network, subnet mask for the destination network, next hop address, and an optional administrative distance. For more information, see the section in Chapter 6 titled “Static Routes.”

32. A, D. sap—Ethernet_802.2 and novell_ether—

Ethernet_802.3 are properly matched encapsulation types. The other two should be arpa—

Ethernet_II and snap—Ethernet_snap. For more information, see the section in Chapter 4 titled

“Novell IPX/SPX Protocol.”

33. C. The Presentation layer performs code conversion and formatting functions. For more information, see the section in Chapter 1 titled

“Presentation Layer.”

34. A, B, D. show interfaces is a very useful command. It displays the primary IP address assigned to an interface, bandwidth assigned to the interface, and Input/Output statistics.

For more information, see the section in Chapter 2 titled “Examining the Components of a Router.”

35. A. The running configuration is stored in RAM.

For more information, see the section in Chapter 2 titled “Understanding Router Elements.”

36. C. show ipx interface will display information on interfaces with assigned IPX networks. For more information, see the section in Chapter 4 titled “Displaying IPX Addresses.”

37. B, D. The terms SPID and 2B+D are related to

ISDN. PVC and DLCI are related to Frame Relay.

For more information, see the section in Chapter 5 titled “Integrated Services Data Network.”

38. D. The MOTD banner is displayed before the password prompt when someone accesses the router. For more information, see the section in

Chapter 2 titled “Message of the Day (MOTD).”

39. A, C. Access list can be applied to incoming or outgoing traffic. For more information, see the section in Chapter 7 titled “Access Lists.”

40. D. Telnet and SMTP are examples of

Application layer implementations. PICT, JPEG,

ASCII, and EBCIDIC are all Presentation layer implementations. NFS and SQL are Session layer implementations. For more information, see the section in Chapter 1 titled “Application Layer.”

41. B. The command boot system ROM placed in the config instructs the router to boot from system ROM. For more information, see the section in Chapter 2 titled “Cisco IOS Command for

Router Startup.”

42. B. The Transport layer uses port numbers to communicate with higher layers. For more information, see the section in Chapter 1 titled “The

Seven Layers.”

43. A. access-list 1 deny 192.168.59.45

denies all traffic from this address. access-list 1 is a simple access list and does not allow the specifying of a protocol as specified in answers C and D.

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For more information, see the section in Chapter

7 titled “Implementing Extended Access Lists.”

44. C, D. Both traceroute and ping can be used to verify that you have properly configured an IP address. For more information, see the section in

Chapter 3 titled “Internet Control Message

Protocol (ICMP).”

45. B. Although most vendors support HDLC, each implementation is slightly different, making it vendor specific. For more information, see the section in Chapter 5 titled “How Routers

Communicate via Serial Links.”

46. A, D. A BRI ISDN line has two B channels and one D channel. A PRI ISDN line has 23 B channels and one D channel. For more information, see the section in Chapter 5 titled

“Integrated Services Data Network (ISDN).”

47. C. To enter a router in the privileged EXEC mode, you enter the login password. Then enter the command enable at the router prompt. Then enter the enable password. For more information, see the section in Chapter 2 titled “Logging In to a Router in Both User and Privileged Modes.”

48. C. To create a subinterface, simply enter the interface command with inteface.subinterface

.

For more information, see the section in Chapter 5 titled “Configuring Frame Relay.”

49. D. The Data Link layer uniquely identifies the networking station on the network media and is usually burned into ROM. For more information, see the section in Chapter 1 titled “The

Seven Layers.”

50. C. IP addresses beginning with 224 are multicast addresses. For more information, see the section in Chapter 3 titled “IP Addresses.”

51. A. Poison reverse updates include a route with a metric that makes the path unreachable. For more information, see the section in Chapter 6 titled “Routing Information Protocol.”

52. B. Novell’s SAP protocol is used to broadcast services available on the network. For more information, see the section in Chapter 4 titled

“Service Advertising Protocol (SAP).”

53. C, D. IPX is a connectionless unreliable protocol that uses the MAC address in the network address.

For more information, see the section in Chapter 4 titled “Internet Packet eXchange (IPX).”

54. B. The local loop is the telephone line between your office and the local CO. For more information, see the section in Chapter 5 titled

“Local Loop.”

55. A. Entering te?

will list all of the commands beginning with the letters te

. For more information, see the section in Chapter 2 titled “Entering

Commands.”

56. D. The IPX network configuration command is ipx network net encapsulation type

. The Cisco

encapsulation type for Ethernet_II is arpa. For more information, see the section in Chapter 4 titled “Configuring IPX Routing.”

57. B. The show version command displays the contents of the config register. For more information, see the section in Chapter 2 titled

“Examining the Components of a Router.”

58. A, D. show frame map and show frame pvc will display DLCI numbers and their status. For more information, see the section in Chapter 5 titled

“Frame Relay.”

59. D. The DE bit indicates that a packet is discard eligible. In times of congestion, frame relay switches may need to drop packets. Packets with the DE bit are dropped first. For more information, see the section in Chapter 5 titled “Frame Relay.”

60. C. Forward Error Congestion Notification

(FECN) and Backward Error Congestion

Notification (BECN) are congestion notification methods used in Frame Relay.

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For more information, see the section in Chapter

5 titled “Frame Relay.”

61. A, C, D. The access list is applied only to incoming traffic on the interface. For more information, see the section in Chapter 7 titled

“Implementing Simple Access Lists.”

62. A. show ipx servers lists the servers found through SAP and the interface they were learned on. For more information, see the section in

Chapter 4 titled “Diagnosing IPX Problems.”

63. D. Configuration changes take effect immediately. For more information, see the section in

Chapter 2 titled “Configuring the Router.”

64. B. copy running-config startup-config copies the running config from RAM to the startup config stored in NVRAM, so the configuration will be used the next time the router is rebooted. A common error is making a configuration change without saving the change. The router may reboot months later and something will stop working. It can take a while to discover this type of problem.

For more information, see the section in Chapter

2 titled “Managing Configuration Files from the

Privileged Exec Mode.”

65. C. Four numbers separated by periods indicates an IP address. The subnet mask separates the network from the node. For more information, see the section in Chapter 3 titled “IP Addresses.”

66. D. In RIP, a distance of 16 is considered unreachable. For more information, see the section in Chapter 7 titled “Routing Information

Protocol (RIP).”

67. C. A routing loop is when multiple routers think they can reach a network through each other when neither actually has a path. In some cases, a packet can be bounced back and forth between the routers until the TTL expires. For more information, see the section in Chapter 6 titled “Routing Information Protocol (RIP).”

68. C. A network 0.0.0.0 0.0.0.0 indicates a default gateway much like the default gateway set in your computer. The default gateway is where packets are sent when an explicit route cannot be found in the router’s routing tables. For more information, see the section in Chapter 6 titled “Static Routes.”

69. B. show start will show the startup configuration file. This is the configuration that will take effect when the router is rebooted. For more information, see the section in Chapter 2 titled

“Managing Configuration Files from the

Privileged Exec Mode.”

70. C. Making an interface passive within a routing protocol means routing updates will not be sent out over that interface. However, the routing protocol will still listen for updates on that interface.

For more information, see the section in Chapter 6 titled “Routing Information Protocol.”

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A

PPENDIXES

A Glossary

B Overview of the Cisco Certification Process

C What’s on the CD-ROM

D How to Use the Top Score Software Suite

P A R T

III

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A P P E N D I X

A

Glossary

P

REFACE

Computer networks are quickly becoming a vital tool in the daily operations of businesses around the world. For example, employees in an accounting department can use a common database to access and share customer account information using DECnet. Using an AppleTalk network, Macintosh users in a marketing department can share product bulletins, data sheets, and slide presentations. In an engineering department, Sun workstation users can share product specifications using TCP/IP over

Ethernet. And in a company’s manufacturing department, IBM devices attached to a Token Ring network can process real-time data about material availability and fill orders sent over links from remote offices.

This glossary assembles and defines the terms and acronyms used in the internetworking industry. Many of the definitions have yet to be standardized, and many terms have several meanings. Multiple definitions and acronym expressions are included where they apply.

The first part of this guide lists terms and acronyms that are specific to Cisco Systems and Cisco IOS. The second part of this guide contains terms and acronyms that are commonly used in the internetworking industry.

This guide also appears on the Cisco Systems, Inc. documentation CD-ROM.

Although many product names and descriptions are included in this glossary, you are encouraged to obtain more specific information from the appropriate vendor.

For information about Cisco products, refer to the

Cisco Product Catalog.

We hope that this glossary adds to your understanding of internetworking technologies and specific Cisco terms. Suggestions for new terms or acronyms and their associated definitions can be submitted by sending an email to [email protected]

.

C

ISCO

S

YSTEMS

T

ERMS AND

A

CRONYMS

A

AIP

ATM Interface Processor. ATM network interface for Cisco 7000 series routers designed to minimize performance bottlenecks at the UNI. The AIP supports

AAL3/4 and AAL5. See also AAL3/4 and AAL5, both in the “Internetworking Terms and Acronyms” section.

APaRT

Automated packet recognition/translation.

Technology that allows a server to be attached to

CDDI or FDDI without requiring the reconfiguration of applications or network protocols. APaRT recognizes specific Data Link layer encapsulation packet types and, when these packet types are transferred from one medium to another, translates them into the native format of the destination device.

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ATG

Address translation gateway. Cisco DECnet routing software function that allows a router to route multiple independent DECnet networks and to establish a user-specified address translation for selected nodes between networks.

ATM network

Traditional Cisco ATM network built around BPX switches.

ATM network interface card

ESP card that is used as the OC-3 interface to the BPX’s BXM.

autonomous switching

Feature on Cisco routers that provides faster packet processing by allowing the ciscoBus to switch packets independently without interrupting the system processor.

B

BIGA

Bus Interface Gate Array. Technology that allows the Catalyst 5000 to receive and transmit frames from its packet-switching memory to its MAC local buffer memory without the intervention of the host processor.

BOBI

Break-out/break-in. VNS feature that allows interworking between Euro-ISDN (ETSI) and other

VNS-supported signaling variants, such as DPNSS and

QSIG.

BPX Service Node

Closely integrated BPX switch,

AXIS interface shelf, and extended services processor designed to support ATM and Frame Relay switched virtual circuits, as well as traditional PVCs.

break-out/break-in

See BOBI.

Bus Interface Gate Array

See BIGA.

C

Call Detail Record

See CDR.

CDP

Cisco Discovery Protocol. Media- and protocol-independent device-discovery protocol that runs on all Cisco-manufactured equipment including routers, access servers, bridges, and switches. Using

CDP, a device can advertise its existence to other devices and receive information about other devices on the same LAN or on the remote side of a WAN. Runs on all media that support SNAP, including LANs,

Frame Relay, and ATM media.

CDR

Call Detail Record. VNS record of voice or data SVCs, which includes calling and called numbers, local and remote node names, date and timestamp, elapsed time, and Call Failure Class fields.

CFRAD

See Cisco FRAD.

Channel Interface Processor

See CIP.

CIP Channel Interface Processor. Channel attachment interface for Cisco 7000 series routers. The CIP is used to connect a host mainframe to a control unit, eliminating the need for an FEP for channel attachment.

ciscoBus controller

See SP.

Cisco Discovery Protocol

See CDP.

Cisco FRAD

Cisco Frame Relay access device. Cisco product that supports Cisco IOS Frame Relay SNA services and can be upgraded to be a full-function multiprotocol router. The Cisco FRAD connects SDLC devices to Frame Relay without requiring an existing

LAN. However, the Cisco FRAD does support attached

LANs and can perform conversion from SDLC to

Ethernet and Token Ring. See also FRAD, in the

“Internetworking Terms and Acronyms” section.

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Cisco Frame Relay access device

See Cisco FRAD.

CiscoFusion Cisco internetworking architecture that

“fuses” together the scalability, stability, and security advantages of the latest routing technologies with the performance benefits of ATM and LAN switching, and the management benefits of VLANs. See also Cisco IOS.

Cisco IOS

Cisco system software that provides common functionality, scalability, and security for all products under the CiscoFusion architecture. Cisco

IOS allows centralized, integrated, and automated installation and management of internetworks, while ensuring support for a wide variety of protocols, media, services, and platforms. See also CiscoFusion.

Cisco Link Services

See CLS.

Cisco Link Services Interface

See CLSI.

CiscoView

GUI-based device-management software application that provides dynamic status, statistics, and comprehensive configuration information for Cisco internetworking devices. In addition to displaying a physical view of Cisco device chassis, CiscoView also provides device monitoring functions and basic troubleshooting capabilities, and can be integrated with several leading SNMP-based network management platforms.

CLS

Cisco Link Services. A front-end for a variety of data-link control services.

CLSI

Cisco Link Services Interface. Messages that are exchanged between CLS and data-link users such as

APPN, SNA service point, and DLSw+.

configuration register

In Cisco routers, a 16-bit, user-configurable value that determines how the router functions during initialization. The configuration register can be stored in hardware or software. In hardware, the bit position is set using a jumper. In software, the bit position is set by specifying a hexadecimal value using configuration commands.

CPP

Combinet Proprietary Protocol.

CxBus

Cisco Extended Bus. Data bus for interface processors on Cisco 7000 series routers. See also SP.

D

Data Movement Processor

See DMP.

Diffusing Update Algorithm

See DUAL.

DistributedDirector

Method of distributing Web traffic by taking into account Web server availability and relative client-to-server topological distances in order to determine the optimal Web server for a client.

DistributedDirector uses the Director Response

Protocol to query DRP server agents for BGP and IGP routing table metrics.

DLSw+

Data-link switching plus. Cisco implementation of the DLSw standard for SNA and NetBIOS traffic forwarding. DLSw+ goes beyond the standard to include the advanced features of the current Cisco

RSRB implementation, and provides additional functionality to increase the overall scalability of data-link switching. See also DLSw, in the “Internetworking

Terms and Acronyms” section.

DMP

Data Movement Processor. Processor on the

Catalyst 5000 that, along with the multiport packet buffer memory interface, performs the frame-switching function for the switch. The DMP also handles translational bridging between the Ethernet and FDDI interfaces, IP segmentation, and intelligent bridging with protocol-based filtering.

DRP

Director Response Protocol. Protocol used by the DistributedDirector feature in IP routing.

DSPU concentration Cisco IOS feature that enables a router to function as a PU concentrator for SNA PU 2 nodes. PU concentration at the router simplifies the task of

PU definition at the upstream host while providing additional flexibility and mobility for downstream PU devices.

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DUAL

Diffusing Update Algorithm. Convergence algorithm in Enhanced IGRP that provides loop-free operation at every instant throughout a route computation. Allows routers involved in a topology change to synchronize at the same time, while not involving routers that are unaffected by the change. See also

Enhanced IGRP.

Ethernet Interface Processor

See EIP.

EXEC

Interactive command processor of Cisco IOS.

Extended Services Processor

See ESP.

E

EIGRP

See Enhanced IGRP.

EIP

Ethernet Interface Processor. Interface processor card on the Cisco 7000 series routers. The EIP provides high-speed (10Mbps) AUI ports that support Ethernet

Version 1 and Ethernet Version 2 or IEEE 802.3 interfaces, and a high-speed data path to other interface processors.

Enhanced IGRP Enhanced Interior Gateway Routing

Protocol. Advanced version of IGRP developed by Cisco.

Provides superior convergence properties and operating efficiency, and combines the advantages of link state protocols with those of distance vector protocols. Compare to IGRP. See also IGP, OSPF, and RIP, all in the

“Internetworking Terms and Acronyms” section.

Enhanced Interior Gateway Routing Protocol

See

Enhanced IGRP.

Enhanced Monitoring Services

Set of analysis tools on the Catalyst 5000 switch, consisting of an integrated

RMON agent and the SPAN. These tools provide traffic monitoring, and network segment analysis and management. See also RMON, in the “Internetworking

Terms and Acronyms” section, and SPAN.

ESP

Extended Services Processor. Rack-mounted adjunct processor that is co-located with a Cisco BPX/

AXIS (all three units comprise a BPX service node) and has IP connectivity to a StrataView Plus Workstation.

F

Fast Ethernet Interface Processor

See FEIP.

Fast Sequenced Transport

See FST.

Fast Serial Interface Processor

See FSIP.

fast switching

Cisco feature whereby a route cache is used to expedite packet switching through a router.

Contrast with process switching.

FDDI Interface Processor

See FIP.

FEIP

Fast Ethernet Interface Processor. Interface processor on the Cisco 7000 series routers. The FEIP supports up to two 100Mbps 100BaseT ports.

FIP

FDDI Interface Processor. Interface processor on the Cisco 7000 series routers. The FIP supports SASs,

DASs, dual homing, and optical bypass, and contains a

16-mips processor for high-speed (100Mbps) interface rates. The FIP complies with ANSI and ISO FDDI standards.

FRAS

Frame Relay access support. Cisco IOS feature that allows SDLC, Token Ring, Ethernet, and Frame

Relay-attached IBM devices to connect to other IBM devices across a Frame Relay network. See also FRAD, in the “Internetworking Terms and Acronyms” section.

FSIP

Fast Serial Interface Processor. Default serial interface processor for Cisco 7000 series routers. The

FSIP provides four or eight high-speed serial ports.

FST

Fast Sequenced Transport. Connectionless, sequenced transport protocol that runs on top of the

IP protocol. SRB traffic is encapsulated inside of IP datagrams and is passed over an FST connection between two network devices (such as routers).

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Speeds up data delivery, reduces overhead, and improves the response time of SRB traffic.

G

Gateway Discovery Protocol

See GDP.

GDP

Gateway Discovery Protocol. Cisco protocol that allows hosts to dynamically detect the arrival of new routers as well as determine when a router goes down. Based on UDP. See also UDP, in the

“Internetworking Terms and Acronyms” section.

generic routing encapsulation

See GRE.

GRE

Generic routing encapsulation. Tunneling protocol developed by Cisco that can encapsulate a wide variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link to Cisco routers at remote points over an IP internetwork. By connecting multiprotocol subnetworks in a single-protocol backbone environment, IP tunneling using GRE allows network expansion across a single-protocol backbone environment.

H

helper address

Address configured on an interface to which broadcasts received on that interface will be sent.

High-Speed Communications Interface

See HSCI.

HIP

HSSI Interface Processor. Interface processor on the Cisco 7000 series routers. The HIP provides one

HSSI port that supports connections to ATM, SMDS,

Frame Relay, or private lines at speeds up to T3 or E3.

Hot Standby Router Protocol

See HSRP.

HSCI

High-Speed Communications Interface.

Single-port interface, developed by Cisco, providing full-duplex synchronous serial communications capability at speeds up to 52Mbps.

I

HSRP

Hot Standby Router Protocol. Provides high network availability and transparent network topology changes. HSRP creates a Hot Standby router group with a lead router that services all packets sent to the

Hot Standby address. The lead router is monitored by other routers in the group, and if it fails, one of these standby routers inherits the lead position and the Hot

Standby group address.

HSSI Interface Processor

See HIP.

IGRP

Interior Gateway Routing Protocol. IGP developed by Cisco to address the issues associated with routing in large, heterogeneous networks. Compare to

Enhanced IGRP. See also IGP, OSPF, and RIP, all in

the “Internetworking Terms and Acronyms” section.

interface processor

Any of a number of processor modules used in the Cisco 7000 series routers. See also

AIP, CIP, EIP, FEIP, FIP, FSIP, HIP, MIP, SIP, and

TRIP.

Interior Gateway Routing Protocol

See IGRP.

Inter-Switch Link

See ISL.

IOS

See Cisco IOS.

ISL

Inter-Switch Link. Cisco-proprietary protocol that maintains VLAN information as traffic flows between switches and routers.

L

local adjacency

Two VNSs that control different

VSN areas, but communicate with one another through a Frame Relay PVC, are considered to be locally adjacent.

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M

MICA

Multiservice IOS Channel Aggregation.

Technology that enables the simultaneous support of remote-access users through both analog modems and

ISDN devices.

MIP

MultiChannel Interface Processor. Interface processor on the Cisco 7000 series routers that provides up to two channelized T1 or E1 connections via serial cables to a CSU. The two controllers on the MIP can each provide up to 24 T1 or 30 E1 channel-groups, with each channel-group presented to the system as a serial interface that can be configured individually.

MultiChannel Interface Processor

See MIP.

N

native client interface architecture

See NCIA.

NCIA

Native client interface architecture. SNA applications-access architecture developed by Cisco that combines the full functionality of native SNA interfaces at both the host and client with the flexibility of leveraging TCP/IP backbones. NCIA encapsulates SNA traffic on a client PC or workstation, thereby providing direct TCP/IP access while preserving the native SNA interface at the end-user level. In many networks, this capability obviates the need for a standalone gateway and can provide flexible TCP/IP access while preserving the native SNA interface to the host.

NetFlow Network flow. A unidirectional sequence of packets between given source and destination endpoints.

Network flows are highly granular: Flow endpoints are identified by IP address as well as by Transport layer application port numbers. (NetFlow also uses IP Protocol, ToS, and the input interface port to uniquely identify flows.)

Conventional Network layer switching handles incoming packets independently, with separate serial tasks for switching, security, services, and traffic measurements applied to each packet. With NetFlow switching, this process is applied only to the first packet of a flow.

Information from the first packet is used to build an entry in the NetFlow cache. Subsequent packets in the flow are handled via a single streamlined task that handles switching, services, and data collection concurrently.

NETscout

Cisco network management application that provides an easy-to-use GUI for monitoring

RMON statistics and protocol analysis information.

NETscout also provides extensive tools that simplify data collection, analysis, and reporting. These tools allow system administrators to monitor traffic, set thresholds, and capture data on any set of network traffic for any segment.

NMP

Network Management Processor. Processor module on the Catalyst 5000 switch used to control and monitor the switch.

NSP

Network Service Point.

P

Physical layer interface module

See PLIM.

PLIM

Physical layer interface module. Interface that allows the AIP to a variety of physical layers, including

TAXI and SONET multimode fiber-optic cable, SDH/

SONET single-mode fiber cable, and E3 coaxial cable.

process switching Operation that provides full route evaluation and per-packet load balancing across parallel

WAN links. Involves the transmission of entire frames to the router CPU, where they are repackaged for delivery to or from a WAN interface, with the router making a route selection for each packet. Process switching is the most resource-intensive switching operation that the

CPU can perform. Contrast with fast switching.

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proxy polling

Technique that alleviates the load across an SDLC network by allowing routers to act as proxies for primary and secondary nodes, thus keeping polling traffic off the shared links. Proxy polling has been replaced by SDLC Transport. See SDLC Transport.

R

Reliable SAP Update Protocol

See RSUP.

Route Processor

See RP.

Route/Switch Processor

See RSP.

RP

Route Processor. Processor module in the Cisco

7000 series routers that contains the CPU, system software, and most of the memory components that are used in the router. Sometimes called a supervisory processor.

RSP

Route/Switch Processor. Processor module in the

Cisco 7500 series routers that integrates the functions of the RP and the SP. See also RP and SP.

RSUP

Reliable SAP Update Protocol. Bandwidthsaving protocol developed by Cisco for propagating services information. RSUP allows routers to reliably send standard Novell SAP packets only when the routers detect a change in advertised services. RSUP can transport network information either in conjunction with or independently of the Enhanced IGRP routing function for IPX.

S

SDLC broadcast

Feature that allows a Cisco router that receives an all-stations broadcast on a virtual multidrop line to propagate the broadcast to each SDLC line that is a member of the virtual multidrop line.

SDLC Transport

Cisco router feature with which disparate environments can be integrated into a single, high-speed, enterprise-wide network. Native SDLC traffic can be passed through point-to-point serial links with other protocol traffic multiplexed over the same links. Cisco routers can also encapsulate SDLC frames inside IP datagrams for transport over arbitrary (non-

SDLC) networks. Replaces proxy polling. See also

proxy polling.

SDLLC

SDLC Logical Link Control. Cisco IOS feature that performs translation between SDLC and

IEEE 802.2 type 2.

serial tunnel

See STUN.

silicon switching Switching based on the SSE, which allows the processing of packets independent of the SSP system processor. Silicon switching provides high-speed, dedicated packet switching. See also SSE and SSP.

silicon switching engine

See SSE.

Silicon Switch Processor

See SSP.

SIP

SMDS Interface Protocol. Used in communications between CPE and SMDS network equipment.

Allows the CPE to use SMDS service for high-speed

WAN internetworking. Based on the IEEE 802.6

DQDB standard. See also DQDB, in the

“Internetworking Terms and Acronyms” section.

SP

Switch Processor. Cisco 7000-series processor module that acts as the administrator for all CxBus activities. Sometimes called ciscoBus controller. See also

CxBus.

SPAN

Switched Port Analyzer. Feature of the Catalyst

5000 switch that extends the monitoring abilities of existing network analyzers into a switched Ethernet environment. SPAN mirrors the traffic at one switched segment onto a predefined SPAN port. A network analyzer attached to the SPAN port can monitor traffic from any of the other Catalyst switched ports.

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SPNNI connection

Frame Relay connection between two VNSs in different areas or domains. The SPNNI connection gets its name from the proprietary

Network-to-Network Interface protocol that operates over this connection.

SSE Silicon switching engine. Routing and switching mechanism that compares the Data Link or Network layer header of an incoming packet to a silicon-switching cache, determines the appropriate action (routing or bridging), and forwards the packet to the proper interface. The SSE is directly encoded in the hardware of the SSP of a Cisco

7000 series router. It can therefore perform switching independently of the system processor, making the execution of routing decisions much quicker than if they were encoded in software. See also silicon switching and SSP.

SSP Silicon Switch Processor. High-performance silicon switch for Cisco 7000 series routers that provides distributed processing and control for interface processors. The

SSP leverages the high-speed switching and routing capabilities of the SSE to dramatically increase aggregate router performance, minimizing performance bottlenecks at the interface points between the router and a highspeed backbone. See also silicon switching and SSE.

STUN Serial tunnel. Router feature allowing two SDLCor HDLC-compliant devices to connect to one another through an arbitrary multiprotocol topology (using Cisco routers) rather than through a direct serial link.

supervisory processor

See RP.

Switch Processor

See SP.

T

TACACS+

Terminal Access Controller Access

Control System Plus. Proprietary Cisco enhancement to Terminal Access Controller Access Control System

(TACACS). Provides additional support for authentication, authorization, and accounting. See also TACACS, in the “Internetworking Terms and Acronyms” section.

THC over X.25

Feature providing TCP/IP header compression over X.25 links, for purposes of link efficiency.

TRIP

Token Ring Interface Processor. High-speed interface processor on the Cisco 7000 series routers.

The TRIP provides two or four Token Ring ports for interconnection with IEEE 802.5 and IBM Token Ring media with ports independently set to speeds of either

4 or 16Mbps.

two-way simultaneous

See TWS.

TWS Two-way simultaneous. Mode that allows a router configured as a primary SDLC station to achieve better utilization of a full-duplex serial line. When TWS is enabled in a multidrop environment, the router can poll a secondary station and receive data from that station while it sends data to or receives data from a different secondary station on the same serial line.

V

Versatile Interface Processor

See VIP.

VIP

1. Versatile Interface Processor. Interface card

used in Cisco 7000 and Cisco 7500 series routers. The

VIP provides multilayer switching and runs Cisco IOS.

The most recent version of the VIP is VIP2. 2. Virtual

IP. Function that enables the creation of logically separated switched IP workgroups across the switch ports of a Catalyst 5000 running Virtual Networking Services software. See also Virtual Networking Services.

virtual IP

See VIP.

Virtual Networking Services

Software on some

Catalyst 5000 switches that enables multiple workgroups to be defined across switches and offers traffic segmentation and access control.

VNS

See Virtual Networking Services.

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I

W

WorkGroup Director

Cisco SNMP-based networkmanagement software tool. WorkGroup Director runs on UNIX workstations either as a standalone application or integrated with another SNMP-based network management platform, providing a seamless, powerful management system for Cisco workgroup products. See also SNMP.

T

N

NTERNETWORKING

ERMS AND

UMERICS

A

I

NDUSTRY

CRONYMS

10Base2

10Mbps baseband Ethernet specification using 50-ohm thin coaxial cable. 10Base2, which is part of the IEEE 802.3 specification, has a distance limit of 606.8 feet (185 meters) per segment. See also

Cheapernet, Ethernet, IEEE 802.3, and Thinnet.

10Base5

10Mbps baseband Ethernet specification using standard (thick) 50-ohm baseband coaxial cable.

10Base5, which is part of the IEEE 802.3 baseband

Physical layer specification, has a distance limit of 1640 feet (500 meters) per segment. See also Ethernet and

IEEE 802.3.

10BaseF

10Mbps baseband Ethernet specification that refers to the 10BaseFB, 10BaseFL, and 10BaseFP standards for Ethernet over fiber-optic cabling. See also

10BaseFB, 10BaseFL, 10BaseFP, and Ethernet.

10BaseFB 10Mbps baseband Ethernet specification using fiber-optic cabling. 10BaseFB is part of the IEEE 10BaseF specification. It is not used to connect user stations, but rather provides a synchronous signaling backbone that allows additional segments and repeaters to be connected to the network. 10BaseFB segments can be up to 1.24 miles (2000 meters) long. See also 10BaseF and Ethernet.

10BaseFL 10Mbps baseband Ethernet specification using fiber-optic cabling. 10BaseFL is part of the IEEE

10BaseF specification and, although able to interoperate with FOIRL, is designed to replace the FOIRL specification. 10BaseFL segments can be up to 3280 feet

(1000 meters) long if used with FOIRL, and up to 1.24

miles (2000 meters) if 10BaseFL is used exclusively. See also 10BaseF, Ethernet, and FOIRL.

10BaseFP

10Mbps fiber-passive baseband Ethernet specification using fiber-optic cabling. 10BaseFP is part of the IEEE 10BaseF specification. It organizes a number of computers into a star topology without the use of repeaters. 10BaseFP segments can be up to 1640 feet (500 meters) long. See also 10BaseF and Ethernet.

10BaseT

10Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Category 3, 4, or 5): one pair for transmitting data and the other for receiving data. 10BaseT, which is part of the IEEE

802.3 specification, has a distance limit of approximately 328 feet (100 meters) per segment. See also

Ethernet and IEEE 802.3.

10Broad36

10Mbps broadband Ethernet specification using broadband coaxial cable. 10Broad36, which is part of the IEEE 802.3 specification, has a distance limit of 2.24 miles (3600 meters) per segment. See also

Ethernet and IEEE 802.3.

100BaseFX

100Mbps baseband Fast Ethernet specification using two strands of multimode fiber-optic cable per link. To guarantee proper signal timing, a

100BaseFX link cannot exceed 1312 feet (400 meters) in length. Based on the IEEE 802.3 standard. See also

100BaseX, Fast Ethernet, and IEEE 802.3.

100BaseT 100Mbps baseband Fast Ethernet specification using UTP wiring. Like the 10BaseT technology on which it is based, 100BaseT sends link pulses over the network segment when no traffic is present.

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However, these link pulses contain more information than those used in 10BaseT. Based on the IEEE 802.3

standard. See also 10BaseT, Fast Ethernet, and IEEE

802.3.

100BaseT4

100Mbps baseband Fast Ethernet specification using four pairs of Category 3, 4, or 5

UTP wiring. To guarantee proper signal timing, a

100BaseT4 segment cannot exceed 328 feet (100 meters) in length. Based on the IEEE 802.3 standard.

See also Fast Ethernet and IEEE 802.3.

100BaseTX 100Mbps baseband Fast Ethernet specification using two pairs of either UTP or STP wiring.

The first pair of wires is used to receive data; the second is used to transmit. To guarantee proper signal timing, a

100BaseTX segment cannot exceed 328 feet (100 meters) in length. Based on the IEEE 802.3 standard.

See also 100BaseX, Fast Ethernet, and IEEE 802.3.

100BaseX

100Mbps baseband Fast Ethernet specification that refers to the 100BaseFX and 100BaseTX standards for Fast Ethernet over fiber-optic cabling.

Based on the IEEE 802.3 standard. See also

100BaseFX, 100BaseTX, Fast Ethernet, and IEEE 802.3.

100VG-AnyLAN

100Mbps Fast Ethernet and Token

Ring media technology using four pairs of Category 3,

4, or 5 UTP cabling. This high-speed transport technology, developed by Hewlett-Packard, can operate on existing 10BaseT Ethernet networks. Based on the

IEEE 802.12 standard. See also IEEE 802.12.

1822

Historic term that refers to the original

ARPANET host-to-IMP interface. The specifications are in BBN report 1822. See also host and IMP.

24th channel signaling

See A&B bit signaling.

2B1Q

2 binary 1 quaternary. Encoding scheme that provides 2 bits per baud, 80 kbaud per second,

160Kbps transfer rate. The most common signaling method on ISDN U interfaces. This protocol is defined in detail in 1988 ANSI spec T1.601.

370 block mux channel See block multiplexer channel.

4B/5B local fiber

4-byte/5-byte local fiber. Fiber channel physical media used for FDDI and ATM.

Supports speeds of up to 100Mbps over multimode fiber. See also TAXI 4B/5B.

4-byte/5-byte local fiber

See 4B/5B local fiber.

6BONE

The Internet’s experimental IPv6 network.

8-byte/10-byte local fiber

See 8B/10B local fiber.

802.x

Set of IEEE standards for the definition of

LAN protocols.

822

Short form of RFC 822. Refers to the format of

Internet style email as defined in RFC 822.

8B/10B local fiber

8-byte/10-byte local fiber. Fiber channel physical media that supports speeds up to

149.76Mbps over multimode fiber.

A

A&B bit signaling

Procedure used in T1 transmission facilities in which each of the 24 T1 subchannels devotes one bit of every sixth frame to the carrying of supervisory signaling information. Also called 24th channel signaling.

AAA

Authentication, authorization, and accounting

(pronounced “triple a”).

AAL

ATM adaptation layer. Service-dependent sublayer of the Data Link layer. The AAL accepts data from different applications and presents it to the ATM layer in the form of 48-byte ATM payload segments.

AALs consist of two sublayers: CS and SAR. AALs differ on the basis of the source-destination timing used, whether they use CBR or VBR, and whether they are used for connection-oriented or connectionless mode data transfer. At present, the four types of AAL recommended by the ITU-T are AAL1, AAL2,

AAL3/4, and AAL5. See also AAL1, AAL2, AAL3/4,

AAL5, ATM, ATM layer, CS, and SAR.

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AAL1 ATM adaptation layer 1. One of four AALs recommended by the ITU-T. AAL1 is used for connectionoriented, delay-sensitive services requiring constant bit rates, such as uncompressed video and other isochronous traffic. See also AAL.

AAL2 ATM adaptation layer 2. One of four AALs recommended by the ITU-T. AAL2 is used for connectionoriented services that support a variable bit rate, such as some isochronous video and voice traffic. See also AAL.

AAL3/4

ATM adaptation layer 3/4. One of four

AALs (merged from two initially distinct adaptation layers) recommended by the ITU-T. AAL3/4 supports both connectionless and connection-oriented links, but is primarily used for the transmission of SMDS packets over ATM networks. See also AAL.

AAL5 ATM adaptation layer 5. One of four AALs recommended by the ITU-T. AAL5 supports connectionoriented VBR services and is used predominantly for the transfer of classical IP over ATM and LANE traffic.

AAL5 uses SEAL and is the least complex of the current

AAL recommendations. It offers low bandwidth overhead and simpler processing requirements in exchange for reduced bandwidth capacity and error-recovery capability. See also AAL and SEAL.

AARP

AppleTalk Address Resolution Protocol.

Protocol in the AppleTalk protocol stack that maps a data-link address to a network address.

AARP probe packets

Packets transmitted by AARP that determine whether a randomly selected node

ID is being used by another node in a nonextended

AppleTalk network. If the node ID is not being used, the sending node uses that node ID. If the node ID is being used, the sending node chooses a different ID and sends more AARP probe packets. See also AARP.

ABM

Asynchronous Balanced Mode. HDLC (and derivative protocol) communication mode supporting peer-oriented, point-to-point communications between two stations, in which either station can initiate transmission.

ABR 1. Available bit rate. QoS class defined by the

ATM Forum for ATM networks. ABR is used for connections that do not require timing relationships between source and destination. ABR provides no guarantees in terms of cell loss or delay, providing only besteffort service. Traffic sources adjust their transmission rate in response to information they receive describing the status of the network and its capability to successfully deliver data. Compare to CBR, UBR, and VBR. 2.

Area border router. Router located on the border of one or more OSPF areas that connects those areas to the backbone network. ABRs are considered members of both the OSPF backbone and the attached areas. They therefore maintain routing tables describing both the backbone topology and the topology of the other areas.

Abstract Syntax Notation One

See ASN.1.

access list

List kept by routers to control access to or from the router for a number of services (for example, to prevent packets with a certain IP address from leaving a particular interface on the router).

access method

1. Generally, the way in which net-

work devices access the network medium. 2. Software within an SNA processor that controls the flow of information through a network.

access server

Communications processor that connects asynchronous devices to a LAN or WAN through network and terminal emulation software. Performs both synchronous and asynchronous routing of supported protocols. Sometimes called a network access server. See also communication server.

access unit

See AU.

accounting management

One of five categories of network management defined by ISO for management of OSI networks. Accounting management subsystems are responsible for collecting network data relating to resource usage. See also configuration management, fault

management, performance management, and security

management.

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ACD Automatic call distribution. Device or service that automatically reroutes calls to customers in geographically distributed locations served by the same CO.

See also CO.

ACELP

Algebraic code excited linear prediction.

ACF

Advanced Communications Function. A group of SNA products that provides distributed processing and resource sharing. See also ACF/NCP.

ACF/NCP Advanced Communications Function/

Network Control Program. The primary SNA NCP.

ACF/NCP resides in the communications controller and interfaces with the SNA access method in the host processor to control network communications. See also

ACF and NCP.

ACK

See acknowledgment.

acknowledgment

Notification sent from one network device to another to acknowledge that some event

(for example, receipt of a message) occurred.

Sometimes abbreviated ACK. Compare to NAK.

ACR

Allowed cell rate. Parameter defined by the

ATM Forum for ATM traffic management. ACR varies between the MCR and the PCR, and is dynamically controlled using congestion control mechanisms. See also MCR and PCR.

ACSE

Association control service element. OSI convention used to establish, maintain, or terminate a connection between two applications.

active hub

Multiported device that amplifies LAN transmission signals.

active monitor

Device responsible for managing a

Token Ring. A network node is selected to be the active monitor if it has the highest MAC address on the ring.

The active monitor is responsible for such management tasks as ensuring that tokens are not lost, or that frames do not circulate indefinitely. See also ring monitor and

standby monitor.

ActiveX

Microsoft’s Windows-specific, non-Java technique for writing applets. ActiveX applets take considerably longer to download than the equivalent Java applets; however, they more fully exploit the features of

Windows 95. ActiveX is sometimes said to be a superset of Java. See also applet and Java.

AD Administrative domain. Group of hosts, routers, and networks operated and managed by a single organization.

adapter

See NIC.

adaptive differential pulse code modulation

See

ADPCM.

adaptive routing

See dynamic routing.

ADCCP

Advanced Data Communications Control

Protocol. ANSI standard bit-oriented data link control protocol.

address

Data structure or logical convention used to identify a unique entity, such as a particular process or network device.

addressed call mode

Mode that permits control signals and commands to establish and terminate calls in

V.25bis. See also V.25bis.

address mapping

Technique that allows different protocols to interoperate by translating addresses from one format to another. For example, when routing IP over X.25, the IP addresses must be mapped to the

X.25 addresses so that the IP packets can be transmitted by the X.25 network. See also address resolution.

address mask

Bit combination used to describe which portion of an address refers to the network or subnet and which part refers to the host. Sometimes referred to simply as mask. See also subnet mask.

address resolution

Generally, a method for resolving differences between computer addressing schemes.

Address resolution usually specifies a method for mapping Network layer (Layer 3) addresses to Data Link layer (Layer 2) addresses. See also address mapping.

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Address Resolution Protocol

See ARP.

address translation gateway

See ATG in the “Cisco

Systems Terms and Acronyms” section.

adjacency

Relationship formed between selected neighboring routers and end nodes for the purpose of exchanging routing information. Adjacency is based upon the use of a common media segment.

adjacent nodes

1. In SNA, nodes that are connected

to a given node with no intervening nodes. 2. In

DECnet and OSI, nodes that share a common network segment (in Ethernet, FDDI, or Token Ring networks).

ADM

Add Drop Multiplexer. In OSS, a multiplexer that allows a signal to be added into or dropped out of a SONET span. See also SONET.

ADMD

Administration Management Domain. An

X.400 Message Handling System public carrier. The

ADMDs in all countries worldwide together provide the X.400 backbone. See also PRMD.

administrative distance

Rating of the trustworthiness of a routing information source. Administrative distance is often expressed as a numerical value between

0 and 255. The higher the value, the lower the trustworthiness rating.

Administrative Domain

See AD.

administrative weight

See AW and PTSP.

admission control

See traffic policing.

ADPCM

Adaptive differential pulse code modulation. Process by which analog voice samples are encoded into high-quality digital signals.

ADSL Asymmetric digital subscriber line. One of four

DSL technologies. ADSL is designed to deliver more bandwidth downstream (from the central office to the customer site) than upstream. Downstream rates range from 1.5 to 9Mbps, whereas upstream bandwidth ranges from 16 to 640Kbps. ADSL transmissions work at distances up to 18,000 feet (5,488 meters) over a single copper twisted pair. See also HDSL, SDSL, and VDSL.

ADSU

ATM data service unit. Terminal adapter used to access an ATM network via an HSSI-compatible device. See also DSU.

Advanced Communications Function

See ACF.

Advanced Communications Function/Network

Control Program

See ACF/NCP.

Advanced CoS Management

Advanced Class-of-

Service Management. Essential for delivering the required QoS to all applications. Cisco switches contain per-VC queuing, per-VC rate scheduling, multiple CoS queuing, and egress queuing. This enables network managers to refine connections to meet specific application needs. Formerly called FairShare and OptiClass.

Advanced Data Communications Control Protocol

See ADCCP.

Advanced Intelligent Network

See AIN.

Advanced Peer-to-Peer Networking

See APPN.

Advanced Program-to-Program Communication

See APPC.

Advanced Research Projects Agency

See ARPA.

Advanced Research Projects Agency Network

See

ARPANET.

advertising

Router process in which routing or service updates are sent at specified intervals so that other routers on the network can maintain lists of usable routes.

AEP

AppleTalk Echo Protocol. Used to test connectivity between two AppleTalk nodes. One node sends a packet to another node and receives a duplicate, or echo, of that packet.

AFI

Authority and format identifier. Portion of an

NSAP-format ATM address that identifies the type and format of the IDI portion of an ATM address. See also

IDI and NSAP.

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AFP

AppleTalk Filing Protocol. Presentation-layer protocol that allows users to share data files and application programs that reside on a file server. AFP supports AppleShare and Mac OS File Sharing.

agent 1. Generally, software that processes queries and returns replies on behalf of an application. 2. In NMSs, process that resides in all managed devices and reports the values of specified variables to management stations.

AIN

Advanced Intelligent Network. In SS7, an expanded set of network services made available to the user, and under user control, that requires improvement in network switch architecture, signaling capabilities, and peripherals. See also SS7.

AIO

Asynchronous I/O.

AIP

See AIP, in the “Cisco Systems Terms and

Acronyms” section.

AIS

Alarm indication signal. In a T1 transmission, an all-ones signal transmitted in lieu of the normal signal to maintain transmission continuity and to indicate to the receiving terminal that there is a transmission fault that is located either at, or upstream from, the transmitting terminal. See also T1.

alarm SNMP message notifying an operator or administrator of a network problem. See also event and trap.

alarm indication signal

See AIS.

a-law

ITU-T companding standard used in the conversion between analog and digital signals in PCM systems. A-law is used primarily in European telephone networks and is similar to the North American mu-law standard. See also companding and mu-law.

algorithm

Well-defined rule or process for arriving at a solution to a problem. In networking, algorithms are commonly used to determine the best route for traffic from a particular source to a particular destination.

alias

See entity.

alignment error

In IEEE 802.3 networks, an error that occurs when the total number of bits of a received frame is not divisible by eight. Alignment errors are usually caused by frame damage due to collisions.

allowed cell rate

See ACR.

all-rings explorer packet See all-routes explorer packet.

all-routes explorer packet

Explorer packet that traverses an entire SRB network, following all possible paths to a specific destination. Sometimes called allrings explorer packet. See also explorer packet, local

explorer packet, and spanning explorer packet.

ALO transaction At-least-once transaction. ATP transaction in which the request is repeated until a response is received by the requester or until a maximum retry count is reached. This recovery mechanism ensures that the transaction request is executed at least once. See also ATP.

alternate mark inversion

See AMI.

AM

Amplitude modulation. Modulation technique whereby information is conveyed through the amplitude of the carrier signal. Compare to FM and PAM.

See also modulation.

AMA

Automatic Messaging Accounting. In OSS, the automatic collection, recording, and processing of information relating to calls for billing purposes.

AMADNS

AMA Data Networking System. In OSS, the next generation Bellcore system for the collection and transport of AMA data from central office switches to a billing system. See also AMA.

AMATPS

AMA Teleprocessing System. In OSS, the

Bellcore legacy system for collecting and transporting

AMA data from central office switches to a billing system. The AMATPS consists of an AMA transmitter and a collector. See also AMA.

American National Standards Institute

See ANSI.

American Standard Code for Information

Interchange

See ASCII.

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AMI

Alternate mark inversion. Line-code type used on T1 and E1 circuits. In AMI, zeros are represented by 01 during each bit cell, and ones are represented by

11 or 00, alternately, during each bit cell. AMI requires that the sending device maintain ones density. Ones density is not maintained independent of the data stream. Sometimes called binary coded alternate mark inversion. Compare to B8ZS. See also ones density.

amplitude

Maximum value of an analog or a digital waveform.

amplitude modulation

See AM.

analog transmission

Signal transmission over wires or through the air in which information is conveyed through variation of some combination of signal amplitude, frequency, and phase.

anonymous FTP

Allows a user to retrieve documents, files, programs, and other archived data from anywhere on the Internet without having to establish a userid and password. By using the special userid of anonymous, the network user will bypass local security checks and will have access to publicly accessible files on the remote system. See also FTP.

ANSI American National Standards Institute.

Voluntary organization composed of corporate, government, and other members that coordinates standardsrelated activities, approves U.S. national standards, and develops positions for the United States in international standards organizations. ANSI helps develop international and U.S. standards relating to, among other things, communications and networking. ANSI is a member of the IEC and the ISO. See also IEC and ISO.

ANSI X3T9.5

See X3T9.5.

anycast In ATM, an address that can be shared by multiple end systems. An anycast address can be used to route a request to a node that provides a particular service.

AOW

Asia and Oceania Workshop. One of the three regional OSI Implementors Workshops. See EWOS.

APaRT

See ApaRT in the “Cisco Systems Terms and

Acronyms” section.

API

Application programming interface. Specification of function-call conventions that defines an interface to a service.

APNIC Asia Pacific Network Information Center.

Nonprofit Internet registry organization for the Asia

Pacific region. The other Internet registries are currently

IANA, RIPE NCC, and InterNIC.

Apollo Domain

Proprietary network protocol suite developed by Apollo Computer for communication on proprietary Apollo networks.

APPC Advanced Program-to-Program

Communication. IBM SNA system software that allows high-speed communication between programs on different computers in a distributed computing environment. APPC establishes and tears down connections between communicating programs, and consists of two interfaces: programming and data-exchange. The programming interface replies to requests from programs requiring communication; the data-exchange interface establishes sessions between programs. APPC runs on

LU 6.2 devices. See also LU 6.2.

applet

Small program, often used in the context of a

Java-based program, that is compiled and embedded in an HTML page. See also ActiveX and Java.

AppleTalk

Series of communications protocols designed by Apple Computer consisting of two phases.

Phase 1, the earlier version, supports a single physical network that can have only one network number and be in one zone. Phase 2, supports multiple logical networks on a single physical network and allows networks to be in more than one zone. See also zone.

AppleTalk Address Resolution Protocol

See AARP.

AppleTalk Filing Protocol

See AFP.

AppleTalk Echo Protocol

See AEP.

AppleTalk Remote Access

See ARA.

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AppleTalk Session Protocol

See ASP.

AppleTalk Transaction Protocol

See ATP.

AppleTalk Update-Based Routing Protocol

See

AURP.

AppleTalk zone

See zone.

application

Program that performs a function directly for a user. FTP and Telnet clients are examples of network applications.

Application layer Layer 7 of the OSI reference model.

This layer provides services to application processes (such as email, file transfer, and terminal emulation) that are outside of the OSI model. The Application layer identifies and establishes the availability of intended communication partners (and the resources required to connect with them), synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. Corresponds roughly with the Transaction Services layer in the SNA model. See also Data Link layer, Network layer, Physical layer,

Presentation layer, Session layer, and Transport layer.

application programming interface

See API.

APPN

Advanced Peer-to-Peer Networking.

Enhancement to the original IBM SNA architecture.

APPN handles session establishment between peer nodes, dynamic transparent route calculation, and traffic prioritization for APPC traffic. Compare to

APPN+. See also APPC.

APPN+

Next-generation APPN that replaces the label-swapping routing algorithm with source routing.

Also called high-performance routing. See also APPN.

APS

Automatic protection switching. SONET switching mechanism that routes traffic from working lines to protect them in case of a line card failure or fiber cut.

ARA

AppleTalk Remote Access. Protocol that provides Macintosh users direct access to information and resources at a remote AppleTalk site.

Archie

System that provides lists of anonymous FTP archives. See also Gopher, WAIS, and WWW.

ARCnet

Attached Resource Computer Network.

2.5Mbps token-bus LAN developed in the late 1970s and early 1980s by Datapoint Corporation.

area

Logical set of network segments (CLNS-,

DECnet-, or OSPF-based) and their attached devices.

Areas are usually connected to other areas via routers, making up a single autonomous system. See also

autonomous system.

area border router

See ABR.

ARIN

American Registry for Internet Numbers.

Nonprofit organization established for the purpose of administrating and registrating IP numbers to the geographical areas currently managed by Network

Solutions (InterNIC). Those areas include, but are not limited to, North America, South America, South

Africa, and the Caribbean.

ARM Asynchronous response mode. HDLC communication mode involving one primary station and at least one secondary station, in which either the primary or one of the secondary stations can initiate transmissions.

See also primary station and secondary station.

ARP

Address Resolution Protocol. Internet protocol used to map an IP address to a MAC address. Defined in RFC 826. Compare to RARP. See also proxy ARP.

ARPA

Advanced Research Projects Agency. Research and development organization that is part of DoD.

ARPA is responsible for numerous technological advances in communications and networking. ARPA evolved into DARPA, and then back into ARPA again

(in 1994). See also DARPA.

ARPANET

Advanced Research Projects Agency

Network. Landmark packet-switching network established in 1969. ARPANET was developed in the 1970s by BBN and funded by ARPA (and later DARPA). It eventually evolved into the Internet. The term

ARPANET was officially retired in 1990. See also

ARPA, BBN, DARPA, and Internet.

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ARQ

Automatic repeat request. Communication technique in which the receiving device detects errors and requests retransmissions.

AS

See autonomous system.

ASBR

Autonomous system boundary router. ABR located between an OSPF autonomous system and a non-OSPF network. ASBRs run both OSPF and another routing protocol, such as RIP. ASBRs must reside in a nonstub OSPF area. See also ABR, non-stub

area, and OSPF.

ASCII

American Standard Code for Information

Interchange. 8-bit code for character representation

(7 bits plus parity).

ASI

ATM service interface.

ASN.1

Abstract Syntax Notation One. OSI language for describing data types independent of particular computer structures and representation techniques.

Described by ISO International Standard 8824. See also BER (basic encoding rules).

ASP AppleTalk Session Protocol. Protocol that uses

ATP to provide session establishment, maintenance, and teardown, as well as request sequencing. See also ATP.

assigned numbers RFC [STD2] documents the currently assigned values from several series of numbers used in network protocol implementations. This RFC is updated periodically, and current information can be obtained from the IANA. If you are developing a protocol or application that will require the use of a link, socket, port, protocol, and so forth, contact the IANA to receive a number assignment. See also IANA and STD.

association control service element

See ACSE.

associative memory

Memory that is accessed based on its contents rather than on its memory address.

Sometimes called content addressable memory (CAM).

AST

Automatic spanning tree. Function that supports the automatic resolution of spanning trees in SRB networks, providing a single path for spanning explorer frames to traverse from a given node in the network to another. AST is based on the IEEE 802.1 standard. See also IEEE 802.1 and SRB.

ASTA

Advanced Software Technology and

Algorithms. Component of the HPCC program intended to develop software and algorithms for implementation on high-performance computer and communications systems. See also HPCC.

async

Subset of tty.

Asynchronous Balanced Mode

See ABM.

asynchronous response mode

See ARM.

asynchronous time-division multiplexing

See

ATDM.

Asynchronous Transfer Mode

See ATM.

asynchronous transmission Term describing digital signals that are transmitted without precise clocking.

Such signals generally have different frequencies and phase relationships. Asynchronous transmissions usually encapsulate individual characters in control bits (called start and stop bits) that designate the beginning and end of each character. Compare to isochronous transmission,

plesiochronous transmission, and synchronous transmission.

ATCP

AppleTalk Control Protocol. Protocol that establishes and configures AppleTalk over PPP, as defined in RFC 1378. See also PPP.

ATDM

Asynchronous time-division multiplexing.

Method of sending information that resembles normal

TDM, except that time slots are allocated as needed rather than preassigned to specific transmitters.

Compare to FDM, statistical multiplexing, and TDM.

ATG

See ATG in the “Cisco Systems Terms and

Acronyms” section.

at-least-once transaction

See ALO transaction.

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ATM

Asynchronous Transfer Mode. International standard for cell relay in which multiple service types

(such as voice, video, or data) are conveyed in fixedlength (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of highspeed transmission media such as E3, SONET, and T3.

ATM adaptation layer

See AAL.

ATM adaptation layer 1

See AAL1.

ATM adaptation layer 2

See AAL2.

ATM adaptation layer 3/4

See AAL3/4.

ATM adaptation layer 5

See AAL5.

ATM ARP server

Device that provides addressresolution services to LISs when running classical IP over ATM. See also LIS.

ATM data service unit

See ADSU.

ATM endpoint Point in an ATM network at which an

ATM connection is initiated or terminated. ATM endpoints include ATM-attached workstations, ATMattached servers, ATM-to-LAN switches, and ATM routers.

ATM Forum International organization jointly founded in 1991 by Cisco Systems, NET/ADAPTIVE,

Northern Telecom, and Sprint that develops and promotes standards-based implementation agreements for

ATM technology. The ATM Forum expands on official standards developed by ANSI and ITU-T, and develops implementation agreements in advance of official standards.

ATM Interface Processor

See AIP in the “Cisco

Systems Terms and Acronyms” section.

ATM layer Service-independent sublayer of the Data

Link layer in an ATM network. The ATM layer receives the 48-byte payload segments from the AAL and attaches a 5-byte header to each, producing standard 53-byte

ATM cells. These cells are passed to the Physical layer for transmission across the physical medium. See also AAL.

ATMM

ATM management. Process that runs on an

ATM switch that controls VCI translation and rate enforcement. See also ATM and VCI.

ATM management

See ATMM.

ATM network

See ATM network in the “Cisco

Systems Terms and Acronyms” section.

ATM NIC

See ATM network interface card in the

“Cisco Systems Terms and Acronyms” section.

ATM service interface

See ASI.

ATM UNI

See UNI.

ATM user-user connection

Connection created by the ATM layer to provide communication between two or more ATM service users, such as ATMM processes.

Such communication can be unidirectional, using one

VCC, or bidirectional, using two VCCs. See also ATM

layer, ATMM, and VCC.

ATP

AppleTalk Transaction Protocol. Transport-level protocol that provides a loss-free transaction service between sockets. The service allows exchanges between two socket clients in which one client requests the other to perform a particular task and to report the results. ATP binds the request and response together to ensure the reliable exchange of request-response pairs.

Attached Resource Computer Network

See ARCnet.

attachment unit interface

See AUI.

attenuation

Loss of communication signal energy.

attribute

Form of information items provided by the

X.500 Directory Service. The directory information base consists of entries, each containing one or more attributes. Each attribute consists of a type identifier together with one or more values.

AU

Access unit. Device that provides ISDN access to

PSNs. See also PSN.

AUI

Attachment unit interface. IEEE 802.3 interface between an MAU and a NIC. The term AUI can also

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A p p e n d i x A G L O S S A RY 373 refer to the rear panel port to which an AUI cable might attach. Also called transceiver cable. See also

IEEE 802.3, MAU, and NIC.

AUP Acceptable use policy. Many transit networks have policies that restrict the use to which the network may be put. Enforcement of AUPs varies with the network.

AURP AppleTalk Update-Based Routing Protocol.

Method of encapsulating AppleTalk traffic in the header of a foreign protocol, allowing the connection of two or more discontiguous AppleTalk internetworks through a foreign network (such as TCP/IP) to form an AppleTalk

WAN. This connection is called an AURP tunnel. In addition to its encapsulation function, AURP maintains routing tables for the entire AppleTalk WAN by exchanging routing information between exterior routers. See also AURP tunnel and exterior router.

AURP tunnel

Connection created in an AURP

WAN that functions as a single, virtual data link between AppleTalk internetworks physically separated by a foreign network (a TCP/IP network, for example).

See also AURP.

AUSM

ATM user service module.

authentication

In security, the verification of the identity of a person or process.

authority zone

Associated with DNS, an authority zone is a section of the domain-name tree for which one name server is the authority. See also DNS.

Automated Packet Recognition/Translation

See

APaRT in the “Cisco Systems Terms and Acronyms”

section.

automatic call distribution

See ACD.

automatic call reconnect

Feature permitting automatic call rerouting away from a failed trunk line.

automatic protection switching

See APS.

automatic repeat request

See ARQ.

Automatic Routing Management

Formerly

AutoRoute. Cisco WAN switches use a connectionoriented mechanism to provide connectivity across the network. The switches perform a connection admission control (CAC) function on all types of connections in the network. Distributed network intelligence enables the CAC function to automatically route and reroute connections over optimal paths, while guaranteeing the required QoS.

automatic spanning tree

See AST.

autonomous confederation Group of autonomous systems that rely on their own network reachability and routing information more than they rely on that received from other autonomous systems or confederations.

autonomous switching

See autonomous switching in the “Cisco Systems Terms and Acronyms” section.

autonomous system

Collection of networks under a common administration sharing a common routing strategy. Autonomous systems are subdivided by areas.

An autonomous system must be assigned a unique 16bit number by the IANA. Sometimes abbreviated as

AS. See also area and IANA.

autonomous system boundary router

See ASBR.

autoreconfiguration

Process performed by nodes within the failure domain of a Token Ring network.

Nodes automatically perform diagnostics in an attempt to reconfigure the network around the failed areas. See also failure domain.

available bit rate

See ABR.

average rate

Average rate, in kilobits per second

(Kbps), at which a given virtual circuit will transmit

AW

Administrative weight. Value set by the network administrator to indicate the desirability of a network link. One of four link metrics exchanged by PTSPs to determine the available resources of an ATM network.

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B

B8ZS

Binary 8-zero substitution. Line-code type, used on T1 and E1 circuits, in which a special code is substituted whenever 8 consecutive zeros are sent over the link. This code is then interpreted at the remote end of the connection. This technique guarantees ones density independent of the data stream. Sometimes called bipolar 8-zero substitution. Compare to AMI.

See also ones density.

backbone

Part of a network that acts as the primary path for traffic that is most often sourced from, and destined for, other networks.

back end

Node or software program that provides services to a front end. See also client, front end, and

server.

backoff

The (usually random) retransmission delay enforced by contentious MAC protocols after a network node with data to transmit determines that the physical medium is already in use.

backplane

Physical connection between an interface processor or card and the data buses and the power distribution buses inside a chassis.

back pressure

Propagation of network congestion information upstream through an internetwork.

backward explicit congestion notification

See

BECN.

backward learning

Algorithmic process used for routing traffic that surmises information by assuming symmetrical network conditions. For example, if node

A receives a packet from node B through intermediate node C, the backward-learning routing algorithm will assume that A can optimally reach B through C.

balanced configuration

In HDLC, a point-to-point network configuration with two combined stations.

balanced, unbalanced

See balun.

balun

Balanced, unbalanced. Device used for matching impedance between a balanced and an unbalanced line, usually twisted-pair and coaxial cable.

bandwidth

Difference between the highest and lowest frequencies available for network signals. The term is also used to describe the rated throughput capacity of a given network medium or protocol.

bandwidth allocation

See bandwidth reservation.

bandwidth reservation

Process of assigning bandwidth to users and applications served by a network.

Involves assigning priority to different flows of traffic based on how critical and delay-sensitive they are. This makes the best use of available bandwidth, and if the network becomes congested, lower-priority traffic can be dropped. Sometimes called bandwidth allocation.

See also call priority.

Banyan VINES

See VINES.

BARRNet

Bay Area Regional Research Network.

Regional network serving the San Francisco Bay Area.

The BARRNet backbone is composed of four

University of California campuses (Berkeley, Davis,

Santa Cruz, and San Francisco), Stanford University,

Lawrence Livermore National Laboratory, and NASA

Ames Research Center. BARRNET is now part of BBN

Planet. See also BBN Planet.

baseband

Characteristic of a network technology in which only one carrier frequency is used. Ethernet is an example of a baseband network. Also called narrowband. Contrast with broadband.

bash

Bourne-again shell. Interactive UNIX shell based on the traditional Bourne shell, but with increased functionality. See also root account.

basic encoding rules

See BER.

Basic Rate Interface

See BRI.

Basic Research and Human Resources

See BRHR.

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baud

Unit of signaling speed equal to the number of discrete signal elements transmitted per second. Baud is synonymous with bits per second (bps) if each signal element represents exactly 1 bit.

Bay Area Regional Research Network

See

BARRNet.

BBN

Bolt, Beranek, and Newman, Inc. Hightechnology company located in Massachusetts that developed and maintained the ARPANET (and later, the Internet) core gateway system. See also BBN Planet.

BBN Planet

Subsidiary company of BBN that operates a nationwide Internet access network composed in part by the former regional networks BARRNET,

NEARNET, and SURAnet. See also BARRNet, BBN,

NEARNET, and SURAnet.

Bc

Committed Burst. Negotiated tariff metric in

Frame Relay internetworks. The maximum amount of data (in bits) that a Frame Relay internetwork is committed to accept and transmit at the CIR. See also Be and CIR.

B channel

Bearer channel. In ISDN, a full-duplex,

64Kbps channel used to send user data. Compare to D

channel, E channel, and H channel.

BCP

Best Current Practices. The newest subseries of

RFCs that are written to describe BCPs in the Internet.

Rather than specifying a protocol, these documents specify the best ways to use the protocols and the best ways to configure options to ensure interoperability between various vendors’ products.

BDCS

Broadband Digital Cross-Connect System.

SONET DCS capable of cross-connecting DS-3,

STS-1 and STS-3c signals. See also DCS.

Be

Excess Burst. Negotiated tariff metric in Frame

Relay internetworks. The number of bits that a Frame

Relay internetwork will attempt to transmit after Bc is accommodated. Be data is, in general, delivered with a lower probability than Bc data because Be data can be marked as DE by the network. See also Bc and DE.

beacon Frame from a Token Ring or FDDI device indicating a serious problem with the ring, such as a broken cable. A beacon frame contains the address of the station assumed to be down. See also failure domain.

bearer channel

See B channel.

Because It’s Time Networking Services

See

BITNET.

BECN

Backward explicit congestion notification. Bit set by a Frame Relay network in frames traveling in the opposite direction of frames encountering a congested path. DTE receiving frames with the BECN bit set can request that higher-level protocols take flow control action as appropriate. Compare to FECN.

Bell Communications Research

See Bellcore.

Bellcore

Bell Communications Research.

Organization that performs research and development on behalf of the RBOCs.

Bellman-Ford routing algorithm

See distance vector

routing algorithm.

Bell operating company

See BOC.

BER

1. Bit error rate. Ratio of received bits that con-

tain errors. 2. Basic encoding rules. Rules for encoding data units described in the ISO ASN.1 standard. See also ASN.1.

Berkeley Internet Name Domain

See BIND.

Berkeley Standard Distribution

See BSD.

BERT

Bit error rate tester. Device that determines the BER on a given communications channel. See also

BER (bit error rate).

best-effort delivery

Describes a network system that does not use a sophisticated acknowledgment system to guarantee reliable delivery of information.

BGP

Border Gateway Protocol. Interdomain routing protocol that replaces EGP. BGP exchanges reachability information with other BGP systems. It is defined by

RFC 1163. See also BGP4 and EGP.

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BGP4

BGP Version 4. Version 4 of the predominant interdomain routing protocol used on the Internet.

BGP4 supports CIDR and uses route aggregation mechanisms to reduce the size of routing tables. See also BGP and CIDR.

BICI

Broadband Inter-Carrier Interface. ITU-T standard that defines the protocols and procedures needed for establishing, maintaining, and terminating broadband switched virtual connections between public networks.

BIGA

See BIGA in the “Cisco Systems Terms and

Acronyms” section.

big-endian

Method of storing or transmitting data in which the most significant bit or byte is presented first.

Compare to little-endian.

binary

Numbering system characterized by ones and zeros (1 = on, 0 = off ).

binary 8-zero substitution

See B8ZS.

binary coded alternate mark inversion

See AMI.

binary synchronous communication

See BSC.

binary synchronous communications protocol

See

bisync.

BIND

Berkeley Internet Name Domain.

Implementation of DNS developed and distributed by the University of California at Berkeley (United States).

Many Internet hosts run BIND, and it is the ancestor of many commercial BIND implementations.

BinHex

Binary Hexadecimal. Method for converting binary files into ASCII for transmission by applications, such as email, that can only handle ASCII.

BIP

Bit interleaved parity. In ATM, a method used to monitor errors on a link. A check bit or word is sent in the link overhead for the previous block or frame.

Bit errors in the payload can then be detected and reported as maintenance information.

biphase coding Bipolar coding scheme originally developed for use in Ethernet. Clocking information is embedded into and recovered from the synchronous data stream without the need for separate clocking leads. The biphase signal contains no direct current energy.

bipolar

Electrical characteristic denoting a circuit with both negative and positive polarity. Contrast with

unipolar.

bipolar 8-zero substitution

See B8ZS.

BISDN

Broadband ISDN. ITU-T communication standards designed to handle high-bandwidth applications such as video. BISDN currently uses ATM technology over SONET-based transmission circuits to provide data rates from 155 to 622 Mbps and beyond.

Contrast with N-ISDN. See also BRI, ISDN, and PRI.

bisync

Binary Synchronous Communications

Protocol. Character-oriented data-link protocol for applications.

bit

Binary digit used in the binary numbering system. Can be 0 or 1.

bit error rate

See BER.

bit error rate tester

See BERT.

bit interleaved parity

See BIP.

BITNET

“Because It’s Time” Networking Services.

Low-cost, low-speed academic network consisting primarily of IBM mainframes and 9600-bps leased lines.

BITNET is now part of CREN. See also CREN.

BITNET III

Dial-up service providing connectivity for members of CREN. See also CREN.

bit-oriented protocol

Class of data link layer communication protocols that can transmit frames regardless of frame content. Compared with byte-oriented protocols, bit-oriented protocols provide full-duplex operation and are more efficient and reliable. Compare to byte-oriented protocol.

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bit rate

Speed at which bits are transmitted, usually expressed in bits per second.

bits per second

Abbreviated bps.

black hole

Routing term for an area of the internetwork where packets enter, but do not emerge, due to adverse conditions or poor system configuration within a portion of the network.

blocking

In a switching system, a condition in which no paths are available to complete a circuit. The term is also used to describe a situation in which one activity cannot begin until another has been completed.

block multiplexer channel

IBM-style channel that implements the FIPS-60 channel, a U.S. channel standard. This channel is also referred to as OEMI channel and 370 block mux channel.

blower

Internal cooling fan used in larger router and switch chassis.

BLSR

Bidirectional Line Switch Ring. SONET ring architecture that provides working and protection fibers between nodes. If the working fiber between nodes is cut, traffic is automatically routed onto the protection fiber. See also SONET.

BNC connector

Standard connector used to connect

IEEE 802.3 10Base2 coaxial cable to an MAU.

BNI

Broadband Network Interface.

BNM

Broadband Network Module.

BNN Boundary network node. In SNA terminology, a subarea node that provides boundary function support for adjacent peripheral nodes. This support includes sequencing, pacing, and address translation. Also called boundary node.

BOC

Bell operating company. 22 local phone companies formed by the breakup of AT&T. See RBOC.

Bolt, Beranek, and Newman, Inc.

See BBN.

BOOTP

Bootstrap Protocol. Protocol used by a network node to determine the IP address of its Ethernet interfaces, in order to affect network booting.

boot programmable read-only memory

See boot

PROM.

boot PROM

Boot programmable read-only memory.

Chip mounted on a printed circuit board used to provide executable boot instructions to a computer device.

Bootstrap Protocol

See BOOTP.

border gateway

Router that communicates with routers in other autonomous systems.

Border Gateway Protocol

See BGP.

boundary function

Capability of SNA subarea nodes to provide protocol support for attached peripheral nodes. Typically found in IBM 3745 devices.

boundary network node

See BNN.

boundary node

See BNN.

BPDU

Bridge Protocol Data Unit. Spanning-Tree

Protocol hello packet that is sent out at configurable intervals to exchange information among bridges in the network. See also PDU.

bps

Bits per second.

BPV

Bipolar violation.

BPX Service Node

See BPX Service Node in the

“Cisco Systems Terms and Acronyms” section.

break-out/break-in

See BOBI in the “Cisco Systems

Terms and Acronyms” section.

BRHR Basic Research and Human Resources.

Component of the HPCC program designed to support research, training, and education in computer science, computer engineering, and computational science. See also HPCC.

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BRI Basic Rate Interface. ISDN interface composed of two B channels and one D channel for circuitswitched communication of voice, video, and data.

Compare to PRI. See also BISDN, ISDN, and N-ISDN.

bridge

Device that connects and passes packets between two network segments that use the same communications protocol. Bridges operate at the Data Link layer (Layer 2) of the OSI reference model. In general, a bridge will filter, forward, or flood an incoming frame based on the MAC address of that frame. See also relay.

bridge forwarding

Process that uses entries in a filtering database to determine whether frames with a given MAC destination address can be forwarded to a given port or ports. Described in the IEEE 802.1 standard. See also IEEE 802.1.

bridge group

Bridging feature that assigns network interfaces to a particular spanning-tree group. Bridge groups can be compatible with the IEEE 802.1 or the

DEC specification.

bridge number

Number that identifies each bridge in an SRB LAN. Parallel bridges must have different bridge numbers.

Bridge Protocol Data Unit

See BPDU.

bridge static filtering

Process in which a bridge maintains a filtering database consisting of static entries. Each static entry equates a MAC destination address with a port that can receive frames with this

MAC destination address and a set of ports on which the frames can be transmitted. Defined in the IEEE

802.1 standard. See also IEEE 802.1.

broadband Transmission system that multiplexes multiple independent signals onto one cable. In telecommunications terminology, any channel having a bandwidth greater than a voice-grade channel (4 kHz). In LAN terminology, a coaxial cable on which analog signaling is used. Also called wideband. Contrast with baseband.

Broadband ISDN

See BISDN.

broadcast

Data packet that will be sent to all nodes on a network. Broadcasts are identified by a broadcast address. Compare to multicast and unicast. See also

broadcast address.

broadcast address

Special address reserved for sending a message to all stations. Generally, a broadcast address is a MAC destination address of all ones.

Compare to multicast address and unicast address. See also broadcast.

broadcast and unknown server

See BUS.

broadcast domain Set of all devices that will receive broadcast frames originating from any device within the set. Broadcast domains are typically bounded by routers because routers do not forward broadcast frames.

broadcast search

Propagation of a search request to all network nodes if the location of a resource is unknown to the requester. See also directed search.

broadcast storm

Undesirable network event in which many broadcasts are sent simultaneously across all network segments. A broadcast storm uses substantial network bandwidth and, typically, causes network time-outs.

brouter

Concatenation of “bridge” and “router.”

Used to refer to devices that perform both bridging and routing functions.

browser GUI-based hypertext client application, such as Internet Explorer, Mosaic, and Netscape Navigator, used to access hypertext documents and other services located on innumerable remote servers throughout the

WWW and Internet. See also hypertext, Internet, Mosaic, and WWW.

BSC

Binary synchronous communication. Characteroriented Data Link layer protocol for half-duplex applications. Often referred to simply as bisync.

BSD

Berkeley Standard Distribution. Term used to describe any of a variety of UNIX-type operating systems based on the UC Berkeley BSD operating system.

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BT

Burst tolerance. Parameter defined by the ATM

Forum for ATM traffic management. For VBR connections, BT determines the size of the maximum burst of contiguous cells that can be transmitted. See also VBR.

BTW

”By the way.” One of many short-hand phrases used in chat sessions and email conversations. See

IMHO.

buffer Storage area used for handling data in transit.

Buffers are used in internetworking to compensate for differences in processing speed between network devices.

Bursts of data can be stored in buffers until they can be handled by slower processing devices. Sometimes referred to as a packet buffer.

burst tolerance

See BT.

BUS

Broadcast and unknown server. Multicast server used in ELANs that is used to flood traffic addressed to an unknown destination and to forward multicast and broadcast traffic to the appropriate clients. See also

ELAN.

bus

1. Common physical signal path composed of

wires or other media across which signals can be sent from one part of a computer to another. Sometimes called highway. 2. See bus topology.

bus and tag channel

IBM channel developed in the

1960s incorporating copper multiwire technology.

Replaced by the ESCON channel. See also ESCON

channel and parallel channel.

Bus Interface Gate Array

See BIGA in the “Cisco

Systems Terms and Acronyms” section.

bus topology Linear LAN architecture in which transmissions from network stations propagate the length of the medium and are received by all other stations.

Compare to ring topology, star topology, and tree topology.

BX.25

An AT&T implementation of X.25. See also

X.25.

BXM

Broadband Switch Module.

bypass mode

Operating mode on FDDI and Token

Ring networks in which an interface has been removed from the ring.

bypass relay

Allows a particular Token Ring interface to be shut down and thus effectively removed from the ring.

byte

Term used to refer to a series of consecutive binary digits that are operated upon as a unit (for example, an 8-bit byte).

byte-oriented protocol

Class of data-link communications protocols that uses a specific character from the user character set to delimit frames. These protocols have largely been replaced by bit-oriented protocols.

Compare to bit-oriented protocol.

byte reversal

Process of storing numeric data with the least-significant byte first. Used for integers and addresses on devices with Intel microprocessors.

C

cable

Transmission medium of copper wire or optical fiber wrapped in a protective cover.

cable range

Range of network numbers that is valid for use by nodes on an extended AppleTalk network.

The cable range value can be a single network number or a contiguous sequence of several network numbers.

Node addresses are assigned based on the cable range value.

cable television

See CATV.

CAC

Connection admission control. Set of actions taken by each ATM switch during connection setup in order to determine whether a connection’s requested

QoS will violate the QoS guarantees for established connections. CAC is also used when routing a connection request through an ATM network.

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caching

Form of replication in which information learned during a previous transaction is used to process later transactions.

California Education and Research Federation

Network

See CERFnet.

Call Detail Record

See CDR in the “Cisco Systems

Terms and Acronyms” section.

call priority

Priority assigned to each origination port in circuit-switched systems. This priority defines the order in which calls are reconnected. Call priority also defines which calls can or cannot be placed during a bandwidth reservation. See also bandwidth reservation.

call reference value

See CRV.

call setup time

Time required to establish a switched call between DTE devices.

CAM

Content-addressable memory. See associative

memory.

Canadian Standards Association

See CSA.

CAP

Competitive access provider. Independent company providing local telecommunications services mainly to business customers in competition with an area’s BOC or IOC. Teleport and MFS are the two major CAPs operating in major metropolitan areas in the U.S. See also BOC and IOC.

carrier

Electromagnetic wave or alternating current of a single frequency, suitable for modulation by another, data-bearing signal. See also modulation.

Carrier Detect

See CD.

carrier sense multiple access collision detect

See

CSMA/CD.

CAS

Channel associated signaling.

Category 1 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard.

Category 1 cabling is used for telephone communications and is not suitable for transmitting data.

Compare to Category 2 cabling, Category 3 cabling,

Category 4 cabling, and Category 5 cabling. See also

EIA/TIA-586 and UTP.

Category 2 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard.

Category 2 cabling is capable of transmitting data at speeds up to 4Mbps. Compare to Category 1 cabling,

Category 3 cabling, Category 4 cabling, and Category 5

cabling. See also EIA/TIA-586 and UTP.

Category 3 cabling One of five grades of UTP cabling described in the EIA/TIA-586 standard. Category 3 cabling is used in 10BaseT networks and can transmit data at speeds up to 10Mbps. Compare to Category 1

cabling, Category 2 cabling, Category 4 cabling, and

Category 5 cabling. See also EIA/TIA-586 and UTP.

Category 4 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard.

Category 4 cabling is used in Token Ring networks and can transmit data at speeds up to 16Mbps. Compare to

Category 1 cabling, Category 2 cabling, Category 3

cabling, and Category 5 cabling. See also EIA/TIA-586

and UTP.

Category 5 cabling

One of five grades of UTP cabling described in the EIA/TIA-586 standard.

Category 5 cabling can transmit data at speeds up to

100Mbps. Compare to Category 1 cabling, Category 2

cabling, Category 3 cabling, and Category 4 cabling. See

also EIA/TIA-586 and UTP.

catenet

Network in which hosts are connected to diverse networks, which themselves are connected with routers. The Internet is a prominent example of a catenet.

CATV

Cable television. Communication system in which multiple channels of programming material are transmitted to homes using broadband coaxial cable.

Formerly called Community Antenna Television.

CBAC

Context-Based Access Control. Protocol that provides internal users with secure access control for each application and for all traffic across network perimeters.

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CBAC enhances security by scrutinizing both source and destination addresses and by tracking each application’s connection status.

CBDS

Connectionless Broadband Data Service.

European high-speed, packet-switched, datagram-based

WAN networking technology. Similar to SMDS. See also SMDS.

CBR Constant bit rate. QoS class defined by the ATM

Forum for ATM networks. CBR is used for connections that depend on precise clocking to ensure undistorted delivery. Compare to ABR (available bit rate), UBR, and

VBR.

CCITT

Consultative Committee for International

Telegraph and Telephone. International organization responsible for the development of communications standards. Now called the ITU-T. See ITU-T.

CCR

Commitment, Concurrency, and Recovery. OSI application service element used to create atomic operations across distributed systems. Used primarily to implement two-phase commit for transactions and nonstop operations.

CCS

Common channel signaling. Signaling system used in telephone networks that separates signaling information from user data. A specified channel is exclusively designated to carry signaling information for all other channels in the system. See also SS7.

CD

Carrier Detect. 1. Signal that indicates whether an interface is active. 2. Signal generated by a modem indicating that a call has been connected.

CDDI Copper Distributed Data Interface.

Implementation of FDDI protocols over STP and UTP cabling. CDDI transmits over relatively short distances

(about 100 meters), providing data rates of 100 Mbps using a dual-ring architecture to provide redundancy. Based on the ANSI TPPMD standard. Compare to FDDI.

CDF

Channel Definition Format. Technology for

“push” applications on the World Wide Web. CDF is an application of XML. See XML.

CDP

See CDP in the “Cisco Systems Terms and

Acronyms” section.

CDPD

Cellular Digital Packet Data. Open standard for two-way wireless data communication over highfrequency cellular telephone channels. Allows data transmissions between a remote cellular link and a

NAP. Operates at 19.2Kbps.

CDV

Cell delay variation. Component of cell transfer delay, which is induced by buffering and cell scheduling. CDV is a QoS delay parameter associated with

CBR and VBR service. See also CBR and VBR.

CDVT Cell delay variation tolerance. In ATM, a QoS parameter for managing traffic that is specified when a connection is set up. In CBR transmissions, CDVT determines the level of jitter that is tolerable for the data samples taken by the PCR. See also CBR and PCR.

CFRAD

See Cisco FRAD in the “Cisco Systems

Terms and Acronyms” section.

cell

Basic data unit for ATM switching and multiplexing. Cells contain identifiers that specify the data stream to which they belong. Each cell consists of a 5byte header and 48 bytes of payload. See also cell relay.

cell delay variation

See CDV.

cell delay variation tolerance

See CDVT.

cell loss priority

See CLP.

cell loss ratio

See CLR.

cell payload scrambling

Technique using an ATM switch to maintain framing on some medium-speed edge and trunk interfaces.

cell relay

Network technology based on the use of small, fixed-size packets, or cells. Because cells are fixedlength, they can be processed and switched in hardware at high speeds. Cell relay is the basis for many highspeed network protocols, including ATM, IEEE 802.6, and SMDS. See also cell.

cells per second

Abbreviated cps.

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cell transfer delay

See CTD.

Cellular Digital Packet Data

See CDPD.

cellular radio

Technology that uses radio transmissions to access telephone-company networks. Service is provided in a particular area by a low-power transmitter.

CELP

Code Excited Linear Prediction Compression.

Algorithm used in low bit-rate voice encoding. Used in

ITU-T Recommendations G.728, G.729, G.723.1.

central office

See CO.

Centrex

LEC service that provides local switching applications similar to those provided by an on-site

PBX. With Centrex, there is no onsite switching; all customer connections go back to the CO. See also CO and LEC (local exchange carrier).

CEPT

Conference Europenne des Postes et des

Telecommunications. Association of the 26 European

PTTs that recommends communication specifications to the ITU-T.

CER

Cell error ratio. In ATM, the ratio of transmitted cells that have errors to the total cells sent in a transmission for a specific period of time.

CERFnet

California Education and Research

Federation Network. TCP/IP network, based in

Southern California, that connects hundreds of highereducation centers internationally while also providing

Internet access to subscribers. CERFnet was founded in

1988 by the San Diego Supercomputer Center and

General Atomics, and is funded by the NSF.

CERN

European Laboratory for Particle Physics.

Birthplace of the World Wide Web.

CERT

Computer Emergency Response Team.

Chartered to work with the Internet community to facilitate its response to computer security events involving Internet hosts, to take proactive steps to raise the community’s awareness of computer security issues, and to conduct research targeted at improving the security of existing systems. The U.S. CERT is based at

Carnegie Mellon University in Pittsburgh (United

States), Regional CERTs are, like NICs, springing up in different parts of the world.

CES Circuit emulation service. Enables users to multiplex or concentrate multiple circuit emulation streams for voice and video with packet data on a single high-speed

ATM link without a separate ATM access multiplexer.

CGI Common Gateway Interface. Set of rules that describes how a Web server communicates with another application running on the same computer and how the application (called a CGI program) communicates with the Web server. Any application can be a CGI program if it handles input and output according to the CGI standard.

chaining

SNA concept in which RUs are grouped together for the purpose of error recovery.

Challenge Handshake Authentication Protocol

See

CHAP.

channel 1. Communication path. Multiple channels can be multiplexed over a single cable in certain environments. 2. In IBM, the specific path between large computers (such as mainframes) and attached peripheral devices.

channel-attached

Pertaining to attachment of devices directly by data channels (input/output channels) to a computer.

Channel Definition Format

See CDF.

Channel Interface Processor

See CIP in the “Cisco

Systems Terms and Acronyms” section.

channel service unit

See CSU.

channelized E1

Access link operating at 2.048Mbps

that is subdivided into 30 B-channels and 1 D-channel.

Supports DDR, Frame Relay, and X.25. Compare to

channelized T1.

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channelized T1

Access link operating at 1.544Mbps

that is subdivided into 24 channels (23 B-channels and

1 D-channel) of 64Kbps each. The individual channels or groups of channels connect to different destinations.

Supports DDR, Frame Relay, and X.25. Also referred to as fractional T1. Compare to channelized E1.

CHAP

Challenge Handshake Authentication

Protocol. Security feature supported on lines using PPP encapsulation that prevents unauthorized access. CHAP does not itself prevent unauthorized access; it merely identifies the remote end. The router or access server then determines whether that user is allowed access.

Compare to PAP.

chat script

String of text that defines the login “conversation” that occurs between two systems. Consists of expect-send pairs that define the string that the local system expects to receive from the remote system and what the local system should send as a reply.

Cheapernet

Industry term used to refer to the IEEE

802.3 10Base2 standard or the cable specified in that standard. Compare to Thinnet. See also 10Base2,

Ethernet, and IEEE 802.3.

checksum

Method for checking the integrity of transmitted data. A checksum is an integer value computed from a sequence of octets taken through a series of arithmetic operations. The value is recomputed at the receiving end and compared for verification.

child peer group

Peer group for which another peer group is the parent peer group. See also LGN, peer

group, and parent peer group.

choke packet

Packet sent to a transmitter to tell it that congestion exists and that it should reduce its sending rate.

CIA

Classical IP over ATM. Specification for running

IP over ATM in a manner that takes full advantage of the features of ATM. Defined in RFC 1577.

CICNet Regional network that connects academic, research, nonprofit, and commercial organizations in the

Midwestern United States. Founded in 1988, CICNet was a part of the NSFNET and was funded by the NSF until the NSFNET dissolved in 1995. See also NSFNET.

CICS

Customer Information Control System. IBM application subsystem allowing transactions entered at remote terminals to be processed concurrently by user applications.

CID

Craft interface device. Terminal or PC-based interface that enables the performance of local maintenance operations.

CIDR

Classless interdomain routing. Technique supported by BGP4 and based on route aggregation.

CIDR allows routers to group routes together in order to cut down on the quantity of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity. With CIDR, IP addresses and their subnet masks are written as 4 octets, separated by periods, followed by a forward slash and a 2-digit number that represents the subnet mask. See also BGP4.

CIP

See CIP in the “Cisco Systems Terms and

Acronyms” section.

CIR

Committed information rate. Rate at which a

Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics. See also Bc.

circuit

Communications path between two or more points.

circuit group Grouping of associated serial lines that link two bridges. If one of the serial links in a circuit group is in the spanning tree for a network, any of the serial links in the circuit group can be used for load balancing. This load-balancing strategy avoids data ordering problems by assigning each destination address to a particular serial link.

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circuit steering

Mechanism used by some ATM switches to eavesdrop on a virtual connection and copy its cells to another port where an ATM analyzer is attached. Also known as port snooping.

circuit switching

Switching system in which a dedicated physical circuit path must exist between sender and receiver for the duration of the “call.” Used heavily in the telephone company network. Circuit switching can be contrasted with contention and token passing as a channel-access method, and with message switching and packet switching as a switching technique.

ciscoBus controller

See SP in the “Cisco Systems

Terms and Acronyms” section.

Cisco Discovery Protocol

See CDP in the “Cisco

Systems Terms and Acronyms” section.

Cisco FRAD

See Cisco FRAD in the “Cisco Systems

Terms and Acronyms” section.

Cisco Frame Relay access device

See Cisco FRAD in the “Cisco Systems Terms and Acronyms” section.

CiscoFusion

See CiscoFusion in the “Cisco Systems

Terms and Acronyms” section.

Cisco Internetwork Operating System software

See

Cisco IOS in the “Cisco Systems Terms and Acronyms”

section.

Cisco IOS

See Cisco IOS in the “Cisco Systems

Terms and Acronyms” section.

Cisco Link Services

See CLS in the “Cisco Systems

Terms and Acronyms” section.

Cisco Link Services Interface

See CLSI in the

“Cisco Systems Terms and Acronyms” section.

CiscoView

See CiscoView in the “Cisco Systems

Terms and Acronyms” section.

CIX

Commercial Internet Exchange. A connection point between the commercial Internet service providers (pronounced “kicks”). See FIX and GIX.

Class A station

See DAS.

Class B station

See SAS (single attachment station).

classical IP over ATM

See CIA.

classless interdomain routing

See CIDR.

class of service

See CoS.

CLAW

Common Link Access for Workstations. Data

Link layer protocol used by channel-attached RISC

System/6000 series systems and by IBM 3172 devices running TCP/IP off-load. CLAW improves efficiency of channel use and allows the CIP to provide the unctionality of a 3172 in TCP/IP environments and support direct channel attachment. The output from

TCP/IP mainframe processing is a series of IP datagrams that the router can switch without modifications.

Clear To Send

See CTS.

clear channel

Channel that uses out-of-band signaling (as opposed to in-band signaling), so the channel’s entire bit rate is available.

CLEC Competitive local exchange carrier. Company that builds and operates communication networks in metropolitan areas and provides its customers with an alternative to the local telephone company. See also CAP.

CLI

Command line interface. Interface that allows the user to interact with the operating system by entering commands and optional arguments. The

UNIX operating system and DOS provide CLIs.

Compare to GUI.

client

Node or software program (front-end device) that requests services from a server. See also back end,

front end, and server.

client/server computing

Term used to describe distributed computing (processing) network systems in which transaction responsibilities are divided into two parts: client (front end) and server (back end). Both terms (client and server) can be applied to software programs or actual computing devices. Also called distributed computing (processing). Compare to peer-to-

peer computing. See also RPC.

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client-server model

Common way to describe network services and the model user processes (programs) of those services. Examples include the nameserver/ nameresolver paradigm of the DNS and fileserver/ file-client relationships such as NFS and diskless hosts.

CLNP Connectionless Network Protocol. OSI

Network layer protocol that does not require a circuit to be established before data is transmitted. See also CLNS.

CLNS

Connectionless Network Service. OSI

Network layer service that does not require a circuit to be established before data is transmitted. CLNS routes messages to their destinations independently of any other messages. See also CLNP.

CLP

Cell loss priority. Field in the ATM cell header that determines the probability of a cell being dropped if the network becomes congested. Cells with CLP = 0 are insured traffic, which is unlikely to be dropped.

Cells with CLP = 1 are best-effort traffic, which might be dropped in congested conditions in order to free up resources to handle insured traffic.

CLR

Cell loss ratio. In ATM, the ratio of discarded cells to cells that are successfully transmitted. CLR can be set as a QoS parameter when a connection is set up.

CLTP

Connectionless Transport Protocol. Provides for end-to-end Transport data addressing (via Transport selector) and error control (via checksum), but cannot guarantee delivery or provide flow control. The OSI equivalent of UDP.

cluster controller

1. Generally, an intelligent device

that provides the connections for a cluster of terminals to a data link. 2. In SNA, a programmable device that controls the input/output operations of attached devices. Typically, an IBM 3174 or 3274 device.

CMI

Coded mark inversion. ITU-T line coding technique specified for STS-3c transmissions. Also used in

DS-1 systems. See also DS-1 and STS-3c.

CMIP

Common Management Information Protocol.

OSI network management protocol created and standardized by ISO for the monitoring and control of heterogeneous networks. See also CMIS.

CMIS

Common Management Information Services.

OSI network management service interface created and standardized by ISO for the monitoring and control of heterogeneous networks. See also CMIP.

CMNS

Connection-Mode Network Service. Extends local X.25 switching to a variety of media (Ethernet,

FDDI, Token Ring). See also CONP.

CMT

Connection management. FDDI process that handles the transition of the ring through its various states (off, active, connect, and so on), as defined by the ANSI X3T9.5 specification.

CO

Central office. Local telephone company office to which all local loops in a given area connect and in which circuit switching of subscriber lines occurs.

CO-IPX

Connection Oriented IPX. Native ATM protocol based on IPX under development by Novell.

coaxial cable

Cable consisting of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. Two types of coaxial cable are currently used in LANs: 50-ohm cable, which is used for digital signaling, and 75-ohm cable, which is used for analog signaling and high-speed digital signaling.

CODEC

Coder-decoder. Device that typically uses pulse code modulation to transform analog signals into a digital bit stream and digital signals back into analog.

coded mark inversion

See CMI.

coder-decoder

See CODEC.

coding

Electrical techniques used to convey binary signals.

CO FRAD

Central office frame relay access device.

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collapsed backbone

Nondistributed backbone in which all network segments are interconnected by way of an internetworking device. A collapsed backbone might be a virtual network segment existing in a device such as a hub, a router, or a switch.

collision

In Ethernet, the result of two nodes transmitting simultaneously. The frames from each device impact and are damaged when they meet on the physical media. See also collision domain.

collision detection

See CSMA/CD.

collision domain

In Ethernet, the network area within which frames that have collided are propagated.

Repeaters and hubs propagate collisions; LAN switches, bridges and routers do not. See also collision.

command line interface

See CLI.

Committed Burst

See Bc.

committed information rate

See CIR.

common carrier

Licensed, private utility company that supplies communication services to the public at regulated prices.

common channel signaling

See CCS.

Common Gateway Interface

See CGI.

Common Link Access for Workstations

See CLAW.

Common Management Information Protocol

See

CMIP.

Common Management Information Services

See

CMIS.

common part convergence sublayer

See CPCS.

Common Programming Interface for

Communications

See CPI-C.

common transport semantic

See CTS.

communication

Transmission of information.

communication controller

In SNA, a subarea node

(such as an IBM 3745 device) that contains an NCP.

communication server

Communications processor that connects asynchronous devices to a LAN or WAN through network and terminal emulation software.

Performs only asynchronous routing of IP and IPX.

Compare to access server.

communications line

Physical link (such as wire or a telephone circuit) that connects one or more devices to one or more other devices.

community

In SNMP, a logical group of managed devices and NMSs in the same administrative domain.

Community Antenna Television

Now known as

CATV. See CATV.

community name

See community string.

community string

Text string that acts as a password and is used to authenticate messages sent between a management station and a router containing an SNMP agent. The community string is sent in every packet between the manager and the agent. Also called a community name.

companding

Contraction derived from the opposite processes of compression and expansion. Part of the

PCM process whereby analog signal values are logically rounded to discrete scale-step values on a nonlinear scale. The decimal step number is then coded in its binary equivalent prior to transmission. The process is reversed at the receiving terminal using the same nonlinear scale. Compare to compression and expansion.

See also a-law and mu-law.

complete sequence number PDU

See CSNP.

Compressed Serial Link Internet Protocol

See

CSLIP.

compression

The running of a data set through an algorithm that reduces the space required to store or the bandwidth required to transmit the data set.

Compare to companding and expansion.

Computer Science Network

See CSNET.

concentrator

See hub.

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Conference Europenne des Postes et des

Telecommunications

See CEPT.

configuration direct VCC In ATM, a bi-directional point-to-point VCC set up by a LEC to an LES. One of three control connections defined by Phase 1 LANE.

Compare to control distribute VCC and control direct VCC.

configuration management One of five categories of network management defined by ISO for management of OSI networks. Configuration management subsystems are responsible for detecting and determining the state of a network. See also accounting management,

fault management, performance management, and security

management.

configuration register

See configuration register in the

“Cisco Systems Terms and Acronyms” section.

congestion

Traffic in excess of network capacity.

congestion avoidance

Mechanism by which an ATM network controls traffic entering the network to minimize delays. In order to use resources most efficiently, lower-priority traffic is discarded at the edge of the network if conditions indicate that it cannot be delivered.

congestion collapse

Condition in which the retransmission of frames in an ATM network results in little or no traffic successfully arriving at the destination.

Congestion collapse frequently occurs in ATM networks composed of switches that do not have adequate and effective buffering mechanisms complimented by intelligent packet discard or ABR congestion feedback mechanisms.

connection admission control

See CAC.

connectionless

Term used to describe data transfer without the existence of a virtual circuit. Compare to

connection-oriented. See also virtual circuit.

Connectionless Broadband Data Service See CBDS.

Connectionless Network Protocol

See CLNP.

Connectionless Network Service

See CLNS.

connection management

See CMT.

Connection-Mode Network Service

See CMNS.

connection-oriented

Term used to describe data transfer that requires the establishment of a virtual circuit. See also connectionless and virtual circuit.

Connection-Oriented Network Protocol See CONP.

CONP

Connection-Oriented Network Protocol. OSI protocol providing connection-oriented operation to upper-layer protocols. See also CMNS.

CONS

Connection-oriented network service.

console

DTE through which commands are entered into a host.

constant bit rate

See CBR.

Consultative Committee for International Telegraph and Telephone

See CCITT.

content-addressable memory

See associative memory.

contention

Access method in which network devices compete for permission to access the physical medium.

Compare to circuit switching and token passing.

Context-Based Access Control

See CBAC.

control direct VCC

In ATM, a bidirectional VCC set up by a LEC to a LES. One of three control connections defined by Phase 1 LANE. Compare to

configuration direct VCC and control distribute VCC.

control distribute VCC

In ATM, a unidirectional

VCC set up from a LES to a LEC. One of three control connections defined by Phase 1 LANE. Typically, the VCC is a point-to-multipoint connection.

Compare to configuration direct VCC and control direct

VCC.

control point

See CP.

convergence

Speed and ability of a group of internetworking devices running a specific routing protocol to agree on the topology of an internetwork after a change in that topology.

convergence sublayer

See CS.

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conversation

In SNA, an LU 6.2 session between two transaction programs.

cookie

Piece of information sent by a Web server to a

Web browser that the browser is expected to save and send back to the Web server whenever the browser makes additional requests of the Web server.

Cooperation for Open Systems Interconnection

Networking in Europe

See COSINE.

Copper Distributed Data Interface

See CDDI.

COPS

Common Open Policy Service. Quality-ofservice (QoS) policy exchange protocol proposed as an

IETF standard for communicating network QoS policy information.

CORBA

Common Object Request Broker

Architecture. OMG’s answer to the need for interoperability among the rapidly proliferating number of hardware and software products available today. Simply stated, CORBA allows applications to communicate with one another no matter where they are located or who has designed them. See IIOP.

core gateway

Primary routers in the Internet.

core router

In a packet-switched star topology, a router that is part of the backbone and that serves as the single pipe through which all traffic from peripheral networks must pass on its way to other peripheral networks.

Corporation for Open Systems

See COS.

Corporation for Research and Educational

Networking

See CREN.

CoS Class of service. Indication of how an upper-layer protocol requires a lower-layer protocol to treat its messages. In SNA subarea routing, COS definitions are used by subarea nodes to determine the optimal route to establish a given session. A CoS definition comprises a virtual route number and a transmission priority field.

Also called ToS.

COS

Corporation for Open Systems. Organization that promulgates the use of OSI protocols through conformance testing, certification, and related activities.

COSINE

Cooperation for Open Systems

Interconnection Networking in Europe. European project financed by the EC to build a communication network between scientific and industrial entities in

Europe. The project ended in 1994.

cost

Arbitrary value, typically based on hop count, media bandwidth, or other measures, that is assigned by a network administrator and used to compare various paths through an internetwork environment.

Cost values are used by routing protocols to determine the most favorable path to a particular destination: the lower the cost, the better the path. Sometimes called path cost. See also routing metric.

count to infinity

Problem that can occur in routing algorithms that are slow to converge, in which routers continuously increment the hop count to particular networks. Typically, some arbitrary hop-count limit is imposed to prevent this problem.

CP

Control point. In SNA networks, element that identifies the APPN networking components of a

PU 2.1 node, manages device resources, and provides services to other devices. In APPN, CPs are able to communicate with logically adjacent CPs by way of

CP-to-CP sessions. See also EN and NN.

CPCS

Common part convergence sublayer. One of the two sublayers of any AAL. The CPCS is serviceindependent and is further divided into the CS and the

SAR sublayers. The CPCS is responsible for preparing data for transport across the ATM network, including the creation of the 48-byte payload cells that are passed to the ATM layer. See also AAL, ATM layer, CS, SAR, and SSCS.

CPE Customer premises equipment. Terminating equipment, such as terminals, telephones, and modems, supplied by the telephone company, installed at customer sites, and connected to the telephone company network.

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CPI-C

Common Programming Interface for

Communications. Platform-independent API developed by IBM and used to provide portability in

APPC applications. See also APPC.

CPP

See CPP (combinet proprietary protocol) in the

“Cisco Systems Terms and Acronyms” section.

cps

Cells per second.

craft interface device

See CID.

crankback

A mechanism used by ATM networks when a connection setup request is blocked because a node along a selected path cannot accept the request.

In this case, the path is rolled back to an intermediate node, which attempts to discover another path to the final destination using GCAC. See also GCAC.

CRC

Cyclic redundancy check. Error-checking technique in which the frame recipient calculates a remainder by dividing frame contents by a prime binary divisor and compares the calculated remainder to a value stored in the frame by the sending node.

CREN

Corporation for Research and Educational

Networking. The result of a merger of BITNET and

CSNET. CREN is devoted to providing Internet connectivity to its members, which include the alumni, students, faculty, and other affiliates of participating educational and research institutions, via BITNET III.

See also BITNET, BITNET III, and CSNET.

CRM Cell rate margin. One of three link attributes exchanged using PTSPs to determine the available resources of an ATM network. CRM is a measure of the difference between the effective bandwidth allocation per traffic class as the allocation for sustainable cell rate.

cross talk

Interfering energy transferred from one circuit to another.

CRV

Call reference value. Number carried in all

Q.931 (I.451) messages that provides an identifier for each ISDN call.

CS

Convergence sublayer. One of the two sublayers of the AAL CPCS, which is responsible for padding and error checking. PDUs passed from the SSCS are appended with an 8-byte trailer (for error checking and other control information) and padded, if necessary, so that the length of the resulting PDU is divisible by 48.

These PDUs are then passed to the SAR sublayer of the

CPCS for further processing. See also AAL, CPCS,

SAR, and SSCS.

CSA

Canadian Standards Association. Agency within

Canada that certifies products that conform to

Canadian national safety standards.

CS-ACELP

Conjugate Structure Algebraic Code

Excited Linear Prediction. CELP voice compression algorithm providing 8Kbps, or 8:1 compression, standardized in ITU-T Recommendation G.729.

CSLIP Compressed Serial Link Internet Protocol.

Extension of SLIP that, when appropriate, allows just header information to be sent across a SLIP connection, reducing overhead and increasing packet throughput on

SLIP lines. See also SLIP.

CSMA/CD

Carrier sense multiple access collision detect. Media-access mechanism wherein devices ready to transmit data first check the channel for a carrier. If no carrier is sensed for a specific period of time, a device can transmit. If two devices transmit at once, a collision occurs and is detected by all colliding devices.

This collision subsequently delays retransmissions from those devices for some random length of time.

CSMA/CD access is used by Ethernet and IEEE 802.3.

CSNET Computer Science Network. Large internetwork consisting primarily of universities, research institutions, and commercial concerns. CSNET merged with

BITNET to form CREN. See also BITNET and CREN.

CSNP

Complete sequence number PDU. PDU sent by the designated router in an OSPF network to maintain database synchronization.

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CSU

Channel service unit. Digital interface device that connects end-user equipment to the local digital telephone loop. Often referred to together with DSU, as CSU/DSU. See also DSU.

CTD Cell transfer delay. In ATM, the elapsed time between a cell exit event at the source UNI and the corresponding cell entry event at the destination UNI for a particular connection. The CTD between the two points is the sum of the total inter-ATM node transmission delay and the total ATM node processing delay.

CTI

Computer telephony integration. Name given to the merger of traditional telecommunications (PBX) equipment with computers and computer applications.

The use of Caller ID to automatically retrieve customer information from a database is an example of a CTI application.

CTS 1. Clear To Send. Circuit in the EIA/TIA-232 specification that is activated when DCE is ready to accept data from DTE. 2. Common transport semantic.

Cornerstone of the IBM strategy to reduce the number of protocols on networks. CTS provides a single API for developers of network software and enables applications to run over APPN, OSI, or TCP/IP.

Customer Information Control System

See CICS.

customer premises equipment

See CPE.

cut-through packet switching

Packet switching approach that streams data through a switch so that the leading edge of a packet exits the switch at the output port before the packet finishes entering the input port.

A device using cut-through packet switching reads, processes, and forwards packets as soon as the destination address is looked up and the outgoing port determined. Also known as on-the-fly packet switching.

Compare to store and forward packet switching.

CxBus

See CxBus in the “Cisco Systems Terms and

Acronyms” section.

Cyberspace

Term coined by William Gibson in his fantasy novel Neuromancer to describe the “world” of computers and the society that gathers around them.

Often used to refer to the Internet, the World Wide

Web, or some combination thereof.

cycles per second

See hertz.

cyclic redundancy check

See CRC.

D

D4 framing

See SF.

DAC

Dual-attached concentrator. FDDI or CDDI concentrator capable of attaching to both rings of an

FDDI or CDDI network. It can also be dual-homed from the master ports of other FDDI or CDDI concentrators.

DACS

Digital Access and Crossconnect System.

AT&T’s term for a digital crossconnect system.

DAP

Directory Access Protocol. Protocol used between a DUA and a DSA in an X.500 directory system. See also LDAP.

DARPA Defense Advanced Research Projects Agency.

U.S. Government agency that funded research for and experimentation with the Internet. Evolved from ARPA, and then, in 1994, back to ARPA. See also ARPA.

DARPA Internet

Obsolete term referring to the

Internet. See Internet.

DAS

1. Dual attachment station. Device attached to

both the primary and the secondary FDDI rings. Dual attachment provides redundancy for the FDDI ring: If the primary ring fails, the station can wrap the primary ring to the secondary ring, isolating the failure and retaining ring integrity. Also called a Class A station.

Compare to SAS. 2. Dynamically assigned socket.

Socket that is dynamically assigned by DDP upon request by a client. In an AppleTalk network, the sockets numbered 128 to 254 are allocated as DASs.

DATABASE2

See DB2.

database object

Piece of information that is stored in a database.

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data bus connector

See DB connector.

data channel

See D channel.

data circuit-terminating equipment

See DCE.

data communications equipment

See DCE.

Data Country Code

See DCC.

data direct VCC In ATM, a bi-directional point-topoint VCC set up between two LECs. One of three data connections defined by Phase 1 LANE. Data direct

VCCs do not offer any type of QOS guarantee, so they are typically used for UBR and ABR connections.

Compare to control distribute VCC and control direct VCC.

Data Encryption Standard

See DES.

Data Exchange Interface

See DXI.

Data Flow Control layer Layer 5 of the SNA architectural model. This layer determines and manages interactions between session partners, particularly data flow. Corresponds to the Session layer of the OSI model. See also Data-Link Control layer, Path Control

layer, Physical Control layer, Presentation Services layer,

Transaction Services layer, and Transmission Control layer.

datagram

Logical grouping of information sent as a

Network layer unit over a transmission medium without prior establishment of a virtual circuit. IP datagrams are the primary information units in the

Internet. The terms cell, frame, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

Datagram Delivery Protocol

See DDP.

Datakit

AT&T proprietary packet switching system widely deployed by the RBOCs.

data-link connection identifier

See DLCI.

Data-Link Control layer

Layer 2 in the SNA architectural model. Responsible for the transmission of data over a particular physical link. Corresponds roughly to the Data-Link layer of the OSI model.

See also Data Flow Control layer, Path Control layer,

Physical Control layer, Presentation Services layer,

Transaction Services layer, and Transmission Control layer.

Data Link layer Layer 2 of the OSI reference model.

Provides reliable transit of data across a physical link.

The Data Link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery of frames, and flow control.

The IEEE divided this layer into two sublayers: the

MAC sublayer and the LLC sublayer. Sometimes simply called Link layer. Roughly corresponds to the Data-Link

Control layer of the SNA model. See also Application

layer, LLC, MAC, Network layer, Physical layer,

Presentation layer, Session layer, and Transport layer.

data-link switching

See DLSw.

data-link switching plus

See DLSw+ in the “Cisco

Systems Terms and Acronyms” section.

Data Movement Processor

See DMP in the “Cisco

Systems Terms and Acronyms” section.

Data Network Identification Code

See DNIC.

data service unit

See DSU.

data set ready

See DSR.

data sink

Network equipment that accepts data transmissions.

data stream

All data transmitted through a communications line in a single read or write operation.

data terminal equipment

See DTE.

data terminal ready

See DTR.

DB2

IBM relational database management system.

dB

Decibels.

DB connector Data bus connector. Type of connector used to connect serial and parallel cables to a data bus.

DB connector names are in the format DB-x, where x represents the number of wires within the connector.

Each line is connected to a pin on the connector, but in many cases, not all pins are assigned a function.

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DB connectors are defined by various EIA/TIA standards.

DCA

Defense Communications Agency. U.S.

Government organization responsible for DDN networks such as MILNET. Now called DISA. See also

DISA.

DCC

Data Country Code. One of two ATM address formats developed by the ATM Forum for use by private networks. Adapted from the subnetwork model of addressing in which the ATM layer is responsible for mapping Network layer addresses to ATM addresses.

Compare to ICD.

DCE 1. Data communications equipment (EIA expansion) 2. Data circuit-terminating equipment (ITU-T expansion). Devices and connections of a communications network that comprise the network end of the user-to-network interface. The DCE provides a physical connection to the network, forwards traffic, and provides a clocking signal used to synchronize data transmission between DCE and DTE devices. Modems and interface cards are examples of DCE. Compare to DTE.

DCOM

Distributed Component Object Model.

Protocol that enables software components to communicate directly over a network. Developed by Microsoft and previously called Network OLE, DCOM is designed for use across multiple network transports, including Internet protocols such as HTTP. See also

IIOP.

DCS

Digital Crossconnect System. Network element providing automatic cross connection of a digital signal or its constituent parts.

D channel

1. Data channel. Full-duplex, 16Kbps

(BRI) or 64Kbps (PRI) ISDN channel. Compare to B

channel, E channel, and H channel. 2. In SNA, a device

that connects a processor and main storage with peripherals.

DCT

Discrete cosine transform.

DDM

Distributed data management. Software in an

IBM SNA environment that provides peer-to-peer communication and file sharing. One of three SNA transaction services. See also DIA and SNADS.

DDN Defense Data Network. U.S. military network composed of an unclassified network (MILNET) and various secret and top-secret networks. DDN is operated and maintained by DISA. See also DISA and MILNET.

DDP

Datagram Delivery Protocol. AppleTalk

Network layer protocol that is responsible for the socket-to-socket delivery of datagrams over an

AppleTalk internetwork.

DDR

Dial-on-demand routing. Technique whereby a router can automatically initiate and close a circuitswitched session as transmitting stations demand. The router spoofs keepalives so that end stations treat the session as active. DDR permits routing over ISDN or telephone lines using an external ISDN terminal adaptor or modem.

DE

Discard eligible. See also tagged traffic.

deadlock

1. Unresolved contention for the use of a

resource. 2. In APPN, when two elements of a process each wait for action by or a response from the other before they resume the process.

decibels

Abbreviated dB.

DECnet

Group of communications products

(including a protocol suite) developed and supported by Digital Equipment Corporation. DECnet/OSI (also called DECnet Phase V) is the most recent iteration and supports both OSI protocols and proprietary

Digital protocols. Phase IV Prime supports inherent

MAC addresses that allow DECnet nodes to coexist with systems running other protocols that have MAC address restrictions. See also DNA.

DECnet routing

Proprietary routing scheme introduced by Digital Equipment Corporation in DECnet

Phase III. In DECnet Phase V, DECnet completed its transition to OSI routing protocols (ES-IS and IS-IS).

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decryption

Reverse application of an encryption algorithm to encrypted data, thereby restoring that data to its original, unencrypted state. See also encryption.

dedicated LAN Network segment allocated to a single device. Used in LAN switched network topologies.

dedicated line

Communications line that is indefinitely reserved for transmissions, rather than switched as transmission is required. See also leased line.

de facto standard

Standard that exists by nature of its widespread use. Compare to de jure standard. See also standard.

default route

Routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table.

Defense Advanced Research Projects Agency

See

DARPA.

Defense Communications Agency

See DCA.

Defense Data Network

See DDN.

Defense Information Systems Agency

See DISA.

de jure standard

Standard that exists because of its approval by an official standards body. Compare to de

facto standard. See also standard.

DEK

Data encryption key. Used for the encryption of message text and for the computation of message integrity checks (signatures).

delay

Time between the initiation of a transaction by a sender and the first response received by the sender.

Also, the time required to move a packet from source to destination over a given path.

demand priority

Media access method used in

100VG-AnyLAN that uses a hub that can handle multiple transmission requests and can process traffic according to priority, making it useful for servicing time-sensitive traffic such as multimedia and video.

Demand priority eliminates the overhead of packet collisions, collision recovery, and broadcast traffic typical in Ethernet networks. See also 100VG-AnyLAN.

demarc

Demarcation point between carrier equipment and CPE.

demodulation

Process of returning a modulated signal to its original form. Modems perform demodulation by taking an analog signal and returning it to its original (digital) form. See also modulation.

demultiplexing Separating of multiple input streams that were multiplexed into a common physical signal back into multiple output streams. See also multiplexing.

dense mode PIM

See PIM dense mode.

Department of Defense

See DoD.

Dependent LU

See DLU.

Dependent LU Requester

See DLUR.

Dependent LU Server

See DLUS.

DES

1. Data Encryption Standard. Standard crypto-

graphic algorithm developed by the U.S. National

Bureau of Standards. 2. Destination end station.

designated bridge

Bridge that incurs the lowest path cost when forwarding a frame from a segment to the root bridge.

designated router

OSPF router that generates LSAs for a multiaccess network and has other special responsibilities in running OSPF. Each multiaccess OSPF network that has at least two attached routers has a designated router that is elected by the OSPF Hello protocol. The designated router enables a reduction in the number of adjacencies required on a multiaccess network, which in turn reduces the amount of routing protocol traffic and the size of the topological database.

destination address

Address of a network device that is receiving data. See also source address.

destination MAC

See DMAC.

destination service access point

See DSAP.

deterministic load distribution Technique for distributing traffic between two bridges across a circuit group.

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Guarantees packet ordering between source-destination pairs and always forwards traffic for a source-destination pair on the same segment in a circuit group for a given circuit-group configuration.

Deutsche Industrie Norm

See DIN.

Deutsche Industrie Norm connector

See DIN

connector.

device

See node.

DHCP

Dynamic Host Configuration Protocol.

Provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.

DIA

Document Interchange Architecture. Defines the protocols and data formats needed for the transparent interchange of documents in an SNA network.

One of three SNA transaction services. See also DDM and SNADS.

dial backup

Feature that provides protection against

WAN downtime by allowing the network administrator to configure a backup serial line through a circuitswitched connection.

dial-on-demand routing

See DDR.

dial-up line

Communications circuit that is established by a switched-circuit connection using the telephone company network.

differential encoding

Digital encoding technique whereby a binary value is denoted by a signal change rather than a particular signal level.

differential Manchester encoding

Digital coding scheme in which a mid-bit-time transition is used for clocking, and a transition at the beginning of each bit time denotes a zero. This coding scheme is used by

IEEE 802.5 and Token Ring networks.

Diffusing Update Algorithm

See DUAL in the

“Cisco Systems Terms and Acronyms” section.

Digital Network Architecture

See DNA.

digital signal level 0

See DS-0.

digital signal level 1

See DS-1.

digital signal level 3

See DS-3.

Dijkstra’s algorithm

See SPF.

DIN

Deutsche Industrie Norm. German national standards organization.

DIN connector

Deutsche Industrie Norm connector.

Multipin connector used in some Macintosh and IBM

PC-compatible computers, and on some network processor panels.

directed search

Search request sent to a specific node known to contain a resource. A directed search is used to determine the continued existence of the resource and to obtain routing information specific to the node.

See also broadcast search.

directed tree

Logical construct used to define data streams or flows. The origin of a data stream is the root. Data streams are unidirectional branches directed away from the root and toward targets, and targets are the leaves of the directed tree.

direct memory access

See DMA.

directory services

Services that help network devices locate service providers.

DISA

Defense Information Systems Agency.

Formerly DCA. U.S. military organization responsible for implementing and operating military information systems, including the DDN. See also DCA and DDN.

discard eligible

See tagged traffic.

discovery architecture

APPN software that enables a machine configured as an APPN EN to automatically find primary and backup NNs when the machine is brought onto an APPN network.

discovery mode Method by which an AppleTalk interface acquires information about an attached network from an operational node and then uses this information to configure itself. Also called dynamic configuration.

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Distance Vector Multicast Routing Protocol

See

DVMRP.

distance vector routing algorithm

Class of routing algorithms that iterate on the number of hops in a route to find a shortest-path spanning tree. Distance vector routing algorithms call for each router to send its entire routing table in each update, but only to its neighbors. Distance vector routing algorithms can be prone to routing loops, but are computationally simpler than link state routing algorithms. Also called

Bellman-Ford routing algorithm. See also link-state

routing algorithm and SPF.

distortion delay

Problem with a communication signal resulting from nonuniform transmission speeds of the components of a signal through a transmission medium. Also called group delay.

distributed computing (processing)

See client/server

computing.

Distributed Data Management

See DDM.

Distributed Queue Dual Bus

See DQDB.

Distributed Relational Database Architecture

See

DRDA.

DIT

Directory Information Tree. Global tree of entries corresponding to information objects in the OSI

X.500 Directory.

DLCI

Data-link connection identifier. Value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are locally significant (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are globally significant

(DLCIs specify individual end devices). See also LMI.

DLSw Data-link switching. Interoperability standard, described in RFC 1434, that provides a method for forwarding SNA and NetBIOS traffic over TCP/IP networks using Data Link layer switching and encapsulation.

DLSw uses SSP instead of SRB, eliminating the major limitations of SRB, including hop-count limits, broadcast and unnecessary traffic, timeouts, lack of flow control, and lack of prioritization schemes. See also SRB and SSP (Switch-to-Switch Protocol).

DLSw+

See DLSw+ in the “Cisco Systems Terms and

Acronyms” section.

DLU

Dependent LU. LU that depends on the SSCP to provide services for establishing sessions with other

LUs. See also LU and SSCP.

DLUR Dependent LU Requester. Client half of the

Dependent LU Requestor/Server enhancement to

APPN. The DLUR component resides in APPN ENs and NNs that support adjacent DLUs by securing services from the DLUS. See also APPN, DLU, and DLUS.

DLUR node

In APPN networks, an EN or NN that implements the DLUR component. See also DLUR.

DLUS

Dependent LU Server. Server half of the

Dependent LU Requestor/Server enhancement to

APPN. The DLUS component provides SSCP services to DLUR nodes over an APPN network. See also

APPN, DLU, and DLUR.

DLUS node

In APPN networks, a NN that implements the DLUS component. See also DLUS.

DMA

Direct memory access. Transfer of data from a peripheral device, such as a hard disk drive, into memory without that data passing through the microprocessor. DMA transfers data into memory at high speeds with no processor overhead.

DMAC

Destination MAC. The MAC address specified in the Destination Address field of a packet.

Compare to SMAC. See also MAC address.

DMP

See DMP in the “Cisco Systems Terms and

Acronyms” section.

DN

Distinguished Name. Global, authoritative name of an entry in the OSI Directory (X.500).

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DNA

Digital Network Architecture. Network architecture developed by Digital Equipment Corporation.

The products that embody DNA (including communications protocols) are collectively referred to as

DECnet. See also DECnet.

DNIC

Data Network Identification Code. Part of an

X.121 address. DNICs are divided into two parts: the first specifying the country in which the addressed PSN is located and the second specifying the PSN itself. See also X.121.

DNS

Domain Name System. System used in the

Internet for translating names of network nodes into addresses. See also authority zone.

DNSIX

Department of Defense Intelligence

Information System Network Security for Information

Exchange. Collection of security requirements for networking defined by the U.S. Defense Intelligence

Agency.

DOCSIS

Data-over-Cable Service Interface

Specifications. Defines technical specifications for equipment at both subscriber locations and cable operators’ headends. Adoption of DOCSIS will accelerate deployment of data-over-cable services and ensure interoperability of equipment throughout system operators’ infrastructures.

Document Interchange Architecture

See DIA.

DoD Department of Defense. U.S. Government organization that is responsible for national defense. The

DoD has frequently funded communication protocol development.

DoD Intelligence Information System Network

Security for Information Exchange

See DNSIX.

domain

1. In the Internet, a portion of the naming

hierarchy tree that refers to general groupings of networks based on organization-type or geography. 2. In

SNA, an SSCP and the resources it controls. 3. In IS-

IS, a logical set of networks.

Domain

Networking system developed by Apollo

Computer (now part of Hewlett-Packard) for use in its engineering workstations.

Domain Name System

See DNS.

domain specific part

See DSP.

dot address Refers to the common notation for IP addresses in the form n.n.n.n, where each number n represents, in decimal, 1 byte of the 4-byte IP address.

Also called dotted notation or four-part dotted notation.

dotted decimal notation

Syntactic representation for a 32-bit integer that consists of four 8-bit numbers written in base 10 with periods (dots) separating them.

Used to represent IP addresses in the Internet as in

192.67.67.20. Also called dotted quad notation.

dotted notation

See dot address.

downlink station

See ground station.

downstream physical unit

See DSPU.

DQDB

Distributed Queue Dual Bus. Data Link layer communication protocol, specified in the IEEE

802.6 standard, designed for use in MANs. DQDB, which permits multiple systems to interconnect using two unidirectional logical buses, is an open standard that is designed for compatibility with carrier transmission standards, and is aligned with emerging standards for BISDN. SIP is based on DQDB. See also MAN.

DRAM

Dynamic random-access memory. RAM that stores information in capacitors that must be periodically refreshed. Delays can occur because DRAMs are inaccessible to the processor when refreshing their contents. However, DRAMs are less complex and have greater capacity than SRAMs. See also SRAM.

DRDA

Distributed Relational Database Architecture.

IBM proprietary architecture.

drop

Point on a multipoint channel where a connection to a networked device is made.

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drop cable

Cable that connects a network device

(such as a computer) to a physical medium. A type of

AUI. See also AUI.

DRP

See DRP in the “Cisco Systems Terms and

Acronyms” section.

DS-0 Digital signal level 0. Framing specification used in transmitting digital signals over a single channel at

64Kbps on a T1 facility. Compare to DS-1 and DS-3.

DS-1

Digital signal level 1. Framing specification used in transmitting digital signals at 1.544Mbps on a

T1 facility (in the United States) or at 2.108Mbps on an E1 facility (in Europe). Compare to DS-0 and DS-3.

See also E1 and T1.

DS-1 domestic trunk interface

See DS-1/DTI.

DS-1/DTI

DS-1 domestic trunk interface. Interface circuit used for DS-1 applications with 24 trunks.

DS-3 Digital signal level 3. Framing specification used for transmitting digital signals at 44.736Mbps on a T3 facility. Compare to DS-0 and DS-1. See also E3 and T3.

DSA

Directory System Agent. Software that provides the X.500 Directory Service for a portion of the directory information base. Generally, each DSA is responsible for the directory information for a single organization or organizational unit.

DSAP

Destination service access point. SAP of the network node designated in the Destination field of a packet. Compare to SSAP. See also SAP (service access

point).

DSL Digital subscriber line. Public network technology that delivers high bandwidth over conventional copper wiring at limited distances. There are four types of DSL: ADSL, HDSL, SDSL, and VDSL. All are provisioned via modem pairs, with one modem located at a central office and the other at the customer site. Because most DSL technologies do not use the whole bandwidth of the twisted pair, there is room remaining for a voice channel. See also ADSL, HDSL, SDSL, and VDSL.

DSP

Domain specific part. Part of an NSAP-format

ATM address that contains an area identifier, a station identifier, and a selector byte. See also NSAP.

DSPU

Downstream physical unit. In SNA, a PU that is located downstream from the host. See also DSPU

concentration in the “Cisco Systems Terms and

Acronyms” section.

DSPU concentration

See DSPU concentration in the

“Cisco Systems Terms and Acronyms” section.

DSR Data set ready. EIA/TIA-232 interface circuit that is activated when DCE is powered up and ready for use.

DSU

Data service unit. Device used in digital transmission that adapts the physical interface on a DTE device to a transmission facility such as T1 or E1. The

DSU is also responsible for such functions as signal timing. Often referred to together with CSU, as

CSU/DSU. See also CSU.

DSX-1

Crossconnection point for DS-1 signals.

DTE Data terminal equipment. Device at the user end of a user-network interface that serves as a data source, destination, or both. DTE connects to a data network through a DCE device (for example, a modem) and typically uses clocking signals generated by the

DCE. DTE includes such devices as computers, protocol translators, and multiplexers. Compare to DCE.

DTL

Designated transit list. List of nodes and optional link IDs that completely specify a path across a single PNNI peer group.

DTMF Dual tone multifrequency. Use of two simultaneous voice-band tones for dialing (such as touch tone).

DTR

Data terminal ready. EIA/TIA-232 circuit that is activated to let the DCE know when the DTE is ready to send and receive data.

DUA

Directory User Agent. Software that accesses the X.500 Directory Service on behalf of the directory user. The directory user may be a person or another software element.

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DUAL

See DUAL in the “Cisco Systems Terms and

Acronyms” section.

dual-attached concentrator

See DAC.

dual attachment station

See DAS.

dual counter-rotating rings

Network topology in which two signal paths, whose directions are opposite each other, exist in a token-passing network. FDDI and

CDDI are based on this concept.

dual-homed station

Device attached to multiple

FDDI rings to provide redundancy.

dual homing

Network topology in which a device is connected to the network by way of two independent access points (points of attachment). One access point is the primary connection, and the other is a standby connection that is activated in the event of a failure of the primary connection.

Dual IS-IS

See Integrated IS-IS.

dual tone multifrequency

See DTMF.

DVMRP

Distance Vector Multicast Routing

Protocol. Internetwork gateway protocol, largely based on RIP, that implements a typical dense mode IP multicast scheme. DVMRP uses IGMP to exchange routing datagrams with its neighbors. See also IGMP.

DXI

Data Exchange Interface. ATM Forum specification, described in RFC 1483, that defines how a network device such as a bridge, router, or hub can effectively act as an FEP to an ATM network by interfacing with a special DSU that performs packet segmentation and reassembly.

dynamic adaptive routing

Automatic rerouting of traffic based on a sensing and analysis of current actual network conditions, not including cases of routing decisions taken on predefined information.

dynamic address resolution

Use of an address resolution protocol to determine and store address information on demand.

Dynamic Buffer Management

Frame Relay and

ATM service modules are equipped with large buffers and the patented Dynamic Buffer Management scheme for allocating and scaling traffic entering or leaving a node on a per-VC basis. The WAN switch dynamically assigns buffers to individual virtual circuits based upon the amount of traffic present and service-level agreements. This deep pool of available buffers readily accommodates large bursts of traffic into the node.

dynamic configuration

See discovery mode.

Dynamic IISP

Dynamic Interim-Interswitch

Signaling Protocol. Basic call routing protocol that automatically reroutes ATM connections in the event of link failures. Dynamic IISP is an interim solution until PNNI Phase 1 is completed. Contrast with IISP.

dynamic random-access memory

See DRAM.

dynamic routing

Routing that adjusts automatically to network topology or traffic changes. Also called adaptive routing.

E

E1

Wide-area digital transmission scheme used predominantly in Europe that carries data at a rate of

2.048 Mbps. E1 lines can be leased for private use from common carriers. Compare to T1. See also DS-1.

E.164

1. ITU-T recommendation for international

telecommunication numbering, especially in ISDN,

BISDN, and SMDS. An evolution of standard telephone numbers. 2. Name of the field in an ATM address that contains numbers in E.164 format.

E2A

Legacy protocols for providing OAM&P functions between a network element and an operations support system. See also OAM&P.

E3

Wide-area digital transmission scheme used predominantly in Europe that carries data at a rate of

34.368 Mbps. E3 lines can be leased for private use from common carriers. Compare to T3. See also DS-3.

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early packet discard

See EPD.

early token release Technique used in Token Ring networks that allows a station to release a new token onto the ring immediately after transmitting, instead of waiting for the first frame to return. This feature can increase the total bandwidth on the ring. See also Token

Ring.

EARN

European Academic Research Network.

European network connecting universities and research institutes. EARN merged with RARE to form

TERENA. See also RARE and TERENA.

EBCDIC

Extended Binary Coded Decimal

Interchange Code. Any of a number of coded character sets developed by IBM consisting of 8-bit coded characters. This character code is used by older IBM systems and telex machines. Compare to ASCII.

EBONE

European Backbone. Pan-European network backbone service.

EC

European Community.

E channel

Echo channel. 64Kbps ISDN circuitswitching control channel. The E channel was defined in the 1984 ITU-T ISDN specification, but was dropped in the 1988 specification. Compare to B

channel, D channel, and H channel.

echo channel

See E channel.

echoplex

Mode in which keyboard characters are echoed on a terminal screen upon return of a signal from the other end of the line indicating that the characters were received correctly.

ECMA

European Computer Manufacturers

Association. Group of European computer vendors who have done substantial OSI standardization work.

edge device

1. Physical device that is capable of

forwarding packets between legacy interfaces (such as Ethernet and Token Ring) and ATM interfaces based on Data Link and Network layer information.

An edge device does not participate in the running of any Network layer routing protocol, but it obtains forwarding descriptions using the route distribution protocol. 2. Any device that is not an ATM switch that can connect to an ATM switch.

EDI

Electronic data interchange. Electronic communication of operational data such as orders and invoices between organizations.

EDIFACT

Electronic Data Interchange for

Administration, Commerce, and Transport. Data exchange standard administered by the United Nations to be a multi-industry EDI standard.

EEPROM

Electrically erasable programmable readonly memory. EPROM that can be erased using electrical signals applied to specific pins. See also EPROM.

EFCI

Explicit Forward Congestion Indication. In

ATM, one of the congestion feedback modes allowed by ABR service. A network element in an impending congestion state or in a congested state can set the

EFCI. The destination end-system can implement a protocol that adaptively lowers the cell rate of the connection based on the value of the EFCI. See also ABR.

EFF

Electronic Frontier Foundation. Foundation established to address social and legal issues arising from the impact on society of the increasingly pervasive use of computers as the means of communication and information distribution.

EGP Exterior Gateway Protocol. Internet protocol for exchanging routing information between autonomous systems. Documented in RFC 904. Not to be confused with the general term exterior gateway protocol. EGP is an obsolete protocol that was replaced by BGP. See also

BGP.

EIA

Electronic Industries Association. Group that specifies electrical transmission standards. The EIA and

TIA have developed numerous well-known communications standards, including EIA/TIA-232 and

EIA/TIA-449. See also TIA.

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EIA-530

Refers to two electrical implementations of

EIA/TIA-449: RS-422 (for balanced transmission) and

RS-423 (for unbalanced transmission). See also RS-

422, RS-423, and EIA/TIA-449.

EIA/TIA-232

Common Physical layer interface standard, developed by EIA and TIA, that supports unbalanced circuits at signal speeds of up to 64 Kbps.

Closely resembles the V.24 specification. Formerly called as RS-232.

EIA/TIA-449

Popular Physical layer interface developed by EIA and TIA. Essentially, a faster (up to

2Mbps) version of EIA/TIA-232 capable of longer cable runs. Formerly called RS-449. See also EIA-530.

EIA/TIA-586 Standard that describes the characteristics and applications for various grades of UTP cabling. See also Category 1 cabling, Category 2 cabling, Category 3

cabling, Category 4 cabling, Category 5 cabling, and UTC.

EIGRP

See Enhanced IGRP in the “Cisco Systems

Terms and Acronyms” section.

EIP

See EIP in the “Cisco Systems Terms and

Acronyms” section.

EISA

Extended Industry-Standard Architecture. 32bit bus interface used in PCs, PC-based servers, and some UNIX workstations and servers. See also ISA.

ELAN

Emulated LAN. ATM network in which an

Ethernet or Token Ring LAN is emulated using a client-server model. ELANs are composed of an LEC, an LES, a BUS, and an LECS. Multiple ELANs can exist simultaneously on a single ATM network. ELANs are defined by the LANE specification. See also BUS,

LANE, LEC (LAN emulation client), LECS, and LES.

ELAP

EtherTalk Link Access Protocol. Link-access protocol used in an EtherTalk network. ELAP is built on top of the standard Ethernet Data Link layer.

electrically erasable programmable read-only memory

See EEPROM.

electromagnetic interference

See EMI.

electromagnetic pulse

See EMP.

electronic data interchange

See EDI.

Electronic Data Interchange for Administration,

Commerce, and Transport

See EDIFACT.

Electronic Frontier Foundation

See EFF.

Electronic Industries Association

See EIA.

electronic mail

See email.

Electronic Messaging Association

See EMA.

electrostatic discharge

See ESD.

ELMI

Enhanced Local Management Interface.

EMA

1. Enterprise Management Architecture. Digital

Equipment Corporation network management architecture, based on the OSI network management model.

2. Electronic Messaging Association. Forum devoted to

standards and policy work, education, and development of electronic messaging systems such as email, voice mail, and facsimile.

email

Electronic mail. Widely used network application in which text messages are transmitted electronically between end users over various types of networks using various network protocols.

EMI

Electromagnetic interference. Interference by electromagnetic signals that can cause reduced data integrity and increased error rates on transmission channels.

EMIF

ESCON Multiple Image Facility. Mainframe

I/O software function that allows one ESCON channel to be shared among multiple logical partitions on the same mainframe. See also ESCON.

EMP

Electromagnetic pulse. Caused by lightning and other high-energy phenomena. Capable of coupling enough energy into unshielded conductors to destroy electronic devices. See also Tempest.

emulated LAN

See ELAN.

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emulation mode

Function of an NCP that enables it to perform activities equivalent to those performed by a transmission control unit.

EN

End node. APPN end system that implements the PU 2.1, provides end-user services, and supports sessions between local and remote CPs. ENs are not capable of routing traffic and rely on an adjacent NN for APPN services. Compare to NN. See also CP.

encapsulation

Wrapping of data in a particular protocol header. For example, Ethernet data is wrapped in a specific Ethernet header before network transit. Also, when bridging dissimilar networks, the entire frame from one network is simply placed in the header used by the Data Link layer protocol of the other network.

See also tunneling.

encapsulation bridging

Carries Ethernet frames from one router to another across disparate media, such as serial and FDDI lines. Contrast with translational

bridging.

encoder

Device that modifies information into the required transmission format.

encryption

Application of a specific algorithm to data so as to alter the appearance of the data making it incomprehensible to those who are not authorized to see the information. See also decryption.

end node

See EN.

end of transmission

See EOT.

end point

Device at which a virtual circuit or virtual path begins or ends.

end system

See ES.

End System-to-Intermediate System

See ES-IS.

Energy Sciences Network

See ESnet.

Enhanced IGRP

See Enhanced IGRP in the “Cisco

Systems Terms and Acronyms” section.

Enhanced Interior Gateway Routing Protocol

See

Enhanced IGRP in the “Cisco Systems Terms and

Acronyms” section.

Enhanced Monitoring Services

See Enhanced

Monitoring Services in the “Cisco Systems Terms and

Acronyms” section.

Enterprise Management Architecture

See EMA.

enterprise network

Large and diverse network connecting most major points in a company or other organization. Differs from a WAN in that it is privately owned and maintained.

Enterprise System Connection

See ESCON.

Enterprise System Connection channel

See

ESCON channel.

entity

Generally, an individual, manageable network device. Sometimes called an alias.

entity identifier

The unique address of an NVEs socket in a node on an AppleTalk network. The specific format of an entity identifier is network-dependent. See also NVE.

entity name

Name that an NVE can assign to itself.

Although not all NVEs have names, NVEs can possess several names (or aliases). An entity name is made up of three character strings: object, entity type, and zone.

For example: Bldg 2 LaserJet 5:[email protected] 2

Zone. See also NVE.

entity type

Part of an entity name that describes the entity’s class. For example, LaserWriter or AFPServer.

See also entity name.

EOM

End of message. Indicator that identifies the last ATM cell containing information from a data packet that was segmented.

EOT

End of transmission. Generally, a character that signifies the end of a logical group of characters or bits.

EPD Early packet discard. Mechanism used by some

ATM switches for discarding a complete AAL5 frame when a threshold condition, such as imminent congestion, is met. EPD prevents congestion that would otherwise jeopardize the switch’s ability to properly support existing connections with a guaranteed service.

Compare to TPD.

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EPROM

Erasable programmable read-only memory.

Nonvolatile memory chips that are programmed after they are manufactured, and, if necessary, can be erased by some means and reprogrammed. Compare to

EEPROM and PROM.

equalization

Technique used to compensate for communications channel distortions.

ER

Explicit rate. In ATM, an RM cell used to limit the ACR for a transmission to a specific value. Usually the source initially sets the ER initially to a requested rate, such as the PCR. Later, any network element in the path can reduce the ER to a value that the element can sustain. See also ACR, PCR, and RM.

erasable programmable read-only memory

See

EPROM.

error control

Technique for detecting and correcting errors in data transmissions.

error-correcting code

Code having sufficient intelligence and incorporating sufficient signaling information to enable the detection and correction of many errors at the receiver.

error-detecting code

Code that can detect transmission errors through analysis of received data based on the adherence of the data to appropriate structural guidelines.

ES

End system. 1. Generally, an end-user device on a network. 2. Nonrouting host or node in an OSI network.

ESCON

Enterprise System Connection. IBM channel architecture that specifies a pair of fiber-optic cables, with either LEDs or lasers as transmitters, and a signaling rate of 200Mbps.

ESCON channel

IBM channel for attaching mainframes to peripherals such as storage devices, backup units, and network interfaces. This channel incorporates fiber channel technology. The ESCON channel replaces the bus and tag channel. Compare to parallel

channel. See also bus and tag channel.

ESCON Multiple Image Facility

See EMIF.

ESD

Electrostatic discharge. Discharge of stored static electricity that can damage electronic equipment and impair electrical circuitry, resulting in complete or intermittent failures.

ESF Extended Superframe Format. Framing type used on T1 circuits that consists of 24 frames of 192 bits each, with the 193rd bit providing timing and other functions. ESF is an enhanced version of SF. See also SF.

ESI

End system identifier. Identifier that distinguishes multiple nodes at the same level when the lower-level peer group is partitioned (usually an IEEE

802 address).

ES-IS

End System-to-Intermediate System. OSI protocol that defines how end systems (hosts) announce themselves to intermediate systems (routers). See also

IS-IS.

ESnet

Energy Sciences Network. Data communications network managed and funded by the U.S.

Department of Energy Office of Energy Research

(DOE/OER). Interconnects the DOE to educational institutions and other research facilities.

ESP

Extended Services Processor.

ESS

Electronic Switching System. AT&T’s term for an electronic central office switch. A 5ESS is AT&T’s digital central office for end office applications. A 4ESS is its digital central office for toll center application.

Ethernet Baseband LAN specification invented by

Xerox Corporation and developed jointly by Xerox,

Intel, and Digital Equipment Corporation. Ethernet networks use CSMA/CD and run over a variety of cable types at 10Mbps. Ethernet is similar to the IEEE 802.3

series of standards. See also 10Base2, 10Base5, 10BaseF,

10BaseT, 10Broad36, Fast Ethernet, and IEEE 802.3.

Ethernet Interface Processor

See EIP in the “Cisco

Systems Terms and Acronyms” section.

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Ethernet meltdown

Event that causes saturation, or near saturation, on an Ethernet. It usually results from illegal or misrouted packets and typically lasts only a short time.

EtherTalk

Apple Computer’s data-link product that allows an AppleTalk network to be connected by

Ethernet cable.

EtherTalk Link Access Protocol

See ELAP.

ETSI

European Telecommunication Standards

Institute. Organization created by the European PTTs and the EC to propose telecommunications standards for Europe.

EUnet

European Internet. European commercial

Internet service provider. EUnet is designed to provide email, news, and other Internet services to European markets.

European Academic Research Network

See EARN.

European Computer Manufacturers Association

See ECMA.

European Internet

See EUnet.

European Telecommunication Standards Institute

See ETSI.

event Network message indicating operational irregularities in physical elements of a network or a response to the occurrence of a significant task, typically the completion of a request for information. See also alarm and trap.

EWOS

European Workshop for Open Systems. The

OSI Implementors Workshop for Europe.

Excess Burst

See Be.

excess rate

In ATM, traffic in excess of the insured rate for a given connection. Specifically, the excess rate equals the maximum rate minus the insured rate.

Excess traffic is delivered only if network resources are available and can be discarded during periods of congestion. Compare to insured rate and maximum rate.

exchange identification

See XID.

EXEC

See EXEC in the “Cisco Systems Terms and

Acronyms” section.

expansion

The process of running a compressed data set through an algorithm that restores the data set to its original size. Compare to companding and compression.

expedited delivery

Option set by a specific protocol layer telling other protocol layers (or the same protocol layer in another network device) to handle specific data more rapidly.

explicit forward congestion indication

See EFCI.

explicit rate

See ER.

explicit route

In SNA, a route from a source subarea to a destination subarea, as specified by a list of subarea nodes and transmission groups that connect the two.

explorer frame

Frame sent out by a networked device in a SRB environment to determine the optimal route to another networked device.

explorer packet

Generated by an end station trying to find its way through a SRB network. Gathers a hopby-hop description of a path through the network by being marked (updated) by each bridge that it traverses, thereby creating a complete topological map. See also

all-routes explorer packet, local explorer packet, and

spanning explorer packet.

Extended Binary Coded Decimal Interchange Code

See EBCDIC.

Extended Industry-Standard Architecture

See

EISA.

Extended Services Processor

See ESP in the “Cisco

Systems Terms and Acronyms” section.

Extended Superframe Format

See ESF.

exterior gateway protocol

Any internetwork protocol used to exchange routing information between autonomous systems. Not to be confused with Exterior

Gateway Protocol (EGP), which is a particular instance of an exterior gateway protocol.

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Exterior Gateway Protocol

See EGP.

exterior router

Router connected to an AURP tunnel, responsible for the encapsulation and deencapsulation of AppleTalk packets in a foreign protocol header (for example, IP). See also AURP and AURP

tunnel.

EXZ

excessive zeros.

F

failure domain

Area in which a failure occurred in a

Token Ring, defined by the information contained in a beacon. When a station detects a serious problem with the network (such as a cable break), it sends a beacon frame that includes the station reporting the failure, its

NAUN, and everything in between. Beaconing in turn initiates a process called autoreconfiguration. See also

autoreconfiguration, beacon, and NAUN.

fallback

Mechanism used by ATM networks when rigorous path selection does not generate an acceptable path. The fallback mechanism attempts to determine a path by selectively relaxing certain attributes, such as delay, in order to find a path that meets some minimal set of desired attributes.

fan-out unit

Device that allows multiple devices on a network to communicate using a single network attachment.

fantail

Panel of I/O connectors that attaches to an equipment rack, providing easy access for data connections to a networking.

FAQ

Frequently asked questions. Usually appears in the form of a “read-me” file in a variety of Internet forums. New users are expected to read the FAQ before participating in newsgroups, bulletin boards, video conferences, and so on.

FARNET

Federation of American Research

NETworks.

Fast Ethernet

Any of a number of 100-Mbps

Ethernet specifications. Fast Ethernet offers a speed increase ten times that of the 10BaseT Ethernet specification, while preserving such qualities as frame format,

MAC mechanisms, and MTU. Such similarities allow the use of existing 10BaseT applications and network management tools on Fast Ethernet networks. Based on an extension to the IEEE 802.3 specification. Compare to Ethernet. See also 100BaseFX, 100BaseT, 100BaseT4,

100BaseTX, 100BaseX, and IEEE 802.3.

Fast Ethernet Interface Processor

See FEIP in the

“Cisco Systems Terms and Acronyms” section.

Fast Sequenced Transport

See FST in the “Cisco

Systems Terms and Acronyms” section.

Fast Serial Interface Processor

See FSIP in the

“Cisco Systems Terms and Acronyms” section.

fast switching

See fast switching in the “Cisco

Systems Terms and Acronyms” section.

fault management

One of five categories of network management defined by ISO for management of OSI networks. Fault management attempts to ensure that network faults are detected and controlled. See also

accounting management, configuration management,

performance management, and security management.

FCC

Federal Communications Commission. U.S.

Government agency that supervises, licenses, and controls electronic and electromagnetic transmission standards.

FCS

Frame check sequence. Extra characters added to a frame for error control purposes. Used in HDLC,

Frame Relay, and other Data Link layer protocols.

FDDI

Fiber Distributed Data Interface. LAN standard, defined by ANSI X3T9.5, specifying a 100Mbps token-passing network using fiber-optic cable, with transmission distances of up to 2km. FDDI uses a dual-ring architecture to provide redundancy. Compare to CDDI and FDDI II.

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FDDI II

ANSI standard that enhances FDDI. FDDI

II provides isochronous transmission for connectionless data circuits and connection-oriented voice and video circuits. Compare to FDDI.

FDDI Interface Processor

See FIP in the “Cisco

Systems Terms and Acronyms” section.

FDDITalk

Apple Computer’s data-link product that allows an AppleTalk network to be connected by FDDI cable.

FDM

Frequency-division multiplexing. Technique whereby information from multiple channels can be allocated bandwidth on a single wire based on frequency. Compare to ATDM, statistical multiplexing, and TDM.

FECN

Forward explicit congestion notification. Bit set by a Frame Relay network to inform DTE receiving the frame that congestion was experienced in the path from source to destination. DTE receiving frames with the FECN bit set can request that higher-level protocols take flow-control action as appropriate. Compare to BECN.

Federal Communications Commission

See FCC.

Federal Networking Council

See FNC.

FEIP

See FEIP (Fast Ethernet Interface Processor) in the “Cisco Systems Terms and Acronyms” section.

FEP

Front-end processor. Device or board that provides network interface capabilities for a networked device. In SNA, typically an IBM 3745 device.

Fiber Distributed Data Interface

See FDDI.

fiber-optic cable

Physical medium capable of conducting modulated light transmission. Compared with other transmission media, fiber-optic cable is more expensive, but is not susceptible to electromagnetic interference, and is capable of higher data rates.

Sometimes called optical fiber.

fiber-optic interrepeater link

See FOIRL.

FID0

Format indicator 0. One of several formats that an SNA TH can use. An FID0 TH is used for communication between an SNA node and a non-SNA node.

See also TH.

FID1

Format indicator 1. One of several formats that an SNA TH can use. An FID1 TH encapsulates messages between two subarea nodes that do not support virtual and explicit routes. See also TH.

FID2

Format indicator 2. One of several formats that an SNA TH can use. An FID2 TH is used for transferring messages between a subarea node and a PU 2, using local addresses. See also TH.

FID3

Format indicator 3. One of several formats that an SNA TH can use. An FID3 TH is used for transferring messages between a subarea node and a PU 1, using local addresses. See also TH.

FID4

Format indicator 4. One of several formats that an SNA TH can use. An FID4 TH encapsulates messages between two subarea nodes that are capable of supporting virtual and explicit routes. See also TH.

field replaceable unit

Hardware component that can be removed and replaced on-site. Typical fieldreplaceable units include cards, power supplies, and chassis components.

file transfer

Category of popular network applications that allow files to be moved from one network device to another.

File Transfer, Access, and Management

See FTAM.

File Transfer Protocol

See FTP.

filter

Generally, a process or device that screens network traffic for certain characteristics, such as source address, destination address, or protocol, and determines whether to forward or discard that traffic based on the established criteria.

finger

Software tool for determining whether a person has an account at a particular Internet site.

Many sites do not allow incoming finger requests.

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FIP

See FIP in the “Cisco Systems Terms and

Acronyms” section.

firewall

Router or access server, or several routers or access servers, designated as a buffer between any connected public networks and a private network. A firewall router uses access lists and other methods to ensure the security of the private network.

firmware

Software instructions set permanently or semipermanently in ROM.

FIX

Federal Internet Exchange. Connection point between the North American governmental internets and the Internet. The FIXs are named after their geographic regions, as in FIX West (Mountain View,

California) and FIX East (College Park, Maryland). See

CIX, GIX, and MAE.

flapping

Routing problem in which an advertised route between two nodes alternates (flaps) back and forth between two paths due to a network problem that causes intermittent interface failures.

Flash memory

Nonvolatile storage that can be electrically erased and reprogrammed so that software images can be stored, booted, and rewritten as necessary. Flash memory was developed by Intel and is licensed to other semiconductor companies.

flash update

Routing update sent asynchronously in response to a change in the network topology. Compare to routing update.

flat addressing

Scheme of addressing that does not use a logical hierarchy to determine location. For example, MAC addresses are flat, so bridging protocols must flood packets throughout the network to deliver the packet to the appropriate location. Compare to

hierarchical addressing.

flooding

Traffic passing technique used by switches and bridges in which traffic received on an interface is sent out all of the interfaces of that device except the interface on which the information was originally received.

flow

Stream of data traveling between two endpoints across a network (for example, from one LAN station to another). Multiple flows can be transmitted on a single circuit.

flow control Technique for ensuring that a transmitting entity, such as a modem, does not overwhelm a receiving entity with data. When the buffers on the receiving device are full, a message is sent to the sending device to suspend the transmission until the data in the buffers has been processed. In IBM networks, this technique is called pacing.

flowspec

In IPv6, the traffic parameters of a stream of IP packets between two applications. See also IPv6.

FLT

Full Line Terminal. Multiplexer that terminates a SONET span. See also SONET.

FM

Frequency modulation. Modulation technique in which signals of different frequencies represent different data values. Compare to AM and PAM. See also

modulation.

FNC

Federal Networking Council. Group responsible for assessing and coordinating U.S. Federal agency networking policies and needs.

FOIRL

Fiber-optic interrepeater link. Fiber-optic signaling methodology based on the IEEE 802.3 fiberoptic specification. FOIRL is a precursor of the

10BaseFL specification, which is designed to replace it.

See also 10BaseFL.

format indicator 0

See FID0.

format indicator 1

See FID1.

format indicator 2

See FID2.

format indicator 3

See FID3.

format indicator 4

See FID4.

forward channel

Communications path carrying information from the call initiator to the called party.

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forward delay interval

Amount of time an interface spends listening for topology change information after that interface has been activated for bridging and before forwarding actually begins.

forward explicit congestion notification

See FECN.

forwarding

Process of sending a frame toward its ultimate destination by way of an internetworking device.

FOTS

Fiber Optics Transmission Systems. Vendorproprietary fiber-optic transmission equipment.

Fourier transform

Technique used to evaluate the importance of various frequency cycles in a time series pattern.

four-part dotted notation

See dot address.

FQDN

Fully qualified domain name. FQDN is the full name of a system, rather than just its host name.

For example, aldebaran is a hostname and aldebaran.interop.com is an FQDN.

fractional T1

See channelized T1.

FRAD Frame Relay access device. Any network device that provides a connection between a LAN and a Frame

Relay WAN. See also Cisco FRAD (Cisco Frame Relay

access device) and FRAS (Frame Relay access support) in

the “Cisco Systems Terms and Acronyms” section.

fragment

Piece of a larger packet that has been broken down to smaller units.

fragmentation

Process of breaking a packet into smaller units when transmitting over a network medium that cannot support the original size of the packet. See also reassembly.

frame

Logical grouping of information sent as a Data

Link layer unit over a transmission medium. Often refers to the header and trailer, used for synchronization and error control, that surround the user data contained in the unit. The terms cell, datagram, message, packet, and segment are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles.

frame check sequence

See FCS.

frame forwarding

Mechanism by which frame-based traffic, such as HDLC and SDLC, traverses an ATM network.

Frame Relay

Industry-standard, switched Data Link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices.

Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement. See also X.25.

Frame Relay access device

See FRAD.

Frame Relay access support

See FRAS in the “Cisco

Systems Terms and Acronyms” section.

Frame Relay bridging

Bridging technique, described in RFC 1490, that uses the same spanning-tree algorithm as other bridging functions, but allows packets to be encapsulated for transmission across a Frame Relay network.

frame switch

See LAN switch.

FRAS

See FRAS (Frame Relay access support) in the

“Cisco Systems Terms and Acronyms” section.

FRASM

Frame Relay access service module.

freenet Community-based bulletin board system with email, information services, interactive communications, and conferencing.

free-trade zone

Part of an AppleTalk internetwork that is accessible by two other parts of the internetwork that are unable to directly access one another.

frequency

Number of cycles, measured in hertz, of an alternating current signal per unit time.

frequency-division multiplexing

See FDM.

frequency modulation

See FM.

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front end Node or software program that requests services of a back end. See also back end, client, and server.

front-end processor

See FEP.

FSIP

See FSIP in the “Cisco Systems Terms and

Acronyms” section.

FST

See FST in the “Cisco Systems Terms and

Acronyms” section.

FTAM

File Transfer, Access, and Management. In

OSI, an Application layer protocol developed for network file exchange and management between diverse types of computers.

FTP File Transfer Protocol. Application protocol, part of the TCP/IP protocol stack, used for transferring files between network nodes. FTP is defined in RFC 959.

full duplex

Capability for simultaneous data transmission between a sending station and a receiving station. Compare to half duplex and simplex.

full mesh

Term describing a network in which devices are organized in a mesh topology, with each network node having either a physical circuit or a virtual circuit connecting it to every other network node.

A full mesh provides a great deal of redundancy, but because it can be prohibitively expensive to implement, it is usually reserved for network backbones. See also

mesh and partial mesh.

fully qualified domain name

See FQDN.

Fuzzball

Digital Equipment Corporation LSI-11 computer system running IP gateway software. The

NSFnet used these systems as backbone packet switches.

G

G.703/G.704

ITU-T electrical and mechanical specifications for connections between telephone company equipment and DTE using BNC connectors and operating at E1 data rates.

G.804

ITU-T framing standard that defines the mapping of ATM cells into the physical medium.

gateway

In the IP community, an older term referring to a routing device. Today, the term router is used to describe nodes that perform this function, and gateway refers to a special-purpose device that performs an Application layer conversion of information from one protocol stack to another. Compare to router.

Gateway Discovery Protocol

See GDP in the “Cisco

Systems Terms and Acronyms” section.

gateway host

In SNA, a host node that contains a gateway SSCP.

gateway NCP

NCP that connects two or more SNA networks and performs address translation to allow cross-network session traffic.

Gateway-to-Gateway Protocol

See GGP.

GB

Gigabyte. Approximately 1,000,000,000 bytes.

GBps

Gigabytes per second.

Gb

Gigabit. Approximately 1,000,000,000 bits.

Gbps

Gigabits per second.

GCAC

Generic connection admission control. In

ATM, a PNNI algorithm designed for CBR and VBR connections. Any node can use GCAC to calculate the expected CAC behavior of another node given than node’s advertised link metrics and the QoS of a connection setup request. See also CAC.

GCRA

Generic cell rate algorithm. In ATM, an algorithm that defines conformance with respect to the traffic contract of the connection. For each cell arrival, the GCRA determines whether the cell conforms to the traffic contract.

GDP

See GDP in the “Cisco Systems Terms and

Acronyms” section.

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generic connection admission control

See GCAC.

generic routing encapsulation

See GRE in the

“Cisco Systems Terms and Acronyms” section.

Get Nearest Server

See GNS.

GGP

Gateway-to-Gateway Protocol. MILNET protocol specifying how core routers (gateways) should exchange reachability and routing information. GGP uses a distributed shortest-path algorithm.

GHz

Gigahertz.

gigabit

Abbreviated Gb.

gigabits per second

Abbreviated Gbps.

gigabyte

Abbreviated GB.

gigabytes per second

Abbreviated GBps.

gigahertz

Abbreviated GHz.

GIX

Global Internet eXchange. Common routing exchange point that allows pairs of networks to implement agreed-upon routing policies. The GIX is intended to allow maximum connectivity to the

Internet for networks all over the world. See CIX, FIX, and MAE.

gleaning

Process by which a router automatically derives AARP table entries from incoming packets.

Gleaning speeds up the process of populating the

AARP table. See also AARP.

GNS

Get Nearest Server. Request packet sent by a client on an IPX network to locate the nearest active server of a particular type. An IPX network client issues a GNS request to solicit either a direct response from a connected server or a response from a router that tells it where on the internetwork the service can be located.

GNS is part of the IPX SAP. See also IPX and SAP

(Service Advertising Protocol).

goodput

Generally referring to the measurement of actual data successfully transmitted from the sender(s) to receiver(s). This is often a more useful measurement than the number of ATM cells per second throughput of an ATM switch if that switch is experiencing cell loss that results in many incomplete, and therefore unusable, frames arriving at the recipient.

Gopher

Distributed document delivery system. The

Internet Gopher allows a neophyte user to access various types of data residing on multiple hosts in a seamless fashion.

GOSIP

Government OSI Profile. U.S. Government procurement specification for OSI protocols. Through

GOSIP, the government mandates that all federal agencies standardize on OSI and implement OSI-based systems as they become commercially available.

Government OSI Profile

See GOSIP.

grade of service

Measure of telephone service quality based on the probability that a call will encounter a busy signal during the busiest hours of the day.

graphical user interface

See GUI.

GRE

See GRE in the “Cisco Systems Terms and

Acronyms” section.

ground station

Collection of communications equipment designed to receive signals from (and usually transmit signals to) satellites. Also called a downlink station.

group address

See multicast address.

group delay

See distortion delay.

guard band

Unused frequency band between two communications channels that provides separation of the channels to prevent mutual interference.

GUI

Graphical user interface. User environment that uses pictorial as well as textual representations of the input and output of applications and the hierarchical or other data structure in which information is stored.

Conventions such as buttons, icons, and windows are typical, and many actions are performed using a pointing device (such as a mouse). Microsoft Windows and the Apple Macintosh are prominent examples of platforms using a GUI.

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H

H.320

Suite of ITU-T standard specifications for videoconferencing over circuit-switched media such as

ISDN, fractional T-1, or switched-56 lines.

H.323

Extension of ITU-T standard H.320 that enables videoconferencing over LANs and other packetswitched networks, as well as video over the Internet.

half duplex

Capability for data transmission in only one direction at a time between a sending station and a receiving station. BSC is an example of a half-duplex protocol. See also BSC. Compare to full duplex and

simplex.

handshake

Sequence of messages exchanged between two or more network devices to ensure transmission synchronization.

hardware address

See MAC address.

HBD3

Line code type used on E1 circuits.

H channel

high-speed channel. Full-duplex ISDN primary rate channel operating at 384Kbps. Compare to B channel, D channel, and E channel.

HDLC

High-Level Data Link Control. Bit-oriented synchronous Data Link layer protocol developed by

ISO. Derived from SDLC, HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums. See also SDLC.

HDSL

High-data-rate digital subscriber line. One of four DSL technologies. HDSL delivers 1.544Mbps of bandwidth each way over two copper twisted pairs.

Because HDSL provides T1 speed, telephone companies have been using HDSL to provision local access to

T1 services whenever possible. The operating range of

HDSL is limited to 12,000 feet (3658.5 meters), so signal repeaters are installed to extend the service.

HDSL requires two twisted pairs, so it is deployed primarily for PBX network connections, digital loop carrier systems, interexchange POPs, Internet servers, and private data networks. Compare to ADSL, SDSL, and VDSL.

headend

End point of a broadband network. All stations transmit toward the headend; the headend then transmits toward the destination stations.

header

Control information placed before data when encapsulating that data for network transmission.

Compare to trailer. See also PCI.

heartbeat

See SQE.

HEC

Header error control. Algorithm for checking and correcting an error in an ATM cell. Using the fifth octet in the ATM cell header, ATM equipment will check for an error and correct the contents of the header. The check character is calculated using a CRC algorithm allowing a single bit error in the header to be corrected or multiple errors to be detected.

HELLO

Interior routing protocol used principally by

NSFnet nodes. HELLO allows particular packet switches to discover minimal delay routes. Not to be confused with the Hello protocol.

hello packet Multicast packet that is used by routers for neighbor discovery and recovery. Hello packets also indicate that a client is still operating and network-ready.

Hello protocol

Protocol used by OSPF systems for establishing and maintaining neighbor relationships.

Not to be confused with HELLO.

HEPnet

High-Energy Physics Network. Research network that originated in the United States, but that has spread to most places involved in high-energy physics. Well-known sites include Argonne National

Laboratory, Brookhaven National Laboratory, Lawrence

Berkeley Laboratory, and the SLAC.

hertz

Measure of frequency. Abbreviated Hz.

Synonymous with cycles per second.

heterogeneous network

Network consisting of dissimilar devices that run dissimilar protocols and in many cases support dissimilar functions or applications.

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HFC

Hybrid fiber-coaxial. Technology being developed by the cable TV industry to provide two-way, high-speed data access to the home using a combination of fiber optics and traditional coaxial cable.

hierarchical addressing

Scheme of addressing that uses a logical hierarchy to determine location. For example, IP addresses consist of network numbers, subnet numbers, and host numbers, which IP routing algorithms use to route the packet to the appropriate location. Compare to flat addressing.

hierarchical routing

The complex problem of routing on large networks can be simplified by reducing the size of the networks. This is accomplished by breaking a network into a hierarchy of networks, where each level is responsible for its own routing.

High-Energy Physics Network

See HEPnet.

High-Level Data Link Control

See HDLC.

High-Performance Computing and

Communications

See HPCC.

High-Performance Computing Systems

See HPCS.

High-Performance Parallel Interface

See HIPPI.

High-Performance Routing

See HPR.

High-Speed Communications Interface

See HSCI in the “Cisco Systems Terms and Acronyms” section.

High-Speed Serial Interface

See HSSI.

highway

See bus.

HIP

See HIP in the “Cisco Systems Terms and

Acronyms” section.

HIPPI

High-Performance Parallel Interface. Highperformance interface standard defined by ANSI.

HIPPI is typically used to connect supercomputers to peripherals and other devices.

holddown

State into which a route is placed so that routers will neither advertise the route nor accept advertisements about the route for a specific length of time (the holddown period). Holddown is used to flush bad information about a route from all routers in the network. A route is typically placed in holddown when a link in that route fails.

homologation

Conformity of a product or specification to international standards, such as ITU-T, CSA,

TUV, UL, or VCCI. Enables portability across company and international boundaries.

hop

Passage of a data packet between two network nodes (for example, between two routers). See also

hop count.

hop count

Routing metric used to measure the distance between a source and a destination. RIP uses hop count as its sole metric. See also hop and RIP.

host Computer system on a network. Similar to node, except that host usually implies a computer system, whereas node generally applies to any networked system, including access servers and routers. See also node.

host address

See host number.

host name

Name given to a machine. See also

FQDN.

host node

SNA subarea node that contains an SSCP.

See also SSCP.

host number

Part of an IP address that designates which node on the subnetwork is being addressed. Also called a host address.

Hot Standby Router Protocol

See HSRP in the

“Cisco Systems Terms and Acronyms” section.

hot swapping

See OIR and power-on servicing.

HPCC

High-Performance Computing and

Communications. U.S. Government-funded program advocating advances in computing, communications, and related fields. The HPCC is designed to ensure

U.S. leadership in these fields through education, research and development, industry collaboration, and implementation of high-performance technology.

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See also the five components of the HPCC: ASTA,

BRHR, HPCS, IITA, and NREN.

HPCS

High-Performance Computing Systems.

Component of the HPCC program designed to ensure

U.S. technological leadership in high-performance computing through research and development of computing systems and related software. See also HPCC.

HPR

High-Performance Routing. Second-generation routing algorithm for APPN. HPR provides a connectionless layer with nondisruptive routing of sessions around link failures, and a connection-oriented layer with end-to-end flow control, error control, and sequencing. Compare to ISR. See also APPN.

HSCI

See HSCI in the “Cisco Systems Terms and

Acronyms” section.

HSRP

See HSRP in the “Cisco Systems Terms and

Acronyms” section.

HSSI

High-Speed Serial Interface. Network standard for high-speed (up to 52Mbps) serial connections over

WAN links.

HSSI Interface Processor

See HIP in the “Cisco

Systems Terms and Acronyms” section.

HTML

Hypertext Markup Language. Simple hypertext document formatting language that uses tags to indicate how a given part of a document should be interpreted by a viewing application, such as a Web browser. See also hypertext and browser.

HTTP

Hypertext Transfer Protocol. The protocol used by Web browsers and Web servers to transfer files, such as text and graphic files.

hub

1. Generally, a term used to describe a device

that serves as the center of a star-topology network. 2.

Hardware or software device that contains multiple independent but connected modules of network and internetwork equipment. Hubs can be active (where they repeat signals sent through them) or passive

(where they do not repeat, but merely split, signals sent

I

through them). 3. In Ethernet and IEEE 802.3, an

Ethernet multiport repeater, sometimes called a concentrator.

hybrid network

Internetwork made up of more than one type of network technology, including LANs and

WANs.

hyperlink

Pointer within a hypertext document that points (links) to another document, which may or may not also be a hypertext document.

hypertext

Electronically stored text that allows direct access to other texts by way of encoded links.

Hypertext documents can be created using HTML, and often integrate images, sound, and other media that are commonly viewed using a browser. See also HTML and

browser.

Hypertext Markup Language

See HTML.

Hypertext Transfer Protocol

See HTTP.

Hz

See hertz.

IAB

Internet Architecture Board. Board of internetwork researchers who discuss issues pertinent to

Internet architecture. Responsible for appointing a variety of Internet-related groups such as the IANA,

IESG, and IRSG. The IAB is appointed by the trustees of the ISOC. See also IANA, IESG, IRSG, and ISOC.

IAHC

Internet International Ad Hoc Committee.

Coalition of participants from the broad Internet community, working to satisfy the requirement for enhancements to the Internet’s global DNS.

Organizations naming members to the committee include Internet Society (ISOC), Internet Assigned

Numbers Authority (IANA), Internet Architecture

Board (IAB), Federal Networking Council (FNC),

International Telecommunication Union (ITU),

International Trademark Association (INTA), and

World Intellectual Property Organization (WIPO).

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IANA

Internet Assigned Numbers Authority.

Organization operated under the auspices of the ISOC as a part of the IAB. IANA delegates authority for IP address-space allocation and domain-name assignment to the InterNIC and other organizations. IANA also maintains a database of assigned protocol identifiers used in the TCP/IP stack, including autonomous system numbers. See also IAB, ISOC, and InterNIC.

ICD

International Code Designator. One of two

ATM address formats developed by the ATM Forum for use by private networks. Adapted from the subnetwork model of addressing in which the ATM layer is responsible for mapping Network layer addresses to

ATM addresses. Compare to DCC.

ICMP

Internet Control Message Protocol. Network layer Internet protocol that reports errors and provides other information relevant to IP packet processing.

Documented in RFC 792.

ICMP Router Discovery Protocol

See IRDP.

ICR

Initial cell rate.

I-D

Internet-Draft. Working documents of the IETF, from its Areas and Working Groups. They are valid for a maximum of six months and might be updated, replaced, or obsoleted by other documents at any time.

Very often, I-Ds are precursors to RFCs.

IDI

Initial domain identifier. Portion of an NSAP or

NSAP-format ATM address that specifies the address allocation and administration authority. See also NSAP.

IDN

International Data Number. See also X.121.

IDP

Initial domain part. Part of a CLNS address that contains an authority and format identifier, and a domain identifier.

IDPR

Interdomain Policy Routing. Interdomain routing protocol that dynamically exchanges policies between autonomous systems. IDPR encapsulates interautonomous system traffic and routes it according to the policies of each autonomous system along the path.

IDPR is currently an IETF proposal. See also policy

routing.

IDRP

IS-IS Interdomain Routing Protocol. OSI protocol that specifies how routers communicate with routers in different domains.

IEC

International Electrotechnical Commission.

Industry group that writes and distributes standards for electrical products and components.

IEEE

Institute of Electrical and Electronics

Engineers. Professional organization whose activities include the development of communications and network standards. IEEE LAN standards are the predominant LAN standards today.

IEEE 802.1

IEEE specification that describes an algorithm that prevents bridging loops by creating a spanning tree. The algorithm was invented by Digital

Equipment Corporation. The Digital algorithm and the IEEE 802.1 algorithm are not exactly the same, nor are they compatible. See also spanning tree, spanning-

tree algorithm, and Spanning-Tree Protocol.

IEEE 802.12

IEEE LAN standard that specifies the

Physical layer and the MAC sublayer of the Data Link layer. IEEE 802.12 uses the demand priority mediaaccess scheme at 100Mbps over a variety of physical media. See also 100VG-AnyLAN.

IEEE 802.2

IEEE LAN protocol that specifies an implementation of the LLC sublayer of the Data Link layer. IEEE 802.2 handles errors, framing, flow control, and the Network layer (Layer 3) service interface.

Used in IEEE 802.3 and IEEE 802.5 LANs. See also

IEEE 802.3 and IEEE 802.5.

IEEE 802.3

IEEE LAN protocol that specifies an implementation of the Physical layer and the MAC sublayer of the Data Link layer. IEEE 802.3 uses

CSMA/CD access at a variety of speeds over a variety

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Physical variations of the original IEEE 802.3 specification include 10Base2, 10Base5, 10BaseF, 10BaseT, and

10Broad36. Physical variations for Fast Ethernet include 100BaseT, 100BaseT4, and 100BaseX.

IEEE 802.4

IEEE LAN protocol that specifies an implementation of the Physical layer and the MAC sublayer of the Data Link layer. IEEE 802.4 uses token-passing access over a bus topology and is based on the token bus LAN architecture. See also token bus.

IEEE 802.5

IEEE LAN protocol that specifies an implementation of the Physical layer and MAC sublayer of the Data Link layer. IEEE 802.5 uses token passing access at 4 or 16Mbps over STP cabling and is similar to IBM Token Ring. See also Token Ring.

IEEE 802.6

IEEE MAN specification based on

DQDB technology. IEEE 802.6 supports data rates of

1.5 to 155Mbps. See also DQDB.

IEPG Internet Engineering Planning Group. Group, primarily composed of Internet service operators, whose goal is to promote a globally coordinated Internet operating environment. Membership is open to all.

IESG

Internet Engineering Steering Group.

Organization, appointed by the IAB, that manages the operation of the IETF. See also IAB and IETF.

IETF

Internet Engineering Task Force. Task force consisting of over 80 working groups responsible for developing Internet standards. The IETF operates under the auspices of ISOC. See also ISOC.

IFIP

International Federation for Information

Processing. Research organization that performs OSI prestandardization work. Among other accomplishments, IFIP formalized the original MHS model. See also M