CCNP Routing Study Guide

CCNP Routing Study Guide
CCNP™
Routing
Study Guide
Todd Lammle
Sean Odom
with Kevin Wallace
San Francisco • Paris • Düsseldorf • Soest • London
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Associate Publisher: Neil Edde
Contracts and Licensing Manager: Kristine O’Callaghan
Acquisitions and Developmental Editor: Jeff Kellum
Editor: Linda Recktenwald
Production Editor: Molly Glover
Technical Editor: Eric Quinn
Book Designer: Bill Gibson
Graphic Illustrators: Tony Jonick, Jerry Williams!
Electronic Publishing Specialist: Nila Nichols
Proofreaders: Nancy Riddiough, Nanette Duffy, Yariv Rabinovitch, Jennifer Campbell
Indexer: Ted Laux
CD Coordinator: Kara Eve Schwartz
CD Technicians: Keith McNeil, Siobhan Dowling
Cover Designer: Archer Design
Cover Photographer: Tony Stone Images
Copyright © 2001 SYBEX Inc., 1151 Marina Village Parkway, Alameda, CA 94501. World rights reserved. The author(s)
created reusable code in this publication expressly for reuse by readers. Sybex grants readers limited permission to reuse the
code found in this publication or its accompanying CD-ROM so long as (author(s)) are attributed in any application containing the reusabe code and the code itself is never distributed, posted online by electronic transmission, sold, or commercially exploited as a stand-alone product. Aside from this specific exception concerning reusable code, no part of this
publication may be stored in a retrieval system, transmitted, or reproduced in any way, including but not limited to photocopy, photograph, magnetic, or other record, without the prior agreement and written permission of the publisher.
Library of Congress Card Number: 00-109137
ISBN: 0-7821-2712-6
SYBEX and the SYBEX logo are either registered trademarks or trademarks of SYBEX Inc. in the United States and/or other
countries.
Screen reproductions produced with FullShot 99. FullShot 99 © 1991-1999 Inbit Incorporated. All rights reserved.
FullShot is a trademark of Inbit Incorporated.
The CD interface was created using Macromedia Director, © 1994, 1997-1999 Macromedia Inc. For more information on
Macromedia and Macromedia Director, visit http://www.macromedia.com.
This study guide and/or material is not sponsored by, endorsed by or affiliated with Cisco Systems, Inc. Cisco ®, Cisco Systems ®, CCDA , CCNA , CCDP , CCNP , CCIE , CCSI , the Cisco Systems logo and the CCIE logo are trademarks or registered trademarks of Cisco Systems, Inc. in the United States and certain other countries. All other trademarks
are trademarks of their respective owners.
TRADEMARKS: SYBEX has attempted throughout this book to distinguish proprietary trademarks from descriptive terms
by following the capitalization style used by the manufacturer.
The author and publisher have made their best efforts to prepare this book, and the content is based upon final release software whenever possible. Portions of the manuscript may be based upon pre-release versions supplied by software manufacturer(s). The author and the publisher make no representation or warranties of any kind with regard to the completeness
or accuracy of the contents herein and accept no liability of any kind including but not limited to performance, merchantability, fitness for any particular purpose, or any losses or damages of any kind caused or alleged to be caused directly or
indirectly from this book.
Manufactured in the United States of America
10 9 8 7 6 5 4 3 2 1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Software License Agreement: Terms and Conditions
The media and/or any online materials accompanying this
book that are available now or in the future contain programs and/or text files (the "Software") to be used in connection with the book. SYBEX hereby grants to you a
license to use the Software, subject to the terms that follow.
Your purchase, acceptance, or use of the Software will constitute your acceptance of such terms.
The Software compilation is the property of SYBEX unless
otherwise indicated and is protected by copyright to SYBEX
or other copyright owner(s) as indicated in the media files
(the "Owner(s)"). You are hereby granted a single-user
license to use the Software for your personal, noncommercial use only. You may not reproduce, sell, distribute, publish, circulate, or commercially exploit the Software, or any
portion thereof, without the written consent of SYBEX and
the specific copyright owner(s) of any component software
included on this media.
In the event that the Software or components include specific
license requirements or end-user agreements, statements of
condition, disclaimers, limitations or warranties ("End-User
License"), those End-User Licenses supersede the terms and
conditions herein as to that particular Software component.
Your purchase, acceptance, or use of the Software will constitute your acceptance of such End-User Licenses.
By purchase, use or acceptance of the Software you further
agree to comply with all export laws and regulations of the
United States as such laws and regulations may exist from
time to time.
Reusable Code in This Book
The authors created reusable code in this publication
expressly for reuse for readers. Sybex grants readers permission to reuse for any purpose the code found in this publication or its accompanying CD-ROM so long as all three
authors are attributed in any application containing the
reusable code, and the code itself is never sold or commercially exploited as a stand-alone product.
Software Support
Components of the supplemental Software and any offers
associated with them may be supported by the specific
Owner(s) of that material but they are not supported by
SYBEX. Information regarding any available support may
be obtained from the Owner(s) using the information provided in the appropriate read.me files or listed elsewhere on
the media.
Should the manufacturer(s) or other Owner(s) cease to
offer support or decline to honor any offer, SYBEX bears
no responsibility. This notice concerning support for the
Software is provided for your information only. SYBEX is
not the agent or principal of the Owner(s), and SYBEX is in
no way responsible for providing any support for the Software, nor is it liable or responsible for any support provided, or not provided, by the Owner(s).
Warranty
SYBEX warrants the enclosed media to be free of physical
defects for a period of ninety (90) days after purchase. The
Software is not available from SYBEX in any other form or
media than that enclosed herein or posted to
www.sybex.com. If you discover a defect in the media during this warranty period, you may obtain a replacement of
identical format at no charge by sending the defective
media, postage prepaid, with proof of purchase to:
SYBEX Inc.
Customer Service Department
1151 Marina Village Parkway
Alameda, CA 94501
(510) 523-8233
Fax: (510) 523-2373
e-mail: [email protected]
WEB: HTTP://WWW.SYBEX.COM
After the 90-day period, you can obtain replacement media
of identical format by sending us the defective disk, proof
of purchase, and a check or money order for $10, payable
to SYBEX.
Disclaimer
SYBEX makes no warranty or representation, either
expressed or implied, with respect to the Software or its
contents, quality, performance, merchantability, or fitness
for a particular purpose. In no event will SYBEX, its distributors, or dealers be liable to you or any other party for
direct, indirect, special, incidental, consequential, or other
damages arising out of the use of or inability to use the Software or its contents even if advised of the possibility of such
damage. In the event that the Software includes an online
update feature, SYBEX further disclaims any obligation to
provide this feature for any specific duration other than the
initial posting.
The exclusion of implied warranties is not permitted by
some states. Therefore, the above exclusion may not apply
to you. This warranty provides you with specific legal
rights; there may be other rights that you may have that
vary from state to state. The pricing of the book with the
Software by SYBEX reflects the allocation of risk and limitations on liability contained in this agreement of Terms
and Conditions.
Shareware Distribution
This Software may contain various programs that are distributed as shareware. Copyright laws apply to both shareware and ordinary commercial software, and the copyright
Owner(s) retains all rights. If you try a shareware program
and continue using it, you are expected to register it. Individual programs differ on details of trial periods, registration, and payment. Please observe the requirements stated
in appropriate files.
Copy Protection
The Software in whole or in part may or may not be copyprotected or encrypted. However, in all cases, reselling or
redistributing these files without authorization is expressly
forbidden except as specifically provided for by the
Owner(s) therein.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
I would like to dedicate this book to all the hard-working staff at GlobalNet
Training, Inc.
Todd Lammle
I would like to dedicate this book to my family—Erin, Mikayla, Sean Jr., and
Hillary.
Sean Odom
To my daughters Stacie and Sabrina, who constantly remind me of the joy
found in learning new things, and to my wife, Vivian, an endless source of
encouragement, support, and love.
Kevin Wallace
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Acknowledgments
This book would not be on the shelf if it were not for the hard work and
dedication of the Sybex editing crew, especially Molly Glover and Jeff Kellum, who kept us all on track. Many thanks! My thanks also to Linda Recktenwald (editor), Eric Quinn (technical editor), Nila Nichols (EPS), Tony
Jonick and Jerry Williams! (illustrators), Nancy Riddiough, Nanette Duffy,
Yariv Rabinovitch, and Jennifer Campbell (proofreaders), and Ted Laux
(indexer).
T.L.
I need to thank Todd Lammle for trusting me to grace the pages of
another one of his books. It’s always exciting when you get to the acknowledgments because that means the book is almost finished. I must thank Erin
for putting up with me during the writing of this book. She is a wonderful
person who is as smart as she is good looking and puts up with a lot of extra
responsibility while I am working on books. I need to also thank some of
those who helped me in the writing process, such as Hanson Nottingham,
Doug Hammond, and John Turner, who have made me consider myself an
expert at BGP and EIGRP. And finally, I’d like to thank everyone at Sybex
who worked so hard at completing this project.
S.O.
I would like to thank John Swartz for introducing me to Todd Lammle
and thanks to Todd for all his encouragement and advice. My family also
deserves acknowledgment for their patience while I was secluded in my
office. Now that the book is finished, Daddy can get back to the really
important things like coloring pictures and working puzzles.
K.W.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
This book is intended to help you continue on your exciting new path
toward obtaining your CCNP and CCIE certification. Before reading this
book, it is important to have at least read the Sybex CCNA: Cisco Certified
Network Associate Study Guide, Second Edition. You can take the CCNP
tests in any order, but you should have passed the CCNA exam before pursuing your CCNP. Many questions in the Routing exam are built upon the
CCNA material. However, we have done everything possible to make sure
that you can pass the Routing exam by reading this book and practicing with
Cisco routers.
The new Cisco certifications reach beyond the popular certifications,
such as the MCSE and CNE, to provide you with an indispensable factor
in understanding today’s network—insight into the Cisco world of
internetworking.
Cisco—A Brief History
A lot of readers may already be familiar with Cisco and what it does. However, those of you who are new to the field just coming in fresh from your
MCSE, or maybe even with 10 or more years in the field but wishing to brush
up on the new technology, may appreciate a little background on Cisco.
In the early 1980s, a married couple who worked in different computer
departments at Stanford University started up cisco Systems (notice the small c).
Their names are Len and Sandy Bosack. They were having trouble getting
their individual systems to communicate (like many married people), so in
their living room they created a gateway server to make it easier for their disparate computers in two different departments to communicate using the IP
protocol.
In 1984, Cisco Systems was founded with a small commercial gateway
server product that changed networking forever. Some people think the
name was intended to be San Francisco Systems, but the paper got ripped on
the way to the incorporation lawyers—who knows? But in 1992, the company name was changed to Cisco Systems, Inc.
The first product it marketed was called the Advanced Gateway Server
(AGS). Then came the Mid-Range Gateway Server (MGS), the Compact
Gateway Server (CGS), the Integrated Gateway Server (IGS), and the AGS+.
Cisco calls these “the old alphabet soup products.”
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xviii
Introduction
In 1993, Cisco came out with the amazing 4000 router and then created
the even more amazing 7000, 2000, and 3000 series routers. These are still
around and evolving (almost daily, it seems).
Cisco Systems has since become an unrivaled worldwide leader in networking for the Internet. Its networking solutions can easily connect users
who work from diverse devices on disparate networks. Cisco products make
it simple for people to access and transfer information without regard to differences in time, place, or platform.
Cisco Systems’ big picture is that it provides end-to-end networking solutions that customers can use to build an efficient, unified information infrastructure of their own or to connect to someone else’s. This is an important
piece in the Internet/networking-industry puzzle because a common architecture that delivers consistent network services to all users is now a functional imperative. Because Cisco Systems offers such a broad range of
networking and Internet services and capabilities, users needing regular
access to their local network or the Internet can do so unhindered, making
Cisco’s wares indispensable.
Cisco answers this need with a wide range of hardware products that are
used to form information networks using the Cisco Internetworking Operating System (IOS) software. This software provides network services, paving the way for networked technical support and professional services to
maintain and optimize all network operations.
Along with the Cisco IOS, one of the services Cisco created to help support the vast amount of hardware it has engineered is the Cisco Certified
Internetworking Expert (CCIE) program, which was designed specifically to
equip people to effectively manage the vast quantity of installed Cisco networks. The business plan is simple: If you want to sell more Cisco equipment
and have more Cisco networks installed, ensure that the networks you
installed run properly.
However, having a fabulous product line isn’t all it takes to guarantee the
huge success that Cisco enjoys—lots of companies with great products are
now defunct. If you have complicated products designed to solve complicated problems, you need knowledgeable people who are fully capable of
installing, managing, and troubleshooting them. That part isn’t easy, so
Cisco began the CCIE program to equip people to support these complicated
networks. This program, known colloquially as the Doctorate of Networking, has also been very successful, primarily due to its extreme difficulty.
Cisco continuously monitors the program, changing it as it sees fit, to make
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
xix
sure that it remains pertinent and accurately reflects the demands of today’s
internetworking business environments.
Building upon the highly successful CCIE program, Cisco Career Certifications permit you to become certified at various levels of technical proficiency, spanning the disciplines of network design and support. So, whether
you’re beginning a career, changing careers, securing your present position,
or seeking to refine and promote your position, this is the book for you!
Cisco’s Installation and Support Certifications
Cisco has created new certifications that will help you get the coveted CCIE,
as well as aid prospective employers in measuring skill levels. Before these
new certifications, you took only one test and were then faced with the lab,
which made it difficult to succeed. With these new certifications that add a
better approach to preparing for that almighty lab, Cisco has opened doors
that few were allowed through before. So, what are these new certifications,
and how do they help you get your CCIE?
Cisco Certified Network Associate (CCNA) 2.0
The CCNA certification is the first certification in the new line of Cisco certifications and it is a precursor to all current Cisco certifications. With the
new certification programs, Cisco has created a type of stepping-stone
approach to CCIE certification. Now, you can become a Cisco Certified Network Associate for the meager cost of the Sybex CCNA: Cisco Certified
Network Associate Study Guide, Second Edition, plus $100 for the test. And
you don’t have to stop there—you can choose to continue with your studies
and achieve a higher certification called the Cisco Certified Network Professional (CCNP). Someone with a CCNP has all the skills and knowledge they
need to attempt the CCIE lab. However, because no textbook can take the
place of practical experience, we’ll discuss what else you need to be ready for
the CCIE lab shortly.
Cisco Certified Network Professional (CCNP) 2.0
This new Cisco certification has opened up many opportunities for the individual wishing to become Cisco-certified but who is lacking the training, the
expertise, or the bucks to pass the notorious and often failed two-day Cisco
torture lab. The new Cisco certifications will truly provide exciting new
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xx
Introduction
opportunities for the CNE and MCSE who just don’t know how to advance
to a higher level.
So, you’re thinking, “Great, what do I do after I pass the CCNA exam?”
Well, if you want to become a CCIE in Routing and Switching (the most popular certification), understand that there’s more than one path to that muchcoveted CCIE certification. The first way is to continue studying and become
a Cisco Certified Network Professional (CCNP). That means four more
tests, and the CCNA certification, to you.
The CCNP program will prepare you to understand and comprehensively
tackle the internetworking issues of today and beyond—not limited to the
Cisco world. You will undergo an immense metamorphosis, vastly increasing your knowledge and skills through the process of obtaining these certifications.
Remember that you don’t need to be a CCNP or even a CCNA to take the
CCIE lab, but to accomplish that, it’s extremely helpful if you already have
these certifications.
What Are the CCNP Certification Skills?
Cisco demands a certain level of proficiency for its CCNP certification. In
addition to those required for the CCNA, these skills include the following:
Installing, configuring, operating, and troubleshooting complex
routed LAN, routed WAN, and switched LAN networks, and Dial
Access Services.
Understanding complex networks, such as IP, IGRP, IPX, Async
Routing, AppleTalk, extended access lists, IP RIP, route redistribution, IPX RIP, route summarization, OSPF, VLSM, BGP, Serial, IGRP,
Frame Relay, ISDN, ISL, X.25, DDR, PSTN, PPP, VLANs, Ethernet,
ATM LAN-emulation, access lists, 802.10, FDDI, and transparent
and translational bridging.
To meet the Cisco Certified Network Professional requirements, you
must be able to perform the following:
Install and/or configure a network to increase bandwidth, quicken
network response times, and improve reliability and quality of service.
Maximize performance through campus LANs, routed WANs, and
remote access.
Improve network security.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
Create a global intranet.
Provide access security to campus switches and routers.
Provide increased switching and routing bandwidth—end-to-end
resiliency services.
Provide custom queuing and routed priority services.
xxi
How Do You Become a CCNP?
After becoming a CCNA, the four exams you must take to get your CCNP
are as follows:
Exam 640-503: Routing This exam continues to build on the fundamentals learned in the CCNA course. It focuses on large multiprotocol
internetworks and how to manage them with access lists, queuing, tunneling, route distribution, router maps, BGP, OSPF, and route summarization. This book covers everything you need to pass the new CCNP
Routing exam.
Exam 640-504: Switching This exam tests your knowledge of the 1900
and 5000 series of Catalyst switches. The Sybex CCNP: Switching Study
Guide covers all the objectives you need to understand for passing the
Switching exam.
Exam 640-506: Support This exam tests you on troubleshooting information. You must be able to troubleshoot Ethernet and Token Ring
LANS, IP, IPX, and AppleTalk networks, as well as ISDN, PPP, and
Frame Relay networks. The Sybex CCNP: Support Study Guide covers all
the exam objectives.
Exam 640-505: Remote Access This exam tests your knowledge of
installing, configuring, monitoring, and troubleshooting Cisco ISDN and
dial-up access products. You must understand PPP, ISDN, Frame Relay,
and authentication. The Sybex CCNP: Remote Access Study Guide covers all the exam objectives.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxii
Introduction
If you hate tests, you can take fewer of them by signing up for the CCNA exam
and the Support exam, and then take just one more long exam called the
Foundation R/S exam (640-509). Doing this also gives you your CCNP—but
beware, it’s a really long test that fuses all the material listed previously into
one exam. Good luck! However, by taking this exam, you get three tests for
the price of two, which saves you $100 (if you pass). Some people think it’s
easier to take the Foundation R/S exam because you can leverage the areas
that you would score higher in against the areas in which you wouldn't.
Remember that test objectives and tests can change at any time without
notice. Always check the Cisco Web site for the most up-to-date information
(www.cisco.com).
Cisco Certified Internetworking Expert (CCIE)
You’ve become a CCNP, and now you fix your sights on getting your CCIE
in Routing and Switching—what do you do next? Cisco recommends that
before you take the lab, you take test 640-025: Cisco Internetwork Design
(CID) and the Cisco authorized course called Installing and Maintaining
Cisco Routers (IMCR). By the way, no Prometric test for IMCR exists at the
time of this writing, and Cisco recommends a minimum of two years of onthe-job experience before taking the CCIE lab. After jumping those hurdles,
you then have to pass the CCIE-R/S Qualification exam (exam 350-001)
before taking the actual lab.
To become a CCIE, Cisco recommends the following:
1. Attend all the recommended courses at an authorized Cisco training
center and pony up around $15,000–$20,000, depending on your corporate discount.
2. Pass the Qualification exam ($200 per exam—so hopefully you’ll pass
it the first time).
3. Pass the two-day, hands-on lab at Cisco. This costs $1,000 per lab,
which many people fail two or more times. (Some never make it
through!) Also, because you can take the exam only in San Jose,
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
xxiii
California; Research Triangle Park, North Carolina; Sydney, Australia; Brussels, Belgium; Sao Paulo, Brazil; Beijing, China; Bangalore,
India; Tokyo, Japan; Seoul, Korea; Halifax, Nova Scotia; Singapore;
or Johannesburg, South Africa, you might just need to add travel costs
to that $1,000. Cisco has added new sites lately for the CCIE lab; it’s
best to check the Cisco Web site at http://www.cisco.com/warp/
public/625/ccie/exam_preparation/lab.html for the most current information.
The CCIE Skills
The CCIE Routing and Switching exam includes the advanced technical
skills that are required to maintain optimum network performance and reliability, as well as advanced skills in supporting diverse networks that use disparate technologies. CCIEs just don’t have problems getting a job. These
experts are basically inundated with offers to work for six-figure salaries!
But that’s because it isn’t easy to attain the level of capability that is mandatory for Cisco’s CCIE. For example, a CCIE will have the following skills
down pat:
Installing, configuring, operating, and troubleshooting complex
routed LAN, routed WAN, switched LAN, and ATM LANE networks, and Dial Access Services.
Diagnosing and resolving network faults.
Using packet/frame analysis and Cisco debugging tools.
Documenting and reporting the problem-solving processes used.
Having general LAN/WAN knowledge, including data encapsulation
and layering; windowing and flow control, and their relation to delay;
error detection and recovery; link-state, distance-vector, and switching algorithms; management, monitoring, and fault isolation.
Having knowledge of a variety of corporate technologies—including
major services provided by Desktop, WAN, and Internet groups—as
well as the functions, addressing structures, and routing, switching,
and bridging implications of each of their protocols.
Having knowledge of Cisco-specific technologies, including router/
switch platforms, architectures, and applications; communication
servers; protocol translation and applications; configuration commands and system/network impact; and LAN/WAN interfaces, capabilities, and applications.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxiv
Introduction
Designing, configuring, installing, and verifying voice over IP and
voice over ATM networks.
Cisco’s Network Design and Installation Certifications
In addition to the Network Installation and Support certifications, Cisco has
created another certification track for network designers. The two certifications within this track are the Cisco Certified Design Associate and Cisco
Certified Design Professional certifications. If you’re reaching for the CCIE
stars, we highly recommend the CCNP and CCDP certifications before
attempting the lab (or attempting to advance your career).
These certifications will give you the knowledge to design routed LAN,
routed WAN, and switched LAN and ATM LANE networks.
Cisco Certified Design Associate (CCDA)
To become a CCDA, you must pass the DCN (Designing Cisco Networks)
test (640-441). To pass this test, you must understand how to do the following:
Design simple routed LAN, routed WAN, and switched LAN and
ATM LANE networks.
Use Network-layer addressing.
Filter with access lists.
Use and propagate VLAN.
Size networks.
The Sybex CCDA: Cisco Certified Design Associate Study Guide is the most
cost-effective way to study for and pass your CCDA exam.
Cisco Certified Design Professional (CCDP) 2.0
If you’re already a CCNP and want to get your CCDP, you can simply take
the CID 640-025 test. If you’re not yet a CCNP, however, you must take the
CCDA, CCNA, Routing, Switching, Remote Access, and CID exams.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
xxv
CCDP certification skills include the following:
Designing complex routed LAN, routed WAN, and switched LAN
and ATM LANE networks
Building upon the base level of the CCDA technical knowledge
CCDPs must also demonstrate proficiency in the following:
Network-layer addressing in a hierarchical environment
Traffic management with access lists
Hierarchical network design
VLAN use and propagation
Performance considerations: required hardware and software; switching engines; memory, cost, and minimization
What Does This Book Cover?
This book covers everything you need to pass the CCNP Routing exam. It
teaches you how to configure and maintain Cisco routers in large internetwork. Each chapter begins with a list of the topics covered, related to the
CCNP Routing test, so make sure to read them over before working through
the chapter.
Chapter 1 covers the introduction to large internetworks and how to clear
up network congestion. This chapter also covers the Cisco three-layer model
and how to use that when designing and maintaining your large routed internetwork. The requirements needed to scale large internetworks are discussed
at the end of the chapter.
Chapter 2 covers the campus network and the basic fundamentals of routing. Both classful and classless routing are discussed, as well as the routing
protocols available with Cisco routers and the differences between them.
Chapter 3 covers advanced IP routing, including VLSM and route summarization. This is important information to understand before reading the
OSPF, EIGRP, and BGP chapters.
Chapter 4 covers Open Shortest Path First (OSPF) and how to configure
OSPF with Cisco routers.
Chapter 5 continues with OSPF, but with more advanced configurations,
such as multiple-area configurations.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxvi
Introduction
Chapter 6 introduces you to the Cisco Enhanced IGRP routing protocol.
This is a proprietary protocol designed by Cisco for large internetworks.
Chapter 7 introduces the Border Gateway Protocol and the terminology
used with it, as well as when to use and not use BGP in an internetwork.
Chapter 8 continues with our BGP discussion and shows how to configure
BGP with Cisco routers.
Chapter 9 is also a continuation of BGP and demonstrates how to scale
BGP to a large Cisco internetwork, including how to connect to two ISPs.
Chapter 10 ends the book with a detailed discussion on route optimization, including redistribution, controlling routing update traffic, and policybased routing.
Each chapter ends with review questions that are specifically designed to
help you retain the knowledge presented. To really nail down your skills,
read each question carefully, and, if possible, work through the hands-on
labs in some of the chapters.
Where Do You Take the Exam?
You may take the exams at any of the Sylvan Prometric or Virtual University
Enterprises (VUE) testing centers around the world. For the location of
a testing center near you, call Sylvan at (800) 755-3926 or VUE at (877)
404-3926. Outside of the United States and Canada, contact your local Sylvan Prometric Registration Center.
To register for a Cisco Certified Network Professional exam:
1. Determine the number of the exam you want to take. (The Routing
exam number is 640-503.)
2. Register with the nearest Sylvan Prometric or VUE testing center. At
this point, you will be asked to pay in advance for the exam. At the
time of this writing, the exams are $100 each and must be taken within
one year of payment. You can schedule exams up to six weeks in
advance or as soon as one working day prior to the day you wish to
take it. If something comes up and you need to cancel or reschedule
your exam appointment, contact the testing center at least 24 hours in
advance. Same-day registration isn’t available for the Cisco tests.
3. When you schedule the exam, you’ll get instructions regarding all
appointment and cancellation procedures, the ID requirements, and
information about the testing-center location.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
xxvii
Tips for Taking Your CCNP Exam
The CCNP Routing test contains about 60 questions to be completed in
about 75 minutes. However, understand that your test may vary.
Many questions on the exam have answer choices that at first glance look
identical—especially the syntax questions! Remember to read through the
choices carefully because “close doesn’t cut it.” If you put commands in the
wrong order or forget one measly character, you’ll get the question wrong.
So, to practice, do the hands-on exercises at the end of the chapters over and
over again until they feel natural to you.
Unlike Microsoft or Novell tests, the exam has answer choices that are
really similar in syntax—although some syntax is dead wrong, it is usually
just subtly wrong. Some other syntax choices may be right, but they’re
shown in the wrong order. Cisco does split hairs, and it is not at all averse
to giving you classic trick questions. Here’s an example:
access-list 101 deny ip any eq 23 denies Telnet access to all
systems.
This item looks correct because most people refer to the port number (23)
and think, “Yes, that’s the port used for Telnet.” The catch is that you can’t
filter IP on port numbers (only TCP and UDP). Another indicator is the use
of an extended access list number but no destination address or “any” for the
destination.
Also, never forget that the right answer is the Cisco answer. In many
cases, more than one appropriate answer is presented, but the correct answer
is the one that Cisco recommends.
Here are some general tips for exam success:
Arrive early at the exam center, so you can relax and review your
study materials.
Read the questions carefully. Don’t just jump to conclusions. Make
sure that you’re clear about exactly what each question asks.
Don’t leave any questions unanswered. They count against you.
When answering multiple-choice questions that you’re not sure about,
use a process of elimination to get rid of the obviously incorrect
answers first. Doing this greatly improves your odds if you need to
make an educated guess.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxviii
Introduction
As of this writing, the written exams still allow you to move forward
and backward. However, it is best to always check the Cisco Web site
before taking any exam to get the most up-to-date information.
After you complete an exam, you’ll get immediate, online notification of
your pass or fail status, a printed Examination Score Report that indicates
your pass or fail status, and your exam results by section. (The test administrator will give you the printed score report.) Test scores are automatically
forwarded to Cisco within five working days after you take the test, so you
don’t need to send your score to them. If you pass the exam, you’ll receive
confirmation from Cisco, typically within two to four weeks.
How to Use This Book
This book can provide a solid foundation for the serious effort of preparing
for the Cisco Certified Network Professional Routing exam. To best benefit
from this book, use the following study method:
1. Take the Assessment Test immediately following this Introduction.
(The answers are at the end of the test.) Carefully read over the explanations for any question you get wrong, and note which chapters the
material comes from. This information should help you plan your
study strategy.
2. Study each chapter carefully, making sure that you fully understand
the information and the test topics listed at the beginning of each
chapter. Pay extra-close attention to any chapter where you missed
questions in the Assessment Test.
3. Complete all hands-on exercises in the chapter, referring to the chap-
ter so that you understand the reason for each step you take. If you do
not have Cisco equipment available, make sure to study the examples
carefully. Also, check www.routersim.com for a router simulator.
Answer the review questions related to that chapter. (The answers
appear at the end of the chapter, after the review questions.)
4. Note the questions that confuse you, and study those sections of the
book again.
5. Take the Practice Exam in this book. You’ll find it in Appendix A. The
answers appear at the end of the exam.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
xxix
6. Before taking the exam, try your hand at the bonus practice exam that
is included on the CD that comes with this book. The questions in this
exam appear only on the CD. This will give you a complete overview
of what you can expect to see on the real thing.
7. Remember to use the products on the CD that is included with this
book. The electronic flashcards, the Boson Software utilities, and the
EdgeTest exam-preparation software have all been specifically picked
to help you study for and pass your exam. Study on the road with the
CCNP: Routing Study Guide eBook in PDF, and be sure to test yourself with the electronic flashcards.
The electronic flashcards can be used on your Windows computer or on your
Palm device.
8. Make sure you read the Key Terms list at the end of each chapter, and
Appendix B includes all the commands used in the book, along with
explanations for each command.
To learn all the material covered in this book, you’ll have to apply yourself regularly and with discipline. Try to set aside the same time period every
day to study, and select a comfortable and quiet place to do so. If you work
hard, you will be surprised at how quickly you learn this material. All the best!
What’s on the CD?
We worked hard to provide some really great tools to help you with your certification process. All of the following tools should be loaded on your workstation when studying for the test.
The EdgeTest for Cisco Routing Test-Preparation Software
Provided by EdgeTek Learning Systems, this test-preparation software prepares you to successfully pass the Routing exam. In this test engine you will
find all of the questions from the book, plus an additional Bonus Exam that
appears exclusively on the CD. You can take the Assessment Test, test yourself by chapter, take the Practice Exam that appears in the book or on
the CD.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxx
Introduction
To find more test-simulation software for all Cisco and NT exams, look
for the exam link on www.lammle.com and www.boson.com.
Electronic Flashcards for PC and Palm Devices
After you read the CCNP: Routing Study Guide, read the review questions
at the end of each chapter and study the practice exams included in the book
and on the CD. But wait, there’s more! Test yourself with the flashcards
included on the CD. If you can get through these difficult questions, and
understand the answers, you’ll know you’ll be ready for the CCNP Routing exam.
The flashcards include more than 100 questions specifically written to hit
you hard and make sure you are ready for the exam. Between the review
questions, practice exam, and flashcards, you’ll be more than prepared for
the exam.
CCNP: Routing Study Guide in PDF
Sybex is now offering the Cisco Certification books on CD so you can read
the book on your PC or laptop. The Dictionary of Networking and the
CCNP: Routing Study Guide are in Adobe Acrobat format. Acrobat Reader 4
with Search is also included on the CD.
This will be extremely helpful to readers who travel and don’t want to
carry a book, as well as to readers who find it more comfortable reading
from their computer.
Boson Software Utilities
Boson Software is an impressive company. It provides many services for free
to help you, the student. Boson has the best Cisco exam-preparation questions on the market, and at a very nice price. On the CD of this book, Boson
has provided for you the following:
IP Subnetter
Superping
System-Logging
Wildcard Mask Checker and Decimal-to-IP Calculator
Router GetPass
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Introduction
xxxi
CCNA Virtual Lab AVI Demo Files
The CCNA Virtual Lab e-trainer provides a router and switch simulator to
help you gain hands-on experience without having to buy expensive Cisco
gear. The demos are .avi files that you can play in RealPlayer, which is
included. The .avi demo files on the CD will help you gain an understanding
of the product features and the labs that the routers and switches can perform. Read more about the CCNA Virtual Lab e-trainer at http://
www.sybex.com/cgi-bin/rd_bookpg.pl?2728back.html. You can
upgrade this product at www.routersim.com.
How to Contact the Authors
You can reach Todd Lammle through Globalnet System Solutions, Inc.
(www.globalnettraining.com)—his training and systems integration
company in Colorado.
To contact Sean Odom, you can e-mail him at [email protected]
You can send e-mail to Kevin Wallace at [email protected]
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
1. What determines the Router ID used in OSPF virtual-link
configuration?
A. A router interface’s MAC address
B. The IP address of the first interface on a router
C. The lowest IP address configured on a router
D. The highest loopback IP address configured on a router
2. Which of the following protocols support VLSM routing? (Choose all
that apply.)
A. RIPv1
B. RIPv2
C. IGRP
D. EIGRP
3. What is the default-metric command used for?
A. It ensures proper metric conversion when redistributing routes
from the same routing protocols.
B. It ensures proper metric conversion when redistributing routes
from different protocols.
C. It changes the administrative weight of a route.
D. It changes the administrative distance of a route.
4. Which of the following are used specifically to break up collision
domains?
A. Repeaters
B. Routers
C. DLC
D. Switches
E. Bridges
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxxiv
Assessment Test
5. What does the MTU metric component indicate?
A. Mean Time Unit
B. Maximum Threshold Unspecified
C. Minimum TCP UNI
D. Maximum Transmission Unit
6. RIPv2 provides which of the following benefits over RIPv1?
A. RIPv2 is link-state.
B. RIPv2 uses a topology table.
C. RIPv2 supports VLSM.
D. RIPv2 uses Hello messages.
7. When an OSPF is not physically adjacent to the backbone area
(Area 0), which of the following offers a solution?
A. A virtual link
B. An NSSA
C. A Summary Link Advertisement
D. A Type 7 LSA
8. Choose the three layers that Cisco uses for building its hierarchical
internetwork model. (Choose all that apply.)
A. Fundamental
B. Distribution
C. IGRP
D. Core
E. Backbone
F. Access
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
xxxv
9. Which of the following pertain to link-state routing protocols?
(Choose all that apply.)
A. They use the Hello protocol to establish adjacencies.
B. They use several components to calculate the metric of a route.
C. Updates are sent only when changes occur in the network.
D. They are better protocols than distance-vector protocols.
10. Which command provides an EIGRP process to run on a Cisco router?
A. ip router eigrp autonomous-system-number
B. router ip eigrp autonomous-system-number
C. router eigrp process-id
D. router eigrp autonomous-system-number
11. Which of the following is the IOS command to set a router's priority?
A. ip ospf no-default priority_number
B. ip ospf no-summary priority_number
C. ip ospf priority priority_number
D. ip ospf-priority priority_number
12. What IP address represents a local loopback?
A. 127.0.0.2
B. 255.255.255.255
C. 127.1.0.0
D. 127.0.0.1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxxvi
Assessment Test
13. Which of the following algorithms is used by EIGRP to determine the
best path?
A. Open Shortest Path First Algorithm
B. Diffusing Update Algorithm
C. Distance-Vector Algorithm
D. Link-State Algorithm
E. Advanced Distance-Vector Algorithm
14. If a route advertised by EIGRP has a load metric of 100, approxi-
mately what percentage of the link is being utilized?
A. 2.5 percent
B. 25 percent
C. 39 percent
D. 100 percent
15. BGP uses which of the following TCP ports to open a session with
another BGP peer?
A. Port 20
B. Port 21
C. Port 179
D. Port 23
16. Which of the following describes the main purpose of the Core layer?
A. To distribute client-server router information
B. To provide an optimized and reliable transport structure
C. To provide access to various parts of the internetwork, as well as
to services
D. To provide access to corporate resources for a workgroup or users
on a local segment
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
xxxvii
17. Which protocols use a topology table? (Choose all that apply.)
A. EIGRP
B. IGRP
C. RIP1
D. OSPF
18. If you wanted to see the status of all BGP connections by using only
one IOS command, which one would that be?
A. show ip bgp
B. show ip bgp status
C. show ip bgp all
D. show ip bgp summary
19. What is the administrative distance of directly connected routes?
A. 0
B. 1
C. 90
D. 100
E. 110
20. What are the first two bits in the first byte that defines a Class B
network?
A. 00
B. 01
C. 10
D. 11
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xxxviii
Assessment Test
21. Which of the following identify the characteristics of a scalable
internetwork?
A. Reliability
B. Responsiveness
C. Efficiency
D. Adaptability
E. Accessibility
F. All of the above
22. Which of the following are ways of managing routes advertised by
BGP routers? (Choose four.)
A. Using route maps
B. Using prefix lists
C. Using distribute lists
D. Using path filters
E. Using re-distribution lists
23. When configuring an OSPF area as a totally stubby area, which of the
following routers need to be configured as totally stubby?
A. All routers in the area
B. Only the ABRs
C. Only the ASBRs
D. Only the internal routers
24. Which subnet mask will support 50 IP addresses?
A. 255.255.255.240
B. 255.255.255.248
C. 255.255.255.192
D. 255.255.255.224
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
xxxix
25. Which IOS command is used to clear all the entries in the BGP table?
A. clear ip route *
B. clear ip bgp *
C. clear route
D. reset bgp table
26. If the seq syntax is not used, in what sequence are numbers assigned
and in what increment?
A. 3 (3,6,9…)
B. 5 (5,10,15…)
C. 15 (15,30,45…)
D. 25 (25,50,75…)
27. Route summarization is best described in which of the following?
A. A router’s ability to take a group of subnetworks and summarize
them as one network advertisement
B. The Cisco IOS feature that permits serial interfaces to borrow an
IP address from another specified interface
C. The ability to tunnel IP address information inside an AURP
encapsulated frame
D. EIGRP’s ability to isolate discontiguous route advertisements from
one AS to another
28. When should BGP be used? (Choose all that apply.)
A. When multi-homing
B. When connecting multiple ISPs
C. When connecting routers within the same AS
D. When configuring backup links
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xl
Assessment Test
29. Which routing algorithm does OSPF use for route calculation?
A. Dijkstra
B. SPF
C. Link-state
D. Distance-vector
30. The IOS command show ip ospf process-id shows which of the
following?
A. The information contained in each OSPF packet, such as Router
ID and Area ID
B. Information about a router’s OSPF database, such as router link
states and network link states
C. Area information, such as the identification of the ABR
D. The status of a router’s virtual links
31. Which of the following are used in confederations?
A. iBGP
B. eBGP
C. Sub-ASes
D. Sequence numbers
E. Confederation identifier
32. When do DR/BDR elections occur? (Choose all that apply.)
A. When two routers are connected via point-to-point.
B. When multiple routers are connected via NBMA point-to-
multipoint.
C. When multiple routers are connected via broadcast multi-access.
D. When multiple routers are connected via NBMA broadcast.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
xli
33. What is the MED command used for in BGP?
A. Provide emergency medical updates on a BGP routing table
B. Provide Medium Extra Documentation on BGP attributes
C. Inform neighboring external AS routers as to which link to use to
receive traffic
D. Inform neighboring internal AS routers as to which link to use to
receive traffic
34. What parameter of IGRP and EIGRP must be the same if automatic
route redistribution is to take place?
A. process-id
B. area
C. metric
D. weight
35. Which IP address is used as the OSPF Router ID?
A. Highest IP address
B. Highest loopback IP address
C. Lowest IP address
D. Lowest loopback IP address
36. When a BGP peer tries to open a session with another endpoint, the
peer is in which of the following states?
A. Active state
B. Connection state
C. Open state
D. Established state
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xlii
Assessment Test
37. The multicast address 224.0.0.5 is assigned to which of the following?
A. AllOSPFRouters
B. AllDR
C. AllRouters
D. AllSPFRouters
38. Which of the following use Type 7 LSAs?
A. Stub areas
B. Not-so-stubby areas
C. Totally stubby areas
D. Internal routers
39. What is the purpose of a passive interface?
A. To stop unwanted route information from entering the specified
interface
B. To allow route information to be filtered by an access list
C. To allow routes to be sent out the specified interface, but deny
route information to enter the interface
D. To allow routes to enter the interface, but deny any route informa-
tion to exit the specified interface
40. How is a BGP session established between two routers?
A. Telnet
B. Hello packets
C. UDP (SYN, ACK, SYN)
D. TCP (SYN, ACK, SYN)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
xliii
41. The neighbor table is used to collect information on which of the
following?
A. Directly connected workstations
B. All routes through the network
C. Neighboring routers in other autonomous systems
D. All directly connected neighboring routers
42. How is the IANA involved in BGP?
A. They are not involved.
B. They assign your Internet security.
C. They provide the IP addresses you use.
D. They are responsible for assigning ASNs.
43. What is the purpose of the set clause in a route map?
A. To test traffic patterns against a specified access list
B. To change such routing parameters as default route
C. To create a specific traffic pattern for the match clause to act upon
D. To translate an entry to the internal port translation table
44. What BGP command syntax identifies the AS of the remote router that
the local router will initiate a session with?
A. remote-as
B. aggregate-paths
C. connect bgp-all
D. network as-10
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xliv
Assessment Test
45. BGP is a non-proprietary protocol. However, Cisco provides some
proprietary attributes. Which of the following is Cisco proprietary?
A. Weight attribute
B. Next-hop attribute
C. MED attribute
D. Atomic Aggregate attribute
46. Which of the following are multi-homing classifications for BGP?
A. Centralized
B. Basic
C. Medium
D. Full
E. Low
47. Which of the following describes the main purpose of the Distribution
layer?
A. To distribute client-server router information
B. To provide an optimized and reliable transport structure
C. To provide access to various parts of the internetwork, as well as
to services
D. To provide access to corporate resources for a workgroup or users
on a local segment
48. Why would a BGP router be called a non-client router?
A. A route reflector not participating in a route reflector cluster in an
iBGP network is a non-client router.
B. A route reflector participating in a route reflector cluster in an
iBGP network is a non-client router.
C. A route reflector not participating in a route reflector cluster is a
non-client router. It usually wouldn’t be the reflector itself.
D. A route reflector participating in a route reflector cluster is a non-
client router.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Assessment Test
xlv
49. What routing protocol is based on the work of Edsger Dijkstra?
A. RIP
B. IGRP
C. OSPF
D. EIGRP
50. Which of the following are considered link-state protocols? (Choose
all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. OSPF
F. IS-IS
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Assessment Test
1. D. In the IOS command area area-id virtual-link router-id,
the router-id is the highest loopback IP number configured on a
router. If a loopback interface has not been configured on the router,
then the router-id is the highest IP address configured on the router.
For more information, see Chapter 5.
2. B, D. VLSM is compatible only with classless routing protocols.
Classless routing protocols have the ability to carry subnet information in their route advertisements. RIPv1 and IGRP are classful,
whereas RIPv2 and EIGRP are classless. For more information, see
Chapter 3.
3. B. The default-metric command ensures proper metric conversion
when redistributing routes from different protocols. See Chapter 6 for
more information.
4. D, E. Even though routers do break up collision domains, only
bridges and switches are used specifically to break up collision
domains. See Chapter 1 for more information on segmentation of a
network.
5. D. The MTU size metric component is the Maximum Transmission
Unit (in bytes) over a specified interface. For example, the default
MTU size for an Ethernet interface is 1,500 bytes. For more information, see Chapter 10.
6. C. RIPv2 is still distance-vector and acts accordingly. However, it
sends prefix routing information in the route updates so it can support
VLSM. See Chapter 2 for more information on RIPv2.
7. A. When designing OSPF networks, each area within an OSPF rout-
ing process should have a link to the backbone area (Area 0). However, when an area is not physically adjacent to Area 0, a virtual link
can be used to connect across the transit area, which separates the area
from Area 0. For more information, see Chapter 5.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Assessment Test
xlvii
8. B, D, F. The Cisco three-layer model includes the Core, Distribution,
and Access layers. See Chapter 1 for more information on the Cisco
three-layer model.
9. A, C. Link-state protocols do not send entire routing table updates
like distance-vector protocols do. Link-state uses Hello messages to
make sure that neighbor routers are still alive, and then when a change
in the network does occur, it sends only the necessary information
about the change. See Chapter 2 for more information on the linkstate routing protocols.
10. D. The command router eigrp is used followed by the AS number
to implement EIGRP. You must then identify the attached networks
using the network command. See Chapter 6 for more information.
11. C. The IOS command to set a router’s priority is ip ospf priority
priority_number, where priority_number is a number from 0 to
255. See Chapter 4 for more information.
12. D. Network 127 is reserved for loopback purposes (e.g., for trouble-
shooting diagnostics). With a local loopback address, a host can send
a test packet to itself without generating network traffic. For more
information, see Chapter 3.
13. B. The Diffusing Update Algorithm (DUAL) is used to calculate
routes in EIGRP. See Chapter 6 for more information.
14. C. Values for the load metric range from 1 through 255. Therefore,
a load metric of 100 indicates an approximate load of 39 percent
(100/255 = 39.2). For more information, see Chapter 10.
15. C. Port 179 is used by BGP to establish a session with another BGP
peer. Ports 20 and 21 are used by FTP, and port 23 is used by Telnet.
For more information, see Chapter 7.
16. B. The Core layer should provide a fast transport between Distribu-
tion layer devices. See Chapter 1 for more information on the Cisco
three-layer model.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
xlviii
Answers to Assessment Test
17. A, D. EIGRP and OSPF both use a topology table to help maintain a
loop-free network. See Chapter 2 for more information on the use of
topology tables.
18. B. The show ip bgp status command displays the status of all BGP
connections. The show ip bgp summary command displays the
BGP configuration. The other two commands are not valid. For more
information, see Chapter 8.
19. A. Directly connected routes have an administrative distance of zero.
See Chapter 2 for more information on administrative distances.
20. C. A leading bit pattern of 0 indicates a Class A network. A leading
bit pattern of 10 indicates a Class B network. A leading bit pattern of
110 indicates a Class C network. For more information, see Chapter 3.
21. F. An internetwork should be reliable, responsive, efficient, adapt-
able, and accessible. See Chapter 1 for more information on scalable
internetworks.
22. A, B, C, D. There is no such item as a re-distribution list using BGP.
The others listed are all valid ways of manipulating routes advertised
by BGP. For more information, see Chapter 9.
23. B. When configuring an area as totally stubby, we are stopping sum-
mary Link State Advertisements from being injected into the area.
Therefore, the IOS router configuration command area area-id
stub no-summary only needs to be issued on the area border routers
(ABRs). However, all of the other routers within the area need to be
configured as stubby. Typically, an area will have only one ABR. For
more information, see Chapter 5.
24. C. The formula 2n – 2 = number of hosts (where n is the number of
host bits in the subnet mask) tells us how many hosts can be supported
for a particular subnet. For more information, see Chapter 3.
25. B. The clear ip bgp * command is used to clear all the entries in
the BGP table. For more information, see Chapter 8.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Assessment Test
xlix
26. B. Sequence numbers are assigned in increments of five when no
sequence number was assigned when the prefix list statements were
configured. See Chapter 9 for more information.
27. A. Route summarization, which works best with contiguous address
space, reduces the memory and processor burden on routers by representing multiple subnets in a single route advertisement. For more
information, see Chapter 3.
28. A, B. BGP should be used when multi-homing and when connecting
multiple ISPs. For more information, see Chapter 7.
29. A. SPF is the type of path created by the Dijkstra algorithm. See
Chapter 4 for more information.
30. C. The IOS command show ip ospf process-id shows area infor-
mation, such as the identity of the area border router (ABR) or autonomous system boundary router (ASBR). For more information, see
Chapter 5.
31. A, B, C, E. Confederations use iBGP on routers in sub-ASes and then
use eBGP to connect the sub-ASes. The sequence number is used in
prefix lists. The confederation identifier is the number assigned to all
the routers to identify that all the routers in the confederation using
sub-ASes reside in the same autonomous system. See Chapter 9 for
more information.
32. C, D. There must always a DR and a BDR for each multi-access seg-
ment. See Chapter 4 for more information.
33. C. The MED attribute is used to inform other external AS routers as
to which route to use in order to receive traffic. For more information,
see Chapter 8.
34. A. If the IGRP and EIGRP processes are both running on the same
router, their routes will be automatically redistributed if their
process-ids are equal. This is possible because IGRP and EIGRP use
very similar metrics. Note that in some of the literature, the processid may be referred to as an Autonomous System. For more information, see Chapter 10.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
l
Answers to Assessment Test
35. B. The highest IP address is used if no loopback interfaces are con-
figured. See Chapter 4 for more information.
36. B. This connection is in the Connection state until a message is sent
to identify each peer. When the connection is established, it transitions
to the Open state. Once the connection is accepted by the other peer,
the connection transitions to Established state. If the connection is
lost, possibly due to a version mismatch, the peer goes to the Active
state and actively tries to reestablish the connection using the proper
version properties. For more information, see Chapter 7.
37. D. AllOSPFRouters does not exist. See Chapter 4 for more
information.
38. B. Not-so-stubby areas (NSSAs) import external routes (Type 7 Link
State Advertisements) via route redistribution and then translate these
Type 7 LSAs into Type 5 LSAs. For more information, see Chapter 5.
39. D. Passive interfaces are used for such interfaces as BRI, where you
do not want to have routing updates sent out the interface. See Chapter 6 for more information.
40. D. A BGP session is established between two routers by using a TCP
SYN, TCP ACK, and then another TCP SYN. For more information,
see Chapter 7.
41. D. The neighbor table tracks all the directly connected routers run-
ning EIGRP. The table also tracks the smooth round-trip timer
(SRTT), the retransmission timer (RTO), and the hold timer, which
are all used by the neighbor table to track its neighboring routers. See
Chapter 6 for more information.
42. D. The Internet Assigned Numbers Authority (IANA) is responsible
for delegating autonomous system numbers. Other organizations may
assign numbers, but only if they are authorized by the IANA. See
Chapter 7 for more information.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Assessment Test
li
43. B. After a traffic pattern has been identified by a route map’s match
clause, the set clause sets route parameters such as next-hop address
or default route. For more information, see Chapter 10.
44. A. The remote-as syntax identifies the peer router that the local
router will enable a session with. The IP address identifies the interface
attached to the peer router. If the ASN is the same number as the internal ASN, it identifies an internal AS; if it is different, it identifies an
external AS. See Chapter 8 for more information.
45. A. The Weight attribute is a Cisco proprietary BGP attribute used as
a metric to find the best routes through the networks. See Chapter 8
for more information.
46. B, C, D. When you use multi-homing with only static routes, it is
considered a Basic classification. When you use static routes and BGP
learned routes, it is considered a Medium classification. When you use
only BGP learned routes, it is considered a Full classification. See
Chapter 9 for more information.
47. C. The Distribution layer connects Access layer devices together and
provides users with network service connections. See Chapter 1 for
more information on the Cisco three-layer model.
48. C. Routers not participating as a route reflector client are called non-
client routers. Non-client refers to any iBGP peer that is not participating in the route reflector cluster as a client. See Chapter 9 for more
information.
49. C. Sometimes referred to as the Dijkstra Algorithm, OSPF uses the
Shortest Path First Algorithm to generate its composite metric. For
more information, see Chapter 10.
50. E, F. Although EIGRP is really a hybrid routing protocol, it is con-
sidered an advanced distance-vector protocol, not link-state. See
Chapter 2 for more information on link-state protocols.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
1
Scaling Large
Internetworks
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
Describe causes of network congestion
List solutions for controlling network congestion
Describe the key requirements of a scalable internetwork
Select a Cisco IOS feature as a solution for a given internetwork
requirement
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
W
e’ll begin this book with a review of internetworks and a
discussion of the typical business requirements for their implementation in
today’s marketplace. This discussion will lead naturally into exploring the
ubiquitous but avoidable problem of network congestion. Examining both
its causes and the solutions for controlling it, we’ll describe the key requirements for a scalable internetwork. We’ll also look to the Cisco three-layer
model for the inherent solutions it provides and unveil helpful Cisco IOS features that will aid us in scaling large internetworks.
Internetworks
A
n internetwork is the communication structure that works to tie
LANs and WANs together. Its primary goal is to efficiently move information anywhere within a corporation quickly, upon demand, and with complete integrity. Today’s users have become increasingly dependent on their
networks—just make a group of users’ server or hub go offline and watch the
chaos that results around the office.
Where this has led—and what this means for corporations that want to
remain capable of competing in today’s global market—is that the networks
they depend on today have to efficiently manage, on a daily basis, some or
all of the following:
Graphics and imaging
Files in the gigabyte range
Client/server computing
High network traffic loads
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Clearing Up Network Congestion
3
To be able to amply meet these needs, the IS department must provide the
following to users:
More bandwidth
Bandwidth on demand
Low delays
Data, voice, and video capabilities on the same media
Also, the network of today must be adaptable in that it must be ready to
suit the applications of tomorrow. In the not-too-distant future, networks
will need to be equipped to handle
High-definition imaging
Full-motion video
Digitized audio
In short, for an internetwork to realize its purpose, it must be able to efficiently connect many different networks together to serve the organizations
that depend on it. This connectivity must happen regardless of the type of
physical media involved. Companies expanding their networks must overcome the limitations of physical and geographic boundaries. The Internet
has served as a model to facilitate this growth.
Clearing Up Network Congestion
With a combination of powerful workstations, audio and video to
the desktop, and network-intensive applications, 10Mbps Ethernet networks no longer offer enough bandwidth to fulfill the business requirements
of the typical large business.
As more and more users are connected to the network, an Ethernet network’s performance begins to wane as users fight for more bandwidth. As
when too many cars try to get onto a freeway at rush hour, this increased utilization causes an increase in network congestion as more users try to access
the existing network resources. Congestion causes users to scream for more
bandwidth. However, simply increasing bandwidth can’t always solve the
problem. A slow server CPU or insufficient memory on the workstations and
servers can also be the culprit, and these need to be considered as well.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
4
Chapter 1
Scaling Large Internetworks
One way to solve congestion problems and increase the networking performance of your LAN is to divide a single Ethernet segment into multiple
network segments, which maximizes the available bandwidth. Some of the
ways to do that are as follows:
Physical segmentation You can segment the network with bridges and
routers, thereby breaking up the collision and broadcast domains. This
minimizes packet collisions by decreasing the number of workstations on
the same physical network.
Network switching technology (microsegmentation) Like a bridge or
router, switches can also provide LAN segmentation capabilities. LAN
switches (for example, the Cisco Catalyst 5000) provide dedicated, pointto-point, packet-switched connections between their ports. Since this
allows simultaneous switching of packets between the ports in the switch,
it increases the amount of bandwidth open to each workstation.
Using full-duplex Ethernet devices Full-duplex Ethernet can provide
almost twice the bandwidth of traditional Ethernet networks. However,
for this to work, both the switch port and the network interface cards
(NICs) must be able to run in Full Duplex mode.
Using Fast or Gigabit Ethernet Using Fast Ethernet and gigabit
switches can provide up to 100 times the amount of bandwidth available
from 10BaseT.
It’s no surprise—reducing the number of users per collision domain
increases the bandwidth on your network segment. By keeping the traffic
local to the network segment, users have more bandwidth available to them
and enjoy a noticeably better response time than if there was simply one
large backbone in place.
Okay, now let’s explore some different ways to clear up nasty network
congestion problems:
Segmentation with bridges
Segmentation with routers
Segmentation with switches
Segmentation with a Bridge
A bridge can segment, or break up, your network into smaller, more manageable pieces. However, if it’s placed incorrectly in your network, it can
cause more harm than good.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Clearing Up Network Congestion
5
Bridges perform at the MAC sublayer of the Data Link layer. They create
both physical and logical separate network segments to reduce the traffic
load. There are solid advantages to bridging—by segmenting a logical network into multiple physical pieces, it secures network reliability, availability,
scalability, and manageability.
As Figure 1.1 shows, bridges work by examining the MAC or hardware
addresses in each frame and, only if necessary, forwarding the frame to the
other physical segments. These devices dynamically build a forwarding table
of information composed of each MAC address and the segment that
address is located on.
FIGURE 1.1
Segmentation with a bridge
Host 1
Host 3
Forwarding
Table
Forwarding
Table
Host Segment
1
1
2
1
3
1
4
2
5
2
6
2
Host Segment
4
2
5
2
6
2
7
3
8
3
9
3
Host 4
Host 6
Bridge
Host 7
Host 9
Bridge
Host 2
Host 5
Host 8
Segment #1
Segment #2
Segment #3
Now for the bad news…. A drawback to using bridges is that if the destination MAC address is unknown to the bridge, it will forward the frame to
all segments except the port from which it received the frame. Also, a 20–30
percent latency period can occur for the processing of frames. This delay can
increase significantly if the frame cannot be immediately forwarded due to
current activity on the destination segment.
Bridges will forward broadcast and multicast packets to all other segments to which they’re attached. Since, by default, the addresses from these
broadcasts are never seen by the bridge, and hence are not filtered, broadcast
storms can result. The same problem can happen with switches because, theoretically, switch ports are bridge ports. A Cisco switch is really a multiport
bridge that runs the Cisco IOS and performs the same functions as a bridge.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
6
Chapter 1
Scaling Large Internetworks
Segmentation with a Router
As you know, routers work at the Network layer and are used to route packets to destination networks. Routers use routing tables to make routing decisions. However, in the routing tables, routers keep information on how to
get to networks in their tables, not to hosts, using that information to route
packets through an internetwork. Routers use logical network addresses
instead of hardware addresses when making their routing decisions. They
maintain a routing table for each protocol on the network—a Cisco router
will keep a routing table for AppleTalk, a different one for IPX, and still
another for IP, as shown in Figure 1.2.
FIGURE 1.2
Routing tables are kept for each Network layer routing protocol.
Here are the pros regarding routers:
Manageability Multiple routing protocols give the network manager
who’s creating an internetwork a lot of flexibility.
Increased functionality Cisco routers provide features that address the
issues of flow, error and congestion control, fragmentation, reassembly,
and control over a packet’s lifetime.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Clearing Up Network Congestion
7
Multiple active paths Using path metrics, routers can make informed
routing decisions. This allows routers to have more than one active path
between networks. Multiple paths can provide load balancing, which provides more bandwidth to remote networks as well as redundancy.
To provide these advantages, routers must be more complex and more
software intensive than bridges. Routers provide a lower level of performance in terms of the number of frames or packets that can be processed
per unit.
Segmentation with LAN Switches
LAN switching is a great strategy for LAN segmentation. LAN switches
improve performance by employing Layer 2 frame switching, which permits
high-speed data exchange.
Just like bridges, switches use the destination MAC address to ensure that
the packet is forwarded to the right outgoing port. Cut-through switches
begin forwarding the packet before reception is complete, keeping latency to
a minimum. Store-and-forward switching receives the entire frame onto its
onboard buffers, runs a CRC, and then forwards the frame out the destination port.
There are three different switching-method terms:
Port configuration-switching Allows a port to be assigned to a physical
network segment under software control. It’s the simplest form of switching.
Frame-switching Increases available bandwidth on the network. Frameswitching allows multiple transmissions to occur in parallel. This is the
type of switching performed by all Catalyst switches.
Cell-switching (ATM) Uses small, fixed-length cells that are switched
on the network, similar to frame-switching. It’s the switching method
used by all Cisco Lightstream switches.
A LAN switch supplies you with considerably higher port density at a
lower cost than standard bridges. Since the largest benefit of LAN switches
is fewer users per segment, the average available bandwidth per user
increases. This fewer-users-per-segment trend is known as microsegmentation, which lets you create dedicated segments. When you have one user per
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
8
Chapter 1
Scaling Large Internetworks
segment, each one enjoys instant access to the full lot of available bandwidth
instead of competing for it with other users. Because of this, the collisions
that are common with shared, medium-sized networks that use hubs (halfduplex) just don’t happen.
A LAN switch bases the forwarding of frames on the frame’s Layer 2
address (Layer 2 LAN switch) or on the Layer 3 address of the packet (multilayer LAN switch). LAN switches are sometimes referred to as LAN frame
switches because they generally forward Layer 2 frames in contrast to an
ATM switch, which forwards cells. Do not confuse this with Frame Relay,
which is a WAN technology.
LAN switches uniquely support some very cool new features, including
the following:
Numerous, simultaneous conversations
High-speed data exchanges
Low latency and high frame-forwarding rates
Dedicated communication between devices
Full-duplex communication
Media rate adaptation (10,100, and 1000Mbps hosts can work on the
same network)
The ability to work with existing 802.3-compliant network interface
cards and cabling
Thanks to dedicated, collision-free communication between network
devices, file-transfer throughput is increased. Many conversations can occur
simultaneously by forwarding or switching several packets at the same time,
which expands the network capacity by the amount of supported conversations.
The Cisco Three-Layer Model
C
isco has created its own three-layer hierarchical model. The Cisco
hierarchical model is used to help you design, implement, and maintain a
scalable, reliable, cost-effective hierarchical internetwork. Cisco defines
three layers of hierarchy, as shown in Figure 1.3, each with specific functionality.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
The Cisco Three-Layer Model
FIGURE 1.3
9
The Cisco hierarchical model
Core
layer
Distribution
layer
Access
layer
The three layers are:
The Core layer
The Distribution layer
The Access layer
Each layer has specific responsibilities. Remember, however, that the
three layers are logical and not necessarily physical. Three layers do not necessarily mean three separate devices. Consider the OSI model, another logical
hierarchy. Its seven layers describe functions but not necessarily protocols,
right? Sometimes a protocol maps to more than one layer of the OSI model,
and sometimes multiple protocols communicate within a single layer. In the
same way, when we build physical implementations of hierarchical networks, we may have many devices in a single layer, or we might have a single
device performing functions at two layers. The definition of the layers is logical, not physical.
Before you learn about these layers and their functions, consider a common hierarchical design, as shown in Figure 1.4. The phrase “keep local traffic local” has almost become a cliché in the networking world. However, the
underlying concept has merit. Hierarchical design lends itself perfectly to fulfilling this concept.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
10
Chapter 1
Scaling Large Internetworks
FIGURE 1.4
Hierarchical network design
Core
layer
FDDI Ring
Distribution
layer
Access
layer
Workgroups
Users’ machines
Users’ machines
Users’ machines
Now, let’s take a closer look at each of the layers.
The Core Layer
The Core layer is literally the core of the network. At the top of the hierarchy, the Core layer is responsible for transporting large amounts of traffic
both reliably and quickly. The only purpose of the Core layer of the network
is to switch traffic as fast as possible. The traffic transported across the core is
common to a majority of users. However, remember that user data is processed at the Distribution layer, and the Distribution layer forwards the
requests to the core if needed.
If there is a failure in the core, every single user can be affected. Therefore,
fault tolerance at this layer is an issue. The core is likely to see large volumes
of traffic, so speed and latency are driving concerns here. Given the function of
the core, we can now consider some design specifics. Let’s start with some
things that we know we don’t want to do:
Don’t do anything to slow down traffic. This includes using access
lists, routing between virtual local area networks (VLANs), and
packet filtering.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
The Cisco Three-Layer Model
11
Don’t support workgroup access here.
Avoid expanding the core when the internetwork grows (i.e., adding
routers). If performance becomes an issue in the core, give preference
to upgrades over expansion.
Now, there are a few things that we want to make sure to do as we design
the core. They include:
Design the core for high reliability. Consider data-link technologies
that facilitate both speed and redundancy, such as FDDI, Fast Ethernet (with redundant links), or even ATM.
Design with speed in mind. The core should have very little latency.
Select routing protocols with lower convergence times. Fast and
redundant data-link connectivity is no help if your routing tables
are shot!
The Distribution Layer
The Distribution layer is sometimes referred to as the workgroup layer and
is the communication point between the Access layer and the Core layer. The
primary function of the Distribution layer is to provide routing, filtering, and
WAN access and to determine how packets can access the core, if needed.
The Distribution layer must determine the fastest way that user requests are
serviced, for example, how a file request is forwarded to a server. After the
Distribution layer determines the best path, it forwards the request to the
Core layer. The Core layer is then responsible for quickly transporting the
request to the correct service.
The Distribution layer is the place to implement policies for the network.
Here, you can exercise considerable flexibility in defining network operation. There are several items that generally should be done at the Distribution
layer. They include
Implementing tools such as access lists, packet filtering, and queuing
Implementing security and network policies, including address translation and firewalls
Redistribution between routing protocols, including static routing
Routing between VLANs and other workgroup support functions
Broadcast and multicast domain definition
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
12
Chapter 1
Scaling Large Internetworks
Things to avoid at the Distribution layer are limited to those functions
that exclusively belong to one of the other layers.
The Access Layer
The Access layer controls user and workgroup access to internetwork
resources. The Access layer is sometimes referred to as the desktop layer. The
network resources that most users need will be available locally. The Distribution layer handles any traffic for remote services. The functions to be
included at this layer include
Continued (from the Distribution layer) access control and policies
Creation of separate collision domains (segmentation)
Workgroup connectivity into the Distribution layer
Technologies such as DDR and Ethernet switching are frequently seen in
the Access layer as well as the Distribution layer. If you are using DDR to
connect to a remote office, then it has to be a Distribution layer device. Static
routing (instead of dynamic routing protocols) is seen here as well.
As already noted, three separate levels does not have to imply three separate routers. It could be fewer, or it could be more. Remember, this is a layered approach.
Requirements of the Scalable Internetwork
T
oday’s internetworks are experiencing extraordinary growth due to
increasing demands for connectivity both in businesses and at home. Therefore, it’s very important for them to be scalable. It’s now vital for administrators to understand what a scalable network is, as well as what is required
to effectively manage its incessant growth.
Since a scalable internetwork undergoes continual growth, it must be
both flexible and easily appended. An ideal design is based on the hierarchical model to simplify management and permit well-planned growth that
honors the network’s requirements. Here are the requirements of a scalable
internetwork:
It must be reliable and available. The Cisco IOS provides features for
implementing redundancy, load balancing, and reachability with protocols such as OSPF and EIGRP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Requirements of the Scalable Internetwork
13
It must be responsive. Because network growth often occurs on a daily
basis, the administrator’s duty to maintain the network’s responsiveness
can become overwhelming. The Cisco IOS provides solutions to provide
Quality of Service (QoS) that will allow multiple protocols to be supported on the network, without compromising QoS requirements.
It must be efficient. Efficiency, in a nutshell, means keeping the bandwidth from becoming saturated. A central goal of this book is to arm you
with information on fine-tuning your router to optimize the existing
bandwidth on your internetwork. You’ll learn how to achieve that objective through innovative techniques such as using access lists, optimizing
route update operations, and scaling IP addresses.
It must be adaptable. The internetwork must be designed to respond
masterfully to change and to accommodate disparate networks as well as
older legacy technologies.
It must be easily accessible while being secure. It is a network administrator’s foremost obligation (obsession?) to meet business requirements
by ensuring that network resources remain available to users at all times,
while managing to keep out any and all hackers. The Cisco IOS provides
dedicated and switched WAN support such as Frame Relay, SMDS, X.25,
and ATM to equip networking professionals with options to meet cost,
location, security, and traffic requirements. The Cisco IOS also provides
exterior routing support with the Exterior Gateway Protocol (EGP) and
Border Gateway Protocol (BGP) to permit routing on the Internet with
maximum security.
We will talk about each of these requirements in the following sections.
Reliability and Availability
Because a network is depended upon so heavily—ideally, it’s up and running
24 hours a day, 365 days a year—failures and downtime must be kept to a
minimum. It’s also vital that when a failure does occur, it’s easy to isolate,
reducing the time needed for troubleshooting. When it comes to reliability,
the internetwork’s Core layer is the most critical. Cisco’s definition of reliable is an internetwork that can respond quickly to changes in the network
topology and accommodate failures by rerouting traffic.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
14
Chapter 1
Scaling Large Internetworks
Some Cisco IOS features that serve to provide stability and availability are
as follows:
Reachability OSPF and EIGRP use expanded metrics that can go
beyond the hop-count limitations of distance-vector routing algorithms.
These routing protocols analyze a combination of factors to establish the
real cost of a path to a network, making Cisco routers able to support very
large internetworks.
Convergence Scalable routing protocols can converge quickly because
of each router’s complete understanding of the internetwork and ability
to detect problems.
Responsiveness
Since it’s the network administrator’s responsibility to make sure users don’t
experience delays in responsiveness as the internetwork grows, they must be
keenly aware of the latency factor that each piece of equipment (routers,
switches, and bridges) contributes to the internetwork. The Cisco IOS provides mitigation for the latency needs of each protocol running on your internetwork, with features such as
Alternate paths routing Because OSPF and EIGRP build a complete
map of the internetwork, a router can easily reroute traffic to an alternate
path if a problem occurs.
Load balancing Through the EIGRP and OSPF routing algorithms, the
Cisco IOS is able to perform load balancing. This allows for redundant
links and for more bandwidth to be available to locations needing more
than just one link. For example, if two T1 WAN links were installed
between buildings, the actual bandwidth between them would reach
approximately 3Mbps.
Tunneling Running a tunneling protocol affords the ability to communicate across WAN links that were previously unreachable. For example,
if you have a WAN link that supports only TCP/IP and you want to manage a Novell NetWare server that supports only IPX, you could tunnel
IPX packets inside of IP packets to achieve your goal.
Dial backup You can configure dial-backup links for redundancy on
your WAN links and to add extra bandwidth whenever it becomes saturated, enhancing the link’s reliability and availability.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Requirements of the Scalable Internetwork
15
Efficiency
The task of creating smoothly running, efficient LANs and internetworks is
obviously very important, but optimizing the bandwidth on a WAN can be
very difficult. The best way to reduce the bandwidth usage is to reduce the
amount of update traffic on the LAN that will be sent over your WAN. The
Cisco IOS features available to help reduce bandwidth usage are as follows:
Access lists Used to permit or deny certain types of traffic from entering
or exiting a specific router interface. They can stop basic traffic, broadcasts, and protocol updates from saturating a particular link. TCP/IP,
IPX, and AppleTalk can all be filtered extensively.
Snapshot routing Commonly used for ISDN connections when running
distance-vector protocols, it allows routers to exchange full distancevector routing information at an interval defined by the administrator.
Compression over WANs The Cisco IOS supports TCP/IP header and
data compression to reduce the amount of traffic crossing a WAN link.
Link compression can be configured, which compresses header and data
information into packets. This is accomplished by the Cisco IOS prior to
sending the frame across the WAN.
DDR (Dial-on-Demand Routing) DDR allows wide area links to be
used selectively. With it, the administrator can define “interesting” traffic
on the router and initiate point-to-point WAN links based upon that traffic. What denotes interesting traffic is defined by access lists, so a great
deal of flexibility is afforded to the administrator. For instance, an expensive ISDN connection to the Internet could be initiated to retrieve e-mail,
but not for a WWW request. DDR is an effective tool in situations where
WAN access is charged according to a quantified time interval—it’s best
to use it in situations where WAN access is infrequent.
Reduction in routing table entries By using route summarization and
incremental updates, you can reduce the number of router processing
cycles by reducing the entries in a routing table. Route summarization
occurs at major network boundaries, which summarize all the routes
advertised into one entry. Incremental updates save bandwidth by sending
only topology changes instead of the entire routing table when transmitting updates.
Switched access Packet-switched networks such as X.25 and Frame
Relay provide global connectivity through a large number of service providers with established circuits to most major cities.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
16
Chapter 1
Scaling Large Internetworks
Adaptability
Another important goal for an administrator is to design an internetwork
that responds well to change. To achieve this goal, internetworks need to be
able to
Pass both routable and nonroutable network protocols Examples
would be TCP/IP, which is routable, and Microsoft’s NetBEUI (NetBIOS
Extended User Interface), which is not routable, only bridgeable.
Create islands of networks using different protocols This allows you to
add protocols used by the network islands to Core layer routers or use
tunneling in the backbone to connect the islands, which keeps you from
having to add unwanted protocols to the core backbone.
Balance between multiple protocols in a network Each protocol has
different requirements, and the internetwork must be able to accommodate the specific issues of each one.
The Cisco IOS also has many different features that contribute to network
adaptability:
EIGRP Cisco’s proprietary EIGRP allows you to use multiple protocols
within one routing algorithm. EIGRP supports IP, IPX, and AppleTalk.
Redistribution Allows you to exchange routing information between
networks that use different routing protocols. For example, you can
update a routing table from a network running IGRP on a router participating in an RIP network.
Accessibility and Security
Access routers must be both accessed and used to access a variety of WAN
services, while maintaining security to keep hackers out.
The Cisco IOS features that support these requirements are as follows:
Dedicated and switched WAN support You can create a direct connection with Cisco routers using basic or digital services (a T1, for example).
Cisco routers also support many different switched services, such as
Frame Relay, SMDS, X.25, and ATM, to give you options to meet cost,
location, and traffic requirements.
Exterior protocol support Both Exterior Gateway Protocol (EGP) and
Border Gateway Protocol (BGP) are supported by the Cisco IOS. BGP
(discussed in detail in Chapters 7 through 9) is used primarily by Internet
Service Providers (ISPs) and has mostly replaced EGP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
17
Access lists Used to filter specific kinds of traffic from either entering or
leaving a Cisco router.
Authentication protocols Cisco supports both Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol
(CHAP) for providing authentication on WAN connections using PPP.
Summary
I
n this chapter, we covered the network congestion issues and showed
how to solve them. For an internetwork to realize its purpose, it must be able
to efficiently connect many different networks together to serve the organizations depending on it. However, the more users and networks that you tie
together, the more network congestion results.
The way to solve congestion problems and increase the networking performance of your LAN is to divide a single Ethernet segment into multiple
network segments using bridges, routers, and switches.
We also discussed in this chapter that the key requirements of a scalable
internetwork are based on an ideal design using the Cisco hierarchical model
to simplify management, which permits well-planned growth that honors
the network’s requirements.
The following issues were discussed as mandatory requirements of a scalable internetwork:
Reliability and availability
Responsiveness
Efficiency
Adaptability
Easy accessibility while maintaining security
Key Terms
Before taking the exam, make sure you’re familiar with the following terms:
Access layer
Core layer
Distribution layer
internetwork
microsegmentation
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
18
Chapter 1
Scaling Large Internetworks
Written Lab
1. Match the following letters to the numbered list below:
A. Reliable and available
B. Responsive
C. Efficient
D. Adaptable
E. Accessible but secure
Numbered Term
Letter
1. Authentication protocols
2. Reachability
3. Create islands of networks using different
protocols
4. DDR (Dial-on-Demand Routing)
5. Convergence
6. Alternate paths routing
7. Compression over WANs
8. Exterior protocol support
9. Balance between multiple protocols in a network
10. Switched access
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
19
Review Questions
1. Which of the following can you use to alleviate congestion in an inter-
network (if used correctly)?
A. Repeaters
B. Routers
C. DLC
D. Switches
E. Bridges
2. Choose the three layers Cisco uses for building its hierarchical inter-
network model.
A. Fundamental
B. Distribution
C. IGRP
D. Core
E. Backbone
F. Access
3. Identify the characteristics of a scalable internetwork.
A. Reliability
B. Responsiveness
C. Efficiency
D. Adaptability
E. Accessibility
F. All of the above
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
20
Chapter 1
Scaling Large Internetworks
4. What is the primary function of the Core layer?
A. To distribute client/server router information
B. To provide an optimized and reliable transport structure
C. To provide access to various parts of the internetwork, as well as
to services
D. To provide access to corporate resources for a workgroup or users
on a local segment
5. What is the primary function of the Distribution layer?
A. To distribute client/server router information
B. To provide an optimized and reliable transport structure
C. To provide access to various parts of the internetwork, as well as
to services
D. To provide access to corporate resources for a workgroup or users
on a local segment
6. What is the purpose of the Access layer?
A. To distribute client/server router information
B. To provide an optimized and reliable transport structure
C. To provide access to various parts of the internetwork, as well as
to services
D. To provide access to corporate resources for a workgroup or users
on a local segment
7. How do LAN switches improve performance on a LAN?
A. By filtering via logical address
B. By regenerating the digital signal
C. By employing packet-switching that permits high-speed data
exchanges
D. By employing frame-switching that permits high-speed data
exchanges
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
8. What is a benefit of bridge segmentation?
A. Regeneration and propagation
B. Segmenting, or breaking up, your network into smaller, more
manageable pieces
C. LAN queuing
D. Forwarding the frame before reception is complete
9. How does cut-through switching provide better performance than
other switching methods?
A. By using LAN queuing
B. By using microsegmentation
C. By receiving the entire frame onto onboard buffers, running a
CRC, and then forwarding the frames out the destination port
D. By forwarding the frame before reception is complete
10. LAN segmentation with switches is also called what?
A. Filtering
B. Microsegmentation
C. Bridging
D. Routing
11. Which Cisco layer governs access to Core layer resources?
A. Distribution
B. Core
C. Backbone
D. Access
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
21
22
Chapter 1
Scaling Large Internetworks
12. Which layer should have the most redundancy?
A. Backbone
B. Core
C. Distribution
D. Access
13. How do bridges filter a network?
A. By logical address
B. By IP address
C. By hardware address
D. By digital signaling
14. How do routers filter a network? (Choose all that apply.)
A. By logical address
B. By IP address
C. By digital signaling
D. By hardware address
E. By IPX address
15. How do switches segment a network?
A. By logical address
B. By IP address
C. By hardware address
D. By IPX address
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
16. What is a drawback of filtering a network with bridges?
A. It segments the network.
B. It creates internetworks.
C. It forwards all broadcasts.
D. It filters frames.
17. How can you reduce routing table entries?
A. Route summarization
B. Incremental updates
C. IP filtering
D. VLANs
18. Which Cisco IOS features are available to help reduce bandwidth
usage? (Choose all that apply.)
A. Access lists
B. Snapshot routing
C. Compression of WANs
D. TTL
E. DDR
F. Incremental updates
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
23
24
Chapter 1
Scaling Large Internetworks
19. Which Cisco IOS features serve to provide stability and availability?
(Choose all that apply.)
A. Reachability
B. Convergence
C. Alternative path routing
D. Snapshot routing
E. Tunneling
F. Dial backup
G. Load balancing
20. Which Cisco layer is responsible for breaking up collision domains?
A. Core
B. Backbone
C. Distribution
D. Access
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Written Lab
Answers to Written Lab
1.
Numbered Term
Letter
1. Authentication protocols
E
2. Reachability
A
3. Create islands of networks using different
protocols
D
4. DDR (Dial-on-Demand Routing)
C
5. Convergence
A
6. Alternate paths routing
B
7. Compression over WANs
C
8. Exterior protocol support
E
9. Balance between multiple protocols in a network D
10. Switched access
Copyright ©2001 SYBEX , Inc., Alameda, CA
C
www.sybex.com
25
26
Chapter 1
Scaling Large Internetworks
Answers to Review Questions
1. B, D, E. Routers, switches, and bridges are used to segment a net-
work and alleviate congestion on a network segment.
2. B, D, F. The Cisco three-layer model includes the Core,
Distribution, and Access layers.
3. F. An internetwork should be reliable, responsive, efficient, adaptable,
and accessible.
4. B. The Core layer should provide a fast transport between Distribu-
tion layer devices.
5. C. The Distribution layer connects Access layer devices together and
provides users with network service connections.
6. D. The Access layer is the connection point for users into the
internetwork.
7. D. LAN switches are Layer 2 devices that filter by hardware address
in a frame.
8. B. Bridges filter the network by using the hardware address in a frame
and create smaller collision domains.
9. D. Cut-through LAN switching begins forwarding the frame to the
destination device as soon as the destination hardware address is read
in the frame.
10. B. Microsegmentation is a term for breaking up collision domains into
smaller segments.
11. A. The Distribution layer is responsible for connecting the Access
layer devices together and managing data flow to the Core layer.
12. B. If there is a failure in the core, every single user can be affected.
Therefore, fault tolerance at this layer is an issue.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
27
13. C. Bridges use the hardware address in a frame to filter a network.
14. A, B, E. Routers use logical network addresses. IP and IPX are exam-
ples of logical network addresses.
15. C. Switches, like bridges, use hardware addresses in a frame to filter
the network.
16. C. Both switches and bridges break up collision domains but are one
large broadcast domain by default. All broadcasts are forwarded to all
network segments with a bridge or switch.
17. A. Route summarization is used to send fewer route entries in an
update. This can reduce the routing table entries.
18. A, B, C, E, F. Access lists, snapshot routing, compression techniques,
Dial-on-Demand Routing (DDR), and incremental updates all can
help reduce bandwidth usage.
19. C, D, E, F. Alternate path routing, which provides redundancy and
load balancing, along with snapshot routing, tunneling, and dial backup,
all provide stability and availability in an internetwork.
20. D. The Access layer is responsible for breaking up collision domains.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
2
Routing Principles
THE CCNP ROUTING EXAM TOPICS
COVERED IN THIS CHAPTER ARE AS FOLLOWS:
List the key information routers need to route data
Describe the use of the fields in a routing table
Describe classful and classless routing protocols
Compare distance-vector and link-state routing protocol
operation
Given a pre-configured laboratory network, discover the
topology, analyze the routing table, and test connectivity using
accepted troubleshooting techniques
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
I
n this chapter, you will learn the difference between distancevector and link-state routing protocols. The idea of this chapter is to provide
you with an overview of the different types of routing protocols available,
not how to configure routers. Distance-vector protocols will be covered in
more detail in this chapter than link-state because link-state routing protocols are covered very thoroughly starting at Chapter 4, “OSPF Areas.”
This is an important chapter to understand before moving on to the linkstate routing protocol chapters. Having a fundamental understanding of the
distance-vector and link-state concepts is important, as it will help you when
you design internetworks and the routing protocol implementation.
Fundamentals of Routing
R
outing is the process of forwarding packets from one network to
another; this is sometimes referred to as a relay system. Logical addressing is
used to identify each network as well as each device on the network. The
actual movement of transient traffic through the router is a separate function; it is actually considered to be the switching function. Routing devices
must perform both a routing and a switching function to be effective.
For a routing decision to take place on a relay system, three major decisions must be made:
Is the logical destination address a known protocol? Is this protocol
enabled on the router and active? This does not have to be IP; IPX,
AppleTalk, and other protocol suites can be used as well.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Fundamentals of Routing
31
Is the destination logical address in the routing table? If not, discard
the packet and send an ICMP (Internet Control Message Protocol)
message to the sender.
If the destination logical address is in the routing table, to which interface will the packet be forwarded? Once this exit, or forwarding interface,
is chosen, the router must have an encapsulation in which to place the
packet. This is called framing and is required to forward the packet to
the next-hop logical device.
Once the packet is framed, it is forwarded from hop to hop until it reaches
the final destination device. Routing tables in each device are used to pass the
packet to the correct destination network.
Routing Tables
All the routing information needed for a router to forward packets to a nexthop relay device can be found in the router’s routing table. Again, if a destination logical address is not found in the table, the router discards the
packets. A gateway of last resort can be set on the router to forward packets
not listed in the routing table. This is called setting the default route.
However, this is not a default gateway, nor does it act as a default gateway, so it is important to not think of setting the gateway of last resort as setting a default gateway. Default gateways are used on hosts to direct packets
to a relay device if the destination logical device is not on the local segment.
Gateway-of-last-resort entries are used to send packets to a next-hop relay
device if the destination logical address is not found in the routing table. If
the destination logical address is in the routing table, then the gateway of last
resort will not be used.
A sample routing table is shown below:
2600B#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
T - traffic engineered route
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
32
Chapter 2
Routing Principles
Gateway of last resort is not set
C
C
S
S
R
R
172.16.0.0/24 is subnetted, 6 subnets
172.16.60.0 is directly connected, BRI0/0
172.16.50.0 is directly connected, Ethernet0/0
172.16.10.0 [1/0] via 172.16.50.1, Ethernet0/0
172.16.11.0 [1/0] via 172.16.50.1, Ethernet0/0
172.16.50.0 [120/3] via 172.16.10.2,
FastEthernet0/0
172.16.40.0 [120/2] via 172.16.10.2,
FastEthernet0/0
2600B#
At the top of the routing table are the different codes that describe the
entries found in a routing table. In the example above, the entries include
both directly connected static routes and RIP entries.
Let’s take a look at a static route entry:
S
172.16.10.0 [1/0] via 172.16.50.1, Ethernet0/0
The list below describe the different parts of the routing table entry:
S The means by which the entry was learned on this router. S is for static
entry, which means that the administrator added the route manually.
172.16.10.0 The logical destination remote network or subnet.
[1 The administrative distance, or trustworthiness, of a route. (We discuss this in the next section.)
/0] The metric value. Since it is a static route, the value is 0 because the
router is not learning the route; thus the router has nothing to compare
the route with. This value will vary widely depending on the routing protocol used.
via 172.16.50.1 The address of the next relay device to forward packets to.
Ethernet0 The interface from which the path was learned and to which
the packets will be forwarded.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Fundamentals of Routing
33
Administrative Distances
When configuring routing protocols, you need to be aware of administrative
distances. These are used to rate the trustworthiness of routing information
received on a router from a neighbor router. An administrative distance is an
integer from 0 to 255, where 0 is the most trusted and 255 means no traffic
will be passed via this route.
Table 2.1 shows the default administrative distances that a Cisco router
will use to decide which route to take to a remote network.
TABLE 2.1
Default Administrative Distances
Route Source
Default Distance
Connected interface
0
Static route
1
EIGRP summary
5
External BGP
20
EIGRP
90
IGRP
100
OSPF
110
IS-IS
115
RIP
120
EDP
140
External EIGRP
170
Internal BGP
200
Unknown
255 (This route will never be used.)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
34
Chapter 2
Routing Principles
If a network is directly connected, it will always use the interface connected to the network. If an administrator configures a static route, the
router will believe that route over any other learned routes. However, you
can change the administrative distance of static routes, but, by default, they
have an administrative distance of 1.
Packet Switching
After a router is started up, the routing protocol tries to establish neighbor
relationships in order to understand the network topology and build the
routing table. All routing protocols perform this differently; for example,
some use broadcast addresses to find the neighbors and some use multicast
addresses.
Once the neighbors are found, the routing protocol creates a peer relationship at Layers 4 through 7 of the OSI model. Routing protocols either
send periodic routing updates or exchange Hello messages to maintain the
relationship.
Only after the topology is completely understood and the best paths to all
remote networks are decided and put in the routing table can the forwarding
of packets begin. This forwarding of packets received on an interface to an
exit interface is known as packet-switching.
There are four basic steps for a router to packet switch:
1. The router receives a frame on an interface, runs a CRC (cyclic redun-
dancy check), and if it is okay, checks the hardware destination
address. If it matches, the packet is pulled from the frame. The frame
is discarded and the packet is buffered in main memory.
2. The packet’s destination logical address is checked. This address is
looked up in the routing table for a match. If there is no match, the
packet is immediately discarded and an ICMP message is sent back to
the originating device. If there is a match, the packet is switched to the
forwarding interface buffer.
3. The hardware address of the next-hop device must be known. The
ARP cache is checked first and if it is not found, an ARP broadcast is
sent to the device. The remote device will respond with its hardware
address.
4. A new frame is created on that interface and the packet is placed in this
frame. The destination hardware address is the address of the nexthop device. Notice that the packet was not altered in any way.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Fundamentals of Routing
35
Dynamic Routing
Dynamic routing is the process of using protocols to find and update routing
tables on routers and to maintain a loop-free, single path to each network.
This is easier than static or default routing, but you use it at the expense of
router CPU processes and bandwidth usage on the network links. A routing
protocol defines the set of rules used by a router when it communicates
between neighbor routers.
Once the router process knows the metric values of each path, then routing decisions are made. When a route is learned from different sources, the
router will first choose the route with the lowest administrative distance. If
two routes have the same AD, then the router will use the routing metrics to
determine the best path to the remote network. If the AD is the same in both
routes, as well as the metrics, then the routing protocol will load balance.
There are two types of dynamic routing protocols used in internetworks:
Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP).
IGP routing protocols are used to exchange routing information with routers
in the same autonomous system (AS). An AS is a collection of networks
under a common administrative domain. EGPs are used to communicate
between ASes. An example of an EGP is the Border Gateway Protocol (BGP),
which is discussed in Chapters 8 through 9.
Routing Protocols
There are two classes of dynamic routing protocols:
Distance-vector The distance-vector protocol uses the distance to a
remote network as a determination of the best path to a remote network.
Each time a packet goes through a router, it’s called a hop. The route with
the least number of hops to the remote network is determined to be the
best route. The vector is the determination of direction to the remote
network.
Examples of a distance-vector protocol are RIP and IGRP.
However, not all distance-vector protocols use hop count in their metric.
IGRP uses bandwidth and delay of the line to determine the best path to a
remote network. It is considered a distance-vector protocol because it sends
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
36
Chapter 2
Routing Principles
out the complete routing table at periodic intervals. The periodic routing
updates from a distance-vector router are sent only to directly connected
routers and sent as a broadcast of 255.255.255.255. Since the updates
include all routes that the sending router knows about, this is sometimes
referred to as “routing by rumor” because a router will accept information
from a neighbor as correct. The disadvantage to distance-vector protocols is
that the periodic updates consume bandwidth even if there are no topology
changes to report.
Link-state Typically called shortest path first, link-state routers create
three separate tables. One of these tables keeps track of directly attached
neighbors, one determines the topology of the entire internetwork, and
one is used for the routing table. Link-state routers know more about the
internetwork than any distance-vector protocol. An example of an IP
routing protocol that is completely link-state is OSPF.
To send routing updates, the link-state router uses a triggered-update type
of announcement. These announcements are sent from a router only
when a topology change has occurred within the network. The advantage
of link-state routing over distance-vector is that when an update occurs,
only the information about the link that changed is contained in the
update.
There is no set way of configuring routing protocols for use with every
business. This task is performed on a case-by-case basis. However, if you
understand how the different routing protocols work, you can make good
business decisions.
Both distance-vector and link-state routing protocols are discussed in
more detail later in this chapter.
Classful Routing
The basic definition of classful routing is that subnet mask information is not carried within the routine, periodic routing updates. This means
that every interface and host on the network must use the same subnet mask.
Examples of classful routing protocols are the Routing Information Protocol
version 1 (RIPv1) and the Interior Gateway Routing Protocol (IGRP).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Classful Routing
37
RIP version 2 (RIPv2) is an example of a classless routing protocol. Classless
routing is discussed later in this chapter.
Devices in an internetwork must know the routing mask associated with
any advertised subnets, or those subnets cannot be advertised. If the subnet
mask does not match the receiving device, then the receiving device must
summarize the received route as a classful boundary and then send the
default routing mask in its own advertisements.
Classful routing protocols must exchange routing information using the
same subnet mask since subnet mask information is not sent in the periodic
updates.
The problem with classful routing protocols is wasted address space. For
example, in Figure 2.1, there is a Class C network address of 192.16.10.0,
using the subnet mask 255.255.255.240. The subnets would be 16, 32, 48,
64, etc. Each subnet has 14 valid hosts. In the figure, each LAN has a requirement of 10 hosts each, which is fine except for the WAN links connecting the
sites. WAN links use only two IP addresses. Since the WAN interfaces must
use the same mask, they waste 12 host addresses.
FIGURE 2.1
Classful routing protocol issues
32
16
48
64
80
96
Another problem with classful routing protocols is the periodic routing
updates sent out all active interfaces of every router. Distance-vector protocols, which we discuss next, are true classful routing protocols that send
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
38
Chapter 2
Routing Principles
complete routing table entries out all active interfaces at periodic time intervals. This can cause congestion on the slower WAN links.
Classless Routing
C
lassless routing protocols include the subnet mask information when
an update is sent. This allows different length subnet masks to be used on the
network, called Variable Length Subnet Masks (VLSM). You must use a
classless routing protocol if you want to have a network design like the one
shown in Figure 2.2.
FIGURE 2.2
Classless network using VLSM
192.168.10.32/30
192.168.10.16/28
192.168.10.48/28
192.168.10.36/30
192.168.10.40/30
192.168.10.64/28
What the classless protocol allows is a subnet mask of 255.255.255.240
on the LANs and a subnet mask of 255.255.255.252 on the WANs, which
saves address space.
VLSM is not the only benefit of classless routing protocols. Classless routing protocols allow summarization at non-major network boundaries,
unlike classful routing protocols, which allow summarization only at major
network boundaries.
Another benefit of classless routing is that less bandwidth is consumed
since no periodic updates are sent out the routers’ interfaces. Updates are
sent only when a change occurs, and then only the change is sent, not the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Distance-Vector Protocols
39
entire routing table as with classful routing protocols. If no changes occur,
classless routing protocols send Hello messages to their directly connected
neighbors. This ensures that the neighbors are still alive. Only if a router
does not receive a Hello message from its neighbor will a convergence of the
network take place.
Distance-Vector Protocols
T
here are four different distance-vector routing algorithms supported
by Cisco routers. Table 2.2 shows the different protocols available along
with their characteristics. RIP and IGRP use the Bellman-Ford algorithm.
EIGRP uses the Diffusing Update-based Algorithm (DUAL).
EIGRP is considered an advanced distance-vector routing algorithm, and
Cisco lists it as a distance-vector routing algorithm in their BSCN course. However, since it uses both the characteristics of distance-vector and link-state, it
is really considered a hybrid routing protocol. EIGRP will be discussed in
detail in Chapter 6, “IGRP and EIGRP.”
TABLE 2.2
Distance-Vector Comparisons
Characteristic
RIPv1
RIPv2
IGRP
Count to infinity
X
X
X
Split horizon with
poison reverse
X
X
X
Hold-down timer
X
X
X
Triggered updates
with route
poisoning
X
X
X
X
Load balancing
with equal paths
X
X
X
X
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
EIGRP
X
40
Chapter 2
Routing Principles
TABLE 2.2
Distance-Vector Comparisons (continued)
Characteristic
RIPv1
RIPv2
Load balancing
with unequal
paths
VLSM support
IGRP
EIGRP
X
X
X
X
Metric
Hops
Hops
Composite
Composite
Hop count limit
16
16
255 (100 by
default)
255 (100 by
default)
Support for size of
network
Medium
Medium
Large
Large
We will discuss RIP and IGRP in detail in the following sections.
RIP
Routing Information Protocol (RIP) is a true distance-vector protocol. It
sends the complete routing table out to all active interfaces every 30 seconds.
RIP uses only hop count to determine the best way to a remote network, but
it has a maximum allowable hop count of 15, meaning that 16 is deemed
unreachable. RIP works well in small networks, but it is inefficient on large
networks with slow WAN links or on networks with a large number of routers installed.
RIP version 1 uses only classful routing, which means that all devices in
the network must use the same subnet mask. This is because RIP version 1
does not send updates with subnet mask information in tow. RIP version 2
provides what is called prefix routing and does send subnet mask information with the route updates. RIPv2 uses classless routing.
To keep a network stable, RIP uses timers.
RIP Timers
RIP uses three different kinds of timers to regulate its performance:
Route update timer Sets the interval (typically 30 seconds) between
periodic routing updates in which the router sends a complete copy of its
routing table out to all neighbors.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Distance-Vector Protocols
41
Route invalid timer Determines the length of time that must expire (90
seconds) before a router determines that a route has become invalid. It
will come to this conclusion if it hasn’t heard any updates about a particular route for that period. When that happens, the router will send out
updates to all its neighbors, letting them know that the route is invalid.
Route flush timer Sets the time between a route becoming invalid and
its removal from the routing table (240 seconds). Before it is removed
from the table, the router notifies its neighbors of that route’s impending
doom. The value of the route invalid timer must be less than that of the
route flush timer. This is to provide the router with enough time to tell its
neighbors about the invalid route before the routing table is updated.
RIP Updates
The distance-vector routing algorithm passes complete routing tables to
neighbor routers. The neighbor routers then combine the received routing
table with their own routing tables to complete the internetwork map. This
is called routing by rumor, as a router receiving an update from a neighbor
router believes the information about remote networks without actually
finding out for itself.
It is possible to have a network with multiple links to the same remote network. If that is the case, the administrative distance is first checked. If the
administrative distance is the same, it will have to use other metrics to determine the best path to use to that remote network.
RIP uses only hop count to determine the best path to an internetwork. If
RIP finds more than one link to the same remote network with the same hop
count, it will automatically perform a round-robin load balance. RIP can
perform load balancing for up to six equal-cost links.
However, a problem with this type of routing metric arises when the two
links to a remote network are different bandwidths but the same hop count.
Figure 2.3, for example, shows two links to remote network 172.16.50.0.
Since network 172.16.30.0 is a T1 link with a bandwidth of 1.544Mbps,
and network 172.16.20.0 is a 56K link, you would want the router to choose
the T1 over the 56K link. However, since hop count is the only metric used
with RIP routing, they would both be seen as equal-cost links. This is called
pinhole congestion.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
42
Chapter 2
Routing Principles
FIGURE 2.3
Pinhole congestion
Router A
Network
172.16.30.0
SO
Router B
SO
T1
Network
172.16.10.0
Network
172.16.20.0
SO
SO
56K
Router C
172.16.50.0
Router D
It is important to understand what happens when a distance-vector protocol starts up. In Figure 2.4, the four routers start off with only their directly
connected networks in the routing table. After a distance-vector protocol is
started on each router, the routing tables are updated with all route information gathered from neighbor routers.
FIGURE 2.4
The internetwork with distance-vector routing
172.16.30.0
172.16.20.0
172.16.10.0
F0/0
E0
S0
E0
S0
2501A
172.16.40.0
S1
S0
172.16.50.0
E0
2501C
2501B
2621A
Routing Table
172.16.10.0 F0/0 0
Routing Table
Routing Table
172.16.10.0
E0
0
172.16.20.0
172.16.20.0
S0
0
Routing Table
S0
0
172.16.40.0
S0
0
172.16.30.0
E0
0
172.16.50.0
E0
0
172.16.40.0
S1
0
As shown in Figure 2.4, each router has only the directly connected networks in each routing table. Each router sends its complete routing table out
to each active interface on the router. The routing table of each router
includes the network number, exit interface, and hop count to the network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Distance-Vector Protocols
43
In Figure 2.5, the routing tables are complete because they include information about all the networks in the internetwork. They are considered converged. (Converging is discussed later in this chapter.) When the routers are
converging, no data is passed. That’s why fast convergence time is a plus,
and that’s one of the problems with RIP: slow convergence time.
FIGURE 2.5
Converged routing tables
172.16.30.0
172.16.20.0
172.16.10.0
F0/0
E0
S0
2501A
E0
S0
172.16.40.0
S1
S0
172.16.50.0
E0
2501C
2501B
2621A
Routing Table
Routing Table
Routing Table
Routing Table
172.16.10.0 F0/0 0
172.16.10.0
E0
0
172.16.20.0
S0
0
172.16.40.0
S0
172.16.20.0 F0/0 1
172.16.20.0
S0
0
172.16.30.0
E0
0
172.16.50.0
E0
0
172.16.30.0 F0/0 2
172.16.30.0
S0
1
172.16.40.0
S1
0
172.16.10.0
S0
2
172.16.40.0 F0/0 2
172.16.40.0
S0
1
172.16.10.0
S0
1
172.16.20.0
S0
1
172.16.50.0 F0/0 3
172.16.50.0
S0
2
172.16.50.0
S1
1
172.16.30.0
S0
1
0
The routing tables in each router keep information regarding the network
number, the interface to which the router will send packets out to get to the
remote network, and the hop count or metric to the remote network.
Verifying RIP
Each routing table should have the routers’ directly connected routes as well
as RIP-injected routes received from neighbor routers. The router output
below shows the contents of an RIP routing table:
2621A#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
Gateway of last resort is not set
R
172.16.0.0/24 is subnetted, 5 subnets
172.16.50.0 [120/3] via 172.16.10.2,
FastEthernet0/0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
44
Chapter 2
Routing Principles
R
R
R
C
2621A#
172.16.40.0 [120/2] via
FastEthernet0/0
172.16.30.0 [120/2] via
FastEthernet0/0
172.16.20.0 [120/1] via
FastEthernet0/0
172.16.10.0 is directly
172.16.10.2,
172.16.10.2,
172.16.10.2,
connected, FastEthernet0/0
In the above output, notice that the R means that the networks were added
dynamically using RIP. The [120/3] is the administrative distance of the
route (120) along with the number of hops to that remote network (3).
IGRP
The Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary distance-vector protocol. This means that all of your routers must be Cisco
routers in order to be able to use IGRP in your network. Cisco created this
routing protocol to overcome the problems associated with RIP.
IGRP has a maximum hop count of 255 with a default of 100. This is
helpful in larger networks and solves the problem of there being only 15
hops possible, as is the case in an RIP network. IGRP also uses a different
metric than RIP. IGRP uses bandwidth and delay of the line by default as a
metric for determining the best route to an internetwork. This is called a
composite metric. Reliability, load, and maximum transmission unit (MTU)
can also be used, although they are not used by default.
IGRP can load balance up to six unequal links to a remote network. RIP
networks must have the same hop count to be able to load balance, whereas
IGRP uses bandwidth to determine how to load balance. To load balance
over unequal-cost links, the variance command controls the load balancing
between the best metric and the worst acceptable metric.
IGRP Timers
To control performance, IGRP includes timers with default settings for each
of the following:
Update timers These specify how frequently routing update messages
should be sent. The default is 90 seconds.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Distance-Vector Protocols
45
Invalid timers These specify how long a router should wait before
declaring a route invalid if it doesn’t receive a specific update about it. The
default is three times the update period.
Hold-down timers These specify the hold-down period. The default is
three times the update timer period plus 10 seconds.
Flush timers These indicate how much time should pass before a route
should be flushed from the routing table. The default is seven times the
routing update period.
Verifying IGRP
The output below is from an IGRP router:
2621A#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
M – [output cut]
T - traffic engineered route
Gateway of last resort is not set
I
I
I
I
C
172.16.0.0/24 is subnetted, 5 subnets
172.16.50.0 [100/160360] via 172.16.10.2,
FastEthernet0/0
172.16.40.0 [100/160260] via 172.16.10.2,
FastEthernet0/0
172.16.30.0 [100/158360] via 172.16.10.2,
FastEthernet0/0
172.16.20.0 [100/158260] via 172.16.10.2,
FastEthernet0/0
172.16.10.0 is directly connected, FastEthernet0/0
The I means IGRP-injected routes. The [100/160360] is the administrative distance of IGRP and the composite metric. The lower the composite
metric, the better the route.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
46
Chapter 2
Routing Principles
Link-State Routing Protocols
Link-state routing protocols are more advanced than distance-vector
protocols because, unlike distance-vector, they do not send periodic routing
updates. When a change in the network occurs, the routers send Link State
Advertisements (LSAs) about the change that has occurred. The whole routing table is not sent as in distance-vector; only the needed information is sent.
Each device that receives the LSA makes a copy, updates its topological
database, and forwards this LSA to all neighbors using multicast addressing.
This flooding of the LSA is to make sure that all devices know about the
change in the internetwork.
Hierarchical design of the physical network is critical in larger networks
to reduce the flooding needed, which reduces the convergence time of the
network. This topic will be covered in more detail in Chapters 4 and 5,
“Interconnecting OSPF Areas.”
Table 2.3 compares the characteristics of the various link-state routing
protocols. Enhanced IGRP is called an advanced distance-vector protocol by
Cisco, but it has link-state features as well. Intermediate System-to-Intermediate
System (IS-IS) is the routing algorithm used by the ISO protocol suite.
TABLE 2.3
Link-State Comparisons
Characteristic
OSPF
IS-IS
Hierarchical topology
needed
X
X
Retains knowledge of
all possible routes
X
X
X
Manual route
summarization
X
X
X
Automatic route
summarization
Event-triggered
announcements
EIGRP
X
X
Copyright ©2001 SYBEX , Inc., Alameda, CA
X
www.sybex.com
X
Convergence
TABLE 2.3
47
Link-State Comparisons (continued)
Characteristic
OSPF
IS-IS
Load balancing with
unequal paths
EIGRP
X
Load balancing with
equal paths
X
X
X
VLSM support
X
X
X
Metric
Cost
Cost
Composite
Hop count limit
200
1024
100 by default
Support for size of
network
Large
Very Large
Large
Chapters 4 and 5 will discuss OSPF in detail, and Chapter 6 will discuss
EIGRP. We will discuss how both EIGRP and OSPF (as well as RIP and
IGRP) provide convergence in a network outage situation in the convergence
sections below.
Convergence
C
onvergence is the time it takes for all routers to agree on the network
topology after a change in the network. The routers basically synchronize
their routing tables.
There are at least two different detection methods used by all routing protocols. The first method is used by the Physical and Data Link layer protocols. When the network interface on the router does not receive three
consecutive keepalives, the link will be considered down. The second method
is that when the routing protocol at the Network and Transport layers fails
to receive three consecutive Hello messages, the link will be considered
down.
After the link is considered down is where the routing protocols differ.
Routing protocols have timers that are used to stop network loops from
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
48
Chapter 2
Routing Principles
occurring on a network when a link failure has been detected. Hold-down
timers are used to give the network stability while new route calculations are
being performed. Since a network cannot converge during this hold-down
period, this can cause a delay in the routing process of the network. Because
of this slow convergence time, link-state routing protocols do not use holddown timers.
The following section will describe the convergence process for both RIP
and IGRP when a link failure occurs in a network.
RIP Convergence
Convergence time is one of the problems associated with distance-vector
protocols. This section details the convergence process of the RIP protocol.
We’ll use Figure 2.6 to help describe the RIP convergence process.
FIGURE 2.6
Convergence
E
A
B
C
D
F
The following list describes the RIP convergence events when a problem
occurs. In Figure 2.6, the WAN between Routers D and F goes down. Here
is what happens:
1. RouterD sends a poisoned route to Routers E and C. RouterC
informs RouterB, which in turn sends the poisoned route to RouterA.
RouterC purges the entry for the downed link from the routing table.
2. RouterD sends a query broadcast. RouterC responds with a poison
reverse, and RouterE responds with a route to that network. Router D
enters this route in the routing table.
3. RouterD does not hold down this route entry since it had just purged
a route entry to this link.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Convergence
49
4. Router D advertises the new route to RouterC, but RouterC ignores
the new route listing because RouterC is in hold-down. RouterC sends
another poison reverse to RouterD.
5. The hold-down timers expire on Routers C, B, and A, which cause
their routing table entries to be updated.
The time required for RouterA to converge is the detection time, plus the
hold-down time, plus two update times, plus another update time. The complete convergence to RouterA could be over 240 seconds.
IGRP Convergence
Using the same Figure 2.6 as an example, let’s take a look at IGRP
convergence:
1. RouterD detects the failure on the link to RouterF. RouterD sends a
poisoned route update to Routers E and C. RouterC sends an update
to RouterB, and RouterB sends an update to RouterA. RouterD
purges the routing table of a link to the downed route.
2. RouterD sends a query broadcast, and RouterC sends back a poison-
reverse response. RouterD sends an update out all interfaces without
the failed link entry.
3. RouterE responds to the broadcast with an entry to the downed
link. RouterD puts this in the routing table and does not put a holddown on the link since it does not have an entry for this link. RouterD
sends a broadcast out all links, which includes this new entry.
4. RouterC receives this update but ignores the new route since it is in
hold-down. It responds again with a poison reverse.
5. Routers C, B, and A hold-down timers expire and their routing tables
will then be updated with the new route.
The time it takes for RouterA to converge is the detection time, plus the holddown time, plus two update times, plus another update time, which is over
490 seconds.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
50
Chapter 2
Routing Principles
EIGRP Convergence
Let’s take a look at the convergence time of Enhanced IGRP (EIGRP). We
will again use Figure 2.6 to help describe the convergence process:
1. RouterD detects the link failure between D and F and immediately
checks the topology table for a feasible successor. RouterD does not
find an alternate route in the topology table and puts the route into
active convergence state.
2. RouterD sends a query out all interfaces looking for a route to the
failed link. Routers C and E acknowledge the query.
3. RouterC responds with no known link.
4. RouterE responds with a route to the failed link, but it is at a higher
cost than the original link.
5. RouterD uses the new link and places it in the topology table, which
is then placed in the routing table.
6. RouterD broadcasts this new route out all interfaces, and Routers C
and E acknowledge the update back to RouterD, with an update of their
own. These updates make sure the routing tables are synchronized.
RouterA convergence time is the total time of detection, plus the query
and reply time, plus the update time—about two seconds total. However, the
time can be higher.
OSPF Convergence
Using Figure 2.6 as a reference, let’s now take a look at the convergence cycle
used in OSPF:
1. RouterD detects the link failure between Routers D and F, and a Des-
ignated Router (DR) election is held on the E0 interface, but no neighbors respond. The route entry is removed from RouterD. A Link State
Advertisement (LSA) is sent out all interfaces on RouterD.
2. Routers C and E receive the LSA and forward the LSA packet out all
interfaces except for the interface where the LSA was received.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
51
3. All routers wait five seconds and then run the shortest path first (SPF)
algorithm. After the algorithm is run, RouterD adds the route through
RouterE, and Routers C, B, and A update the metric in their routing
table to that route.
4. After another 30 seconds, RouterF sends an LSA out all interfaces
after timing out the link to RouterD. All routers wait five seconds and
then run the SPF algorithm, and all routers now know that the route
to the failed link is through RouterE.
RouterA convergence time is the time of detection, plus the LSA forwarding time, plus five seconds. This is about six seconds. However, if RouterF’s
time to converge is considered, then the time is about 36 seconds.
Summary
T
his chapter described the difference between distance-vector and
link-state routing protocols. The idea of this chapter was to provide you with
an overview of the different types of routing protocols available, not how to
configure routers.
Both link-state and distance-vector routing algorithms were covered, as
well as how they create routing tables and regulate performance with timers,
and their different convergence methods.
The difference between classful and classless routing protocols is very
important to understand before continuing on with the other chapters in this
book. We discussed both types of routing protocols in detail.
Key Terms
Before taking the exam, make sure you’re familiar with the following terms:
administrative distances
autonomous systems (AS)
composite metric
distance-vector protocol
dynamic routing
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
52
Chapter 2
Routing Principles
framing
hop
link-state routers
packet-switching
pinhole congestion
prefix routing
relay system
routing by rumor
routing table
setting the default route
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
53
Written Lab
In this lab, write in the routing protocol that is described. The possibilities are RIP, RIPv2, IGRP, EIGRP, IS-IS, and OSPF. More than one
answer may be possible per characteristic.
Protocol
Characteristic
Maximum of 15 hops by default
Administrative distance of 120
Administrative distance of 100
Administrative distance of 90
Administrative distance of 110
Uses composite metric to determine the best path to a
remote network
Uses only hop count to determine the best path to a
remote network
VLSM support
Non-proprietary
Floods network with LSAs to prevent loops
Must have hierarchical network design
Uses only bandwidth as a metric
Uses more than one table to assist in rapid
convergence
Route update contains only needed information
Automatic route summarization at major network
boundaries
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
54
Chapter 2
Routing Principles
Review Questions
1. What are two benefits of using a link-state routing protocol?
A. It uses the Hello protocol to establish adjacencies.
B. It uses several components to calculate the metric of a route.
C. Updates are sent only when changes occur in the network.
D. It is a better protocol than the distance-vector protocol.
2. Which protocols do not use a topology table?
A. EIGRP
B. IGRP
C. RIPv1
D. OSPF
3. RIPv2 provides which of the following benefits over RIPv1?
A. RIPv2 is link-state.
B. RIPv2 uses a topology table.
C. RIPv2 supports VLSM.
D. RIPv2 uses Hello messages.
4. What is the administrative distance of directly connected routes?
A. 0
B. 1
C. 90
D. 100
E. 110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
55
5. Which of the following are considered distance-vector? (Choose all
that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. OSPF
F. IS-IS
6. What is the default administrative distance of OSPF?
A. 0
B. 1
C. 90
D. 100
E. 110
7. Which is true regarding classless routing?
A. It sends periodic subnet mask information.
B. It sends incremental subnet mask information.
C. It sends prefix mask information.
D. All devices on the network must use the same mask.
8. What is the default administrative distance of EIGRP?
A. 0
B. 1
C. 90
D. 100
E. 110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
56
Chapter 2
Routing Principles
9. Which is true regarding classful routing?
A. It sends periodic subnet mask information.
B. It sends incremental subnet mask information.
C. It sends prefix mask information.
D. All devices on the network must use the same mask.
10. What is the default administrative distance of static routes?
A. 0
B. 1
C. 90
D. 100
E. 110
11. Which of the following protocols support VLSM? (Choose all that
apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
12. What is the default administrative distance of IGRP?
A. 0
B. 1
C. 90
D. 100
E. 110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
57
13. Which of the following protocols use only hop count to determine the
best path to a remote network? (Choose all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
14. What is the default administrative distance of RIP?
A. 0
B. 1
C. 90
D. 100
E. 120
15. Which of the following routing protocols use a composite metric?
(Choose all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
58
Chapter 2
Routing Principles
16. Which of the following consider only bandwidth as a metric? (Choose
all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
17. Which of the following must have a hierarchical network design to
operate properly? (Choose all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
18. Which of the following routing protocols are non-proprietary?
(Choose all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
59
19. Which of the following are proprietary routing protocols? (Choose all
that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
20. Which of the following automatically summarize at classful bound-
aries? (Choose all that apply.)
A. RIP
B. RIPv2
C. IGRP
D. EIGRP
E. IS-IS
F. OSPF
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
60
Chapter 2
Routing Principles
Answer to Written Lab
Protocol
Characteristic
RIP, RIPv2
Maximum of 15 hops by default
RIP, RIPv2
Administrative distance of 120
IGRP
Administrative distance of 100
EIGRP
Administrative distance of 90
OSPF
Administrative distance of 110
IGRP, EIGRP
Uses composite metric to determine the best path to a
remote network
RIP, RIPv2
Uses only hop count to determine the best path to a
remote network
RIPv2, EIGRP,
IS-IS, OSPF
VLSM support
RIP, RIPv2,
IS-IS, OSPF
Non-proprietary
OSPF
Floods network with LSAs to prevent loops
IS-IS, OSPF
Must have hierarchical network design
IS-IS, OSPF
Uses only bandwidth as a metric
EIGRP, IS-IS,
OSPF
Uses more than one table to assist in rapid
convergence
EIGRP, IS-IS,
OSPF
Route update contains only needed information
RIP, RIPv2,
IGRP, EIGRP
Automatic route summarization at major network
boundaries
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
61
Answers to Review Questions
1. A, C. Link-state does not send entire routing table updates like dis-
tance-vector does. Link-state uses Hello messages to make sure that
neighbor routers are still alive, and then when a change in the network
does occur, it sends only the necessary information about the change.
2. B, C. IGRP and RIP are distance-vector protocols and do not use a
topology table.
3. C. RIPv2 is still distance-vector and acts accordingly. However, it
sends prefix routing information in the route updates so that it can
support VLSM.
4. A. Directly connected routes have an administrative distance of zero.
5. A, B, C, D. Although EIGRP is really a hybrid routing
protocol, it is considered an advanced distance-vector protocol.
6. E. OSPF has an administrative distance of 110.
7. C. Classless routing protocols send prefix routing information with
each update.
8. C. EIGRP routes have a default administrative distance
of 90.
9. D. Classful routing protocols send no subnet mask information with
the routing updates, so all devices on the network must use the same
subnet mask.
10. B. Static routes have a default administrative distance of 1.
11. B, D, E, F. RIPv2, EIGRP, IS-IS, and OSPF all send prefix routing
information with the route updates.
12. D. IGRP routes have a default administrative distance of 100.
13. A, B. Both RIP and RIPv2 consider only hop count as a metric.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
62
Chapter 2
Routing Principles
14. E. RIP routes have a default administrative distance of 120.
15. C, D. IGRP and EIGRP use a composite metric of bandwidth and
delay of the line.
16. E, F. IS-IS and OSPF use only bandwidth as a metric.
17. E, F. Both IS-IS and OSPF must be run on a hierarchical network
design to properly work.
18. A, B, E, F. IGRP and EIGRP are Cisco proprietary routing protocols.
19. C, D. IGRP and EIGRP are Cisco proprietary routing protocols.
20. A, B, C, D. RIP, RIPv2, IGRP, and EIGRP auto-summarize at classful
boundaries. EIGRP, OSPF, and IS-IS can be manually configured to
summarize at non-classful boundaries.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
3
IP Addressing
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
Review the fundamental concepts of IP addressing
Gain an understanding of how IP addresses can be depleted if
used inefficiently
Understand the benefits of VLSM (Variable-Length
Subnet Mask)
Learn how to calculate VLSM
Explain how OSPF supports the use of VLSM
Explain how EIGRP supports the use of VLSM
Become familiar with CIDR (Classless Interdomain Routing)
Recognize the benefits of route summarization
Detail how to disable automatic route summarization for
classless routing protocols
Examine how to use IP unnumbered interfaces
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
I
n this chapter, we will discuss IP addressing. However, we will
assume a basic understanding of IP addressing and subnetting.
Even though this chapter does cover a review of IP addressing, you must have
a fundamental understanding of IP subnetting before reading this chapter.
The CCNA Study Guide, 2nd Edition by Sybex has a complete chapter on IP
addressing and subnetting. Please read that chapter prior to reading this
chapter.
After we review IP addressing, we will provide detailed descriptions and
examples of advanced IP addressing techniques that you can use on your
production networks. First, we’ll discuss Variable-Length Subnet Masks
(VLSMs) and provide an example to show how VLSMs can be used to help
save precious address space on your network.
After discussing VLSMs, we will provide an understanding of Classless
Interdomain Routing (CIDR) as well as summarization techniques.
After you have read the chapter, you can use both the written and handson labs to help you better prepare for using the advanced IP addressing techniques found in this chapter. Also, to help you study for the CCNP Routing
exam, be sure to read the review questions at the end of this chapter.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review of IP Addressing
65
Review of IP Addressing
One of the most important topics in any discussion of TCP/IP is IP
addressing. An IP address is a numeric identifier assigned to each machine on
an IP network. It designates the location of a device on the network. An IP
address is a software address, not a hardware address. A hardware address
is hard-coded on a network interface card (NIC) and used for finding hosts
on a local network. IP addressing was designed to allow a host on one network to communicate with a host on a different network, regardless of the
type of LANs in which the hosts are participating.
Before we get into the more difficult aspects of IP addressing, let’s look at
some of the basics.
IP Terminology
In this chapter, we’ll introduce you to a number of terms that are fundamental to an understanding of TCP/IP. We’ll start by defining a few that are the
most important:
Bit One digit; either a 1 or a 0.
Byte Seven or eight bits, depending on whether parity is used. For the
rest of this chapter, always assume that a byte is eight bits.
Octet Always eight bits; the Base 8 addressing scheme.
Network address The designation used in routing to send packets to a
remote network; for example, 172.16.0.0 and 10.0.0.0.
Broadcast address Used by applications and hosts to send information
to all nodes on a network; for example, 172.16.255.255 and
10.255.255.255.
The Hierarchical IP Addressing Scheme
An IP address is made up of 32 bits of information. These are divided into
four sections, referred to as octets or bytes, containing one byte (eight bits)
each. You can depict an IP address using three methods:
Dotted-decimal, as in 172.16.30.56
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
66
Chapter 3
IP Addressing
Binary, as in 10101100.00010000.00011110.00111000
Hexadecimal, as in AC 10 1E 38
All of these examples represent the same IP address. Although hexadecimal is
not used as often as dotted-decimal or binary when IP addressing is discussed, you might find an IP address stored as hexadecimal in some programs. The 32-bit IP address is a structured, or hierarchical, address, as
opposed to a flat, or nonhierarchical, address. Although either type of
addressing scheme could have been used, the hierarchical variety was chosen
for a good reason.
The advantage of the hierarchical scheme is that it can handle a large
number of addresses, namely 4.2 billion (a 32-bit address space with two
possible values for each position—either 0 or 1—gives you approximately
4.2 billion). The disadvantage of the flat address scheme, and the reason it’s
not used for IP addressing, relates to routing. If every address were unique,
all routers on the Internet would need to store the address of each and every
machine on the Internet. This would make efficient routing impossible, even
if only a fraction of the possible addresses were used.
The solution to this flat address dilemma is to use a two- or three-level
hierarchical addressing scheme that is structured by network and host or by
network, subnet, and host. A two- or three-level hierarchy is comparable to
the sections of a telephone number. The first section, the area code, designates a very large area. The second section, the prefix, narrows the scope to
a local calling area. The final segment, the customer number, zooms in on the
specific connection. IP addresses use the same type of layered structure.
Rather than all 32 bits being treated as a unique identifier, as in flat addressing, one part of the address is designated as the network address, and the
other part is designated as either the subnet or host address. Note that in
some literature, the host address may be referred to as the node address.
In the following sections, we will discuss network addressing and the
three different address classes:
Class A addresses
Class B addresses
Class C addresses
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review of IP Addressing
67
Network Addressing
The network address uniquely identifies each network. Every machine on the
same network shares that network address as part of its IP address. In the IP
address 172.16.30.56, for example, 172.16 is the network address by
default.
The host address is assigned to, and uniquely identifies, each machine on
a network. This part of the address must be unique because it identifies a particular machine—an individual—as opposed to a network, which is a group.
In the sample IP address 172.16.30.56, .30.56 is the host address by
default.
The designers of the Internet decided to create classes of networks based
on network size. For the small number of networks possessing a very large
number of nodes, they created the rank Class A network. At the other
extreme is the Class C network, reserved for the numerous networks with a
small number of nodes. The class distinction for networks between very
large and very small is predictably called a Class B network. Subdividing an
IP address into a network and node address is determined by the class designation of a network. Table 3.1 provides a summary of the three classes of
networks, which will be described in much more detail throughout this
chapter.
TABLE 3.1
The Three Classes of IP Addresses Used in Networks Today
Class
Leading
Bit
Pattern
Default
Subnet Mask
Address Range
Number of
Addresses
A
0
255.0.0.0
1.0.0.0–126.0.0.0
16,777,214
B
10
255.255.0.0
128.1.0.0–
191.254.0.0
65,534
C
110
255.255.255.0
192.0.1.0–
223.255.254.0
254
To ensure efficient routing, Internet designers defined a mandate for the
leading bits section of the address for each network class. For example, since
a router knows that a Class A network address always starts with 0, the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
68
Chapter 3
IP Addressing
router might be able to speed a packet on its way after reading only the first
bit of its address. This is where the address schemes define the difference
between a Class A, a Class B, and a Class C address.
Some IP addresses are reserved for special purposes, and network administrators shouldn’t assign them to nodes. Table 3.2 lists the members of this
exclusive little club and explains why they’re included in it.
TABLE 3.2
Reserved IP Addresses
Address
Function
Network address of all zeros
Interpreted to mean “this network
or segment.”
Network address of all ones
Interpreted to mean “all networks.”
Network 127
Reserved for loopback tests. Designates the local node and allows that
node to send a test packet to itself
without generating network traffic.
Node address of all zeros
Interpreted to mean “this network.”
Node address of all ones
Interpreted to mean “all nodes” on
the specified network; for example,
128.2.255.255 means “all nodes”
on network 128.2 (Class B address).
Entire IP address set to all zeros
Used by Cisco routers to designate
the default route.
Entire IP address set to all ones
(same as 255.255.255.255)
Broadcast to all nodes on the current network; sometimes called an
“all ones broadcast.”
We will now take a look at the different network address classes:
Class A addresses
Class B addresses
Class C addresses
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review of IP Addressing
69
Class A Addresses
In a Class A address, the first byte is assigned to the network address, and the
three remaining bytes are used for the node addresses. The Class A format is
Network.Node.Node.Node
For example, in the IP address 49.22.102.70, 49 is the network address and
22.102.70 is the node address. Every machine on this particular network
would have the distinctive network address of 49.
Class A network addresses are one byte long, with the first bit of that byte
reserved and the seven remaining bits available for manipulation. Thus, the
maximum number of Class A networks that can be created is 128. Why?
Because each of the seven bit positions can either be 0 or 1, thus 27 or 128.
But to complicate things further, it was also decided that the network
address of all zeros (0000 0000) would be reserved to designate the default
route (see Table 3.1, earlier in this chapter). Thus, the actual number of
usable Class A network addresses is 128 minus 1, or 127. However, the
address 127 is reserved for diagnostics, so that can’t be used, which means
that you can use only numbers 1 through 126 to designate Class A networks.
Each Class A address has three bytes (24-bit positions) for the host
address of a machine. Thus, there are 224—or 16,777,216—unique combinations and, therefore, precisely that many possible unique node addresses
for each Class A network. Because addresses with the two patterns of all
zeros and all ones are reserved, the actual maximum usable number of nodes
for a Class A network is 224 minus 2, which equals 16,777,214.
Here is an example of how to figure out the valid host IDs in a Class A
network.
10.0.0.0
All host bits off is the network address.
10.255.255.255
All host bits on is the broadcast address.
The valid hosts are the numbers in between the network address and the
broadcast address: 10.0.0.1 through 10.255.255.254. Notice that zeros and
255s are valid host IDs. All you need to remember when trying to find valid
host addresses is that the host bits cannot all be turned off or on at the
same time.
When you go out to request a network number from the NIC, don’t
expect to be assigned a Class A address. These have all been taken for quite
some time. Big names such as HP and IBM got in the game early enough to
have their own Class A network. However, a check of the IANA records
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
70
Chapter 3
IP Addressing
shows that several corporations were handed Class A addresses back in 1995
and that Stanford University’s Class A was revoked in July 2000. The
records also indicate that the IANA has control of many Class A addresses,
ones that have not been allocated to regional ISPs. A company can also buy
another company to get a Class A network ID. For example, Compaq got the
16 network by acquiring Digital.
Class B Addresses
In a Class B address, the first two bytes are assigned to the network address,
and the remaining two bytes are used for host addresses. The format is
Network.Network.Node.Node
For example, in the IP address 172.16.30.56, the network address is 172.16,
and the host address is 30.56.
With a network address being two bytes of eight bits each, there would be
65,536 unique combinations. But the Internet designers decided that all
Class B addresses should start with the binary digits 1 and 0. This leaves 14
bit positions to manipulate; therefore, there are 16,384 unique Class B
addresses.
A Class B address uses two bytes for node addresses. This is 216 minus the
two reserved patterns (all zeros and all ones), for a total of 65,534 possible
node addresses for each Class B network.
Here is an example of how to find the valid hosts in a Class B network:
172.16.0.0
All host bits turned off is the network address.
172.16.255.255
All host bits turned on is the broadcast address.
The valid hosts would be the numbers in between the network address and
the broadcast address: 172.16.0.1 through 172.16.255.254.
Just as we saw with Class A addresses, all Class B addresses have also been
assigned. Many universities, which were connected to the Internet in the
early ’90s, in addition to many big-name organizations such as Microsoft,
Cisco, Xerox, Novell, and Sun Microsystems, have all of these addresses
consumed. However, they are available under the right circumstances.
Class C Addresses
The first three bytes of a Class C address are dedicated to the network portion of the address, with only one measly byte remaining for the host
address. The format is
Network.Network.Network.Node
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
71
Using the example IP address 192.168.100.102, the network address is
192.168.100, and the host address is 102.
In a Class C address, the first three bit positions are always the binary
110. The calculation is as follows: 3 bytes, or 24 bits, minus 3 reserved positions, equals 21 positions. There are, therefore, 221, or 2,097,152, possible
Class C networks.
Each unique Class C network uses one byte for node addresses. This leads
to 28, or 256, minus the two reserved patterns of all zeros and all ones, for
a total of 254 node addresses for each Class C network.
Here is an example of how to find a valid host ID in a Class C network:
192.168.100.0
All host bits turned off is the network ID.
192.168.100.1
The first host.
192.168.100.254
The last host.
192.168.100.255
All host bits turned on is the broadcast address.
Extending IP Addresses
I
n the “old days,” when the Network Information Center (NIC)
assigned a network number to an organization, it either assigned the first
octet (a Class A network), the first two octets (a Class B network), or the
first three octets (a Class C network). The organization could take this one
network number and further subdivide it into smaller networks through a
process called subnetting.
To illustrate, let’s say that our organization has been assigned the Class B
network 172.16.0.0. We have several different network segments, each of
which needs a unique network number. So, we decide to subnet our network.
We use a subnet mask of 255.255.255.0. The subnet mask determines which
portion of our IP address belongs to the network portion and which part
belongs to the host portion. If we write our subnet mask out in binary, as
illustrated in Table 3.3, the ones correspond to the network portion of the
address, and the zeros correspond to the node portion of the address.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
72
Chapter 3
IP Addressing
TABLE 3.3
IP Address Example
Decimal
172
16
0
0
Binary
10101100
00010000
00000000
00000000
Decimal
255
255
255
0
Binary
11111111
11111111
11111111
00000000
So, in our case, instead of having one network (172.16.0.0) with 65,534
available hosts numbers, we have 254 networks (172.16.1.0–172.16.254.0)
with 254 available host numbers in each subnet.
We can calculate the number of hosts available on a subnet by using the
formula 2n – 2 = number of available host IPs, where n is the number
of hosts bits (in our example, 8). The minus 2 (–2) represents all host bits on
and all hosts bits off, which are reserved.
Similarly, the number of networks (or subnets) can be calculated with
nearly the same formula: 2n – 2 = number of available networks, where
n is the number of subnet bits (in our example, 8). So, with subnetting we
have balanced our need for available network and host numbers. However,
there may be instances where we need fewer host numbers on a particular
subnet and more host numbers on another. The –2 represents all subnet bits
on and all subnet bits off.
Let’s extend our example to include a serial link between two routers, as
shown in Figure 3.1.
FIGURE 3.1
IP address example
Network 172.16.10.0/24
172.16.10.1
172.16.10.2
Since these are routers and not switches, each interface belongs to a different network. The interfaces need to share a network to talk. How many
IP numbers do we really need on the network interconnecting the two routers? We only need two IP numbers, one for each serial interface, as shown in
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
73
Table 3.3. Unfortunately, we have an eight-bit subnet mask (i.e.,
255.255.255.0), so we are wasting 252 of the 254 available numbers on the
subnet. One possible solution to this dilemma is to use Variable-Length
Subnet Masks (VLSMs).
Variable-Length Subnet Masks
As the name suggests, with Variable-Length Subnet Masks we can have different subnet masks for different subnets. So, for the serial link in our example above, we could have a subnet mask of 255.255.255.252. If we do the
math and look at our subnet in binary, we see that we have only two host
bits, as shown in Table 3.4.
TABLE 3.4
VLSM Example 1
Decimal
255
255
255
252
Binary
11111111
11111111
11111111
11111100
Therefore, this subnet mask will give us only two host IPs (22 – 2 = 2),
which is exactly what we need for our serial link.
As another example, consider what would happen if we were running out
of IP numbers on a particular subnet. Perhaps we have several Web servers
on network 172.16.10.0. With our subnet mask of 255.255.255.0 (which
we could also write as 172.16.0.0/24, due to the 24 bits in our subnet mask), we
have only 254 available host addresses. However, in our Web server implementation, each URL needs its own IP number, and our need for unique IP
numbers is about to grow beyond the 254 numbers that we have available.
It is possible to support several URLs to an IP address, but for our example
we’ll use only one address per URL.
Yet again, VLSMs can provide a solution. Instead of making our subnet
mask longer, as in the previous example, we can make our subnet mask
shorter, sometimes called supernetting. In Table 3.5, let’s examine what
would happen if we backed our subnet mask off from 24 bits to 23 bits.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
74
Chapter 3
IP Addressing
TABLE 3.5
VLSM Example 2
Decimal
255
255
254
0
Binary
11111111
11111111
11111110
00000000
We now have a network of 172.16.10.0 with a subnet mask of
255.255.254.0, which can also be written as 172.16.10.0/23. Again, by
doing the math (29 – 2 = 510, since we have nine host bits), we see that we
now have 510 available IP addresses instead of 254.
Let’s work through a VLSM design example, as depicted in Figure 3.2.
FIGURE 3.2
VLSM design example
Class B Address: 172.16.0.0
Server Farm
(300 Addresses Needed)
Ethernet User Segment
(200 Addresses Needed)
Router Interconnections
(4 Addresses Needed)
Point to Point HDLC Link
(2 Addresses Needed)
In our example, we have the following set of requirements for our network addressing:
A server farm requires 300 IP addresses.
A user segment requires 200 IP addresses.
A serial link between two routers requires two IP addresses.
A hub segment interconnecting four routers requires four IP addresses.
We have been assigned the Class B network of 172.16.0.0.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
75
We will now go through a simple three-step process for efficiently calculating the IP address ranges to be assigned to each segment. We say “efficiently,” because we will be using the minimum number of IP addresses
required to accomplish our goal.
1. Create a table detailing the segments and the number of hosts required
on each segment, as shown in Table 3.6.
TABLE 3.6
Number of IP Addresses Used in Figure 3.2
Description of Segment
Number of IP Addresses
Required
Server farm
300
Ethernet user segment
200
Serial link
2
Router interconnection switched
segment
4
2. Determine the subnet mask required to support the requirements
defined in step 1, and expand the table to list the subnet masks.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
76
Chapter 3
IP Addressing
We can use our formula 2n – 2 = number of hosts to create a handy reference
chart to quickly determine how many hosts can be supported for any given
subnet mask, as shown in Table 3.7. The table goes up to only 1022 hosts,
because Cisco has a design recommendation that you should not have a nonrouted network segment with more than 500 hosts (due to performance problems caused by broadcast traffic). To confuse you further, understand that
some Cisco documentation states that the number of hosts on a segment can
be up to 800! Just keep in mind the amount of traffic when making this
decision.
TABLE 3.7
Number of Hosts Needed in Figure 3.2
Maximum Number of Hosts
Bits in Subnet Mask
Subnet Mask
2
30
255.255.255.252
6
29
255.255.255.248
14
28
255.255.255.240
30
27
255.255.255.224
62
26
255.255.255.192
126
25
255.255.255.128
254
24
255.255.255.0
510
23
255.255.254.0
1022
22
255.255.252.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
77
Referring to our table, we can easily determine the subnet mask required
for each of the segments in our example, as shown in Table 3.8.
TABLE 3.8
Subnet Masks for Figure 3.2
Description of
Segment
Number of IP
Addresses Required
Subnet Mask (Number
of Subnet Bits)
Server farm
300
255.255.254.0 (23)
Ethernet user segment
200
255.255.255.0 (24)
Serial link
2
255.255.255.252 (30)
Router interconnection
switched segment
4
255.255.255.248 (29)
3. Starting with the segment requiring the greatest number of subnet bits,
begin allocating addresses.
Let’s begin with the serial link, which has 30 bits of subnetting. Since all
of our addresses are going to start with 172.16, we will examine only the last
16 bits of the IP address. In Table 3.9, we see the subnet mask, in binary and
the first and last IP number in the range. Remember that the host portion of
the address cannot be all ones (which is the broadcast address) or all zeros
(which is the address of the network, or wire).
TABLE 3.9
Networks, Hosts, and Subnets for Figure 3.2
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet Mask
1
1 1 1 1111
1
1 1 1 1100
255.255.255.252
Network
0
0 0 0 0000
0
0 0 0 0100
172.16.0.4
First IP in
range
0
0 0 0 0000
0
0 0 0 0101
172.16.0.5
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
78
Chapter 3
IP Addressing
TABLE 3.9
Networks, Hosts, and Subnets for Figure 3.2 (continued)
3rd Octet
4th Octet
Decimal IP Address
Last IP in
range
0
0 0 0 0000
0
0 0 0 0110
172.16.0.6
Broadcast
0
0 0 0 0000
0
0 0 0 0111
172.16.0.7
After picking the first available network number (172.16.0.4) given our
30-bit subnet mask and eliminating host IP addresses that are all ones and all
zeros, we have the following range of numbers: 172.16.0.5–172.16.0.6. The
broadcast address is all host bits on, or 172.16.0.7. We can take one of these
numbers and assign it to one side of the serial link. The other number can be
assigned to the other end of the serial link.
Next, we will calculate the range of IP addresses to use for our hub
segment, containing four router interfaces. We pick the first available network address given our 29-bit subnet mask. In this case, the first available
network is 172.16.0.8, as shown in Table 3.10.
TABLE 3.10
IP Address Range for Switched Segment in Figure 3.2
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet Mask
1
1 1 1 1111
1
1 1 1 1000
255.255.255.248
Network
0
0 0 0 0000
0
0 0 0 1000
172.16.0.8
First IP in
range
0
0 0 0 0000
0
0 0 0 1001
172.16.0.9
Last IP in
range
0
0 0 0 0000
0
0 0 0 1110
172.16.0.14
Broadcast
0
0 0 0 0000
0
0 0 0 1111
172.16.0.15
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
79
Eliminating host IP addresses that contain all ones and all zeros as before,
we discover that our IP address range for this segment is 172.16.0.9–
172.16.0.14. The broadcast address is all host bits on, or 172.16.0.15. Now
we will perform the same steps on the Ethernet user segment, as shown in
Table 3.11, and the server farm segment, as shown in Table 3.12.
TABLE 3.11
Valid Addresses for Ethernet Segment in Figure 3.2
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet mask
1
1 1 1 1111
0
0 0 0 0000
255.255.255.0
Network
0
0 0 0 0001
0
0 0 0 0000
172.16.1.0
First IP in
range
0
0 0 0 0001
0
0 0 0 0001
172.16.1.1
Last IP in
range
0
0 0 0 0001
1
1 1 1 1110
172.16.1.254
Broadcast
0
0 0 0 0001
1
1 1 1 1111
172.16.1.255
TABLE 3.12
Valid Addresses for Server Farm Segment in Figure 3.2
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet mask
1
1 1 1 1110
0
0 0 0 0000
255.255.254.0
Network
0
0 0 0 0010
0
0 0 0 0000
172.16.2.0
First IP in
range
0
0 0 0 0010
0
0 0 0 0001
172.16.2.1
Last IP in
range
0
0 0 0 0011
1
1 1 1 1110
172.16.3.254
Broadcast
0
0 0 0 0011
1
1 1 1 1111
172.16.3.255
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
80
Chapter 3
IP Addressing
From these tables, we see that our IP address range for the Ethernet user
segment is 172.16.1.1–172.16.1.254. Also, we see that the IP address range
for the server farm segment is 172.16.2.1–172.16.3.254.
In summary, we have defined the following address ranges for our four
segments, as detailed in Table 3.13.
TABLE 3.13
Valid IP Addresses for All Four Segments Used in Figure 3.2
Description of Segment
Address Range
Server farm
172.16.2.1–172.16.3.254
Ethernet user segment
172.16.1.1–172.16.1.254
Serial link
172.16.0.5–172.16.0.6
Router interconnection switched segment
172.16.0.9–172.16.0.14
We can now take our VLSM address ranges and apply them to our network diagram, as shown in Figure 3.3.
FIGURE 3.3
VLSM example with IP addresses
Class B Address: 172.16.0.0
Server Farm
(172.16.2.1—172.16.3.254)
Ethernet User Segment
(172.16.1.1—172.16.1.254)
Router Interconnections
(172.16.0.9—172.16.0.14)
Point-to-Point HDLC Link
(172.16.0.5—172.16.0.6)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
81
Design Considerations with VLSM
Now that we have seen how valuable VLSMs can be in preserving those
precious IP addresses, be aware that there is a catch. Specifically, if you use
a classful routing protocol (a protocol that advertises routes at the Class A,
Class B, and Class C boundaries) such as RIPv1 or IGRP, then VLSMs are
not going to work.
RIPv1 and IGRP routing protocols do not have a field for subnet information. Therefore, the subnet information gets dropped. This means that if
a router running RIP has a subnet mask of a certain value, it assumes that all
interfaces within the classful address space have the same subnet mask.
Classless routing protocols, however, do support the advertisement of subnet information. So, you can use VLSM with routing protocols such as
RIPv2, EIGRP, or OSPF.
Another important point to consider when assigning addresses is to not
have discontiguous networks. Specifically, if you have two subnets of the
same network separated by a different network, some of your hosts could
become unreachable. Consider the network shown in Figure 3.4.
FIGURE 3.4
Discontiguous networking example
RouterA
RouterB
10.1.1.1
10.1.1.2
WAN Cloud
172.16.1.0/24
172.16.2.0/24
If routes are being summarized—a technique called route summarization—then both RouterA and RouterB will be advertising to the WAN cloud
that they are the route to network 172.16.0.0/16. While there are techniques
to overcome this behavior, which will be discussed in the “Route Summarization” section later in this chapter, it makes for better network design to not
separate a network’s subnets by another network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
82
Chapter 3
IP Addressing
Classless Interdomain Routing
Classless Interdomain Routing (CIDR) is an industry standard for displaying the number of subnet bits used with the IP address of a host or a network.
Let’s say you have a 172.16.10.1 address with a 255.255.255.0 mask.
Instead of writing the IP address and subnet mask separately, you can combine them. For example, 172.16.10.1/24 means that the subnet mask has 24
out of 32 bits on.
The following list shows all the possible CIDRs:
255.0.0.0 = /8
255.128.0.0 = /9
255.192.0.0 = /10
255.224.0.0 = /11
255.240.0.0 = /12
255.248.0.0 = /13
255.252.0.0 = /14
255.254.0.0 = /15
255.255.0.0 = /16
255.255.128.0 = /17
255.255.192.0 = /18
255.255.224.0 = /19
255.255.240.0 = /20
255.255.248.0 = /21
255.255.252.0 = /22
255.255.254.0 = /23
255.255.255.0 = /24
255.255.255.128 = /25
255.255.255.192 = /26
255.255.255.224 = /27
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
83
255.255.255.240 = /28
255.255.255.248 = /29
255.255.255.252 = /30
Notice that the CIDR list starts at a minimum of /8 and can’t go higher
than /30. This is because the mask must at least be a Class A default, and you
must leave two hosts at a minimum.
Let’s now take a look at how Cisco handles CIDR.
Cisco and CIDR
Cisco has not always followed the CIDR standard. Take a look at the way
a Cisco 2500 series router asks you to put the subnet mask in the configuration when using the Setup mode:
Configuring interface Ethernet0:
Is this interface in use? [yes]: return
Configure IP on this interface? [yes]: return
IP address for this interface: 1.1.1.1
Number of bits in subnet field [0]: 8
Class A network is 1.0.0.0, 8 subnet bits; mask is /16
Notice that the router asks for the number of bits used only for subnetting, which does not include the default mask. When dealing with these questions, remember that your answers involve the number of bits used for
creating subnets, not the number of bits in the subnet mask. The industry
standard is that you count all bits used in the subnet mask and then display
that number as a CIDR, for example, /25 is 25 bits.
The newer IOS that runs on Cisco routers, however, runs a Setup script
that no longer asks you to enter the number of bits used only for subnetting.
Here is an example of a new 1700 series router in Setup mode:
Configure IP on this interface? [no]: y
IP address for this interface: 1.1.1.1
Subnet mask for this interface [255.0.0.0]: 255.255.0.0
Class A network is 1.0.0.0, 16 subnet bits; mask is /16
Notice that the Setup mode asks you to enter the subnet mask address. It
then displays the mask using the slash notation format. Much better.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
84
Chapter 3
IP Addressing
Route Summarization
In the “Design Considerations with VLSM” section, we briefly mentioned
the concept of route summarization. So, what is it, and why do we need it?
On very large networks, there may be hundreds or even thousands of individual networks and subnetworks being advertised. All these routes can be
very taxing on a router’s memory and processor.
In many cases, the router doesn’t even need specific routes to each and
every subnet (e.g., 172.16.1.0/24). It would be just as happy if it knew how
to get to the major network (e.g., 172.16.0.0/16) and let another router take
it from there. A router’s ability to take a group of subnetworks and summarize them as one network (i.e., one advertisement) is called route summarization, as shown in Figure 3.5.
In some of the literature, you may find route summarization referred to as
route aggregation.
FIGURE 3.5
Route summarization
172.16.1.0/24
I am the way to
get to network
172.16.0.0/16
172.16.2.0/24
172.16.3.0/24
Besides reducing the number of routing entries that a router must keep
track of, route summarization can also help protect an external router from
making multiple changes to its routing table, due to instability within a particular subnet. For example, let’s say that we were working on a router that
connected to 172.16.2.0/24. As we were working on the router, we rebooted
it several times. If we were not summarizing our routes, an external router
would see each time 172.16.2.0/24 went away and came back. Each time, it
would have to modify its own routing table. However, if our external router
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
85
were receiving only a summary route (i.e., 172.16.0.0/16), then it wouldn’t
have to be concerned with our work on one particular subnet.
We will get the most benefit from route summarization when the networks or subnetworks that we are summarizing are contiguous. To illustrate
this point, let’s look at an example.
Route Summarization Example 1
We have the following networks that we want to advertise as a single summary route:
172.16.100.0/24
172.16.101.0/24
172.16.102.0/24
172.16.103.0/24
172.16.104.0/24
172.16.105.0/24
172.16.106.0/24
To determine what the summary route would be for these networks, we
can follow a simple two-step process.
1. Write out each of the numbers in binary, as shown in Table 3.14.
TABLE 3.14
Summary Example
IP Network Address
Binary Equivalent
172.16.100.0
10101100.0001000.01100100.0
172.16.101.0
10101100.0001000.01100101.0
172.16.102.0
10101100.0001000.01100110.0
172.16.103.0
10101100.0001000.01100111.0
172.16.104.0
10101100.0001000.01101000.0
172.16.105.0
10101100.0001000.01101001.0
172.16.106.0
10101100.0001000.01101010.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
86
Chapter 3
IP Addressing
2. Examine the table to determine the maximum number of bits (starting
from the left) that all of the addresses have in common (where they
stop lining up; we bolded them to make them easier for you to see).
The number of common bits is the subnet mask for the summarized
address (/20).
In our example, we can see from the table that all of the addresses have the
first 20 bits in common. The decimal equivalent of these first 20 bits is
172.16.96.0. So, we can write our new summarized address as 172.16.96.0/20.
If we were to later add a network 172.16.98.0, it would need to come off the
router summarizing this address space. If we didn’t, it could cause problems.
Okay, this is confusing, we know. This is why we’re going to give you three
more examples.
Route Summarization Example 2
In this example, we will summarize 10.1.0.0 through 10.7.0.0. First, put
everything into binary, and then follow the bits, starting on the left and stopping when the bits do not line up. Notice where we stopped boldfacing the
following:
10.1.0.0
00001010.00000001.00000000.00000000
10.2.0.0
00001010.00000010.00000000.00000000
10.3.0.0
00001010.00000011.00000000.00000000
10.4.0.0
00001010.00000100.00000000.00000000
10.5.0.0
00001010.00000101.00000000.00000000
10.6.0.0
00001010.00000110.00000000.00000000
10.7.0.0
00001010.00000111.00000000.00000000
Now, create the network number using only the boldfaced bits. Do not
count the bits that are not in boldface. The second octet has no bits on (bits
in the bolded section), so we get this:
10.0.0.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
87
To come up with the summary mask, count all the bolded bits as ones.
Because eight bits are boldface in the first octet and five bits in the second,
we’ll get this:
255.248.0.0
Route Summarization Example 3
This example will show you how to summarize 172.16.16.0 through
172.16.31.0. First, let’s put the network addresses into binary and then line
up the bits.
172.16.16.0
10101100.0001000.00010000.00000000
172.16.17.0
10101100.0001000.00010001.00000000
172.16.18.0
10101100.0001000.00010010.00000000
172.16.19.0
10101100.0001000.00010011.00000000
172.16.20.0
10101100.0001000.00010100.00000000
172.16.21.0
10101100.0001000.00010101.00000000
172.16.22.0
10101100.0001000.00010110.00000000
172.16.23.0
10101100.0001000.00010111.00000000
172.16.24.0
10101100.0001000.00011000.00000000
172.16.25.0
10101100.0001000.00011001.00000000
172.16.26.0
10101100.0001000.00011010.00000000
172.16.27.0
10101100.0001000.00011011.00000000
172.16.28.0
10101100.0001000.00011100.00000000
172.16.29.0
10101100.0001000.00011101.00000000
172.16.30.0
10101100.0001000.00011110.00000000
172.16.31.0
10101100.0001000.00011111.00000000
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
88
Chapter 3
IP Addressing
Notice where the bits stop lining up (in boldface). Count only the bits that
are on (ones) to get the network address:
172.16.0.0
Now, create the summary mask by counting all the bits that are in boldface up to the point where they stop lining up. We have eight bits in the first
octet, eight bits in the second octet, and four bits in the third octet. That is
a /20 or
255.255.240.0
Boy, that sure seems like a pain in the pencil, huh? Try this shortcut. Take
the first number and the very last number, and put them into binary:
172.16.16.0
10101100.0001000.00010000.00000000
172.16.31.0
10101100.0001000.00011111.00000000
Can you see that we actually came up with the same numbers? It is a lot
easier than writing out possibly dozens of addresses. Let’s do another example, but let’s use our shortcut.
Route Summarization Example 4
In this example, we will show you how to summarize 192.168.32.0 through
192.168.63.0. By using only the first network number and the last, we’ll save
a lot of time and come up with the same network address and subnet mask:
First number: 192.168.32.0 =
11000000.10101000.00100000.00000000
Last number: 192.168.63.0 =
11000000.10101000.00111111.00000000
Network address: 192.168.32.0
Subnet mask: 255.255.224.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
89
Design Considerations for Route Summarization
Keep the following information in mind when designing your network summarization points:
Only classless routing protocols support route summarization. Examples of classless routing protocols include RIPv2, EIGRP, and OSPF.
Therefore, if you are working in a RIPv1 or IGRP environment, route
summarization is not going to work for you.
Classless and classful protocols were discussed in Chapter 2, “Routing
Principles.”
Route summarization is most effective when the addresses have been
organized in a hierarchy (i.e., “hierarchical addressing”). When we
speak of addresses being hierarchical, we mean that the IP subnets at
the “bottom of the tree” (i.e., the ones with the longest subnet masks)
are subsets of the subnets at the “top of the tree” (i.e., the ones with
the shortest subnet masks). Figure 3.6 will be used to illustrate hierarchical versus non-hierarchical addressing.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
90
Chapter 3
IP Addressing
FIGURE 3.6
Discontiguous networking example
Hierarchical Adressing
10.1.0.0/16
10.1.1.0/24
10.1.1.4/30
10.1.2.0/24
10.1.1.8/30
10.1.2.4/30
10.1.2.8/30
Non-Hierarchical Adressing
10.1.0.0/16
172.16.1.0/24
172.16.2.0/24
10.2.0.0/16
10.3.0.0/16
10.1.2.4/30
10.1.2.8/30
In the VLSM section of this chapter, we discussed how route summarization in discontiguous networks could cause some hosts to become unreachable, as we saw in Figure 3.4. If both RouterA and RouterB are sending out
advertisements to the WAN cloud advertising that they are the path to network 172.16.0.0/16, then devices in the WAN cloud will not know which
advertisement to believe.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Extending IP Addresses
91
Remember that you can avoid this situation by proper address planning
ahead of time. However, you may find yourself in a situation where you are
dealing with a legacy installation, and you need to overcome this issue of discontiguous networks.
One solution is to turn off route summarization on the routers. To keep
routing protocols such as RIPv2 and EIGRP from automatically summarizing routes, we can explicitly disable route summarization in the Cisco IOS.
Following are examples of IOS configurations, where we are disabling automatic route summarization. As the OSPF chapters will show, OSPF does not
automatically summarize.
To turn off auto-summarization for RIP version 2 routed networks, use
the following router configuration:
router rip
version 2
network 10.0.0.0
network 172.16.0.0
no auto-summary
To turn off auto-summarization for EIGRP routed networks, use the following router configuration:
router eigrp 100
network 10.0.0.0
network 172.16.0.0
no auto-summary
Another way to allow discontiguous networks to be interconnected over
a serial link is to use Cisco’s IOS feature called IP unnumbered. We’ll look
at this next.
IP Unnumbered
With IP unnumbered, a serial interface is not on a separate network, as all
router interfaces tend to be. Instead, the serial port “borrows” an IP address
from another interface. In the following router configuration example, interface Serial 0 is using a borrowed IP address from interface Ethernet 0:
interface serial 0
ip unnumbered ethernet 0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
92
Chapter 3
IP Addressing
Therefore, by using IP unnumbered, the apparently discontiguous subnets,
shown in Figure 3.4, are actually supported. Understand that both sides of
the network must be the same address class. In other words, you can’t borrow an IP address on one side from a 10.0.0.0 network and then from
172.16.0.0 on the other side of the point-to-point link.
There are a few things to be aware of before using IP unnumbered interfaces.
For example, IP unnumbered is not supported on X.25 or SMDS networks.
Also, since the serial interface has no IP number, you will not be able to ping
the interface to see if it is up, although you can determine the interface status
with SNMP. In addition, IP security options are not supported on an IP unnumbered interface.
Decimal-to-Binary Conversion Chart
F
or your convenience, Table 3.15 provides a decimal-to-binary chart
to help you with your IP addressing.
TABLE 3.15
Decimal-to-Binary Chart
Decimal
Binary
Decimal
Binary
Decimal
Binary
Decimal
Binary
0
00000000
16
00010000
32
00100000
48
00110000
1
00000001
17
00010001
33
00100001
49
00110001
2
00000010
18
00010010
34
00100010
50
00110010
3
00000011
19
00010011
35
00100011
51
00110011
4
00000100
20
00010100
36
00100100
52
00110100
5
00000101
21
00010101
37
00100101
53
00110101
6
00000110
22
00010110
38
00100110
54
00110110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Decimal-to-Binary Conversion Chart
TABLE 3.15
93
Decimal-to-Binary Chart (continued)
Decimal
Binary
Decimal
Binary
Decimal
Binary
Decimal
Binary
7
00000111
23
00010111
39
00100111
55
00110111
8
00001000
24
00011000
40
00101000
56
00111000
9
00001001
25
00011001
41
00101001
57
00111001
10
00001010
26
00011010
42
00101010
58
00111010
11
00001011
27
00011011
43
00101011
59
00111011
12
00001100
28
00011100
44
00101100
60
00111100
13
00001101
29
00011101
45
00101101
61
00111101
14
00001110
30
00011110
46
00101110
62
00111110
15
00001111
31
00011111
47
00101111
63
00111111
64
01000000
80
01010000
96
01100000
112
01110000
65
01000001
81
01010001
97
01100001
113
01110001
66
01000010
82
01010010
98
01100010
114
01110010
67
01000011
83
01010011
99
01100011
115
01110011
68
01000100
84
01010100
100
01100100
116
01110100
69
01000101
85
01010101
101
01100101
117
01110101
70
01000110
86
01010110
102
01100110
118
01110110
71
01000111
87
01010111
103
01100111
119
01110111
72
01001000
88
01011000
104
01101000
120
01111000
73
01001001
89
01011001
105
01101001
121
01111001
74
01001010
90
01011010
106
01101010
122
01111010
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
94
Chapter 3
IP Addressing
TABLE 3.15
Decimal-to-Binary Chart (continued)
Decimal
Binary
Decimal
Binary
Decimal
Binary
Decimal
Binary
75
01001011
91
01011011
107
01101011
123
01111011
76
01001100
92
01011100
108
01101100
124
01111100
77
01001101
93
01011101
109
01101101
125
01111101
78
01001110
94
01011110
110
01101110
126
01111110
79
01001111
95
01011111
111
01101111
127
01111111
128
10000000
144
10010000
160
10100000
176
10110000
129
10000001
145
10010001
161
10100001
177
10110001
130
10000010
146
10010010
162
10100010
178
10110010
131
10000011
147
10010011
163
10100011
179
10110011
132
10000100
148
10010100
164
10100100
180
10110100
133
10000101
149
10010101
165
10100101
181
10110101
134
10000110
150
10010110
166
10100110
182
10110110
135
10000111
151
10010111
167
10100111
183
10110111
136
10001000
152
10011000
168
10101000
184
10111000
137
10001001
153
10011001
169
10101001
185
10111001
138
10001010
154
10011010
170
10101010
186
10111010
139
10001011
155
10011011
171
10101011
187
10111011
140
10001100
156
10011100
172
10101100
188
10111100
141
10001101
157
10011101
173
10101101
189
10111101
142
10001110
158
10011110
174
10101110
190
10111110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Decimal-to-Binary Conversion Chart
TABLE 3.15
95
Decimal-to-Binary Chart (continued)
Decimal
Binary
Decimal
Binary
Decimal
Binary
Decimal
Binary
143
10001111
159
10011111
175
10101111
191
10111111
192
11000000
208
11010000
224
11100000
240
11110000
193
11000001
209
11010001
225
11100001
241
11110001
194
11000010
210
11010010
226
11100010
242
11110010
195
11000011
211
11010011
227
11100011
243
11110011
196
11000100
212
11010100
228
11100100
244
11110100
197
11000101
213
11010101
229
11100101
245
11110101
198
11000110
214
11010110
230
11100110
246
11110110
199
11000111
215
11010111
231
11100111
247
11110111
200
11001000
216
11011000
232
11101000
248
11111000
201
11001001
217
11011001
233
11101001
249
11111001
202
11001010
218
11011010
234
11101010
250
11111010
203
11001011
219
11011011
235
11101011
251
11111011
204
11001100
220
11011100
236
11101100
252
11111100
205
11001101
221
11011101
237
11101101
253
11111101
206
11001110
222
11011110
238
11101110
254
11111110
207
11001111
223
11011111
239
11101111
255
11111111
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
96
Chapter 3
IP Addressing
Summary
After a review of fundamental IP addressing concepts, which detailed
the various classes of IP numbers in addition to the concepts of subnetting
and CIDR, this chapter discussed how to preserve IP addresses by using
VLSMs (Variable-Length Subnet Masks). It also examined various design
considerations, such as using contiguous network addressing and using
classless routing protocols (e.g., RIPv2 and EIGRP).
Next, we introduced the concept of route summarization. We saw how
router resources, such as memory and processor cycles, could be preserved
by representing contiguous network address space by a single route advertisement. We also showed how to overcome the caveat of having discontiguous address space by using such methods as disabling automatic
summarization on our routers and by using IP unnumbered.
Key Terms
Before you take the exam, be sure you are familiar with the following terms:
bytes
Classless Interdomain Routing (CIDR)
IP address
IP unnumbered
octets
route summarization
subnet mask
Variable-Length Subnet Mask (VLSM)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
97
Commands Used in This Chapter
Here is the list of commands used in this chapter:
Command
Description
no auto-summary
Used to disable the automatic route
summarization performed by various
classless routing protocols, such as RIPv2
and EIGRP.
ip unnumbered
Allows serial interfaces to borrow an IP
number from another router interface
(which may or may not be specified), so that
it can join two contiguous address spaces.
Written Lab
Given the following set of address requirements, the available Class B
network address, and the topology map shown in the graphic below, use
VLSM to efficiently assign addresses to each of the four network segments.
Server Farm Switch
(Requires 50 IP Addresses)
Ethernet User Segment
(Use Class C Subnet Mask)
Public Access Computer Lab Switch
(Requires 400 IP Addresses)
Point-to-Point Serial Connection
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
98
Chapter 3
IP Addressing
Design Requirements
You have been given the Class B address of 172.16.0.0/16 to use.
The first segment connects to a server farm requiring no more than 50
IP addresses.
The second segment is a serial connection to a remote router. Due to
security concerns, you should not use IP unnumbered.
The third segment is a large publicly accessible computer laboratory
containing 400 PCs, each of which requires its own unique IP address.
The forth segment is an Ethernet user LAN. To simplify management,
the network administrator has requested that the LAN have a Class C
subnet mask.
Solution to Written Exercise
Although there are multiple ways that the given address space (172.16.0.0/
16) could be divided up, here is one possible solution based on the methodology presented in this chapter.
1. Create a table detailing the segments and the number of hosts required
on each segment, as shown in the following table:
Description of Segment
Number of IP Addresses Required
Server farm
50 (Because the maximum number of
servers is 50)
Ethernet user segment
254 (Because a Class C subnet was
specified)
Serial link
2 (Because each of the two routers needs
one IP address)
Computer lab
400 (Because each PC needs its own
IP address)
2. Determine the subnet mask required to support the requirements
defined in step 1, and expand the table to list the subnet masks. We
will use the table listed earlier in the chapter (Table 3.7), which tells
the maximum number of hosts permitted by each subnet mask. The
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
99
following table shows the number of IP addresses required and the
subnet masks needed to support the network.
Description of
Segment
Number of IP
Addresses Required
Subnet Mask (Number
of Bits in Subnet)
Server farm
50 (Because the
maximum number of
servers is 50)
255.255.255.192 (26)
Ethernet user
segment
254 (Because a
Class C subnet was
specified)
255.255.255.0 (24)
Description of
Segment
Number of IP
Addresses Required
Subnet Mask (Number
of Bits in Subnet)
Serial link
2 (Because each of the
two routers needs one
IP address)
255.255.255.252 (30)
Computer lab
400 (Because each PC
needs its own IP
address)
255.255.254 (23)
3. Beginning with the segment requiring the greatest number of subnet
bits, begin allocating addresses.
We’ll do the serial link first, since it has 30 bits of subnetting. Since all of
our addresses begin with 172.16, we will examine only the last 16 bits of the
IP address. In the following table, we show the subnet mask, in binary, and
the first and last IP number in the range. Remember that the host portion of
the address cannot be all ones or all zeros.
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet mask
1
1 1 1 1111
1
1 1 1 1100
255.255.255.252
Network
0
0 0 0 0000
0
0 0 0 0100
172.16.0.4
First IP in
range
0
0 0 0 0000
0
0 0 0 0101
172.16.0.5
Last IP in
range
0
0 0 0 0000
0
0 0 0 0110
172.16.0.6
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
100
Chapter 3
IP Addressing
After picking the first available network number (172.16.0.4) given our
30-bit subnet mask and eliminating host IP addresses that are all ones and all
zeros, we have the following range of numbers: 172.16.0.5–172.16.0.6.
Each of these numbers in the range can be assigned to one side of the serial link.
Next, as shown in the following table, we will calculate the range of IP
addresses to use for our server farm segment, which needs 50 IP addresses.
We pick the first available network address, given our 26-bit subnet mask.
In this case, the first available network is 172.16.0.64.
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet mask
1
1 1 1 1111
1
1 0 0 0000
255.255.255.248
Network
0
0 0 0 0000
0
1 0 0 0000
172.16.0.64
First IP in
range
0
0 0 0 0000
0
1 0 0 0001
172.16.0.65
Last IP in
range
0
0 0 0 0000
0
1 1 1 1110
172.16.0.126
Eliminating host IP addresses that contain all ones and all zeros, as before,
we discover that our IP address range for this segment is: 172.16.0.65–
172.16.0.126.
We now perform the same steps for the Ethernet user segment, as shown
in the table below:
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet mask
1
1 1 1 1111
0
0 0 0 0000
255.255.255.0
Network
0
0 0 0 0001
0
0 0 0 0000
172.16.1.0
First IP in
range
0
0 0 0 0001
0
0 0 0 0001
172.16.1.1
Last IP in
range
0
0 0 0 0001
1
1 1 1 1110
172.16.1.254
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
101
We now perform the same steps for the public lab segment, as shown in
the following table:
3rd Octet
4th Octet
Decimal IP Address
128 64 32 16 8 4 2 1
128 64 32 16 8 4 2 1
(Last 16 bits in bold)
Subnet mask
1
1 1 1 1110
0
0 0 0 0000
255.255.254.0
Network
0
0 0 0 0010
0
0 0 0 0000
172.16.2.0
First IP in
range
0
0 0 0 0010
0
0 0 0 0001
172.16.2.1
Last IP in
range
0
0 0 0 0011
1
1 1 1 1110
172.16.3.254
In summary, we have defined the address ranges for our four segments
shown in the following table:
Description of Segment
Address Range
Server farm
172.16.0.65–172.16.0.126
Ethernet user segment
172.16.1.1–172.16.1.254
Serial link
172.16.0.5–172.16.0.6
Computer lab
172.16.2.1–172.16.3.254
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
102
Chapter 3
IP Addressing
We can now take our VLSM address ranges and apply them to our network diagram, as shown in the following graphic.
Server Farm Switch
(172.16.0.65—172.16.0.126)
Ethernet User Segment
(172.16.1.1—172.16.1.254)
Public Access Computer Lab Switch
(172.16.2.1—172.16.3.254)
Point-to-Point Serial Connection
(172.16.0.5—172.16.0.6)
Hands-on Lab
For this lab, you will need the following:
Two Cisco routers running IOS 11.2 or later, each with at least one
serial interface
A serial crossover cable (or connect a DTE cable to a DCE cable to
make your own crossover cable)
A terminal (or a PC running terminal emulation software) with the
appropriate console connection hardware for the routers
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
103
Using IP Unnumbered
Following are the steps required to complete the lab:
1. Physically connect the routers, as shown in the diagram.
Interface S0
DTE
IP unnumbered
Interface S0
DCE
IP unnumbered
RouterA
RouterB
Loopback Interface: Io0
172.16.1.1/24
Loopback Interface: Io0
172.16.2.1/24
2. Configure a loopback interface on RouterA with an IP number of
172.16.1.1 and a 24-bit subnet mask:
hostname RouterA
interface lo0
ip address 172.16.1.1 255.255.255.0
no shut
3. Configure a loopback interface on RouterB with an IP number of
172.16.2.1 and a 24-bit subnet mask:
hostname RouterB
interface lo0
ip address 172.16.2.1 255.255.255.0
no shut
4. Interconnect the serial ports on the routers with a serial crossover
cable (RouterA connected to the DTE side of the cable and RouterB
connected to the DCE side of the cable).
5. For RouterA, configure for RIPv2:
router rip
version 2
network 172.16.0.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
104
Chapter 3
IP Addressing
6. For RouterB, configure for RIPv2:
router rip
version 2
network 172.16.0.0
7. Configure the serial interfaces on RouterA for IP unnumbered:
interface s0
ip unnumbered lo0
no shut
8. Configure the serial interfaces on RouterB for IP unnumbered:
interface s0
ip unnumbered lo0
clockrate 56000
no shut
9. To test your configuration, from RouterA, ping 172.16.2.1.
10. To further test your configuration, from RouterB, ping 172.16.1.1.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
105
Review Questions
1. A router can determine that an IP address is part of a Class B network
by examining the first two bits in the IP address. What are the first two
bits for a Class B network?
A. 00
B. 01
C. 10
D. 11
2. What does an IP address of 127.0.0.1 indicate?
A. A local broadcast
B. A directed multicast
C. The local network
D. A local loopback
3. Which of the following subnet masks will support 50 IP addresses?
(Choose all that apply.)
A. 255.255.255.240
B. 255.255.255.0
C. 255.255.255.192
D. 255.255.255.224
4. VLSM is compatible with which of the following routing protocols?
(Choose all that apply.)
A. RIPv1
B. RIPv2
C. IGRP
D. EIGRP
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
106
Chapter 3
IP Addressing
5. Which of the following best describes route summarization?
A. A router’s ability to take a group of subnetworks and summarize
them as one network advertisement
B. The Cisco IOS feature that permits serial interfaces to borrow an
IP address from another specified interface
C. The ability to tunnel IP address information inside an AURP
encapsulated frame
D. EIGRP’s ability to isolate discontiguous route advertisements from
one AS to another
6. Which of the following best summarizes the networks 172.16.100.0/24
and 172.16.106.0/24?
A. 172.16.0.0/24
B. 172.16.100.0/20
C. 172.16.106.0/20
D. 172.16.96.0/20
7. Which of the following is a good design practice for implementing
route summarization?
A. Use primarily with discontiguous networks.
B. Use primarily with contiguous networks.
C. Do not use with VLSM.
D. Use with non-hierarchical addressing.
8. Which of the following router-configuration commands would you
use to disable automatic route summarization in an EIGRP
environment?
A. no summary
B. no auto-summary
C. no summary stub
D. no route-summary
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
107
9. Which of the following are caveats of Cisco’s IP unnumbered IOS fea-
ture? (Choose all that apply.)
A. Does not work over HDLC networks.
B. Is not compatible with SNMP.
C. Does not work over X.25 networks.
D. You cannot ping an unnumbered interface.
10. If you have a subnet mask of 255.255.255.248, what is another way
of displaying this mask?
A. /17
B. /23
C. /27
D. /29
11. Given the VLSM address 172.16.1.8/30, what are the two IP
addresses in the range that may be assigned to hosts?
A. 172.16.1.8
B. 172.16.1.9
C. 172.16.1.10
D. 172.16.1.11
12. Given an IP address of 172.16.0.10/29, what is the network address?
A. 172.16.0.8
B. 172.16.0.9
C. 172.16.0.11
D. 172.16.0.12
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
108
Chapter 3
IP Addressing
13. Which of the following may be done to overcome problems associated
with discontiguous networks?
A. Use route summarization.
B. Use VLSM.
C. Use IP unnumbered.
D. Disable route summarization.
14. Which of the following IP addresses do Cisco routers use to designate
the default route?
A. 1.1.1.1
B. 0.0.0.0
C. 255.255.255.255
D. 127.0.0.1
15. If you decide not to use IP unnumbered on a serial link, to best pre-
serve IP addresses, what should your subnet mask be?
A. 255.255.255.255
B. 255.255.255.0
C. 255.255.255.252
D. 255.255.252.0
16. What are the two methods that are most commonly used to represent
an IP address?
A. Dotted-decimal
B. Octal
C. Binary
D. Hexadecimal
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
109
17. An IP address is made up of what? (Choose all that apply.)
A. four octets
B. 32 bits
C. four bytes
D. eight bits
18. Route summarization is particularly effective in which of the follow-
ing environments? (Choose all that apply.)
A. When a large number of contiguous network numbers are being
advertised
B. When IGRP is being used as the routing protocol
C. When EIGRP is being used as the routing protocol
D. When VLSMs are in use
19. If you have an IP address of 172.16.1.10/25, what is the broadcast
address that the host will use?
A. 172.16.255.255
B. 172.16.1.255
C. 172.16.1.0
D. 172.16.1.127
20. How many hosts will the 255.255.255.224 subnet mask support?
A. 6
B. 14
C. 30
D. 62
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
110
Chapter 3
IP Addressing
Answers to Review Questions
1. C. A leading bit pattern of 0 indicates a Class A network. A leading bit
pattern of 10 indicates a Class B network. A leading bit pattern of 110
indicates a Class C network.
2. D. Network 127 is reserved for loopback purposes (e.g., for trouble-
shooting diagnostics). With a local loopback address, a host can send
a test packet to itself without generating network traffic.
3. B, C. The formula 2n – 2 = number of hosts (where n is the number of
host bits in the subnet mask) tells us how many hosts can be supported
for a particular subnet. Here are some examples:
255.255.255.240 => 4 host bits => 14 hosts
255.255.255.0 => 8 host bits => 254 hosts
255.255.255.192 => 6 host bits => 62 hosts
255.255.255.224 => 5 host bits => 30 hosts
4. B, D. VLSM is compatible only with classless routing protocols.
Classless routing protocols have the ability to carry subnet information in their route advertisements. RIPv1 and IGRP are classful,
whereas RIPv2 and EIGRP are classless.
5. A. Route summarization, which works best with contiguous address
space, reduces the memory and processor burden on routers by representing multiple subnets in a single route advertisement.
6. D. If you write out the networks 172.16.100.0/24 and 172.16.106.0/
24 in binary and see how many leading bits they have in common, you
will find that the first 20 bits are the same for both networks. If you
then convert these 20 bits back into decimal, you will have the address
of the summarized route.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
111
7. B. Route summarization is most effective when used with contiguous
address space, because contiguous address space tends to have the
most higher-order bits in common.
8. B. The command no auto-summary is a router-configuration com-
mand that disables the automatic summarization of routes.
9. C, D. IP unnumbered is not supported on X.25 or SMDS networks.
Since the serial interface has no IP number, you will not be able to ping
the interface to see if it is up. However, you can determine the interface
status with SNMP. Also, IP security options are not supported on an
IP unnumbered interface.
10. D. If you write out 255.255.255.248 in binary, you will find that the
first 29 bits are ones, and the remaining three bits are zeros. Therefore,
we say that it is a /29.
11. B, C. If we look at each of these IP addresses and the subnet mask in
binary, we see that the host portion of the address for 172.16.1.8 is all
zeros, which refers to the network. The host portion of 172.16.1.9 is
01, which is valid for a host address. The host portion of 172.16.1.10 is
10, which is also valid for a host address. The host portion of
172.16.1.11 is 11, which refers to the broadcast address.
12. A. If you write out the last octet in binary, you have 00001010. Since
we are using a 29-bit subnet mask, the last three bits are not part of the
network address, which leaves us with a network address of
00001000. If we convert this number back to decimal, we get 8.
Therefore, the network address is 172.16.0.8.
13. C and D. IP unnumbered can, in some cases, make a discontiguous
network appear as contiguous across a serial link. Also, if you disable
route summarization, then a router will advertise each individual subnet. However, answer D only works with routing protocols that can
carry subnet information, otherwise you still end up with a discontiguous network, for example EIGRP and RIP V2.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
112
Chapter 3
IP Addressing
14. B. The command to set a default route in Cisco IOS is set ip route
0.0.0.0 0.0.0.0 next_hop (where next_hop can be a local interface or the IP of an adjacent router interface).
15. C. On a serial link, we need only two IP addresses, one for each side
of the link. A subnet mask of 255.255.255.252 has only two host bits,
which gives us a maximum of two host IP addresses (22 – 2 = 2), which
is exactly what we need for our serial link.
16. A, C. Although an IP number can be represented in practically any
base of numbering system, dotted-decimal and binary are the most
common representations.
17. A, B, C. An IP address is 32 bits in length. An octet is eight bits in
length. Therefore, four octets equal 32 bits. Since a byte is eight bits,
four bytes also equal 32 bits.
18. A, C, D. Route summarization is not compatible with IGRP, because
IGRP is a classful routing protocol, meaning that it does not carry subnet information in its routing updates.
19. D. With 25 bits of subnetting, we have the last seven bits to use as the
host address. If we set each of these last seven bits to 1 (the definition
of a broadcast address), then we get 172.16.1.127.
20. C. If you convert 224 to binary, you get 11100000. With five host bits
(i.e., bits set to 0 in the subnet mask), you can support 30 host
addresses (25 – 2 = 30, where 5 is the number of host bits).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
4
OSPF Areas
THE CCNP ROUTING TOPICS COVERED IN THIS
CHAPTER ARE AS FOLLOWS:
Introduction to OSPF terminology
Introduction to OSPF functionality
Discussion of OSPF areas, routers, and link-state
advertisements
Discussion of choosing and maintaining routes, in particular in
multi-access, PPP, and non-broadcast multi-access networks
Configuration and verification of OSPF operation
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
T
his chapter is the introduction to Open Shortest Path First
(OSPF) areas. It will introduce the term OSPF areas and discuss their role in
OSPF routing. It is very important that you take the time to learn the terminology used in OSPF. Without this knowledge, the remaining sections of the
chapter will be difficult to follow.
Open Shortest Path First
Open Shortest Path First (OSPF) is an open standards routing protocol. It is important to recognize that Cisco’s implementation of OSPF is a
standards-based version. This means that Cisco based its version of OSPF on
the open standards. While doing so, Cisco also has added features to its version of OSPF that may not be found in other implementations of OSPF. This
becomes important when interoperability is needed.
John Moy heads up the working group of OSPF. Two RFCs define
OSPF: Version 1 is defined by RFC 1131, and Version 2 is defined by
RFC 2328. Version 2 is the only version to make it to an operational status.
However, many vendors modify OSPF. OSPF is known as a link-state routing protocol (link-state routing protocols were discussed in Chapter 2,
“Routing Principles”). The Dijkstra algorithm is used to calculate the shortest path through the network. Within OSPF, links become synonymous with
interfaces.
OSPF is a robust protocol, and due to the robustness, you must learn
many terms in order to understand the operation of OSPF. The next section
covers the terminology necessary to enable you to understand the many
operations and procedures performed by the OSPF process.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Open Shortest Path First
115
OSPF Terminology
The most basic of terms that are related to OSPF are related to many routing
protocols. We begin by defining relationships among routers. From there, we
will move on to defining terms relating to OSPF operations.
Neighbor A neighbor refers to a connected (adjacent) router that is running an OSPF process with the adjacent interface assigned to the same
area. Neighbors are found via Hello packets. No routing information is
exchanged with neighbors unless adjacencies are formed.
Adjacency An adjacency refers to the logical connection between a
router and its corresponding designated routers and backup designated
routers. The formation of this type of relationship depends heavily on the
type of network that connects the OSPF routers.
Link In OSPF, a link refers to a network or router interface assigned to
any given network. Within OSPF, link is synonymous with interface.
Interface An interface is the physical interface on a router. When an
interface is added to the OSPF process, it is considered by OSPF as a link.
If the interface is up, then the link is up. OSPF uses this association to
build its link database.
Link State Advertisement Link State Advertisement (LSA) is an OSPF
data packet containing link-state and routing information that is shared
among OSPF routers.
Designated router A designated router (DR) is used only when the
OSPF router is connected to a broadcast (multi-access) network. To minimize the number of adjacencies formed, a DR is chosen to disseminate/
receive routing information to/from the remaining routers on the broadcast network or link.
Backup designated router A backup designated router (BDR) is a hot
standby for the DR on broadcast (multi-access) links. The BDR receives
all routing updates from OSPF adjacent routers but does not flood LSA
updates.
OSPF areas OSPF areas are similar to EIGRP Autonomous Systems.
Areas are used to establish a hierarchical network. OSPF uses four types
of areas, all of which will be discussed later in this chapter.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
116
Chapter 4
OSPF Areas
Area border router An area border router (ABR) is a router that has
multiple area assignments. An interface may belong to only one area. If a
router has multiple interfaces and if any of these interfaces belong to different areas, the router is considered an ABR.
Autonomous system boundary router An autonomous system boundary router (ASBR) is a router with an interface connected to an external
network or a different AS. An external network or autonomous system
refers to an interface belonging to a different routing protocol, such as
EIGRP. An ASBR is responsible for injecting route information learned by
other Interior Gateway Protocols (IGPs) into OSPF.
Non-broadcast multi-access Non-broadcast multi-access (NMBA) networks are networks such as Frame Relay, X.25, and ATM. This type of
network allows for multi-access but has no broadcast ability like Ethernet. NBMA networks require special OSPF configuration to function
properly.
Broadcast (multi-access) Networks such as Ethernet allow multiple
access as well as provide broadcast ability. A DR and BDR must be elected
for multi-access broadcast networks.
Point-to-point This type of network connection consists of a unique
NMBA configuration. The network can be configured using Frame Relay
and ATM to allow point-to-point connectivity. This configuration eliminates the need for DRs or BDRs.
Router ID The Router ID is an IP address that is used to identify the
router. Cisco chooses the Router ID by using the highest IP address of all
configured loopback interfaces. If no loopback addresses are configured,
OSPF will choose the highest IP address of the functional physical
interfaces.
All of these terms play an important part in understanding the operation
of OSPF. You must come to know and understand each of these terms. As
you read through the chapter, you will be able to place the terms in their
proper context.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Open Shortest Path First
117
OSPF Operation
OSPF operation can be divided into three categories:
Neighbor and adjacency initialization
LSA flooding
SPF tree calculation
We will discuss each in the following sections.
Neighbor and Adjacency Initialization
We begin with neighbor/adjacency formation. This is a very big part of OSPF
operation. These relationships are often easily formed over point-to-point
connections, but much more complex procedures are required when multiple
OSPF routers are connected via a broadcast multi-access media.
The Hello protocol is used to discover neighbors and establish adjacencies. Hello packets contain a great deal of information regarding the originating router. Hello packets are multicast out every interface on a 10-second
interval by default. The data contained in the Hello packet can be seen in
Table 4.1. It is important to remember that the Router ID, Area ID, and
authentication information are carried in the common OSPF header. The
Hello packet uses the common OSPF header.
TABLE 4.1
OSPF Hello Packet Information
Originating Router
Characteristic
Description
Router ID
The highest active IP address on the router.
(Loopback addresses are used first. If no loopback interfaces are configured, OSPF will choose
from physical interfaces.)
Area ID
The area to which the originating router interface
belongs.
Authentication
information
The authentication type and corresponding
information.
Network mask
The IP mask of the originating router’s interface
IP address.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
118
Chapter 4
OSPF Areas
TABLE 4.1
OSPF Hello Packet Information (continued)
Originating Router
Characteristic
Description
Hello interval
The period between Hello packets.
Options
OSPF options for neighbor formation.
Router priority
An 8-bit value used to aid in the election of the
DR and BDR. (Not set on point-to-point links.)
Router dead interval
The length of time allotted for which a Hello
packet must be received before considering the
neighbor down—four times the Hello interval,
unless otherwise configured.
DR
The Router ID of the current DR.
BDR
The Router ID of the current BDR.
Neighbor router IDs
A list of the Router IDs for all the originating
router’s neighbors.
Neighbor States
There are a total of eight states for OSPF neighbors:
Down No Hello packets have been received on the interface.
Attempt Neighbors must be configured manually for this state. It
applies only to NBMA network connections. (Note: This state is not represented in Figure 4.1)
Init Hello packets have been received from other routers.
2Way Hello packets have been received that include their own Router
ID in the Neighbor field.
ExStart Master/Slave relationship is established in order to form an
adjacency by exchanging Database Description (DD) packets. (The router
with the highest Router ID becomes the Master.)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Open Shortest Path First
119
Exchange Routing information is exchanged using DD and LSR
packets.
Loading Link State Request packets are sent to neighbors to request any
new LSAs that were found while in the Exchange state.
Full All LSA information is synchronized among adjacent neighbors.
To gain a better understanding of how an adjacency is formed, let’s consider the formation of an adjacency in a broadcast multi-access environment.
Figure 4.1 displays a flow chart that depicts each step of the initialization
process. The process starts by sending out Hello packets. Every listening
router will then add the originating router to the neighbor database. The
responding routers will reply with all of their Hello information so that the
originating router can add them to its own neighbor table.
FIGURE 4.1
OSPF peer initialization
Down
Init State
Multicast
Hello packets
Listening routers add
the new router to
the adjacency table.
Choose DR
and BDR.
2Way state
Link type is
broadcast
multi-access.
Originating router adds
all replying routers
to neighbor table.
Adjacencies must be
established (depends
on link type).
Compare all
Router Priority
values.
Exchange
link-state
information.
Exchange
Compare
Router IDs.
Any final LSAs
are also
exchanged.
Loading
Assign as DR.
Exchange Hello packets
every 10s LSR/LSU exchanges.
(Full routing information.)
ExStart state
Yes
Is there a tie?
Routers reply to Hello
packets with information
contained in Table 4.1.
No
Take
highest value.
Full state
Take secondhighest value.
Assign as BDR.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
120
Chapter 4
OSPF Areas
Adjacency Requirements
Once neighbors have been identified, adjacencies must be established so that
routing (LSA) information can be exchanged. There are two steps required
to change a neighboring OSPF router into an adjacent OSPF router:
Two-way communication (achieved via the Hello protocol)
Database synchronization—this consists of three packet types being
exchanged between routers:
Database Description (DD) packets
Link State Request (LSR) packets
Link State Update (LSU) packets
Once the database synchronization has taken place, the two routers are
considered adjacent. This is how adjacency is achieved, but you must also
know when an adjacency will occur.
When adjacencies form depends on the network type. If the link is pointto-point, the two neighbors will become adjacent if the Hello packet information for both routers is configured properly.
On broadcast multi-access networks, adjacencies are formed only
between the OSPF routers on the network and the DR and BDR. Figure 4.2
gives an example. Three types of routers are pictured: DR, BDR, and
DROther. DROther routers are routers that belong to the same network as
the DR and BDR but do not represent the network via LSAs.
FIGURE 4.2
OSPF adjacencies for multi-access networks
DR
DROther
DROther
Ethernet
DROther
Copyright ©2001 SYBEX , Inc., Alameda, CA
BDR
www.sybex.com
Open Shortest Path First
121
You will notice the dotted lines connecting the DROther routers to the
DR and BDR routers. Notice also that there are no dotted lines between any
of the DROther routers. The dotted lines represent the formation of adjacencies. DROther routers form only two adjacencies on a broadcast multiaccess network—one with the DR and the other with the BDR. The following router output indicates the assignments of routers connected via a broadcast multi-access network as well as two Frame Relay (non-broadcast multiaccess, or NBMA) network connections.
Note that the Frame Relay connections displayed below do not have DR/BDR
assignments. DR/BDR roles and election will be covered more fully in the following section, “DR and BDR Election Procedure.”
RouterA>sho ip ospf neighbor
Neighbor ID
172.16.22.101
172.16.247.1
172.16.245.1
172.16.244.1
172.16.247.1
172.16.249.1
172.16.248.1
172.16.245.1
172.16.241.1
172.16.248.1
RouterA>
Pri
State
1 FULL/DROTHER
1 FULL/DR
1 2WAY/DROTHER
1 2WAY/DROTHER
1 FULL/BDR
1 FULL/DR
1 2WAY/DROTHER
1 FULL/ 1 FULL/ 1 FULL/ -
Dead Time
00:00:32
00:00:34
00:00:32
00:00:37
00:00:34
00:00:34
00:00:36
00:00:34
00:00:34
00:00:35
Address
172.16.22.101
172.16.22.9
172.16.12.8
172.16.12.13
172.16.12.9
172.16.12.15
172.16.12.12
172.16.1.105
172.16.202.2
172.16.1.41
Interface
FastEthernet0/0
FastEthernet0/0
FastEthernet1/0
FastEthernet1/0
FastEthernet1/0
FastEthernet1/0
FastEthernet1/0
Serial3/0.1
Serial3/1
Serial3/3.1
We need to bring up a few important points about this output. Notice that
four different interfaces are configured to use OSPF.
Interface Fast Ethernet 0/0 shows only a DROther and a DR. You know
that there must always be a DR and a BDR for each multi-access segment.
Deductively, you can ascertain that RouterA must be the BDR for this segment.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
122
Chapter 4
OSPF Areas
It is also important to recognize that this command displays OSPF neighbors and not adjacencies. To learn adjacency formations, study the following
summarization:
Point-to-point valid neighbors form adjacencies.
NBMA neighbors require special configuration (e.g., point-to-point
subinterfaces) for adjacency formation.
Broadcast multi-access neighbors require the election of a DR and a
BDR. All other routers form adjacencies with only the DR and BDR.
DR and BDR Election Procedure
Each OSPF interface (multi-access only) possesses a configurable Router Priority. The Cisco default is 1. If you don’t want a router interface to participate in the DR/BDR election, set the Priority to 0 using the ip ospf
priority command in Interface Configuration mode. Here is a sample (the
Priority field is bolded for ease of identification):
RouterA>show ip ospf interface
FastEthernet0/0 is up, line protocol is up
Internet Address 172.16.22.14/24, Area 0
Process ID 100, Router ID 172.16.246.1, Network Type
BROADCAST, Cost: 1
Transmit Delay is 1 sec, State BDR, Priority 1
Designated Router (ID) 172.16.247.1, Interface address
172.16.22.9
Backup Designated router (ID) 172.16.246.1, Interface
address 172.16.22.14
Timer intervals configured, Hello 10, Dead 40, Wait 40,
Retransmit 5
Hello due in 00:00:08
Neighbor Count is 2, Adjacent neighbor count is 2
Adjacent with neighbor 172.16.22.101
Adjacent with neighbor 172.16.247.1 (Designated
Router)
Suppress hello for 0 neighbor(s)
Message digest authentication enabled
Youngest key id is 10
RouterA>
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Open Shortest Path First
123
This value is key when electing the DR and BDR. Let’s go through the
steps that occur when the DR and BDR are elected.
1. A list of eligible routers is created. The criteria for eligible routers are:
Priority ≥ 1.
OSPF state of 2Way.
DR or BDR IP address is the same as the participating interface’s
IP address.
2. A list of all routers not claming to be the DR (the DR IP address is the
same as the participating interface’s IP address) is compiled from the
list of eligible routers.
3. The BDR is chosen from the list in Step 2 based on the following
criteria:
The BDR IP address is the same as the participating interface’s IP
address.
The router with the highest Router Priority becomes the BDR.
If all Router Priorities are equal, the router with the highest
Router ID becomes the BDR.
or
If none of the above criteria hold true, the router with the highest
Router Priority is chosen, and in case of a tie, the router with the
highest Router ID is chosen as BDR.
4. The DR is chosen from the remaining eligible routers based on the fol-
lowing criteria:
The DR field is set with the router’s interface IP address.
The router with the highest Router Priority is chosen DR. If all
Router Priorities are equal, the router with the highest Router ID
is chosen.
or
If none of the remaining eligible routers claim to be the DR, the
BDR that was chosen in Step 3 becomes the DR. Step 3 would
then be repeated to choose another BDR.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
124
Chapter 4
OSPF Areas
You should remember that the previous process occurs when multiple
routers become active at the same time on a segment. If a DR and BDR
already exist on the segment, any new interfaces accept the DR and BDR
regardless of their own Router ID or Router Priority.
To further the example, if initially there is only one OSPF router interface
active on the segment, it becomes the DR. The next router would become the
BDR. Subsequent routers would all accept the existing DR and BDR and
form adjacencies with them.
LSA Flooding
LSA flooding is the method by which OSPF shares routing information. Via
LSU packets, LSA information containing link-state data is shared with all
OSPF routers. The network topology is created from the LSA updates.
Flooding is used so that all OSPF routers have the topology map from which
SPF calculations may be made.
Efficient flooding is achieved through the use of a reserved multicast
address, 224.0.0.5 (AllSPFRouters). LSA updates (indicating that something
in the topology changed) are handled somewhat differently. The network
type determines the multicast address used for sending updates. Table 4.2
contains the multicast address associated with LSA flooding. Point-tomultipoint networks use the adjacent router’s unicast IP address. Figure 4.3
depicts a simple update and flood scenario on a broadcast multi-access network.
TABLE 4.2
LSA Update Multicast Addresses
Network Type
Multicast Address
Description
Point-to-point
224.0.0.5
AllSPFRouters
Broadcast
224.0.0.6
AllDR
Point-to-multipoint
NA
NA
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Open Shortest Path First
FIGURE 4.3
125
LSA updates and flooding
RouterG
RouterF
Frame
Relay
Frame
Relay
RouterA
DR
3
3
fe1/0
RouterB
DROther
2
RouterC
DROther
fe1/0
1
fe1/0
fe1/0
fe1/0
DROther
RouterD
s0/0
BDR
RouterE
Ethernet
2
1. Link s0/0 goes down.
2. RouterC sends LSU containing the LSA
for int s0/0 on multicast AIIDRouters
(224.0.0.6) to the DR and BDR.
3. RouterA floods the LSA to AIISPFRouters
(224.0.0.5) out all interfaces.
Once the LSA updates have been flooded throughout the network, each
recipient must acknowledge that the flooded update was received. It is also
important that the recipient validate the LSA update.
LSA Acknowledgement and Validation
Routers receiving LSA updates must acknowledge the receipt of the LSA, but
they can do it using two forms:
Explicit acknowledgement The recipient sends a Link State Acknowledgement packet to the originating interface.
Implicit acknowledgement A duplicate of the flooded LSA is sent back
to the originator.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
126
Chapter 4
OSPF Areas
Here is a packet decode of an Explicit acknowledgement:
IP Header - Internet Protocol Datagram
Version:
4
Header Length:
5
Precedence:
6
Type of Service:
%000
Unused:
%00
Total Length:
84
Identifier:
1285
Fragmentation Flags: %000
Fragment Offset:
0
Time To Live:
1
IP Type:
0x59 OSPF (Hex value for protocol
number)
Header Checksum:
0x8dda
Source IP Address:
131.31.194.140
Dest. IP Address:
224.0.0.6
No Internet Datagram Options
OSPF - Open Shortest Path First Routing Protocol
Version:
2
Type:
5 Link State Acknowledgement
Packet Length:
64
Router IP Address:
142.42.193.1
Area ID:
1
Checksum:
0x6699
Authentication Type:
0 No Authentication
Authentication Data:
........
00 00 00 00 00 00 00 00
Link State Advertisement Header
Age:
3600 seconds
Options:
%00100010
No AS External Link State Advertisements
Type:
3 Summary Link (IP Network)
ID:
0x90fb6400
Advertising Router:
153.53.193.1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
SPF Tree Calculation
127
Sequence Number:
2147483708
Checksum:
0x3946
Link State Length:
28
Link State Advertisement Header
Age:
3600 seconds
Options:
%00100010
No AS External Link State Advertisements
Type:
3 Summary Link (IP Network)
ID:
0x90fb6400
Advertising Router:
131.31.193.1
Sequence Number:
2147483650
Checksum:
0x25c0
Link State Length:
28
Frame Check Sequence: 0x00000000
You can tell that this is a Link State Acknowledgement packet based on
the OSPF header information. You will see that it is a type 5 OSPF packet,
or a Link State Acknowledgement packet.
There are two methods by which an implicit acknowledgement may be made:
Direct method The acknowledgement, either explicit or implicit, is sent
immediately. The following criteria must be met before the Direct method
is used:
A duplicate flooded LSA is received.
LSA age equals MaxAge (one hour).
Delayed method The recipient waits to send the LSA acknowledgement
with other LSA acknowledgements that need to be sent.
Validation occurs through the use of the sequencing, checksum, and aging
data contained in the LSA update packet. This information is used to make
sure that the router possesses the most recent copy of the link-state database.
SPF Tree Calculation
S
hortest Path First (SPF) trees are paths through the network to any
given destination. A separate path exists for each known destination. There
are two destination types recognized by OSPF: network and router. Router
destinations are specifically for area border routers (ABRs) and autonomous
system boundary routers (ASBRs).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
128
Chapter 4
OSPF Areas
Once all of the OSPF routers have synchronized link-state databases, each
router is responsible for calculating the SPF tree for each known destination.
This calculation is done using the Dijkstra algorithm. In order to do calculations, metrics for each link are required.
OSPF Metrics
OSPF uses a metric referred to as cost. A cost is associated with every outgoing interface along an SPF tree. The cost of the entire path is the sum of
costs of the outgoing interfaces along the path. Since cost is an arbitrary
value as defined in RFC 2338, Cisco had to implement its own method of
calculating the cost for each OSPF-enabled interface. Cisco uses a simple
equation of 108 /bandwidth. The bandwidth is the configured bandwidth
for the interface.
This value may be overridden by using the ip ospf cost command. The
cost is manipulated by changing the value to a number within the range of
1 to 65,535. Since the cost is assigned to each link, the value must be changed
on each interface.
Cisco bases link cost on bandwidth. Other vendors may use other metrics to
calculate the link’s cost. When connecting links between routers from different vendors, you may have to adjust the cost to match the other router. Both
routers must assign the same cost to the link for OSPF to work.
NBMA Overview
N
on-broadcast multi-access networks (e.g., Frame Relay and ATM)
present a special challenge for OSPF. As you know, multi-access networks
use an election process to select a DR and a BDR to represent all OSPF routers on the network. This election process requires the participation of all
routers on the multi-access network. However, Hello packets are used to
facilitate the communication for the election process. This works fine on
broadcast multi-access because the connected devices on the network can
hear the AllSPFRouters multicast address for the subnet.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
NBMA Environments
129
When you move to a non-broadcast form of multi-access network, you
lose the assurance that all connected devices are receiving the Hello packets
and are participating in the DR/BDR election.
Because of the difficulty in running OPSF on NBMA networks, it is
important to know which configuration, or environment, will be the most
effective solution. The following section, “NBMA Environments,” discusses
some possible solutions for implementing OSPF over NBMA networks.
NBMA Environments
E
arlier, we mentioned that there are three types of networks: broadcast multi-access, non-broadcast multi-access, and point-to-point. Although
NBMA requires somewhat more configuration to make OSPF operational, it
also gives you the option of deciding how you want it to behave.
With extended configurations on NBMA interfaces, an administrator can
cause OSPF to behave as if it were running on one of the following four network types:
Broadcast
Non-broadcast
Point-to-point
Point-to-multipoint
Broadcast
In order to achieve a broadcast implementation of OSPF on an NBMA network, a full mesh must exist among the routers. Figure 4.4 depicts what the
NBMA network would have to look like. You can see that each router has
a permanent virtual circuit (PVC) configured with all of the other routers.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
130
Chapter 4
OSPF Areas
FIGURE 4.4
NBMA broadcast implementation
RouterA
RouterB
BDR
DR
RouterC
RouterD
This configuration guarantees that all routers have connectivity and that
all will be able to participate in the DR/BDR election process. Once the DR
and BDR have been chosen, the meshed networks act as a broadcast network. All LSA updates are sent to the DR and BDR, and the DR floods the
updates out every interface.
One of the major weaknesses with this configuration is that if one of the
PVCs fails (especially if it is a PVC between a DROther and the DR), then
communication is also halted between the two adjacent peers.
Non-broadcast
This environment requires that all OSPF neighbors be manually configured. This is the default setting for the router. By manually configuring each
neighbor, OSPF knows exactly which neighbors need to participate and
which neighbor is identified as the DR. Also, communication between neighbors is done via unicast instead of multicast. This configuration also requires
a full mesh and has the same weakness as the broadcast environment.
Point-to-Point
This environment uses subinterfaces on the physical interface to create pointto-point connections with other OSPF neighbors. No DR or BDR is elected
since the link is treated as a point-to-point circuit. This allows for faster
convergence.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
131
A full mesh is not required when implementing this environment. PVCs
on the subinterface may fail, but there is still OSPF connectivity to other
PVCs on the same physical interface.
The drawback of this environment is inefficient flooding. Because of multiple PVCs per interface and depending on the mesh of the PVCs, one LSA
update can be flooded multiple times.
Point-to-Multipoint
This environment is very similar to the point-to-point environment. No DR
or BDR is chosen. All PVCs are treated as point-to-point links. The only difference is that all the PVCs go back to a single router. Figure 4.5 depicts the
difference between a true point-to-point environment and a point-tomultipoint deployment.
FIGURE 4.5
Point-to-point vs. point-to-multipoint
NBMA (point-to-point)
NBMA (point-to-multipoint)
RouterA
RouterB
RouterA
RouterB
RouterC
RouterD
RouterC
RouterD
Configuring OSPF
C
onfiguring OSPF is a simple task. There are many options that are
allowed within OSPF, such as statically configuring neighbors, creating a virtual link between an area that is not physically connected to Area 0, neighbor/adjacency encryption, and many more. The following sections describe
how to configure OSPF in different environments.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
132
Chapter 4
OSPF Areas
Enabling OSPF is common for all implementations of OSPF; the difference comes when you configure parameters to make OSPF behave in the
desired fashion. We’ll cover parameters for NBMA as well.
The basic elements of OSPF configuration are:
Enabling OSPF
Configuring OSPF for different network types
Configuring the OSPF area
Route summarization
Route redistribution (covered in detail in Chapter 10, “Route
Optimization”)
Interface parameters
We will start with basic configuration of OSPF, then introduce commands
relating to NBMA, as well as the methods and commands used to verify
proper configuration and operation of OSPF.
Discovering the Network with OSPF
The moment OSPF is enabled on a router and networks are added to the
OSPF process, the router will try to discover the OSPF neighbors on the connected links. Here is a sample of what OSPF events transpire when the interface is added to an OSPF process:
RouterA(config-router)#network 172.16.10.5 0.0.0.0 area 0
RouterA(config-router)#
OSPF: Interface Serial0 going Up
OSPF: Tried to build Router LSA within MinLSInterval
OSPF: Tried to build Router LSA within MinLSInterval^Z
RouterA#
OSPF: rcv. v:2 t:1 l:44 rid:172.16.20.1
aid:0.0.0.0 chk:3B91 aut:0 auk: from Serial0
OSPF: rcv. v:2 t:2 l:32 rid:172.16.20.1
aid:0.0.0.0 chk:2ECF aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x71A opt
0x2 flag 0x7 len 32 state INIT
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
133
OSPF: 2 Way Communication to 172.16.20.1 on Serial0, state
2WAY
OSPF: Send DBD to 172.16.20.1 on Serial0 seq 0x2E opt 0x2
flag 0x7 len 32
OSPF: First DBD and we are not SLAVE
OSPF: rcv. v:2 t:2 l:52 rid:172.16.20.1
aid:0.0.0.0 chk:A641 aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x2E opt 0x2
flag 0x2 len 52 state EXSTART
OSPF: NBR Negotiation Done. We are the MASTER
OSPF: Send DBD to 172.16.20.1 on Serial0 seq 0x2F opt 0x2
flag 0x3 len 52
OSPF: Database request to 172.16.20.1
OSPF: sent LS REQ packet to 172.16.10.6, length 12
OSPF: rcv. v:2 t:2 l:32 rid:172.16.20.1
aid:0.0.0.0 chk:35C1 aut:0 auk: from Serial0
OSPF: rcv. v:2 t:3 l:36 rid:172.16.20.1
aid:0.0.0.0 chk:5A1 aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x2F opt 0x2
flag 0x0 len 32 state EXCHANGE
OSPF: Send DBD to 172.16.20.1 on Serial0 seq 0x30 opt 0x2
flag 0x1 len 32
OSPF: rcv. v:2 t:4 l:64 rid:172.16.20.1
aid:0.0.0.0 chk:F4EA aut:0 auk: from Serial0
OSPF: rcv. v:2 t:2 l:32 rid:172.16.20.1
aid:0.0.0.0 chk:35C0 aut:0 auk: from Serial0
OSPF: Rcv DBD from 172.16.20.1 on Serial0 seq 0x30 opt 0x2
flag 0x0 len 32 state EXCHANGE
OSPF: Exchange Done with 172.16.20.1 on Serial0
OSPF: Synchronized with 172.16.20.1 on Serial0, state FULL
This simple debug output describes exactly what we talked about earlier
in this chapter regarding LSA exchanges and the state of adjacent OSPF
neighbors. We bolded the state information for your convenience.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
134
Chapter 4
OSPF Areas
We used the OSPF debugging commands to produce this output. The configuration commands consisted of two simple OSPF commands:
router ospf 1 This command starts the OSPF process on RouterA.
The number 1 indicates the OSPF process ID.
network 172.16.10.5 0.0.0.0 area 0 This command adds the network (link) 172.16.10.5. The wildcard mask indicates that only this single IP
address is going to be part of the link. Area 0 indicates that the interface
with the address 172.16.10.5 is assigned to Area 0.
The generic IOS syntax for the commands is router ospf process-id
and network ip-address wildcard-mask area area-id, respectively.
Point-to-Point
Since the link described by the previous output is point-to-point, no DR/
BDR election occurred; instead, each router decided which would be the
Master and which would be the Slave. Once the Master/Slave roles had been
established, DBD packets containing LSA information for each router were
exchanged.
LSA exchanges continue until the link-state databases for each router are
identical (synchronized). Once that happens, the OSPF state changes to Full.
Broadcast
Discovering the neighbors on a broadcast network is done somewhat differently. Here you will see what happens on a broadcast multi-access network:
RouterA(config-if)#router ospf 1
RouterA(config-router)#network 172.16.230.0 0.0.0.255
area 0
OSPF: Interface Ethernet0 going Up
OSPF: Tried to build Router LSA within MinLSInterval
OSPF: Tried to build Router LSA within MinLSInterval
RouterA(config-router)#
OSPF: end of Wait on interface Ethernet0
OSPF: DR/BDR election on Ethernet0
OSPF: Elect BDR 172.16.240.1
OSPF: Elect DR 172.16.240.1
OSPF: Elect BDR 0.0.0.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
135
OSPF: Elect DR 172.16.240.1
DR: 172.16.240.1 (Id)
BDR: none
OSPF: Build router LSA for area 0, router ID 172.16.240.1
We end the output here, because we know that once adjacencies have
been established, the link-state databases must synchronize during the
Exchange state and the transfer of DBD packets containing LSA updates.
Of interest in this output is the election of the DR and BDR. Initially, the
value for the BDR was 0.0.0.0. This was the first router on the network to
become active. Therefore, the first election to take place is that of BDR,
because Ethernet 0 is the only active OSPF interface at the moment, and the
Router ID of 172.16.240.1 (the loopback 0 IP address) is chosen to be the BDR.
When the process goes on to elect the DR, the only router capable is itself.
The role of DR is taken by 172.16.240.1, and the BDR is reset to 0.0.0.0
because there are no other routers active on this multi-access network.
No new commands were used to create this output. The only difference
was that the network 172.16.230.0 was configured on a broadcast multiaccess network.
Configuring OSPF—Single Area
The easiest (and least scalable) way to configure OSPF is to simply use Area 0.
If all you want to configure is one area, it must be Area 0. Creating a single
backbone area makes it easy to understand what OSPF is doing, but once
you get a number of routers in the area with all the interfaces assigned to
Area 0, processing time is going to be much greater and convergence slower.
To start learning, however, a single area is the perfect place to start. You
have already seen the command that is used for assigning an interface to an
area. Let’s look at the configuration of a few routers to get a good feeling for
how it is done. Figure 4.6 depicts the physical layout of a test network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
136
Chapter 4
OSPF Areas
FIGURE 4.6
OSPF area topology
RouterA
RouterB
172.16.10.4/24
172
.16
172.16.32.0/24
.10
.8/
30
172.16.20.0/24
RouterD
RouterC
172.16.64.0/24
RouterE
Only two of the five configurations are shown—otherwise you would just
see a lot of redundant information. Notice the very specific wildcard masks
in the network statements. These facilitate the removal or addition of specific links when troubleshooting. If you have a link that is flapping, you can
easily remove it so that it does not cause LSA flooding within the area. After
the link has stabilized, it will be very easy to add the interface back in.
For example, if all of the router’s interfaces could be summarized by a network statement of 172.16.0.0 0.0.255.255, then you would need only one
network statement to add all interfaces to the OSPF process. However, if one
out of the many interfaces was flapping, you could not easily isolate that
interface so that it would not cause unnecessary LSA flooding. Let’s examine
the IOS configuration for this topology:
RouterA#show running-config
Building configuration...
Current configuration:
!
version 11.2
no service password-encryption
no service udp-small-servers
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
no service tcp-small-servers
!
hostname RouterA
!
enable password cisco
!
!
interface Loopback0
ip address 172.16.240.1 255.255.255.0
!
interface Ethernet0
ip address 172.16.230.20 255.255.255.0
!
interface Serial0
ip address 172.16.10.5 255.255.255.252
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial1
ip address 172.16.10.9 255.255.255.252
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial2
no ip address
shutdown
!
interface Serial3
no ip address
shutdown
!
interface BRI0
no ip address
shutdown
!
router ospf 1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
137
138
Chapter 4
OSPF Areas
network 172.16.230.0 0.0.0.255 area 0
network 172.16.10.5 0.0.0.0 area 0
network 172.16.10.9 0.0.0.0 area 0
!
RouterB#wr t
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname RouterB
!
enable password cisco
!
ip subnet-zero
!
!
!
interface Loopback0
ip address 172.16.241.1 255.255.255.0
no ip directed-broadcast
!
interface Ethernet0
no ip address
no ip directed-broadcast
shutdown
!
interface Serial0
ip address 172.16.10.6 255.255.255.252
no ip directed-broadcast
no ip mroute-cache
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
139
no fair-queue
!
interface Serial1
ip address 172.16.20.1 255.255.255.0
no ip directed-broadcast
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial2
no ip address
no ip directed-broadcast
shutdown
!
interface Serial3
no ip address
no ip directed-broadcast
shutdown
!
interface BRI0
no ip address
no ip directed-broadcast
shutdown
!
router ospf 1
network 172.16.10.6 0.0.0.0 area 0
network 172.16.20.0 0.0.0.255 area 0
!
As you can see, these are very simple, straightforward configurations. All
interfaces are assigned to Area 0. Another interesting fact about creating a
single area is that there are no ABRs or ASBRs. It is possible to have an ASBR
without having an ABR. If external routes are injected into the area, the
router injecting them will be considered an ASBR. In order to activate an
ABR, any interface on the router must be assigned to a different area.
It is also important to recognize that the neighbor discovery was automatic in this single-area configuration. Now let’s move on to an environment
where sometimes neighbors must be configured manually.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
140
Chapter 4
OSPF Areas
Configuring OSPF—Single Area (NBMA Environment)
Previously, we mentioned three different possible ways to configure NBMA
network interfaces. They are:
Broadcast
Non-broadcast
Point-to-multipoint (a version of point-to-point)
We’ll outline all three methods in this section. The key configuration
statement that is common to all configuration methods is the ip ospf
network command.
The command has the options of specifying broadcast, non-broadcast,
and point-to-multipoint network types. The IOS senses the media type for all
interfaces and assigns the default network type accordingly. If you wish to
change that assignment, you would do so via the ip ospf network
command.
Broadcast Configuration
A full mesh among all OSPF routers is required for this environment to be
configured and work properly. A full explanation of the PVC configuration
is beyond the scope of this chapter, but here is a sample configuration:
RouterA#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterA(config)#int serial 1
RouterA(config-if)#ip ospf network broadcast
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#frame-relay map ip 172.16.11.2 102
broadcast
RouterA(config-if)#frame-relay map ip 172.16.11.3 103
broadcast
RouterA(config-if)#frame-relay map ip 172.16.11.4 104
broadcast
RouterA(config-if)#router ospf 1
RouterA(config-router)#network 172.16.11.0 0.0.0.255
area 0
RouterA(config-router)#^Z
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
RouterA#show running-config
Building configuration...
Current configuration:
!
version 11.2
no service password-encryption
no service udp-small-servers
no service tcp-small-servers
!
hostname RouterA
!
enable password cisco
!
!
interface Loopback0
ip address 172.16.240.1 255.255.255.0
!
interface Ethernet0
ip address 172.16.230.20 255.255.255.0
!
interface Serial0
ip address 172.16.10.5 255.255.255.252
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial1
no ip address
encapsulation frame-relay
ip ospf network broadcast
frame-relay map ip 172.16.11.2 102 broadcast
frame-relay map ip 172.16.11.3 103 broadcast
frame-relay map ip 172.16.11.4 104 broadcast
!
interface Serial2
no ip address
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
141
142
Chapter 4
OSPF Areas
shutdown
!
interface Serial3
no ip address
shutdown
!
interface BRI0
no ip address
shutdown
!
router ospf 1
network 172.16.10.5 0.0.0.0 area 0
network 172.16.11.0 0.0.0.255 area 0
!
Connected routers would have similar configurations. The key to this
configuration is to override the default network type by using the ip ospf
network broadcast command.
Non-broadcast Configuration
This environment requires all neighbors to be statically configured so that a
DR may be chosen from the attached routers on the network segment. We
use the same commands as for the configuration of a broadcast network,
with the exception of the neighbor statements used under the OSPF routing
process. Here is a sample configuration:
RouterB#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterB(config)#interface serial1
RouterB(config-if)#ip ospf network non-broadcast
RouterB(config-if)#encapsulation frame-relay ietf
RouterB(config-if)#frame-relay map ip 172.16.25.10 210
broadcast
RouterB(config-if)#frame-relay map ip 172.16.25.11 211
broadcast
RouterB(config-if)#frame-relay map ip 172.16.25.12 212
broadcast
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
143
RouterB(config-if)#router ospf 1
RouterB(config-router)#neighbor 172.16.25.10 priority 1
RouterB(config-router)#neighbor 172.16.25.11 priority 1
RouterB(config-router)#neighbor 172.16.25.12 priority 1
RouterB(config-router)#network 172.16.25.0 0.0.0.255 area
0
RouterB(config-router)#^Z
RouterB#
Point-to-Multipoint
This configuration does away with the assumption that there are PVCs configured for all routers creating a full mesh. The same ip ospf network
broadcast command is used to specify that the network type is point-tomultipoint non-broadcast. This tells the router that no DR/BDR needs to be
elected and that the interfaces are treated as individual point-to-point links.
Here is a sample configuration:
RouterC#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterC(config)#interface serial2
RouterC(config-if)#ip ospf network point-to-multipoint
non-broadcast
RouterC(config-if)#encapsulation frame-relay ietf
RouterC(config-if)#frame-relay local dlci 300
RouterC(config-if)#frame-relay map ip 172.16.26.12 312
broadcast
RouterC(config-if)#frame-relay map ip 172.16.26.13 313
broadcast
RouterC(config-if)#router ospf 1
RouterC(config-router)#neighbor 172.16.26.12 priority 1
RouterC(config-router)#neighbor 172.16.26.13 priority 1
RouterC(config-router)#network 172.16.25.0 0.0.0.255
area 0
RouterC(config-router)#^Z
RouterC#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
144
Chapter 4
OSPF Areas
Once the configuration has been created, it is time to test it and make sure
it works. There are several show commands that facilitate this task, and we
discuss them in the following section.
Verifying OSPF Configuration
This section describes several ways in which to verify proper OSPF configuration and operation. Table 4.5 contains a list of OSPF show commands.
TABLE 4.3
OSPF Show Commands
Command
Description
show ip ospf
Summarizes all relative OSPF information, such
as OSPF processes, Router ID, area assignments,
authentication, and SPF statistics.
show ip ospf
process-id
Shows the same information as the show ip ospf
command, but only for the specified process.
show ip ospf
border-routers
Displays the Router IDs of all ABRs and ASBRs
within the autonomous system.
show ip ospf
database
Displays the link-state database.
show ip ospf
interface
Displays interface OSPF parameters and other
OSPF information specific to the interface.
show ip ospf
neighbor
Displays each OSPF neighbor and adjacency
status.
show ip ospf
This command is used to display OSPF information for one or all OSPF processes running on the router. Information contained therein includes the
Router ID, area information, SPF statistics, and LSA timer information.
Here is a sample output:
RouterA#sho ip ospf
Routing Process "ospf 1" with ID 172.16.240.1
Supports only single TOS(TOS0) routes
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
145
SPF schedule delay 5 secs, Hold time between two SPFs 10
secs
Number of DCbitless external LSA 0
Number of DoNotAge external LSA 0
Number of areas in this router is 1. 1 normal 0 stub 0
nssa
Area BACKBONE(0)
Number of interfaces in this area is 3
Area has no authentication
SPF algorithm executed 17 times
Area ranges are
Link State Update Interval is 00:30:00 and due in
00:17:52
Link State Age Interval is 00:20:00 and due in
00:07:52
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
RouterA#
show ip ospf border-routers
This command displays the process ID on the router, the route to the ABR
or ASBR, and the SPF information. Here is a sample output:
RouterC#show ip ospf border-routers
OSPF Process 1 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 172.16.240.1 [65] via 172.16.1.106, Serial1, ABR,
Area 0, SPF 582
i 172.16.241.1 [65] via 172.16.1.94, Serial11, ASBR,
Area 0, SPF 582
RouterC#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
146
Chapter 4
OSPF Areas
This is a simple output that shows only one ABR and one ASBR. In order
to have an ABR, you must have multiple areas configured. In order to have
an ASBR, external routes on an external autonomous system must be connected to the router.
show ip ospf database
The information displayed by this command indicates the number of links
and the neighboring Router ID. The output is broken down by area. Here is
a sample output:
RouterA#show ip ospf database
OSPF Router with ID (172.16.240.1) (Process ID 1)
Router Link States (Area 0)
Link ID
172.16.240.1
172.16.241.1
RouterA#
ADV Router
172.16.240.1
172.16.241.1
Age
1530
667
Seq#
0x80000016
0x80000008
Checksum
0x9C7C
0x3AFF
Link count
4
3
show ip ospf interface
This command displays all interface-related OSPF information. Data is displayed about OSPF information for all interfaces or for specified interfaces.
Information includes the interface IP address, area assignment, Process ID,
Router ID, network type, cost, priority, DR/BDR (if applicable), timer intervals, and adjacent neighbor information. Here is a sample output:
RouterA#show ip ospf interface
BRI0 is administratively down, line protocol is down
OSPF not enabled on this interface
BRI0:1 is administratively down, line protocol is down
OSPF not enabled on this interface
BRI0:2 is administratively down, line protocol is down
OSPF not enabled on this interface
Ethernet0 is up, line protocol is up
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring OSPF
147
Internet Address 10.11.230.20/24, Area 0
Process ID 1, Router ID 172.16.240.1, Network Type
BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.16.240.1, Interface address
10.11.230.20
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40,
Retransmit 5
Hello due in 00:00:08
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)
Loopback0 is up, line protocol is up
Internet Address 172.16.240.1/24, Area 0
Process ID 1, Router ID 172.16.240.1, Network Type
LOOPBACK, Cost: 1
Loopback interface is treated as a stub Host
Serial0 is up, line protocol is up
Internet Address 172.16.10.5/30, Area 0
Process ID 1, Router ID 172.16.240.1, Network Type
POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40,
Retransmit 5
Hello due in 00:00:02
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.16.241.1
Suppress hello for 0 neighbor(s)
Serial1 is administratively down, line protocol is down
OSPF not enabled on this interface
show ip ospf neighbor
This is a very useful command. It summarizes the pertinent OSPF information regarding neighbors and the adjacency state. If a DR or BDR exists, that
information is also displayed. Here is a sample:
RouterA#show ip ospf neighbor
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
148
Chapter 4
OSPF Areas
Neighbor ID Pri
172.16.241.1
1
RouterA#
State
FULL/
Dead Time
- 00:00:39
Address
Interface
172.16.10.6
Serial0
Summary
T
his chapter contains a great deal of information about OSPF. It is difficult to include everything about OSPF because so much of it falls outside
the scope of this chapter and book.
We have discussed the following topics:
OSPF terminology
OSPF operation
OSPF configuration
Of course, each of the preceding bullet points encompasses quite a bit of
information. We also explained all of the important and pertinent terms
required to fully understand OSPF’s operation. Several processes fall under
OSPF operation, such as DR/BDR election, adjacency formation, etc. OSPF
configuration is actually very simple. Once you understand how OSPF
works, it is easy to configure it.
Key Terms
Before taking the exam, make sure you are familiar with the following terms:
area border router (ABR)
autonomous system boundary router (ASBR)
backup designated router (BDR)
designated router (DR)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
Link State Advertisement (LSA)
LSA acknowledgement
LSA flooding
non-broadcast multi-access (NMBA)
Open Shortest Path First (OSPF)
OSPF areas
Shortest Path First (SPF) trees
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
149
150
Chapter 4
OSPF Areas
Written Lab
1. Write the command that will enable OSPF process 101 on a router.
2. Write the command that will display details of all OSPF routing pro-
cesses enabled on a router.
3. Write the command that enables OSPF on an NBMA network for a
non-broadcast configuration.
4. Write the command that enables OSPF on an NBMA network for a
broadcast configuration.
5. Write the command that will display interface-specific OSPF
information.
6. Write the command that will display all OSPF neighbors.
7. Write the command that will display the SPF information to the ABR
and ASBR.
8. Write the command that will display all different OSPF route types
that are currently known by the router.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
151
Hands-on Lab
Due to the content of this chapter, you will only be asked to enable OSPF
routing on three routers. The following graphic depicts the physical layout of
the network. It also includes IP assignments and hostnames.
.5
s0
s1
.5
172.16.10.4/30
172.16.20.4/30
.6
.6
s0
s0
.20 e0
e0 .21
10.11.230.0/24
This section includes the following lab exercises:
Lab 4.1: Enabling the OSPF Process
Lab 4.2: Configuring OSPF Neighbors
Lab 4.3: Verifying OSPF Operation
LAB 4.1
Enabling the OSPF Process
1. Enable OSPF process 100 on RouterA.
2. Enable OSPF process 101 on RouterB.
3. Enable OSPF process 102 on RouterC.
LAB 4.2
Configuring OSPF Neighbors
1. Configure the network between RouterA and RouterB. Assign it to
Area 0.
2. Configure the network between RouterA and RouterC. Assign it to
Area 0.
3. Configure the network between RouterB and RouterC. Assign it to
Area 0.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
152
Chapter 4
OSPF Areas
LAB 4.3
Verifying OSPF Operation
1. Execute a show ip ospf neighbors command from each router.
What are the results?
2. Execute a show ip route command to verify that all other routers
are learning all routes.
Answer to Lab 4.1
RouterA#conf t
Enter configuration commands, one per line.
CNTL/Z.
RouterA(config)#router ospf 100
RouterA(config-router)#^Z
RouterB#conf t
Enter configuration commands, one per line.
CNTL/Z.
RouterB(config)#router ospf 101
RouterB(config-router)#^Z
RouterB#
RouterC#conf t
Enter configuration commands, one per line.
CNTL/Z.
Router(config)#router ospf 102
RouterC(config-router)#^Z
RouterC#
End with
End with
End with
Answer to Lab 4.2
RouterA#conf t
Enter configuration commands, one per line.
CNTL/Z.
RouterA(config)#router ospf 100
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
End with
Hands-on Lab
153
RouterA(config-router)#network 172.16.10.5 0.0.0.0 area 0
RouterA(config-router)#network 172.16.20.5 0.0.0.0 area 0
RouterA(config-router)#^Z
RouterA#
RouterB#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterB(config)#router ospf 101
RouterB(config-router)#network 172.16.10.6 0.0.0.0 area 0
RouterB(config-router)#network 10.11.230.0 0.0.0.255
area 1
RouterB(config-router)#^Z
RouterB#exit
RouterC#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterC(config)#router ospf 102
RouterC(config-router)#network 172.16.20.6 0.0.0.0 area 0
RouterC(config-router)#network 10.11.230.0 0.0.0.255
area 1
RouterC(config-router)#^Z
RouterC#
Answer to Lab 4.3
RouterA#sho ip ospf neig
Neighbor ID
172.16.241.1
172.16.20.6
Pri
1
1
State
FULL/
FULL/
-
Dead Time
00:00:31
00:00:38
Address
172.16.10.6
172.16.20.6
Interface
Serial0
Serial1
RouterA#sho ip route
Codes: C - connected, S - static, I - IGRP, R - RIP,
M - mobile, B - BGP
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
154
Chapter 4
OSPF Areas
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
O IA
10.11.230.0 [110/74] via 172.16.20.6, 00:00:24,
Serial1
172.16.0.0/16 is variably subnetted, 4 subnets, 3
masks
O
172.16.241.1/32 [110/65] via 172.16.10.6,
00:01:28, Serial0
C
172.16.240.0/24 is directly connected, Loopback0
C
172.16.20.4/30 is directly connected, Serial1
C
172.16.10.4/30 is directly connected, Serial0
RouterA#
RouterB#sho ip ospf neig
Neighbor ID
172.16.20.6
172.16.240.1
Pri
1
1
State
FULL/DR
FULL/ -
Dead Time
00:00:33
00:00:32
Address
10.11.230.21
172.16.10.5
Interface
Ethernet0
Serial0
RouterB#sho 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
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
155
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
172.16.0.0/16 is variably subnetted, 3 subnets, 2
masks
C
172.16.241.0/24 is directly connected, Loopback0
O IA
172.16.20.4/30 [110/74] via 10.11.230.21,
00:00:48, Ethernet0
C
172.16.10.4/30 is directly connected, Serial0
10.0.0.0/24 is subnetted, 1 subnets
C
10.11.230.0 is directly connected, Ethernet0
RouterB#
RouterC#sho ip ospf neigh
Neighbor ID
172.16.10.6
172.16.240.1
Pri
1
1
State
FULL/BDR
FULL/ -
Dead Time
00:00:34
00:00:36
Address
10.11.230.20
172.16.20.5
Interface
Ethernet0
Serial0
RouterC#sho 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
172.16.0.0/16 is variably subnetted, 3 subnets, 2
masks
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
156
Chapter 4
OSPF Areas
O
172.16.241.1/32 [110/129] via 172.16.20.5,
00:03:04, Serial0
C
172.16.20.4/30 is directly connected, Serial0
O
172.16.10.4/30 [110/128] via 172.16.20.5,
00:03:04, Serial0
10.0.0.0/24 is subnetted, 1 subnets
C
10.11.230.0 is directly connected, Ethernet0
RouterC#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
157
Review Questions
1. A router chooses the Router ID based on which of the following?
A. Lowest IP address from any interface
B. Highest IP address from any interface
C. Lowest IP address from any loopback interface
D. Highest IP Address from any loopback interface
2. What are the three areas of OSPF operation? (Choose three.)
A. Link-state routing
B. SPF calculation
C. LSA flooding
D. Neighbor discover and adjacency formation
3. Which of the following is the IOS command to set the cost on an OSPF
interface?
A. ip ospf no-default cost
B. ip ospf no-summary cost
C. ip ospf cost cost
D. ip ospf-cost cost
4. In what type of topology do all routers have a virtual connection to all
other routers?
A. Full-mesh
B. Star
C. Hub-and-spoke
D. Bus
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
158
Chapter 4
OSPF Areas
5. What does an OSPF neighbor status of down mean?
A. The connected interfaces are in a “line down, line protocol down”
state.
B. No Hello packets have been transmitted from the interface.
C. The interface is administratively shut down.
D. No Hello packets have been received on the interface.
6. What does the OSPF neighbor status init mean?
A. Hello packets have been received from the OSPF neighbor.
B. The router is going to exchange LSA information.
C. The interface has been assigned to an area.
D. Adjacency information has been exchanged between neighbors.
7. What does the OSPF neighbor status 2Way mean?
A. That a router has received a Hello packet with its own Router ID
listed as a neighbor.
B. That a router has received a Hello packet from the DR.
C. That a router is exchanging LSU packets.
D. That a router is waiting for the LSU from the DR.
8. What does the OSPF neighbor status ExStart mean?
A. The OSPF process is starting on the interface.
B. The router is establishing the Master/Slave roles for Database
Description packet exchange.
C. All routing information is beginning to be exchanged between
routers.
D. An LSA flood is about to start.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
159
9. What does the OSPF neighbor status Loading mean?
A. Routers are loading (exchanging) full DD and LSR packets.
B. Routers are loading the topology database.
C. Routers are loading the link-state database.
D. Routers are sending LSR packets to request new LSA information.
10. What does the OSPF neighbor status Exchange mean?
A. Exchange of Hello packets
B. Exchange of routing updates
C. Exchange of full route information via LSR and Database Descrip-
tion packets
D. Exchange of ABR and ASBR information
11. What does the OSPF neighbor status Full indicate?
A. The OSPF topology database has been filled.
B. The OSPF topology databases are synchronized.
C. The neighbor database is synchronized.
D. The OSPF link-state table is full.
12. Which of the following network types have a DR and a BDR assigned?
(Choose all that apply.)
A. Broadcast
B. Point-to-point
C. NBMA broadcast
D. NBMA point-to-point
E. NBMA point-to-multipoint
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
160
Chapter 4
OSPF Areas
13. Which routers form adjacencies with routers designated as DROther
on a broadcast multi-access network? (Choose all that apply.)
A. DROther
B. BDR
C. DR
D. RP
14. Which IP multicast address corresponds with AllSPFRouters?
A. 224.0.0.4
B. 224.0.0.5
C. 224.0.0.6
D. 224.0.0.7
15. Which of the following OSPF terms refers to a connected (or adjacent)
router that is running an OSPF process, with the adjacent interface
assigned to the same area?
A. Link
B. Neighbor
C. LSA
D. STP
16. What is the valid range for the cost metric for OSPF interfaces?
A. 1–255
B. 1–2046
C. 1–63,535
D. 1–65,535
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
161
17. Which method does Cisco use to calculate the cost of a link?
A. 1 x 108 / bandwidth
B. bandwidth / 1 x 108
C. Dijkstra’s Algorithm
D. 1 / bandwidth
18. What OSPF term refers to a network or router interface assigned to
any given interface?
A. Link
B. Area
C. LSA
D. STP
19. All OSPF networks must contain which of the following?
A. Route redistribution configuration
B. Area 0
C. A designated controller
D. A manually defined interface cost
20. Which of the following are advantages of OSPF over RIP? (Choose all
that apply.)
A. Speed of convergence
B. Simplicity to configure
C. Support for VLSMs
D. Scalability
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
162
Chapter 4
OSPF Areas
Answers to Written Lab
1. Write the command that will enable OSPF process 101 on a router.
2. Write the command that will display details of all OSPF routing pro-
cesses enabled on a router.
3. Write the command that enables OSPF on an NBMA network for a
non-broadcast configuration.
4. Write the command that enables OSPF on an NBMA network for a
broadcast configuration.
5. Write the command that will display interface-specific OSPF
information.
6. Write the command that will display all OSPF neighbors.
7. Write the command that will display the SPF information to the ABR
and ASBR.
8. Write the command that will display all different OSPF route types
that are currently known by the router.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
163
Answers to Review Questions
1. D. The Router ID is determined by the highest IP address configured
on a loopback interface. If a router does not have a loopback interface,
then the Router ID is determined by the highest IP address configured
on the router.
2. B, C, D. Link-state routing is the type of routing performed by OSPF;
however, it is not an area of operation.
3. C. The IOS command to set the cost of an OSPF interface is ip ospf
cost cost, where cost is a number from 1 to 65,535.
4. A. In a full-mesh topology, all routers have a virtual connection to all
other routers. The configuration of a fully meshed network can
quickly become administratively prohibitive, because as the number
of full-meshed routers grows, the number of required virtual links
grows exponentially.
5. D. This status could result from an interface being down, but the spe-
cific OSPF definition is the lack of Hello packets received from the
neighbor.
6. A. The init state is simply the state of receiving Hello packets on the
interface; no adjacencies or other information have been exchanged at
this point.
7. A. Hello packets contain Router ID information. Once a router sees
its own Router ID, it is in a 2Way state.
8. B. ExStart is the step prior to exchanging all route information. LSA
floods occur for routing updates after adjacencies have been formed.
9. D. This process follows the Exchange state and verifies that no new
LSA information became available during the exchange
process.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
164
Chapter 4
OSPF Areas
10. C. Although there are continuous route exchanges, the Exchange state
occurs at the time adjacencies are established.
11. B. When a neighbor reaches Full status, it has synchronized its data-
base with all of the adjacent routers.
12. A, C. No DR is assigned on any type of point-to-point link. No DR/
BDR is assigned on the NBMA point-to-multipoint due to the hub/
spoke topology.
13. B, C. DROther routers form adjacencies only with the DR and BDR.
An RP is a rendezvous point for multicast routing.
14. B. 224.0.0.6 is used for AllDRs.
15. B. Found via Hello packets, a neighbor is an adjacent OSPF router.
Note that no routing information is exchanged with neighbors unless
adjacencies are formed.
16. D. The 1–255 range often describes the load or reliability metric for
distance-vector algorithms.
17. A. The correct equation gives values for Cisco-derived metrics,
although this can be modified.
18. A. Within OSPF, link is synonymous with interface.
19. B. Every OSPF network must contain a backbone area, which is num-
bered as Area 0.
20. A, C, and D. While OSPF has more configuration complexity than
RIP, OSPF does offer far speedier convergence, the support of Variable Length Subnet Masks, and greater scalability (overcoming RIP’s
15 hop-count limitation).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
5
Interconnecting OSPF
Areas
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
OSPF scalability considerations
Definitions of multi-area components (e.g., classifications of
routers, Link State Advertisements, and areas)
Step-by-step guide to multi-area OSPF configuration
Guidelines for establishing stub, totally stubby, and not-sostubby areas
Virtual link configuration
Strategies for monitoring and troubleshooting multi-area OSPF
networks
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
I
n this chapter, we will illustrate the scalability constraints of an
OSPF network with a single area. The concept of multi-area OSPF will be
introduced as a solution to these scalability limitations. This chapter will
also identify the various categories of routers used in multi-area configurations. These router categories include a backbone router, internal router,
area border router (ABR), and autonomous system boundary router (ASBR).
We’ll explore how these routers can use summarization and default routes to
reduce the amount of route information that is injected into an area, thus
reducing a router’s memory and processor overhead.
The functions of different OSPF Link State Advertisements (LSAs) are
very important to understand for the Routing exam, and we will detail the
types of LSAs used by OSPF. We will see how these LSAs can be minimized
through the effective implementation of specific OSPF area types.
Specifically, we will examine stub areas, totally stubby areas, and not-sostubby areas and show how these areas can be used to minimize the number
of LSAs advertised into an area. We’ll also provide a set of design guidelines
and configuration examples as well as the syntax required to configure route
summarization at both area border routers and autonomous system boundary routers.
You’ll learn that all areas need to have a link to Area 0. If an area is not
attached to Area 0, virtual links can be used to span transit areas in OSPF
networks where all areas are not physically adjacent to the backbone area.
We then will conclude with a collection of debug and show commands that
can be used to effectively monitor and troubleshoot a multi-area OSPF
implementation.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
OSPF Scalability
167
OSPF Scalability
In the previous chapter, we examined the configuration of OSPF networks that contained a single area. We saw that OSPF had significant advantages over distance-vector protocols, such as RIP, due to OSPF’s ability to
represent an entire network within its link state database, thus vastly reducing the time required for convergence.
However, let’s consider what the router does in order to give us such great
performance. Each router recalculates its database every time there is a
topology change, requiring CPU overhead. Each router has to hold the entire
link state database, which represents the topology of the entire network,
requiring memory overhead. Furthermore, each router contains a complete
copy of the routing table, requiring more memory overhead. Keep in mind
that the number of entries in the routing table may be significantly greater
than the number of networks in the routing table because we may have multiple routes to multiple networks.
With these OSPF behavioral characteristics in mind, it becomes obvious
that in very large networks, single area OSPF has some serious scalability
considerations. Fortunately, OSPF gives us the ability to take a large OSPF
topology and break it down into multiple, more manageable areas, as illustrated in Figure 5.1.
FIGURE 5.1
OSPF areas
Multi-area OSPF network
Single area OSPF network
Area 0
Area 0
Copyright ©2001 SYBEX , Inc., Alameda, CA
Area 10
www.sybex.com
Area 20
168
Chapter 5
Interconnecting OSPF Areas
Consider the advantages of this hierarchical approach. First of all, routers
that are internal to a defined area need not worry about having a link state
database for the entire network, only their own areas, thus reducing memory
overhead. Second, routers that are internal to a defined area now only have
to recalculate their link state database when there is a topology change
within their particular area. Topology changes in one area will not cause global OSPF recalculations, thus reducing processor overhead. Finally, since
routes can be summarized at area boundaries, the routing tables on each
router need not be as large as they would be in a single area environment.
Of course, as we start subdividing our OSPF topology into multiple areas,
we introduce some complexity into our configuration. Therefore, in this
chapter we will examine these various configuration subtleties, in addition to
strategies for effectively troubleshooting multi-area OSPF networks.
Categories of Multi-area Components
T
his section covers the various roles that routers play in an OSPF large
network. These include backbone routers, internal routers, area border routers, and autonomous system boundary routers. We’ll also discuss the different types of advertisements that are used in an OSPF network and the
different types of areas that can be configured.
OSPF Router Roles
As we alluded to earlier, routers within a multi-area OSPF network fall into
different categories. To gain an understanding of the various roles that our
routers can play, let’s consider Figure 5.2.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Categories of Multi-area Components
FIGURE 5.2
169
Router roles
EIGRP
Autonomous
System
Internal Router
Backbone Router
Internal Router
Area 10
RouterC
Area 0
RouterB
RouterA
Area Border Router
Autonomous System Boundary Router
Backbone Router
Starting at the core of the given network and working our way outward,
consider RouterA. Notice that RouterA is part of Area 0. As we learned in
the previous chapter, Area 0 is referred to as the backbone area. Therefore,
we can make the following definition:
Backbone router A backbone router is any router that exists (wholly or
in part) in OSPF Area 0.
Another distinction that we can make about RouterA is that it is contained completely within a single area, in this case Area 0. Since all of
RouterA’s interfaces are internal to a single area, we can make the following
definition:
Internal router An internal router is any router that has all of its interfaces as members of the same area.
Remember that a router can play more than one role. In our example, RouterA
is both a backbone router and an internal router.
Now consider RouterB. Notice that RouterB meets the requirement to be
classified as a backbone router (i.e., RouterB has one or more interfaces that
are part of Area 0). However, unlike RouterA, RouterB is partially in Area 0 and
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
170
Chapter 5
Interconnecting OSPF Areas
partially in Area 10. There is yet another term to define routers that have
interfaces in more than one area:
Area border router An area border router is any router that is connected
to multiple OSPF areas.
Recall that the topology of an OSPF area is contained in a link state database. Therefore, if a router is connected to multiple areas, it will contain multiple link state databases. This should be a design consideration when sizing
a router that will function as an area border router.
Also notice that RouterB is connected to an EIGRP network. Whether an
OSPF network is connected to an EIGRP network, a BGP network, an OSPF
network with a different Process ID, or a network running any other such
external routing process, this external network may be referred to as an
autonomous system. The scenario of an OSPF router sitting at the boundary
of an external routing process leads us to a fourth category of OSPF router:
Autonomous system boundary router An autonomous system boundary router is any OSPF router that is connected to an external routing process.
The ability of an ASBR to exchange routing information between its
OSPF routing process and the external routing process to which the router
is connected is not an automatic process. Such routes are exchanged through
a process called route redistribution, which is the focus of Chapter 10,
“Route Optimization.”
Link State Advertisements
Recall that a router’s link state database is made up of Link State Advertisements (LSAs). However, just as we had multiple OSPF router categories to
consider, we also have multiple types of LSAs to consider. Specifically, there
are five types of LSAs that we need to be concerned with. While the importance of LSA classification may not be immediately apparent, we will see its
application when we examine the various types of OSPF areas. Let’s examine
the function of the various LSA types:
Type 1 LSA Referred to as a Router Link Advertisement (RLA), the
Type 1 LSA is an advertisement sent by a router to other routers in its
area. The advertisement contains the status of a router’s link to the area
it is connected to. If a router is connected to multiple areas, then it will
send a Type 1 LSA for each of the areas it is connected to.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Categories of Multi-area Components
171
Type 2 LSA Referred to as a Network Link Advertisement (NLA), the
Type 2 LSA is generated by designated routers (DRs). Recall that a designated router is elected to represent other routers in its network, and it
has established adjacencies with each of the routers within its network.
The DR uses the Type 2 LSA to send out information about the state of
other routers that are part of the same network. Note that the Type 2 LSA
is only sent to routers that are in the area containing the specific network.
Type 3 and Type 4 LSAs Referred to as Summary Link Advertisements
(SLAs), the Type 3 and Type 4 LSAs are generated by area border routers.
These ABRs send Type 3 and Type 4 LSAs to all routers within an area. These
LSAs advertise intra-area routes to the backbone area (Area 0) and both
intra-area and inter-area routes to non-backbone areas.
Type 5 LSA Referred to as AS External Link Advertisements, Type 5
LSAs are sent by autonomous system boundary routers (ASBRs). These
ASBRs use Type 5 LSAs to advertise routes that are external to the OSPF
autonomous system.
OSPF Area Types
One of our main motivations for subdividing a single OSPF area into multiple
areas was to reduce router overhead. We decided that all routers didn’t need
to have the entire network topology in their link state databases. Let’s
now examine the types of non-standard areas we can use to reduce router
overhead:
Stub area Routers in a stub area do not receive Type 5 LSAs. Instead,
they receive a default route that is used to reach external networks. Therefore, stub area routers have reduced overhead since they do not have to
process Type 5 LSAs.
Totally stubby area To further reduce the number of LSAs that an internal router will need to process, the router can be configured as a totally
stubby area. In addition to not receiving Type 5 LSAs, a totally stubby
area does not receive summary LSAs. The function of a totally stubby area
is Cisco-specific, which is an important concept to remember when
designing an OSPF network in a multi-vendor routing environment.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
172
Chapter 5
Interconnecting OSPF Areas
Not-so-stubby area (NSSA) Like a stub area, a not-so-stubby area does
not receive Type 5 LSAs. However, sometimes there is a need, on a limited
basis, to import external routes. Such a situation is where NSSAs are useful. The NSSA imports external routes (Type 7 LSAs), via route redistribution, and then translates these Type 7 LSAs into Type 5 LSAs.
Basic Multi-area Configuration
C
onsider the multi-area OSPF network shown in Figure 5.3. To
review some of the router classifications that we previously discussed, notice
that RouterA would be classified as both an internal router and a backbone
router. Also, RouterB would be classified as both a backbone router and an
area border router. Finally, RouterC would be classified as an internal
router.
FIGURE 5.3
Sample multi-area configuration
Area 1
Area 0
1.1.3.2/24
1.1.2.1/24
1.1.4.1/24
e0
e0
e1 1.1.3.1/24
Ethernet
RouterC
1.1.1.1/24
e1
1.1.2.2/24
e0
RouterB
RouterA
interface Ethernet0
ip address 1.1.1.1 255.255.255.0
!
interface Ethernet1
ip address 1.1.2.1 255.255.255.0
!
router ospf 70
network 1.1.1.0 0.0.0.255 area 0
network 1.1.2.0 0.0.0.255 area 0
Copyright ©2001 SYBEX , Inc., Alameda, CA
e1
RouterA
www.sybex.com
Ethernet
Basic Multi-area Configuration
173
RouterB
interface Ethernet0
ip address 1.1.3.1 255.255.255.0
!
interface Ethernet1
ip address 1.1.2.2 255.255.255.0
!
router ospf 70
network 1.1.2.0 0.0.0.255 area 0
network 1.1.3.0 0.0.0.255 area 1
RouterC
interface Ethernet0
ip address 1.1.4.1 255.255.255.0
!
interface Ethernet1
ip address 1.1.3.2 255.255.255.0
!
router ospf 70
network 1.1.3.0 0.0.0.255 area 1
network 1.1.4.0 0.0.0.255 area 1
Let’s examine the syntax to configure OSPF on RouterA. First, we need to
enable the OSPF process on the router:
RouterA (config)#router ospf 70
where 70 is the Process ID.
Next, we need to identify each of the networks connected to the router
that we want to participate in the OSPF process. In this example, we have
two networks connected to RouterA (1.1.1.0/24 and 1.1.2.0/24):
RouterA(config-router)#network 1.1.1.0 0.0.0.255 area 0
where 1.1.1.0 0.0.0.255 is the network and wildcard mask of a network
connected to RouterA and where 0 is the area that network 1.1.1.0/24 is a
member of.
RouterA(config-router)#network 1.1.2.0 0.0.0.255 area 0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
174
Chapter 5
Interconnecting OSPF Areas
The syntax for RouterB is similar to that used for RouterA. The primary
difference is that RouterB is connected to two areas:
RouterB(config)#router ospf 70
RouterB(config-router)#network 1.1.2.0 0.0.0.255 area 0
RouterB(config-router)#network 1.1.3.0 0.0.0.255 area 1
The syntax for RouterC is very similar to that of RouterA. The difference
is that RouterA is internal to Area 0, thereby classifying it as a backbone
router:
RouterC(config)#router ospf 70
RouterC(config-router)#network 1.1.3.0 0.0.0.255 area 1
RouterC(config-router)#network 1.1.4.0 0.0.0.255 area 1
Stub Area Configuration
Since the main purpose of having stub areas (and totally stubby areas)
is to keep such areas from carrying external routes, we need to review some
design guidelines before configuring a stub area or a totally stubby area:
Do not make the backbone area (Area 0) a stub area.
Since external routes are injected by autonomous system boundary
routers, do not make any area containing an ASBR a stub area.
Since routers within a stub area use a default route to get out of the
stub area, typically there is only one route out of the stub area. Therefore, a stub area should usually only contain a single area border
router. Keep in mind that since a default route is being used, if a stub
area contains more than one ABR, a non-optimal path may be used.
If you decide to make a particular area a stub area, be sure to configure
all the routers in the area as stubby. If a router within a stub area has
not been configured as stubby, it will not be able to correctly form
adjacencies and exchange OSPF routes.
With these guidelines in mind, let’s examine a sample configuration for a
stub area. Consider the network shown in Figure 5.4. We’re going to make
Area 25 a stub area. In this example, we won’t be concerned with the configuration of RouterA, since it does not participate in Area 25. We will then
examine the syntax for RouterB, RouterC, and RouterD.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Stub Area Configuration
FIGURE 5.4
OPSF configuration example continued—stub area configuration
Stub Area
Area 25
e0
Area 0
Summary Route
Information
Summary Route
Information
RouterC 10.1.1.2/24
e1
1.1.1.1/24
10.1.1.1/24
e0
10.1.2.2/24
e2
RouterA
10.1.2.1/24 RouterB
e0
Default Route
External Route
RouterD
Information
Information
RouterB
interface Ethernet0
ip address 1.1.1.1 255.255.255.0
!
interface Ethernet1
ip address 10.1.1.1 255.255.255.0
!
interface Ethernet2
ip address 10.1.2.1 255.255.255.0
!
router ospf 10
network 1.0.0.0 0.255.255.255 area 0
network 10.0.0.0 0.255.255.255 area 25
area 25 stub
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
175
176
Chapter 5
Interconnecting OSPF Areas
RouterC
interface Ethernet0
ip address 10.1.2.2 255.255.255.0
!
router ospf 10
network 10.0.0.0.0 255.255.255 area 25
area 25 stub
RouterD
interface Ethernet0
ip adress 10.1.1.2 255.255.255.0
!
router ospf 10
network 10.0.0.0 0.255.255.255 area 25
area 25 stub
First, we’ll configure RouterB. Notice that RouterB is an ABR and that it
is the only ABR in Area 25, as recommended in our stub area design guidelines. When configuring an ABR that is a member of a stub area, be cautious
to only configure the stub area as stubby:
RouterB(config)#router ospf 10
where 10 is the Process ID.
RouterB(config-router)#network 1.0.0.0 0.255.255.255 area 0
where 1.0.0.0 0.255.255.255 is the network and wildcard mask of a network connected to RouterB and where 0 is the area that network 1.1.1.0/24
is a member of.
RouterB(config-router)#network 10.0.0.0 0.255.255.255 area 25
where 10.0.0.0 0.255.255.255 is a summary network and wildcard
mask of networks connected to RouterB and where 25 is the area that networks 10.1.1.0/24 and 10.1.2.0/24 are members of.
RouterB(config-router)#area 25 stub
where 25 is the area that we have designated as stubby.
Notice that instead of using two network statements to represent networks 10.1.1.0/24 and 10.1.2.0/24, we used a single network statement
specifying network 10.0.0.0/8, which includes, or summarizes, these two
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Totally Stubby Area Configuration
177
networks. By using these summary routes where possible, we can reduce the
size of a router’s routing tables, thus lowering memory and processor overhead.
We will also use the 10.0.0.0/8 summary when we configure RouterC and
RouterD. Remember that it is critical that all routers that are members of a
stub area be configured as stubby for that area. Therefore, RouterC and
RouterD will have identical OSPF configurations:
RouterC(config)#router ospf 10
RouterC(config-router)#network 10.0.0.0 0.255.255.255 area
25
RouterC(config-router)#area 25 stub
RouterD(config)#router ospf 10
RouterD(config-router)#network 10.0.0.0 0.255.255.255 area
25
RouterD(config-router)#area 25 stub
Let’s review some key elements of our stub area configuration example:
The syntax to make a router stubby is area area-id stub.
All routers that are part of Area 25 are configured as stubby.
Area 25 has only one ABR (i.e., only one path out of the area).
The ABR used the area area-id stub command only for Area 25,
not for Area 0, which is not stubby.
Totally Stubby Area Configuration
U
sing the same network topology as we had for the stub area configuration, let’s examine how to make Area 25 a totally stubby area. Remembering that the difference between a stub area and a totally stubby area is
that a totally stubby area doesn’t have summary routes injected into it, we
only need to change the configuration of RouterB. Since RouterB is the ABR,
it is the router that will have the responsibility for blocking summary routes
from entering the stub area. So, again consider our network, as illustrated in
Figure 5.5.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
178
Chapter 5
Interconnecting OSPF Areas
FIGURE 5.5
OPSF configuration example continued—totally stubby area configuration
Totally Stubby
Area
Area 25
e0
Default Route
Information
Area 0
Summary Route
Information
RouterC 10.1.1.2/24
e1
1.1.1.1/24
10.1.1.1/24
10.1.2.2/24
e0
e2
RouterA
10.1.2.1/24 RouterB
e0
Default Route
External Route
RouterD
Information
Information
RouterB
interface Ethernet0
ip address 1.1.1.1 255.255.255.0
!
interface Ethernet1
ip address 10.1.1.1 255.255.255.0
!
interface Ethernet2
ip address 10.1.2.1 255.255.255.0
!
router ospf 10
network 1.0.0.0 0.255.255.255 area 0
network 10.0.0.0 0.255.255.255 area 25
area 25 stub no-summary
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Totally Stubby Area Configuration
179
RouterC
interface Ethernet0
ip address 10.1.2.2 255.255.255.0
!
router ospf 10
network 10.0.0.0 0.255.255.255 area 25
area 25 stub
RouterD
interface Ethernet0
ip address 10.1.1.2 255.255.255.0
!
router ospf 10
network 10.0.0.0 0.255.255.255 area 25
area 25 stub
Notice that we only have to change, from the previous example, the configuration of RouterB. We simply add the no-summary argument to the area
area-id stub command:
RouterB(config)#router ospf 10
where 10 is the Process ID.
RouterB(config-router)#network 1.0.0.0 0.255.255.255 area 0
where 1.0.0.0 0.255.255.255 is the network and wildcard mask of a network connected to RouterB and where 0 is the area that network 1.1.1.0/24
is a member of.
RouterB(config-router)#network 10.0.0.0 0.255.255.255 area 25
where 10.0.0.0 0.255.255.255 is a summary network and wildcard mask
of networks connected to RouterB and where 25 is the area that networks
10.1.1.0/24 and 10.1.2.0/24 are members of.
RouterB(config-router)#area 25 stub no-summary
where the no-summary argument makes Area 25 totally stubby.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
180
Chapter 5
Interconnecting OSPF Areas
Not-So-Stubby Area Configuration
Recall that a not-so-stubby area (NSSA) is useful when we have an
area that requires the injection of external routes, although we still want to
eliminate the injection of Type 5 LSAs. Figure 5.6 presents such a scenario.
In Area 1, we want to prevent Area 0 from injecting Type 5 LSAs, yet we still
need external routes from the RIP routing process to be injected into Area 1.
The solution to these requirements is to make Area 1 an NSSA.
FIGURE 5.6
OPSF configuration example continued—not-so-stubby area configuration
Not-So-Stubby Area
RIP
OSPF
OSPF
Area 1
172.16.2.1/24
172.16.1.2/24
e1
RouterD
10.1.2.2/24
1.1.1.2/24
e1
e1
Ethernet
Area 0
e0
172.16.1.1/24
e0
e0
1.1.1.1/24
RouterC
10.1.1.1/24
e0
10.1.2.1/24 RouterA
RouterB
RouterA
interface Ethernet0
ip address 10.1.1.1 255.255.255.0
!
interface Ethernet1
ip address 10.1.2.1 255.255.255.0
!
router ospf 24
network 10.0.0.0 0.255.255.255 area 0
RouterB
interface Ethernet0
ip address 10.1.2.2 255.255.255.0
!
Copyright ©2001 SYBEX , Inc., Alameda, CA
e1
www.sybex.com
Ethernet
Not-So-Stubby Area Configuration
interface Ethernet1
ip address 1.1.1.1 255.255.255.0
!
router ospf 24
network 10.0.0.0 0.255.255.255 area 0
network 1.0.0.0 0.255.255.255 area 1
area 0 range 10.0.0.0 255.255.0.0
area 1 nssa
RouterC
interface Ethernet0
ip address 1.1.1.2 255.255.255.0
!
interface Ethernet1
ip address 172.16.1.1 255.255.255.0
!
router ospf 24
redistribute rip
network 1.0.0.0 0.255.255.255 area 1
default-metric 128
area 1 nssa
!
router rip
redistribute ospf 24
network 172.16.0.0
default-metric 3
RouterD
interface Ethernet0
ip address 172.16.1.2 255.255.255.0
!
interface Ethernet1
ip address 172.16.2.1 255.255.255.0
!
router rip
network 172.16.0.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
181
182
Chapter 5
Interconnecting OSPF Areas
Let’s examine the configuration of each of these routers, beginning with
RouterA. RouterA is a backbone router (and an internal router), which does
not participate in our NSSA (Area 1). Therefore, RouterA doesn’t need any
special NSSA configuration. However, by way of review, we will still examine its syntax:
RouterA(config)#router ospf 24
where 24 is the Process ID.
RouterA(config-router)#network 10.0.0.0 0.255.255.255
area 0
where 10.0.0.0 0.255.255.255 is a network and wildcard mask summarization of the networks connected to RouterA and where 0 is the area that
networks 10.1.1.0/24 and 10.1.2.0/24 are members of.
RouterB does participate in the NSSA. Therefore, it will require a special
configuration:
RouterB(config)#router ospf 24
RouterB(config-router)#network 10.0.0.0 0.255.255.255
area 0
RouterB(config-router)#network 1.0.0.0 0.255.255.255
area 1
RouterB(config-router)#area 0 range 10.0.0.0 255.0.0.0
where 10.0.0.0 255.0.0.0 is the network number and subnet mask of a
network that summarizes the individual networks within Area 0, thus reducing the number of a router’s routing table entries.
RouterB(config-router)#area 1 nssa
where 1 is the area that is being designated as a not-so-stubby area.
Notice that the configuration for RouterB included the command area
area-id range network_address network_mask, which can be used on
area border routers to summarize the IP address space being used by routers
within a given area. Also notice the area area-id nssa command. This
command tells the router that the specified area the router is connected to is
a not-so-stubby area. As we saw when configuring stub areas, all routers
within a not-so-stubby area must agree that they are connected to a NSSA
(i.e., be configured with the area area-id nssa command).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Not-So-Stubby Area Configuration
183
To expand upon the idea of advertising summarized routes, the area areaid range network_address network_mask command is used to summarize
intra-area routes on an ABR. Similarly, we can summarize external routes on
an autonomous system boundary router (ASBR) with the command summaryaddress network_address network_mask. Proper use of these summarization
tools can greatly reduce the number of routes that have to be maintained by
a router, thus reducing memory and processor overhead.
RouterC will be an even more complex configuration. Not only is
RouterC part of an NSSA, it also participates in a RIP routing process. In
order to exchange its OSPF and RIP routes, RouterC must perform route
redistribution (route redistribution is the focus of Chapter 10):
RouterC(config)#router ospf 24
RouterC(config-router)#redistribute rip
where rip is the routing protocol whose routes are being injected into the
OSPF routing process.
RouterC(config-router)#network 1.0.0.0 0.255.255.255 area 1
RouterC(config-router)#default-metric 128
where 128 is the OSPF metric value to be assigned to routes being redistributed into the OSPF routing process.
RouterC(config-router)#area 1 nssa
RouterC(config-router)#router rip
This enables the RIP routing process on the router.
RouterC(config-router)#redistribute ospf 24
where ospf 24 is the routing process whose routes are being injected into the
RIP routing process.
RouterC(config-router)#network 172.16.0.0
RouterC(config-router)#default-metric 3
where 3 is the RIP metric value (hop count) to be assigned to routes being
redistributed into the RIP routing process.
RouterD is internal to the RIP routing process. Therefore, RouterD does
not require any NSSA-specific configuration:
RouterD(config)#router rip
RouterD(config-router)#network 172.16.0.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
184
Chapter 5
Interconnecting OSPF Areas
OSPF Virtual Links
When designing a multi-area OSPF network, all areas should be connected to the backbone area. However, there may be instances when an area
has to cross another area to reach the backbone area, as shown in Figure 5.7.
Since, in this example, Area 20 does not have a direct link to Area 0, we need
to create a virtual link.
FIGURE 5.7
OSPF virtual link
Area 0
Area 10
Area 20
Lo0:6.6.6.1/24
Lo0:5.5.5.1/24
Lo0:2.2.2.1/24
1.1.1.1/24
3.3.3.1/24
e0
Ethernet
4.4.4.1/24
e0
e1
RouterA
e1
3.3.3.2/24
e0
7.7.7.1/24
4.4.4.2/24
e1
Ethernet
RouterB
RouterC
The syntax for creating a virtual link across an area is
area area-id virtual-link router-id
where area-id is the number of the transit area, in this example, Area 10,
and router-id is the IP address of the highest loopback interface configured
on a router. If a loopback interface has not been configured on the router,
then the router-id is the highest IP address configured on the router. Note
that a virtual link has area border routers as the endpoints of the link.
As shown in Figure 5.8, we are going to create a virtual link from Area 20
to Area 0, with Area 10 acting as the transit area. Let’s examine the configuration of RouterB and RouterC, since RouterA does not have any virtuallink-specific configuration.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
OSPF Virtual Links
FIGURE 5.8
185
OSPF virtual link
Area 0
Area 10
Lo0:5.5.5.1/24
Area 20
Lo0:6.6.6.1/24
Lo0:2.2.2.1/24
1.1.1.1/24
3.3.3.1/24
e0
Ethernet
e1
e0
e0
4.4.4.1/24
4.4.4.2/24
3.3.3.2/24
7.7.7.1/24
e1
e1
RouterA
Ethernet
RouterB
RouterC
Virtual Link
Here is the configuration of RouterB and RouterC:
RouterB(config)#router ospf 10
RouterB(config-router)#network 3.0.0.0 0.255.255.255
area 0
RouterB(config-router)#network 4.0.0.0 0.255.255.255
area 10
RouterB(config-router)#area 10 virtual-link 6.6.6.1
where 10 is the Area ID of the transit area and where 6.6.6.1 is the highest
loopback address of the ABR joining the transit area to Area 20.
RouterC(config)#router ospf 10
RouterC(config-router)#network 4.0.0.0 0.255.255.255
area 10
RouterC(config-router)#network 7.0.0.0 0.255.255.255
area 20
RouterC(config-router)#area 10 virtual-link 5.5.5.1
where 10 is the Area ID of the transit area and where 5.5.5.1 is the highest
loopback address of the ABR joining the transit area to the backbone area.
RouterA
interface Loopback0
ip address 2.2.2.1 255.255.255.0
!
internet Ethernet0
ip address 1.1.1.1 255.255.255.0
!
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
186
Chapter 5
Interconnecting OSPF Areas
interface Ethernet1
ip address 3.3.3.1 255.255.255.0
!
router ospf 10
network 1.0.0.0 0.255.255.255 area 0
network 3.0.0.0 0.255.255.255 area 0
RouterB
interface Loopback0
ip address 5.5.5.1 255.255.255.0
!
interface Ethernet0
ip address 3.3.3.2 255.255.255.0
!
interface Ethernet1
ip address 4.4.4.1 255.255.255.0
!
router ospf 10
network 3.0.0.0 0.255.255.255 area 0
network 4.0.0.0 0.255.255.255 area 10
area 10 virtual-link 6.6.6.1
RouterC
interface Loopback0
ip address 6.6.6.1 255.255.255.0
!
interface Ethernet0
ip address 4.4.4.2 255.255.255.0
!
interface Ethernet1
ip address 7.7.7.1 255.255.255.0
!
router ospf 10
network 4.0.0.0 0.255.255.255 area 10
network 7.0.0.0 0.255.255.255 area 20
area 10 virtual-link 5.5.5.1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
187
Monitoring and Troubleshooting Multi-area
OSPF Networks
C
isco’s IOS has several debug and show commands that can be useful
in monitoring and troubleshooting OSPF networks. Following is a sampling
of these commands, which can be used to gain information about various
OSPF characteristics:
debug ip ospf events Shows information concerning OSPF events,
such as the selection of a designated router and the formation of router
adjacencies.
debug ip ospf packet Shows information contained in each OSPF
packet, such as Router ID and Area ID.
show ip ospf border-routers Shows an ABR’s internal routing
table.
show ip ospf virtual-links Shows the status of a router’s
virtual link.
show ip ospf neighbor Shows neighbor router information, such as
Neighbor ID and the state of adjacency with the neighboring router.
show ip ospf process-id Shows area information, such as identifying the area’s area border router or autonomous system boundary router.
show ip ospf database Shows information about a router’s OSPF
database, such as a router’s router link states and network link states.
Summary
I
n this chapter, we illustrated the scalability constraints of an OSPF network with a single area. We introduced the concept of multi-area OSPF as a
solution to these scalability limitations.
We identified the different categories of routers used in multi-area configurations. These router categories include backbone router, internal router,
area border router, and autonomous system boundary router. We explored
how these routers can use summarization and default routes to reduce the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
188
Chapter 5
Interconnecting OSPF Areas
amount of route information that is injected into an area, thus reducing a
router’s memory and processor overhead.
We detailed the function of different OSPF Link State Advertisements
(LSAs). We saw how these LSAs could be minimized through the effective
implementation of specific OSPF area types.
Specifically, we examined stub areas, totally stubby areas, and not-sostubby areas and showed how these areas can be used to minimize LSAs
advertised into an area. We provided a set of design guidelines and configuration examples. We also showed the syntax required to configure route
summarization at both area border routers and autonomous system boundary routers.
Since all areas need to have a link to the backbone area, we explained how
virtual links could be used to span transit areas in OSPF networks where all
areas are not physically adjacent to the backbone area. We then concluded
with a collection of debug and show commands that can be used to effectively monitor and troubleshoot a multi-area OSPF implementation.
Key Terms
Before taking the exam, make sure you’re familiar with the following terms:
transit area
virtual link
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
189
Written Lab
Given the network diagram in the following graphic, write out the
OSPF-specific configuration to create the virtual link that would be required
for this topology.
Area 0
Area 2
Lo0:3.3.3.3/24
Area 1
Lo0:6.6.6.6/24
Lo0:1.1.1.1/24
Lo0:8.8.8.8/24
2.2.2.2/24
e0
RouterA
e0
4.4.4.4/24
e1
4.4.4.5/24
e0
2.2.2.3/24
RouterB
7.7.7.7/24
e1
e0
7.7.7.8/24
RouterD
RouterC
Solution
Recall that all areas need a link back to the backbone area. If an area is not
physically adjacent to the backbone area, then a virtual link can be used to
span the transit area that separates the area from the backbone area. In this
exercise, since Area 1 does not attach directly to Area 0, we need to create a
virtual link across the transit area (Area 2).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
190
Chapter 5
Interconnecting OSPF Areas
Since a virtual link requires that only the ABRs on each side of the transit
area be configured, we will examine only the configuration of RouterB and
RouterC.
Area 0
Area 2
Lo0:3.3.3.3/24
Area 1
Lo0:6.6.6.6/24
Lo0:1.1.1.1/24
Lo0:8.8.8.8/24
2.2.2.2/24
e0
RouterA
e0
4.4.4.4/24
e1
4.4.4.5/24
e0
2.2.2.3/24
RouterB
7.7.7.7/24
e1
e0
7.7.7.8/24
RouterD
RouterC
Virtual Link
As shown in the above graphic, RouterB uses the following command to
create its side of the virtual link:
area 2 virtual-link 6.6.6.6
where 2 is the transit area and where 6.6.6.6 is the Router ID (the highest
loopback IP address) of the ABR at the other side of the transit area.
Similarly, RouterC uses the following command to create its side of the
virtual link:
area 2 virtual-link 3.3.3.3
where 2 is the transit area and where 3.3.3.3 is the Router ID (the highest
loopback IP address) of the ABR at the other side of the transit area.
RouterB
interface Loopback0
ip address 3.3.3.3 255.255.255.0
!
internet Ethernet0
ip address 2.2.2.3 255.255.255.0
!
interface Ethernet1
ip address 4.4.4.4 255.255.255.0
!
router ospf 20
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
191
network 2.0.0.0 0.255.255.255 area 0
network 4.0.0.0 0.255.255.255 area 2
area 2 virtual-link 6.6.6.6
RouterC
interface Loopback0
ip address 6.6.6.6 255.255.255.0
!
interface Ethernet0
ip address 4.4.4.5 255.255.255.0
!
interface Ethernet1
ip address 7.7.7.7 255.255.255.0
!
router ospf 20
network 4.0.0.0 0.255.255.255 area 2
network 7.0.0.0 0.255.255.255 area 1
area 2 virtual-link 3.3.3.3
Hands-on Lab
I
nterconnect three routers, as shown in the graphic below. Configure
Area 10 as a stub area, and configure Area 20 as a totally stubby area.
Area 10
1.1.1.1/24
Area 0
2.2.2.1/24 e0
e0
e1
2.2.2.2/24
Ethernet
RouterA
Area 20
3.3.3.3/24 e0
e1
3.3.3.2/24
RouterB
Copyright ©2001 SYBEX , Inc., Alameda, CA
RouterC
www.sybex.com
e1
4.4.4.4/24
Ethernet
192
Chapter 5
Interconnecting OSPF Areas
Solution
The configuration given below is a suggested solution to the lab. Notice that
RouterB uses the command area 10 stub to specify that Area 10 is a stub
area. Also notice that RouterC uses the command area 20 stub nosummary to specify that Area 20 is a totally stubby area. Recall that the difference between a stub area and a totally stubby area is that a totally stubby
area does not receive summary Link State Advertisements. Another important design distinction is that the concept of a totally stubby area is specific
to Cisco routers.
Totally Stubby
Area
Stub Area
Area 10
Area 0
1.1.1.1/24
Area 20
2.2.2.1/24 e0
e0
e1
Ethernet
2.2.2.2/24
RouterA
3.3.3.3/24 e0
e1
3.3.3.2/24
RouterB
RouterC
RouterA
!
version 11.2
no services udp-small-servers
no services tcp-small-servers
!
hostname RouterA
!
interface Ethernet0
ip address 2.2.2.2 255.255.255.0
!
interface Ethernet1
ip address 3.3.3.3 255.255.255.0
!
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
e1
4.4.4.4/24
Ethernet
Hands-on Lab
!
router ospf 50
network 2.0.0.0 0.255.255.255 area 0
network 3.0.0.0 0.255.255.255 area 0
!
no ip classless
!
!
line con 0
line aux 0
line vty 0 4
login
!
end
RouterB
!
version 11.2
no services udp-small-servers
no services tcp-small-servers
!
hostname RouterB
!
interface Ethernet0
ip address 1.1.1.1 255.255.255.0
!
interface Ethernet1
ip address 2.2.2.1 255.255.255.0
!
!
router ospf 50
network 1.0.0.0 0.255.255.255 area 10
network 2.0.0.0 0.255.255.255 area 0
area 10 stub
!
no ip classless
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
193
194
Chapter 5
Interconnecting OSPF Areas
!
!
line con 0
line aux 0
line vty 0 4
login
!
end
RouterC
!
version 11.2
no services udp-small-servers
no services tcp-small-servers
!
hostname RouterC
!
interface Ethernet0
ip address 3.3.3.2 255.255.255.0
!
interface Ethernet1
ip address 4.4.4.4 255.255.255.0
!
!
router ospf 50
network 3.0.0.0 0.255.255.255 area 0
network 4.0.0.0 0.255.255.255 area 20
area 20 stub no-summary
!
no ip classless
!
!
line con 0
line aux 0
line vty 0 4
login
!
end
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
195
Review Questions
1. Which of the following are scalability issues with single area OSPF
networks? (Choose all that apply.)
A. Size of the routing table
B. Size of the OSPF database
C. Maximum hop count limitation
D. Recalculation of the OSPF database
2. Which of the following describes a router that connects to an external
routing process (e.g., EIGRP)?
A. ABR
B. ASBR
C. Type 2 LSA
D. Stub router
3. Which of the following makes use of route redistribution?
A. Stub area
B. Totally stubby area
C. ABR
D. NSSA
4. Which of the following describes the syntax used to summarize intra-
area routes on an ABR?
A. ip route summarize area-id network-address network-
mask
B. summary-address network-address network-mask
C. area area-id range network-address network-mask
D. summary area-id network-address network-mask
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
196
Chapter 5
Interconnecting OSPF Areas
5. Which IOS command shows information about a router’s OSPF data-
base, such as router link states and network link states?
A. debug ospf events
B. show ip ospf border-routers
C. show ip ospf neighbor
D. show ip ospf database
6. What is the term given to describe a router connected to multiple
OSPF areas?
A. ABR
B. ASBR
C. Type 2 LSA
D. Stub router
7. As compared to a standard OSPF area, what is unique about a stub area?
A. It does not receive Summary Link Advertisements.
B. It does not receive Type 5 LSAs.
C. It makes use of route redistribution.
D. It is a concept specific to Cisco routers.
8. What is the IOS syntax to designate an area as a not-so-stubby area?
A. area area-id nssa
B. area area-id stub
C. area area-id no-summary
D. area area-id stub no-summary
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
197
9. What is an autonomous system boundary router (ASBR)?
A. Any OSPF router that is connected to an external routing process
B. Any OSPF router that is connected to an internal routing process
C. Any router that is connected to multiple OSPF areas
D. Any router that is connected to single OSPF areas
10. Which of the following can be described as a router that has all of its
interfaces in the same area?
A. Area border router
B. Internal router
C. Autonomous System Boundary Router
D. Designated router
11. Which of the following IOS commands shows an ABR’s internal
router routing table?
A. debug ospf events
B. show ip ospf border-routers
C. show ip ospf neighbor
D. show ip ospf database
12. As compared to an OSPF stub area, what is unique about an OSPF
totally stubby area? (Choose all that apply.)
A. It does not receive Type 5 LSAs.
B. It is a Cisco-specific feature.
C. It does not receive summary Link State Advertisements.
D. It makes use of route redistribution.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
198
Chapter 5
Interconnecting OSPF Areas
13. Which of the following are design guidelines for setting up stub areas?
(Choose all that apply.)
A. Use only when a router within an area connects to an external
routing process (e.g., EIGRP).
B. Do not make the backbone area a stub area.
C. If you decide to make a particular area a stub area, be sure to con-
figure all the routers in the area as stubby.
D. Configure only the area’s ABR as stubby.
14. What is the IOS syntax to create a virtual link?
A. area source-area-id virtual-link destination-area-id
B. link area-id router-id
C. ospf map area-id router-id
D. area area-id virtual-link router-id
15. Which of the following IOS commands shows the state of adjacency
with neighbor routers?
A. debug ospf events
B. show ip ospf border-routers
C. show ip ospf neighbor
D. show ip ospf database
16. Which of the following statements are true regarding Summary Link
Advertisements (SLAs)? (Choose all that apply.)
A. Type 3 LSAs are Summary Link Advertisements.
B. Totally stubby areas do not receive Summary Link Advertise-
ments.
C. Type 4 LSAs are Summary Link Advertisements.
D. Stub areas do not receive Summary Link Advertisements.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
199
17. Which of the following can be defined as a router that exists (wholly
or in part) in OSPF Area 0?
A. Backbone router
B. Autonomous system boundary router
C. Designated router
D. Area border router
18. Which of the following describes the syntax used to summarize exter-
nal routes on an ASBR?
A. ip route summarize area-id network-address network-
mask
B. summary-address network-address network-mask
C. area area-id range network-address network-mask
D. summary area-id network-address network-mask
19. Which of the following IOS commands shows the formation of router
adjacencies?
A. debug ospf events
B. debug ip ospf border-routers
C. debug ip ospf neighbor
D. debug ip ospf database
20. Which of the following LSA types is referred to as an AS External Link
Advertisement?
A. Type 1 LSA
B. Type 2 LSA
C. Type 3 or Type 4 LSA
D. Type 5 LSA
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
200
Chapter 5
Interconnecting OSPF Areas
Answers to Review Questions
1. A, B, and D. As the size of a single area OSPF network grows, so
does the size of the routing table and OSPF database that has to be
maintained. Also, if there is a change in network topology, the OSPF
algorithm has to be rerun for the entire network.
2. B. An autonomous system boundary router (ASBR) is any OSPF
router that is connected to an external routing process.
3. D. A not-so-stubby area (NSSA) imports external routes (Type 7
LSAs) via route redistribution and then translates these Type 7 LSAs into
Type 5 LSAs.
4. C. The command area area-id range network-address
network-mask is used on an ABR to summarize routes before they are
injected into an area.
5. D. The IOS command show ip ospf database shows details about
a router’s OSPF database. Some of these details include router link
states and network link states.
6. A. An area border router (ABR) is any OSPF router that is connected
to more than one OSPF area.
7. B. Routers in a stub area do not receive Type 5 LSAs (referred to as
AS External Link Advertisements). Instead, they use a default route to
reach external networks.
8. A. The router configuration command area area-id nssa is used
to designate an area as a not-so-stubby area (NSSA).
9. A. An autonomous system boundary router (ASBR) is any OSPF
router that is connected to an external routing process.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
201
10. B. The definition of an internal router is a router that has all of its
interfaces within the same OSPF area. Keep in mind that a router can
fall into multiple categories. For example, if an internal router is in
Area 0, it would also be considered a backbone router.
11. B. The routing table of area border routers can be shown using the
IOS command show ip ospf border-routers. This table shows
which border router will get a packet to the network.
12. B and C. The concept of an OSPF totally stubby area is specific to
Cisco routers. In addition to not receiving Type 5 LSAs as a stub area
does, a totally stubby area does not receive summary Link State
Advertisements.
13. B and C. The backbone area cannot be a stub area. Also, only rout-
ers configured as stubby will be able to form adjacencies within a
stubby area.
14. D. The syntax for creating a virtual link is area area-id virtual-
link router-id, where area-id is the number of the transit area
and where router-id is the IP address of the highest loopback interface configured on a router. If a loopback is not configured, then the
router-id is the highest IP address configured on the router.
15. C. The IOS command show ip ospf neighbor shows neighbor
router information, such as Neighbor ID and the state of adjacency
with the neighboring router.
16. A, B, and C. Both Type 3 LSAs and Type 4 LSAs are considered to be
Summary Link Advertisements. While totally stubby areas do not
receive Summary Link Advertisements, stub areas do.
17. A. A backbone router has at least one of its interfaces connected to
OSPF Area 0.
18. B. The command summary-address network-address network-
mask is used on an autonomous system boundary router (ASBR) to
summarize external routes before they are injected into OSPF.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
202
Chapter 5
Interconnecting OSPF Areas
19. A. The IOS command debug ospf events shows such events as the
selection of a designated router and the formation of router adjacencies.
20. D. Type 5 LSAs are referred to as AS External Link Advertisements.
Type 5 LSAs are blocked by stub areas, totally stubby areas, and notso-stubby areas.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
6
IGRP and EIGRP
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
Describe IGRP features and operation
Configure IGRP
Verify IGRP operation
Describe Enhanced IGRP features and operation
Explain how metrics are used with EIGRP
Explain how DUAL is used with EIGRP
Explain the features supported by EIGRP
Learn how EIGRP discovers, decides, and maintains routes
Explain EIGRP process identifiers
Explain EIGRP troubleshooting commands
Configure EIGRP and verify its operation
Verify route redistribution
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
S
o far in this book, we have taken an in-depth look at the routing protocol OSPF and shown how a routing protocol is used to find routes
through the network. We also learned how routing protocols are used to
exchange IP address information between routers in an enterprise network.
IP addressing schemes establish a hierarchy that makes path information
both distinct and efficient. A router receives this routing information via a
given interface. It then advertises the information it knows out the other
physical interfaces. This routing process occurs at Layer 3 of the OSI model.
In this chapter, in order to decide on the best routing protocol or protocols to
use, we’ll take a look at both the Interior Gateway Routing Protocol
(IGRP) and its big brother, the Enhanced Interior Gateway Routing Protocol (EIGRP).
Unlike OSPF, IGRP and EIGRP are proprietary Cisco protocols and run
on Cisco routers and internal route processors found in the Cisco Distribution and Core layer switches. (I need to note here that Cisco has licensed
IGRP to be used on other vendors’ equipment, such as Compaq.) Each of
these routing protocols also has its own identifiable functions, so we’ll discuss each routing protocol’s features and differences. Once you understand
how these protocols differ from OSPF and how they calculate routes, you
will learn how to configure these protocols and fine-tune them with configuration changes to make each perform at peak efficiency.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Scalability Features of Routing Protocols
205
Scalability Features of Routing Protocols
Several times in this book, as we look at the different routing protocols—OSPF, IGRP, EIGRP, and BGP—we will refer back to distance-vector
and link-state routing protocol differences. It is important to identify how
these protocols differ from one another.
As networks grow and administrators implement or use Cisco-powered
networks, OSPF might not be the most efficient or recommended protocol to
use. OSPF does have some advantages of IGRP, EIGRP, and BGP, including:
It is versatile.
It uses a very scalable routing algorithm.
It allows the use of a routing protocol that is compatible with nonCisco routers.
BGP will be discussed in Chapters 7 through 9.
Cisco provides two other proprietary solutions that allow better scaling
and convergence, which can be very critical issues. These are the Interior
Gateway Routing Protocol (IGRP) and Enhanced IGRP (EIGRP). Network
growth imposes a great number of changes on the network environment and
takes into consideration the following factors:
The number of hops between end systems
The number of routes in the routing table
The different ways a route was learned
Route convergence
IGRP and EIGRP can be used to maintain a very stable routing environment,
which is absolutely crucial in larger networks.
As the effects of network growth start to manifest themselves, whether or
not your network’s routers can meet the challenges faced in a larger scaled
network is completely up to the routing protocol the routers are running. If
you use a protocol that’s limited by the number of hops it can traverse, the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
206
Chapter 6
IGRP and EIGRP
number of routes it can store in its table, or even the inability to communicate with other protocols, then you have a protocol that will likely hinder the
growth of your network.
All the issues we’ve brought up so far are general scalability considerations. Before we look at IGRP and EIGRP, let’s take another look at the differences between link-state routing protocols and distance-vector protocols
and the scalability issues of each.
Link-state routing and distance-vector protocols are discussed in detail in
Chapter 2, and are discussed in Chapter 7 as they relate to BGP.
Distance-Vector Protocol Scalability Issues
In small networks—meaning those with fewer than 100 routers and an environment that’s much more forgiving of routing updates and calculations—
distance-vector protocols perform fairly well. However, you’ll run into several problems when attempting to scale a distance-vector protocol to a larger
network—convergence time, router overhead (CPU utilization), and bandwidth utilization all become factors that hinder scalability.
A network’s convergence time is determined by the ability of the protocol
to propagate changes within the network topology. Distance-vector protocols
don’t use formal neighbor relationships between routers. A router using
distance-vector algorithms becomes aware of a topology change in two ways:
When a router fails to receive a routing update from a directly connected router
When a router receives an update from a neighbor notifying it of a
topology change somewhere in the network
Routing updates are sent out on a default or specified time interval. So
when a topology change occurs, it could take up to 90 seconds before a
neighboring router realizes what’s happened. When the router finally recognizes the change, it recalculates its routing table and sends the whole thing
out to all its neighbors.
Not only does this cause significant network convergence delay, it also
devours bandwidth—just think about 100 routers all sending out their entire
routing table and imagine the impact on your bandwidth. It’s not exactly a
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Scalability Features of Routing Protocols
207
sweet scenario, and the larger the network, the worse it gets, because a
greater percentage of bandwidth is needed for routing updates.
As the size of the routing table increases, so does CPU utilization, because
it takes more processing power to calculate the effects of topology changes
and then converge using the new information. Also, as more routes populate
a routing table, it becomes increasingly complex to determine the best path
and next hop for a given destination. The following list summarizes the scalability limitations inherent in distance-vector algorithms:
Network convergence delay
Increased CPU utilization
Increased bandwidth utilization
Scalability Limitations of Link-State Routing Protocols
Link-state routing protocols assuage the scalability issues faced by distancevector protocols because the algorithm uses a different procedure for route
calculation and advertisement. This enables them to scale along with the
growth of the network.
Addressing distance-vector protocols’ problem with network convergence, link-state routing protocols maintain a formal neighbor relationship
with directly connected routers that allows for faster route convergence.
They establish peering by exchanging Hello packets during a session, which
cements the neighbor relationship between two directly connected routers.
This relationship expedites network convergence because neighbors are
immediately notified of topology changes. Hello packets are sent at short
intervals (typically every 10 seconds), and if an interface fails to receive Hello
packets from a neighbor within a predetermined hold time, the neighbor is
considered down, and the router will then flood the update out all physical
interfaces. This occurs before the new routing table is calculated, so it saves
time. Neighbors receive the update, copy it, flood it out their interfaces, and
then calculate the new routing table. The procedure is followed until the
topology change has been propagated throughout the network.
It’s noteworthy that the router sends an update concerning only the new
information—not the entire routing table. So the update is a lot smaller,
which saves both bandwidth and CPU utilization. Plus, if there aren’t any
network changes, updates are sent out only at specified, or default, intervals,
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
208
Chapter 6
IGRP and EIGRP
which differ among specific routing protocols and can range from 30 minutes to two hours.
These are key differences that permit link-state routing protocols to function well in large networks—they really have no limitations when it comes to
scaling, other than the fact that they’re a bit more complex to configure than
distance-vector protocols.
Interior Gateway Routing Protocol
I
nterior Gateway Routing Protocol (IGRP) is a Cisco proprietary routing protocol that uses a distance-vector algorithm. It uses this algorithm
because it uses a vector (a one-dimensional array) of information to calculate
the best path. This vector consists of four elements:
Bandwidth
Delay
Load
Reliability
We’ll describe each element in detail shortly.
Maximum transfer unit (MTU) information is included in the final route information, but it’s used as part of the vector of metrics.
IGRP is intended to replace RIP and create a stable, quickly converging
protocol that will scale with increased network growth. As we mentioned,
it’s preferable to implement a link-state routing protocol in large networks
because of the overhead and delay that results from using a distance-vector
protocol.
In the next few sections, we will quickly take you through the features of
IGRP and show how to implement this routing protocol in your network.
We will also cover the types of metrics, unequal-cost load balancing, and the
limitations of redistribution.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Interior Gateway Routing Protocol
209
IGRP Features and Operation
IGRP has several features included in the algorithm—these features and a
brief description can be found below in Table 6.1. Most of these features
were added to make IGRP more stable, and a few were created to deal with
routing updates and make network convergence happen faster.
TABLE 6.1
IGRP Features
Feature
Description
Configurable metrics
The user can configure metrics involved
in the algorithm responsible for
calculating route information.
Flash update
Updates are sent out prior to the default
time setting. This occurs when the
metrics for a route change.
Poison reverse updates
Implemented to prevent routing loops,
these updates place a route in holddown. Hold-down means that the router
won’t accept any new route information
on a given route for a certain period
of time.
Unequal-cost load balancing
Allows packets to be shared or
distributed across multiple paths.
IGRP is a classful protocol, which means it doesn’t include any subnet
information about the network with route information.
Classful protocols are discussed in Chapter 2.
IGRP recognizes three types of routes:
Interior Networks directly connected to a router interface.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
210
Chapter 6
IGRP and EIGRP
System Routes advertised by other IGRP neighbors within the same
autonomous system (AS). The AS number (ASN) identifies the IGRP session, because it’s possible for a router to have multiple IGRP sessions.
Exterior Routes learned via IGRP from a different ASN, which provide
information used by the router to set the gateway of last resort. The gateway of last resort is the path a packet will take if a specific route isn’t
found on the router.
When we talked about the scalability of distance-vector protocols, we
told you that they don’t establish a formal neighbor relationship with
directly connected routers and that routing updates are sent at designated
intervals. IGRP’s interval is 90 seconds, which means that every 90 seconds
IGRP will broadcast its entire routing table to all directly connected IGRP
neighbors.
IGRP Metrics
Metrics are the mathematics used to select a route. The higher the metric
associated with a route, the less desirable it is. The overall metric assigned to
a route is created by the Bellman-Ford algorithm, using the following
equation:
metric = [K1 × Bw + (K2 × Bw) / (256 – Load) + K3 × Delay] × [K5 /
(Rel + K4)]
By default: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0.
Delay is the sum of all the delays of the links along the paths.
Delay = [Delay in 10s of microseconds] × 256.
BW is the lowest bandwidth of the links along the paths.
BW = [10000000 / (bandwidth in Kbps)] × 256.
By default, metric = bandwidth + delay.
The formula above is used for the non-default setting, when K5 does not equal
0. If K5 equals the default value of 0, then this formula is used: metric = K1 ×
bandwidth + (K2 × bandwidth) / (256 − Load) + K3 × Delay].
If necessary, you can adjust metrics within the router configuration interface. Metrics are tuned to change the manner in which routes are calculated.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Interior Gateway Routing Protocol
211
After you enable IGRP on a router, metric weights can be changed using the
following command:
metric weights tos K1 K2 K3 K4 K5
Table 6.2 shows the relationship between the constant and the metric it
affects.
TABLE 6.2
Metric Association of K Values
Constant
Metric
K1
Bandwidth (Be)
K2
Delay (Dc)
K3
Reliability (r)
K4
Load (utilization on path)
K5
MTU
Each constant is used to assign a weight to a specific variable. This means
that when the metric is calculated, the algorithm will assign a greater importance to the specified metric. By assigning a weight, you are able to specify
what is most important. If bandwidth is of greatest concern to a network
administrator, then a greater weight would be assigned to K1. If delay is
unacceptable, then the K2 constant should be assigned a greater weight. The
tos variable is the type of service.
As well as tuning the actual metric weights, you can do other tunings. All
routing protocols have an administrative distance associated with the protocol type. If multiple protocols are running on one router, the administrative
distance value helps the router decide which path is best. The protocol with
the lowest administrative distance will be chosen. IGRP has a default administrative distance of 100. The tuning of this value is accomplished with the
distance command, like this:
distance 1–255
Valid values for the administrative distance range from 1 to 255. Again, the
lower the value, the better.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
212
Chapter 6
IGRP and EIGRP
When redistributing static routes or other protocol types within IGRP,
metrics may be set for these routes as well by using the default-metric
command:
default-metric bandwidth delay reliability load MTU
The words in italics in the command above are just placeholders for variables
and should be replaced with numbers.
Bandwidth and delay have a range of values from 0 to 4,294,967,295 (in
Kbps) and 0 to 4,294,967,295 (in 10-microsecond units), respectively. Reliability ranges from 0 to 255, with 255 being the most reliable. Load ranges
from 0 to 255; however, a value of 255 means that the link is completely
loaded. Finally, the value of MTU has the same range as the bandwidth variable: 0 to 4,294,967,295.
When a router receives multiple routes for a specific network, one of the
routes must be chosen as the best route from all of the advertisements. The
router still knows that it is possible to get to a given network over multiple
interfaces, yet all data default to the best route.
IGRP provides the ability of unequal-cost load balancing. The variance
command is used to assign a weight to each feasible successor. A feasible successor is a predetermined route to use should the most optimal path be lost.
The feasible successor can also be used as long as the secondary route conforms to the following three criteria, and an unequal-cost load balancing session may be established:
A limit of four feasible successors may be used for load balancing.
Four is the default; the maximum number of feasible successors is six
for IOS version 11.0 and later.
The feasible successor’s metric must fall within the specified variance
of the local metric.
The local metric must be greater than the metric for the next-hop
router.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
213
A lower metric signifies a better route.
Redistribution Limitations
As an enterprise network grows, there is a possibility that more than one
protocol will run on the router. An example is when a company acquires
another company and needs to merge the two existing networks. The problem surfaces when the routes of the purchasing company need to be advertised to the newly acquired company. IGRP solves the problem with route
redistribution.
When multiple protocols run on a router, you can configure IGRP to
redistribute routes from specified protocols. Since different protocols calculate metrics distinctly, adjustments must be made when redistributing protocols. These adjustments cause some limitations in how the redistribution
works. The adjustments are made by using the default-metric command,
as shown previously.
IGRP may also be redistributed to other routing protocols such as RIP,
other IGRP sessions, EIGRP, and OSPF. Metrics are also configured using
the default-metric command.
Enhanced Interior Gateway Routing Protocol
Enhanced Interior Gateway Routing Protocol (EIGRP) is better than
its little brother, IGRP. EIGRP allows for equal-cost load balancing, incremental routing updates, and formal neighbor relationships, which overcome
the limitations of IGRP. The enhanced version uses the same distance-vector
information as IGRP, yet with a different algorithm. EIGRP uses DUAL
(Diffusing Update Algorithm) for metric calculation, which permits rapid
convergence. This algorithm allows for the following:
Backup route determination if one is available
Support of Variable-Length Subnet Masks (VLSM)
Dynamic route recoveries
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
214
Chapter 6
IGRP and EIGRP
Querying neighbors for unknown alternate routes
Sending out queries for an alternate route if no route can be found
EIGRP fixes many of the problems associated with IGRP, such as the
propagation of the entire routing table, which is sent when changes occur in
the network topology. One unique characteristic of EIGRP is that it is both
a link-state routing and a distance-vector protocol. How can this be? Let’s
look at how this protocol combines the best from both routing protocol
types.
Along with rapid convergence discussed above, EIGRP reduces bandwidth usage. It does this by not making scheduled updates but sending
updates only when topology changes occur. When EIGRP does send an
update, the update contains information only on the change in the topology,
which requires a path or metric change. Another plus is the fact that only the
routers that need to know about the change receive the update.
One of the best features is that the routing protocol supports all of the
major Layer 3 routed protocols using protocol-dependent modules (PDMs),
those being IP, IPX, and AppleTalk. At the same time, EIGRP can maintain
a completely loop-free routing topology and very predictable behavior, even
when using all three routed protocols over multiple redundant links.
With all these features, EIGRP must be hard to configure, right? Guess
again. Cisco has made this part easy as well and allows you to implement
load balancing over equal-cost links. So why would you use anything else?
Well, I guess you might if all your routers weren’t Cisco routers. Remember,
EIGRP is proprietary and only runs over Cisco routers and internal route
processors.
Now that we have mentioned all this, we’ve sold you on EIGRP, right?
Well, if we stopped right here, you would miss out on many other important
details of the route-tagging process, neighbor relationships, route calculation, and the metrics used by EIGRP, which will be discussed in the next few
sections. Following that discussion, we will look at how to configure EIGRP,
tune EIGRP, load balance, redistribute routes, and troubleshoot.
Route Tagging
Route tagging is used to distinguish routes learned by the different EIGRP
sessions. By defining a different AS number, EIGRP can run multiple sessions
on a single router. Routers using the same ASN speak to each other and share
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
215
routing information, which includes the routes learned and the advertisement of topology changes.
Route redistribution, which will be covered in its own section later in this
chapter, allows routes learned by one AS EIGRP session to be shared with
another session. When route distribution occurs, the routes are tagged as
being learned from an external EIGRP session. Each type of route is assigned
its own administrative distance value.
Neighbor Relationships
Using Hello messages, EIGRP sessions establish and maintain neighbor relationships with neighboring routers. This is a quality of a link-state routing
protocol. EIGRP uses the Hello protocol just like OSPF does, as discussed in
Chapter 5, to establish and maintain the peering relationships with directly
connected routers. The Hello packets sent between EIGRP neighboring routers determine the state of the connection between them. Once the neighbor
relationship is established using the Hello protocol, the routers then
exchange route information.
Each EIGRP session running on a router establishes a neighbor table in
which each router stores information on all the routers known to be directly
connected neighbors. The neighboring routers’ IP address, hold time interval, smooth round-trip timer (SRTT), and queue information are all kept in
the table, which is used to help determine when there are topology changes
that need to be propagated to the neighboring routers.
The only time EIGRP advertises its entire routing table is when two neighbors initiate communication. When this happens, both neighbors advertise
their entire routing tables to one another. After each has learned its neighbor’s directly connected or known routes, only changes to the routing table
are propagated.
When Hello messages are sent out each of the routers’ interfaces, replies
to the Hello packets are sent with the neighboring router’s topology table
(which is not the routing table) and include each route’s metric information
with the exception of any routes that were already advertised by the router
receiving the reply. As soon as the reply is received, the receiving router sends
out what is called an ACK (acknowledgement) packet to acknowledge
receipt, and the routing table is updated if any new information is received
from the neighboring router. Once the topology table has been updated, the
originating router will then advertise its entire table to any new neighbors
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
216
Chapter 6
IGRP and EIGRP
that come online. Then when the originating router receives information from
its neighbors, the route calculation process begins. Let’s now take a look at
how EIGRP uses metrics to calculate the best routes through the network.
Route Calculation
EIGRP uses multicasts instead of broadcasts. Therefore, only identified stations are affected by routing updates or queries. Where IGRP updates use a
24-bit format, EIGRP uses a 32-bit format for granularity. Only changes in
the network topology are advertised instead of the entire topology table.
EIGRP is called an advanced distance-vector protocol although it contains properties of both distance-vector and link-state routing protocols
when calculating routes. DUAL is much faster and calculates new routes
only when updates or Hello messages cause a change in the routing table.
And then recalculation occurs only when the changes directly affect the
routes contained in the routing table.
This last statement may be confusing. If a change occurs to a network that
is directly connected to a router, all of the relevant information is used to calculate a new metric and route entry for it. If a link between two EIGRP peers
becomes congested, both routers would have to calculate a new route metric,
then advertise the change to any other directly connected routers.
Now that we understand the difference between a route update and a
route calculation, we can summarize the steps that a router takes to calculate, learn, and propagate route update information.
Redundant Link Calculation
The topology database stores all known routes to a destination and the metrics used to calculate the least-cost path. Once the best routes have been calculated, they are moved to the routing table. The topology table can store up
to six routes to a destination network, meaning that EIGRP can calculate the
best path for up to six redundant paths. Using the known metrics to the destination, the router must make a decision as to which path to make its primary path and which path to use as a standby or secondary path to a
destination network. Once the decision is made, the primary route will be
added to the routing table as the active route, or successor, and the standby
will be listed as a passive route, or the feasible successor, to the destination.
The path-cost calculation decisions are made from information contained
in the routing table using the bandwidth and delay from both the local and
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
217
adjacent routers. Using this information, a composite metric is calculated. The
local router adds its cost to the cost advertised by the adjacent router. The total
cost is the metric. Figure 6.1 shows how cost is used to select the best route
(successor) and the backup route (feasible successor).
FIGURE 6.1
The best-route selection process
Host
Cost 30
RouterB
172.3.4.4/30
RouterC
Cost 20 172.5.6.4/30
172.10.10.0/24
Cost 35
Host X
172.7.8.0/24
172.1.2.4/30
RouterA
Cost 35
172.6.7.4/30
RouterD
Host Y
Cost 20
172.11.12.4/30
WAN
CO
Using RouterA as a starting point, we see that there are three different
routes to Host Y. Each link has been assigned a cost. Numbers in bold represent advertised distances, and numbers in italics represent feasible distances. Advertised distances are costs that routers advertise to neighbors.
In this example, RouterD and the WAN all have advertised costs that they
send to RouterA. In turn, RouterA has a feasible distance for every router to
which it is connected. The feasible distance is the cost assigned to the link
that connects adjacent routers.
The feasible and advertised costs are added together to provide a total
cost to reach a specific network. Let’s calculate the lowest cost for Host X to
get to Host Y. We will use the path from Host X to RouterA to RouterB to
Router C and finally to Host Y for our first path calculation. To calculate the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
218
Chapter 6
IGRP and EIGRP
total cost, we add 20 (RouterA to RouterB) to 30 (RouterB to RouterC), for
a final value of 50. For the feasible successor calculation, RouterA tells
RouterB the cost of 35, which is the advertised cost. B then adds its cost to
get to RouterA. This becomes 35 + 20, for a total path cost of 55.
The next path calculated is from Host X to RouterA to RouterD to Host
Y. In this case, there is no advertised cost, so the final value consists of only
the feasible cost of 35. The final path is calculated in the same manner to give
us the result of 55.
Since the lowest cost was 35, the route to 172.10.10.0/24 learned via
RouterD will be chosen as the successor or primary route. The other two
routes remain in the topology table as feasible successors and are used if the
successor to Host Y fails.
Information given in Table 6.4 closely represents what is contained in an
actual topology table, though not exactly. The Status field shows whether a
new route is being calculated or if a primary route has been selected. In our
example, the route is in passive state because it has already selected the primary route.
TABLE 6.3
Topology Table Information
Status
P
Route—Adjacent Router’s
Address (Metrics)
Number of
Successors
Feasible
Distance
172.10.10.0/24 via 172.1.2.6
(3611648/3609600) via 172.5.6.6
(4121600/3609600) via 172.6.7.6
(5031234/3609600)
1
(Router C)
3611648
The route with the best metric contains the lowest metric value and is chosen as the primary route. If there is more than one route to a destination, the
route with the second-lowest metric will be chosen as the feasible successor,
as long as the advertised distance of the potential feasible successor is not
greater than the distance of the successor. Primary routes are moved to the
routing table after selection. More than one route can be made a primary
route in order to load balance. This will be discussed in the “Load Balancing” section later in this chapter.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
219
EIGRP uses the same metrics as IGRP. Those metrics are:
Bandwidth
Delay
Reliability
Load
Just as with IGRP, there is no specific calculation for the maximum transmission unit (MTU) as a metric. The MTU, however, is used as a tiebreaker for
equal metric paths.
Bandwidth and delay are the two metrics used by default. The other metrics can be configured manually. When you configure reliability, load and
MTU can cause the topology table to be calculated more often.
Updates and Changes
EIGRP also has link-state properties. One of these properties is that it propagates only changes in the routing table instead of sending an entire new
routing table to its neighbors. EIGRP relies on IP to deliver updates to its
neighbors, as shown in a breakdown of an EIGRP packet in Figure 6.2.
When changes occur in the network, a regular distance-vector protocol will
send the entire routing table to neighbors. By avoiding sending the entire
routing table, less bandwidth is consumed. Neighboring routers don’t have
to re-initialize the entire routing table; all the routers need to do is insert
the new route changes. This is one of the big advantages that EIGRP has
over IGRP.
FIGURE 6.2
An IP frame showing the protocol type to be EIGRP
88 = EIGRP
IP Header
Frame Header
Copyright ©2001 SYBEX , Inc., Alameda, CA
Protocol
Packet Paycash
Frame Payload
www.sybex.com
CRC
220
Chapter 6
IGRP and EIGRP
Updates can follow two paths. If a route update contains a better metric
or a new route, the routers simply exchange the information. If the update
contains information that a network is unavailable or that the metric is
worse than before, an alternate path must be found. When a new path must
be found, the router first searches the topology database for feasible successors. If no feasible successors are found, a multicast request is sent to all adjacent routers. Each router will then respond to the query. Depending on how
the router answers, different paths will be taken. After the intermediate steps
are taken, two final actions can occur:
1. If route information is eventually found, the route is added to the rout-
ing table, and an update is sent.
2. If the responses from the adjacent routers do not contain any route
information, the route is removed from the topology and routing
tables.
After the routing table has been updated, the new information is sent to
all adjacent routers via a multicast.
EIGRP Metrics
EIGRP utilizes several databases or tables of information to calculate routes.
These databases are as follows:
The route database (routing table) where the best routes are stored
The topology database (topology table) where all route information
resides
A neighbor table that is used to house information concerning other
EIGRP neighbors
Each of these databases exists separately for each routed protocol configured for EIGRP. The following characteristics identify each session of
EIGRP:
The IP session is called IP-EIGRP.
The IPX session is called IPX-EIGRP.
The AppleTalk session is called AT-EIGRP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
221
Therefore, it is possible for EIGRP to have nine active databases when all
three protocols are configured on the router.
As stated above, the metrics used by EIGRP are the same as those used by
IGRP. As with IGRP, metrics decide how routes are selected. The higher the
metric associated with a route, the less desirable the route is. The overall metric
assigned to a route is created by the Bellman-Ford algorithm, using the
following equation:
metric = [K1 × Bw + (K2 × Bw) / (256 – Load) + K3 × Delay] × [K5 /
(Rel + K4)]
By default: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0.
Delay is the sum of all the delays of the links along the paths.
Delay = [Delay in 10s of microseconds] × 256.
BW is the lowest bandwidth of the links along the paths.
BW = [10000000 / (bandwidth in Kbps)] × 256.
By default, metric = bandwidth + delay.
Just as with IGRP, you can set the metrics manually from within the Configuration mode. We’ll provide the details on how to change metrics after we
discuss how EIGRP is configured.
Configuring EIGRP
Although EIGRP can be configured for IP, IPX, and AppleTalk, as a Cisco
Certified Network Professional, you should focus on the configuration of IP.
An autonomous system must be defined for each EIGRP session on a router.
To start an EIGRP session on a router, use the router eigrp command followed by the autonomous system number of your network. You must then
enter the network numbers connected to the router using the network command followed by the network number. The network mask is optional for
network statements entered on the Cisco IOS 12.0 or later.
Let’s look at an example of enabling EIGRP on a router connected to two
networks with the network numbers 10.0.0.0 and 172.16.0.0:
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router#router eigrp 20
Router(config-router)#network 172.16.0.0
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
222
Chapter 6
IGRP and EIGRP
Router(config-router)#network 10.0.0.0
Router(config-router)#^Z
Router#
Unfortunately, EIGRP assumes that all serial connections use T1 speeds.
In order to identify slower links, such as a 128K link, you must identify it
manually. Bandwidth is one of the two default metrics used to calculate a
route’s metric. If the bandwidth is slower or faster than T1 speeds, use the
bandwidth command followed by the bandwidth in kilobits in Interface
Configuration mode. The possible values are between 1 and 10,000,000.
If you need to stop routing updates from being sent on an interface, such
as a BRI interface, you can flag the interface as a passive interface. To do this
from an EIGRP session, use the passive-interface interface-type
interface-number command. The interface-type portion defines the
type of interface, and the interface-number portion defines the number of
the interface.
EIGRP Tuning
The metrics used with EIGRP are tuned in the same manner as the metrics for
IGRP. Metrics are tuned to change the manner in which routes are calculated. The same command as for IGRP is also used:
metric weights tos K1 K2 K3 K4 K5
Each constant is used to assign a weight to a specific variable. This means
that when the metric is calculated, the algorithm will assign a greater importance to the specified metric. By assigning a weight, you are able to specify
what is most important. If bandwidth is of greatest concern to a network
administrator, a greater weight should be assigned to K1. If delay is unacceptable, the K2 constant should be assigned a greater weight. The tos variable is the type of service. Refer back to Table 6.2 for the relationship
between the constant and the metric it affects. Also, remember that EIGRP
uses bandwidth and delay by default only when calculating routes.
Other tuning is possible. All routing protocols have an administrative distance associated with the protocol type. If multiple protocols are running on
one router, the administrative distance value helps the router decide which
path is best. The protocol with the lower administrative distance will be chosen. EIGRP has a default administrative distance of 90 for internal routes
and 170 for external routes. Use the following command to make changes:
distance 1–255
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
223
Valid values for the administrative distance range from 1 to 255. Again, the
lower the value, the better. If an administrative distance of 255 is chosen,
routes will be considered unreachable and will be ignored.
When redistributing static routes or other protocol types within EIGRP,
metrics may be set for these routes as well by using the default-metric
command:
default-metric bandwidth delay reliability load mtu
Bandwidth and delay have a range of values from 0 to 4,294,967,295 (in
Kbps) and 0 to 4,294,967,295 (in 10-microsecond units), respectively. Reliability ranges from 0 to 255, with 255 being the most reliable. Load ranges
from 0 to 255; however, a value of 255 means that the link is completely
loaded. Finally, the value of MTU has the same range as the bandwidth variable: 0 to 4,294,967,295.
Most of this information should be a review, since it’s basically the same
information associated with IGRP.
Load Balancing
One of EIGRP’s major enhancements is its ability to select more than one
primary route or successor. We have discussed how route costs are calculated and shown that up to six routes for every destination can be stored in
the topology database. EIGRP capitalizes on this information.
By using multiple LAN or WAN connections from one router to another,
multiple routes can exist to the next-hop address. When the links are symmetric (meaning they have the same circuit type and the same bandwidth
capacity), the same local cost is assigned to each link.
Since both links have the same feasible distance, the metrics for destinations accessible via the links will be equal. As EIGRP chooses the successor
for a route, it looks for the route with the lowest cost. When it sees multiple
routes with the same metric, it selects them all as successors. EIGRP will then
share traffic loads across each of the multiple links. This is called load
balancing.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
224
Chapter 6
IGRP and EIGRP
Let’s look at an example of a topology table with multiple routes:
IP-EIGRP topology entry for 172.10.10.0/24
State is Passive, Query origin flag is 1, 1
Successor(s),
FD is 283648
Routing Descriptor Blocks:
172.16.1.6 (Serial1), from 172.16.1.6, Send flag is 0x0
Composite metric is (283648/281600), Route is
Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 1080 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
172.16.1.10 (Serial2), from 172.16.1.10, Send flag is 0x0
Composite metric is (283648/281600), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 1080 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
172.16.1.14 (Serial3),from 172.16.1.14, Send flag is 0x0
Composite metric is (283648/281600), Route is Internal
Vector metric:
Minimum bandwidth is 1544 Kbit
Total delay is 1080 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
225
In this output, the feasible distance is the same for all three links. This
means that traffic will be load balanced (shared) across all three links
equally.
There will also be situations where there are multiple links to a given destination, but the links have different next-hops. The metric for these links
will not likely be the same. Even though each link may have a different cost
assigned to it, EIGRP does allow for unequal-cost load balancing.
This is achieved by using the variance command—the same command
used in IGRP for unequal-cost load balancing:
variance multiplier
The variance command uses a multiplier, which can be a value from 1 to
128. The default setting for the multiplier is 1. This command must be used
inside the EIGRP protocol configuration.
Route Redistribution
When a router has more than one routed protocol configured, each EIGRP
session is defined by the autonomous system number used when enabling
EIGRP. With all of the different protocols and sessions running on a router,
it becomes important that the information learned by each session can be
shared with the other protocols and sessions. Route redistribution is the feature that allows for the exchange of route information among multiple protocols and multiple sessions.
The router where multiple protocols or sessions meet is called the Autonomous System Boundary Router (ASBR). When routes from one protocol or
session are injected or redistributed into another protocol or session, the
routes are tagged as external routes. Let’s look at a simple example of a routing table that has external routes:
Router#show ip route eigrp
172.16.0.0/16 is variably subnetted, 301 subnets, 10 masks
D EX
172.16.27.230/32
[170/24827392] via 172.16.131.82, 11:39:32, ATM6/0/0.3114
D EX
172.16.237.16/29
[170/40542208] via 172.16.131.82, 11:41:32, ATM6/0/0.3114
[170/40542208] via 172.16.131.74, 11:41:32, ATM6/0/0.3113
D EX
172.16.237.24/29
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
226
Chapter 6
IGRP and EIGRP
D
D
D
D
D
D
[170/40542208] via 172.16.131.82, 11:40:32, ATM6/0/0.3114
[170/40542208] via 172.16.131.74, 11:41:32, ATM6/0/0.3113
EX
172.16.52.192/26
[170/2202112] via 172.16.131.82, 11:41:27, ATM6/0/0.3114
EX
172.16.41.216/29
[170/46232832] via 172.16.131.82, 11:41:28, ATM6/0/0.3114
EX
172.16.38.200/30
[170/2176512] via 172.16.131.82, 11:41:27, ATM6/0/0.3114
EX
172.16.237.0/29
[170/40542208] via 172.16.131.82, 11:41:32, ATM6/0/0.3114
[170/40542208] via 172.16.131.74, 11:41:32, ATM6/0/0.3113
172.16.236.0/24
[90/311808] via 172.16.131.82, 11:41:32, ATM6/0/0.3114
[90/311808] via 172.16.131.74, 11:41:32, ATM6/0/0.3113
172.16.235.0/24
[90/311808] via 172.16.131.82, 11:41:32, ATM6/0/0.3114
There are internal routes and external routes in this routing table. The
external routes are flagged with EX, while the internal routes have no flag.
The D stands for an EIGRP learned route.
While redistribution allows multiple protocols to share routing information, it can cause routing loops, slow convergence, and inconsistent route
information. This is caused by the different algorithms and methods used by
each protocol. It is not a good practice to redistribute bi-directionally. For
example, if you have both EIGRP 10 using IP-EIGRP and EIGRP 20 using
AT-EIGRP routing sessions, then bi-directional redistribution would occur if
you entered redistribution commands under each protocol session. Here is
an example:
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router eigrp 10
Router(config-router)#redistribute eigrp 20
Router(config-router)#router eigrp 20
Router(config-router)#redistribute eigrp 10
Router(config-router)#^Z
Router#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Enhanced Interior Gateway Routing Protocol
227
If a route from RIP, IGRP, or OSPF is injected into EIGRP, the route loses its
identity, and its metrics are converted from the original format to EIGRP’s format. This can cause confusion within the router.
You can reset EIGRP metrics to help alleviate certain problems by using
the default-metric command, as follows:
default-metric bandwidth delay reliability load MTU
This command takes the metrics for the protocol being injected into EIGRP
and converts them directly to values that EIGRP can use. The bandwidth is
the capacity of the link. The delay is the time in microseconds, and reliability
and load are values from 1 to 255. The MTU is the maximum transmission
unit size in bytes.
Finally, you can change the distance values that are assigned to EIGRP (90
internal and 170 external). The distance value tells the router which protocol
to believe. The lower the distance value, the more believable the protocol.
The distance values for EIGRP are changed with the following command
from within the EIGRP session:
distance eigrp internal-distance external-distance
Internal-distance and external-distance have a range of values from
1 to 255.
Remember that a value of 255 tells the router to ignore the route. So
unless you want the routes from the protocol to be ignored, never use the
value 255.
Troubleshooting EIGRP
There are several commands that can be used on a router to aid in troubleshooting EIGRP. Table 6.5 contains all of the commands that are used in
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
228
Chapter 6
IGRP and EIGRP
conjunction with verifying EIGRP operation and offers a brief description of
what each command does.
TABLE 6.4
EIGRP Troubleshooting Commands
Command
Description/Function
show ip route eigrp
Shows EIGRP entries in the routing table.
show ip eigrp
neighbors
Shows all EIGRP neighbors.
show ip eigrp topology
Shows entries in the EIGRP topology table.
show ip eigrp traffic
Shows the packet count for EIGRP packets
sent and received.
show ip protocols
Shows information about the active protocol
sessions.
show ip eigrp events
Shows a log of EIGRP events. These are
routes being added or removed from the
routing table.
When troubleshooting an EIGRP problem, it is always a good idea to get
a picture of the network. The most relevant picture is provided by the show
ip eigrp neighbors command. This command shows all adjacent routers
that share route information within a given autonomous system. If neighbors
are missing, check the configuration and link status on both routers to verify
that the protocol has been configured correctly.
If all neighbors are present, verify the routes learned. By executing the
show ip route eigrp command, you gain a quick picture of the routes in
the routing table. If the route does not appear in the routing table, verify the
source of the route. If the source is functioning properly, check the topology
table.
The topology table is displayed by using the show ip eigrp topology
command. If the route is in the topology table, it is safe to assume that there
is a problem between the topology database and the routing table. There
must be a reason why the topology database is not injecting the route into the
routing table.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
229
Other commands such as show ip eigrp traffic can be used to see if
updates are being sent. If the counters for EIGRP input and output packets
don’t increase, no EIGRP information is being sent between peers.
The show ip eigrp events command is an undocumented command.
This command displays a log of every EIGRP event—when routes are
injected and removed from the routing table, and when EIGRP adjacencies
reset or fail. This information can be used to see if there are routing instabilities in the network.
All of these commands are intended to be used at the discretion of the system administrator when troubleshooting a problem in the network. The
information provided by these commands can be used for many more issues
than have been discussed in this section.
Summary
This chapter has been fairly extensive. Just to refresh your memory, we
discussed the differences between and limitations of distance-vector and
link-state routing protocols. Link-state routing protocols or a hybrid of linkstate routing and distance-vector protocols provide for greater scalability
and stability.
EIGRP, which was the main focus of the chapter, is a hybrid of link-state
routing and distance-vector protocols. It provides greater stability than
IGRP, which was also discussed, and allows for equal-cost load balancing,
controlled routing updates, and formal neighbor adjacencies.
We also looked at how to configure, tune, and troubleshoot EIGRP, how
to load balance, and how to redistribute routes using EIGRP.
So far in this book we have covered OSPF, a touch of RIP, IGRP, and
EIGRP. So what’s left? The Border Gateway Protocol and its components.
This protocol is so complex that it will be covered in not one, not two, but
the next three chapters.
Key Terms
Before taking the exam, make sure you’re familiar with the following terms:
active route
Autonomous System Boundary Router (ASBR)
Enhanced IGRP (EIGRP)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
230
Chapter 6
IGRP and EIGRP
feasible successor
gateway of last resort
Interior Gateway Routing Protocol (IGRP)
load balancing
passive route
route redistribution
Commands Used in This Chapter
Command
Description
distance
When multiple protocols are
running, this command allows a
distance value from 1 to 255 to
decide which path is the best. The
lowest value wins.
passive-interface
interface-type interfacenumber
Identifies interfaces that do not
participate in EIGRP updates.
router eigrp
Starts EIGRP processes on a
router.
show ip route eigrp
Shows all EIGRP neighbors.
show ip eigrp neighbors
Shows directly connected EIGRPenabled routers.
show ip eigrp topology
Shows entries in the EIGRP
topology table.
show ip eigrp traffic
Shows the packet count for EIGRP
packets sent and received.
show ip protocols
Shows information about the
active protocol sessions.
show ip eigrp events
Shows a log of EIGRP events.
These are routes being added to or
removed from the routing table.
variance
Assigns a weight to each feasible
successor.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
231
Written Lab
1. What three routed protocols are supported by EIGRP?
2. When is redistribution required for EIGRP?
3. What command would be used to enable EIGRP with an ASN of 300?
4. What command will tell EIGRP that it is connected to network
172.10.0.0?
5. What type of EIGRP interface will listen to routing updates but not
propagate them?
6. What command allows you to make an interface passive?
7. Which command is used to ensure proper metric conversion when
redistributing routes from different protocols?
8. Which two commands show all known EIGRP routes?
9. Which command can be used to see all the router’s known EIGRP
neighbors?
10. Which command can be used for troubleshooting EIGRP events?
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
232
Chapter 6
IGRP and EIGRP
Hands-on Lab
Using the graphic shown below, we will walk through configuring all
three routers for EIGRP and then view the configuration.
Autonomous System 100
172.16.30.0/24
E2
E0
E1
RouterA
E0
E1
E0
RouterC
E1
RouterB
10.1.6.0/24
192.168.1.0/24
Router
IP Address
Interface
A
172.16.20.5/24
E1
B
172.16.20.6
E0
B
172.16.40.5
E1
C
172.16.40.6
E0
1. Implement EIGRP on RouterA, as shown here:
RouterA#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#router eigrp 100
RouterA(config-router)#network 10.0.0.0
RouterA(config-router)#network 172.16.0.0
RouterA(config-router)#^Z
RouterA#
2. Implement EIGRP on RouterB, as shown here:
RouterB#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
233
RouterB(config)#router eigrp 100
RouterB(config-router)#network 172.16.0.0
RouterB(config-router)#exit
RouterB#
3. Implement EIGRP on RouterC, as shown here:
RouterC#conf t
Enter configuration commands, one per line. End with CNTL/Z.
RouterC(config)#router eigrp 100
RouterC(config-router)#network 172.16.0.0
RouterC(config-router)#network 192.168.1.0
RouterC(config-router)#^Z
RouterC#
4. Display the topology table for RouterB, as shown here:
RouterB#show ip eigrp topology
Codes: P - Passive, A - Active, U - Update, Q - Query,
R - Reply, r - Reply status
P 172.0.0.0/8, 1 successors, FD is 307200
via 172.16.20.5 (307200/281600), Ethernet0/0
P 192.168.1.0/24, 1 successors, FD is 307200
via 172.16.40.6 (307200/281600), Ethernet0/1
P 172.16.40.4/30, 1 successors, FD is 281600
via Connected, Ethernet0/1
P 172.16.20.4/30, 1 successors, FD is 281600
via Connected, Ethernet0/0
RouterB#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
234
Chapter 6
IGRP and EIGRP
Review Questions
1. When does EIGRP recalculate its topology table?
A. On a synchronized schedule
B. When an administrator uses the redirect command
C. Automatically every 120 seconds
D. Only when there is a change in the network topology
2. The neighbor table uses which of the following timers? (Choose all
that apply.)
A. SRTT
B. RTO
C. Hold timer
D. FwdDelay timer
E. MaxAge timer
3. When there are no feasible successors and only one link to a destina-
tion network, even if the link cost is set to 100,000, the link will
always be in which of the following modes?
A. On
B. Standby
C. Active
D. Sending
4. Which of the following are not routed protocols supported by EIGRP?
A. TCP
B. IP
C. IPX
D. AppleTalk
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
235
5. What are benefits of using a link-state routing protocol? (Choose all
that apply.)
A. It uses the Hello protocol to establish adjacencies.
B. It uses several components to calculate the metric of a route.
C. Updates are sent only when changes occur in the network.
D. It is a better protocol than distance-vector is.
6. Which route type must be redistributed by a routing protocol if other
routers are to learn about it?
A. RIP
B. Default routes
C. Connected routes
D. Static routes
7. Why are passive interfaces used on interfaces where the router partic-
ipates in EIGRP Global mode processes?
A. To stop unwanted route information from entering the specified
interface
B. To allow route information to be filtered by an access list
C. To allow routes to be sent out the specified interface, but deny
route information to enter the interface
D. To allow routes to enter the interface, but deny any route informa-
tion to exit the specified interface
8. How is a feasible successor chosen when the successor fails (assuming
that a redundant route exists)? (Choose all that apply.)
A. The route with the next-lowest metric is chosen.
B. If a router doesn’t have a feasible successor, queries are multicast
to neighboring routers in search of a feasible successor.
C. The route is removed from the routing table.
D. The route is flagged as an active state.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
236
Chapter 6
IGRP and EIGRP
9. Which command should be used to ensure proper metric conversion
when redistributing routes from different protocols?
A. distance distance-value
B. default-metric
C. distribute-list
D. default-information
10. How is EIGRP implemented on a router?
A. ip router eigrp autonomous-system-number
B. router ip eigrp autonomous-system-number
C. router eigrp process-id
D. router eigrp autonomous-system-number
11. Which of the following are not features of EIGRP? (Choose all that
apply.)
A. Incremental updates
B. Only one route per destination
C. Support for IP, IPX, and AT
D. Hybrid distance-vector and link-state routing protocol
E. Not a scalable protocol
F. Hello protocol used to establish adjacencies
12. Which of the following problems may occur if route redistribution
occurs?
A. Non-optimal route choices
B. Slow convergence
C. Routing loops
D. All of the above
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
237
13. When using the show ip route command, which of the following codes
indicate an EIGRP learned route?
A. D
B. R
C. S
D. I
14. When using EIGRP, the process number indicates which of the
following?
A. Link-state value
B. Autonomous system number
C. Path cost
D. Number of ACKs
15. Which of the following commands can be used to learn the number of
EIGRP packets sent and received?
A. show ip eigrp mail
B. show ip eigrp sent
C. show ip eigrp traffic
D. show ip eigrp data
E. show ip eigrp counters
16. Which of the following is not a route type recognized by IGRP?
A. Network
B. Interior
C. System
D. Exterior
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
238
Chapter 6
IGRP and EIGRP
17. Which of the following are used by IGRP to calculate the best path to
a destination network? (Choose all that apply.)
A. Bandwidth
B. Load
C. Delay
D. Reliability
18. By default, what is the maximum number of feasible links that IGRP
may use to load balance over unequal-cost links?
A. Two
B. Four
C. Six
D. Eight
19. What is the maximum number of feasible successors that EIGRP can
place in its routing table?
A. Two
B. Four
C. Six
D. Eight
20. Which of the following algorithms is used by EIGRP to determine the
best path?
A. Open Shortest Path First
B. DUAL
C. Distance-vector
D. Link-state routing
E. Advanced Distance Vector
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Written Lab
239
Answers to Written Lab
1. The three EIGRP routed protocols supported by EIGRP are IP, IPX,
and AppleTalk.
2. Redistribution is required when more than one EIGRP session is run-
ning and they are identified with different ASNs. Redistribution shares
topology information between EIGRP sessions.
3. router eigrp 300
4. network 172.10.0.0
5. Passive interface
6. passive-interface interface-type interface-number
7. default-metric
8. show ip route eigrp and show ip route
9. show ip eigrp neighbors
10. show ip eigrp events
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
240
Chapter 6
IGRP and EIGRP
Answers to Review Questions
1. D. One of the great benefits of EIGRP is that it advertises only
changes, and only when there is a change in the network topology
does it recalculate routes. Hello packets continue to be sent in order
to verify that all the attached links are still connected and did not go
down.
2. A, B, C. The neighbor table uses the smooth round-trip timer
(SRTT), the retransmission timer (RTO), and the hold timer to track
its neighboring routers. The FwdDelay and MaxAge timers are both
used by the Spanning Tree Protocol to keep Layer 2 switches from creating data loops.
3. C. The link will always be in Active mode regardless of the link cost
because there is no other feasible successor. If the link goes down,
there is no other redundant link to use.
4. A. This is a trick question. IP is a routed protocol but TCP is not.
Both IPX and AppleTalk are examples of routed protocols.
5. A, C. Link-state routing protocols use the Hello protocol and update
neighbors of changes without sending the entire routing or topology
table.
6. D. Static routes must always be redistributed by a routing protocol
and always have the smallest administrative distance.
7. D. Passive interfaces are used for such interfaces as BRI, where you
do not want to have routing updates sent out the interface. If routing
updates were sent out of a BRI interface, the interface would never disconnect. You can also configure the routing traffic to be uninteresting
traffic to perform a similar function.
8. A, B. The feasible successor, which would be the path with the next-
lowest metric, would be chosen. Or, if the router has not learned of
any secondary routes, the router will query its neighbors to see if they
know of any routes.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
241
9. B. Use the default-metric command to ensure proper metric con-
version when redistributing routes from different protocols.
10. D. The command router eigrp followed by the ASN is used to
implement EIGRP. You must then identify the attached networks
using the network command.
11. B, E. Answer B is not a feature because redundant paths are sup-
ported, and answer E is not a feature because EIGRP is the most scalable routing protocol.
12. D. All of these problems may occur when using route
redistribution.
13. A. EIGRP uses D, RIP uses R, S identifies a static route, and I indi-
cates IGRP.
14. B. The EIGRP process number is always the number assigned to an
autonomous system. Multiple processes can run simultaneously on a
router.
15. C. The show ip eigrp traffic command shows the sent and
received packets. The other commands are not real commands that
can be used on a router.
16. A. The network route is not recognized by IGRP. An interior route is
a network directly connected to a router interface. A system route is a
route advertised by other IGRP neighbors within the same AS. An exterior route is learned using IGRP from a different ASN.
17. A, B, C, D. All of these are used by IGRP by the distance-vector algo-
rithm to determine the best path to a destination network. By default
however, only bandwidth and delay are used.
18. B. IGRP can use up to four feasible successors to load balance. The
default is four, and the maximum is six.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
242
Chapter 6
IGRP and EIGRP
19. C. There may be more routes in the topology table, but the max-
imum number of feasible successors listed in the routing table is six.
20. B. The Diffusing Update Algorithm (DUAL) is used to calculate
routes in EIGRP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
7
BGP’s Basic
Components
THE CCNP ROUTING EXAM TOPICS
COVERED IN THIS CHAPTER ARE AS FOLLOWS:
Describe how to connect to another autonomous system using
an alternative to BGP, static routes
Describe BGP operations and features
Describe and configure external and internal BGP
Compare distance-vector and link-state protocol operation
Explain how BGP peering functions
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
T
his chapter covers BGP, which stands for Border Gateway
Protocol. BGP will be discussed in great detail, not only in this chapter but
in the two following this one as well. Here we’ll focus on BGP terminology
and its basics components. Chapter 8, “Configuring Basic BGP,” will focus
on how BGP works and configuring BGP. Chapter 9, “Monitoring, Troubleshooting, and Scaling BGP,” will focus on the more advanced uses of
BGP, including scaling, policy implementation, and optimization techniques.
For some time now, Cisco has required an understanding of BGP as a
requirement for obtaining your CCIE. But in order to fulfill the CCNP
requirements, you needed only a basic overview and never had to deal with
BGP configurations or advanced configurations. There is no way that we can
project to you the actual complexities of configuring BGP for an ISP needing
20 or more paths going through ISPs. This is not an uncommon scenario,
and we will prepare you to configure and support BGP in a real Internet
environment.
BGP is one of the most complex routing protocols I have ever seen. It is
used to connect multiple autonomous systems, which we’ll discuss in detail.
In this chapter, we’ll focus on the following:
Autonomous systems, including stud autonomous systems and transit
autonomous systems
BGP peers
Internal BGP
External BGP
Routing protocols
When to use and when not to use BGP
Ingress filtering
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Autonomous Systems
245
BGP update messages
BGP has been used for quite some time on routers connecting to the Internet. The Internet can be really thought of as the backbone of thousands of
small and large companies. This book focuses on the latest version of BGP:
BGP version 4(BGPv4). BGPv4 is an exterior routing protocol. Interior routing protocols such as RIP, IGRP, and OSPF run inside a company’s network.
BGP is the glue that connects the different networks to the Internet. BGP also
helps in finding and distributing route information.
Are you ready for all of this? Let’s get started and see what an autonomous system is.
Autonomous Systems
You can imagine the Internet as a Lego castle. In order to build a
Lego™ castle, you need many pieces. The same goes for the Internet. The
Internet is built with many autonomous systems, which we will think of as
Lego pieces. These pieces are then assembled to form a much larger piece.
Autonomous systems (AS) are the basic building blocks of network-tonetwork routing. An autonomous system can be the entire corporate network comprised of multiple locations connecting to the network.
An AS uses BGP to advertise routes in its network that need to be visible
outside of the network; it also uses BGP to learn about the reachability and
routes by listening to advertisement announcements from other autonomous
systems. Each AS can have a specific policy regarding the routes it wishes to
advertise externally. These policies can be different for every point in which
the AS attaches to the outside world.
The Internet consists of a number of commercial networks that connect to
each other via tier-one providers, such as Sprint, Qwest, WorldCom/MCI,
UUNet, and many others. Each enterprise network or ISP must be identified
by an autonomous system number (ASN). This number allows a hierarchy to
be maintained when sharing route information.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
246
Chapter 7
BGP’s Basic Components
RFC 1930 defines an autonomous system as a set of routers under one or
more administrations that presents a common routing policy to the internet
(lowercase i). By definition, an internet is a set of interconnected networks that
cooperate with each other, advertise policies, and contain a specified level of
independence. There are many organizations, such as the government, state
departments, and financial institutions, with networks large enough to need
BGP and to split into multiple autonomous systems.
Inside autonomous networks, interior routing protocols called interior
gateway protocols (IGP) are used to discover the connectivity among a set of
IP subnets. IGPs are well-known protocols such as the Routing Information
Protocol (RIP), Interior Gateway Routing Protocol (IGRP), Open Shortest
Path First (OSPF), and Enhanced Interior Gateway Routing Protocol
(EIGRP). In Figure 7.1, we see an example of two corporate networks being
connected by BGP.
FIGURE 7.1
Two corporate networks being connected by BGP
Company A
Internet
Company B
Autonomous
System 51,064
Autonomous
System 204
ISP
Customers all in
their own
Autonomous
Systems
Autonomous
System 64,042
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Autonomous Systems
247
Variations of BGP terms will be used in the next two chapters. These terms are
internal BGP (iBGP) and external BGP (eBGP), also known as an interdomain
routing protocol. These are the same BGP protocol, but iBGP runs inside an
AS while eBGP runs outside an AS and connects one AS to another AS.
So what type of routing protocol is used to find the paths and connect
these autonomous systems? An Exterior Gateway Protocol (EGP). That is
exactly what BGP is, an External Gateway Protocol used to connect and find
routes to and from autonomous systems.
BGP is defined in many Requests For Comments (RFCs), which include 17711774, 1863, 1965-1966, 1997-1998, 2042, 2283, 2385, and 2439. BGPv4, the latest version, and autonomous systems are defined in RFC 1771.
Now we have to remember another routing protocol that learns the networks and can keep loops from forming in the network. In this book, there
are three mapping protocol types to remember that help to determine paths
and eliminate data loops:
Internal routing protocols These are protocols like OSPF, IGRP,
EIGRP, and RIP, which operate at Layer 3 of the OSI Reference Model.
They are used to learn the network topology on the internal network and
IP subnets to create routes that guarantee that there are no data loops in
the Layer 3 network.
External routing protocols These protocols are used to learn the network topology of multiple autonomous systems or networks and connect
them with loop-free paths.
Spanning Tree Protocol This protocol is used at Layer 2 inside a segment of an AS. It ensures that the internal network topology is learned at
Layer 2 and verifies that there is a path through the network without data
loops.
BGP uses reliable session management, using TCP port 179 for triggered
UPDATE and KEEPALIVE messages to its neighbors to propagate and
update the BGP routing table. Triggered updates are updates that are sent for
a certain reason and not on a schedule.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
248
Chapter 7
BGP’s Basic Components
A router can be a member of only one autonomous system and must
appear to other autonomous systems to have one routing plan. All the destinations must be reachable through that plan.
There are 65,535 available autonomous system numbers that can be
assigned, from 1 to 65,535. Autonomous system numbers (ASN) are 16-bit
integers. Of those 65,535 ASNs, the numbers 64,512 to 65,535 are reserved
for private use. This obviously means that there must be some authority
available to assign these numbers to those who need them, right? Yes, there is.
RFC 1930 provides guidelines for assigning BGP autonomous system
numbers. When you request a BGP ASN, you will be required to provide the
following information:
All administrative contacts in the company.
The Internet address of your routers.
A preferred autonomous system name.
A hardware profile of your routing hardware and the software being
used. In the case of Cisco, this would be the model and IOS version.
Expected deployment schedule when you will begin using two or more
upstream providers.
All networks in your organization connected by the routers.
The Internet Assigned Numbers Authority (IANA) is the organization
that assigns BGP autonomous system numbers. The IANA allows the American Registry for Internet Numbers (ARIN) to assign autonomous system
numbers for North America, South America, the Caribbean, and Africa.
Reseaux IP Eurpeennes-Network Information Center (RIPE-NIC) assigns
the AS numbers for Europe, and the Asia Pacific-NIC (AP-NIC) assigns the
numbers for Asia.
Stub AS
A stub AS is a single-homed network with only one entry and exit point, as
shown in Figure 7.2. In this type of network, the stub network does not need
to learn Internet routes. The reason? The local service provider or Internet
Service Provider is the next hop, and all the traffic is sent to one exit interface
to the provider. The provider then can have the responsibility for advertising
its customers’ static routes. This type of situation works well if there are relatively few static routes to manually configure and advertise. If there are
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Autonomous Systems
249
many routes, then taking the time to manually configure these static routes
can become burdensome.
FIGURE 7.2
A stub AS
Stub AS
Local Service Provider
Intranet
In this situation, where there are many routes through the network, you
have some choices. You can maintain static routes, use an IGP to determine
the network topology and choose the most efficient paths between the AS
and the service provider, use eBGP between the customer and the local service provider, or use any combination of these, as shown in Figure 7.3.
FIGURE 7.3
A stub AS to an ISP
1
IGP
2
Static
3
eBGP
4
eBGP
IGP
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
250
Chapter 7
BGP’s Basic Components
Obtaining an AS number for a stub network may be somewhat difficult
when using an ISP. The ISP considers your AS an extension of their AS, and
it must abide by the ISP’s AS policies. What we have seen in most cases is that
the ISP assigns the customer a number out of the private pool discussed earlier. The private pool of numbers runs from 64,512 to 65,535.
Transit AS
A transit AS is an AS through which data from one AS must travel to get to
another AS. A non-transit AS is an AS that does not pass data through
to another AS. A non-transit AS can be used to pass data from two service
providers but never between them, as shown in Figure 7.4.
FIGURE 7.4
A non-transit AS connected to three ISPs
AS
ISP1
ISP2
ISP3
An enterprise network can have a transit AS if the network uses multiple
ASes. In this situation, we would look at this as a backbone of backbones in
the enterprise network. A good example of a transit AS is a local service provider. Local service providers carry traffic for many other ASes as this is the
local service provider’s primary business.
Now let’s take a more in-depth look at eBGP and iBGP.
BGP Peers
In BGP, the word peer is somewhat confusing because it has two meanings. It can be used at both the protocol level and the policy level. The first
usage is simple: Two BGP routers that have a BGP session running between
them over a TCP connection are called peers or neighbors.
The second usage occurs at the BGP policy level and refers to a relationship within an entire AS. Peering is used to pair two ASes of the same status
or an AS at one level with an AS at a higher level. If two ASes decide that they
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BGP Peers
251
are peers, then they assume that they are equal in relationship. Usually an
administrator has decided that it is beneficial for his customers to reach one
another. These peers advertise their customers’ routes to one another. This
does not mean that they exchange their full Internet routing tables.
Let’s take a more in-depth look at iBGP and eBGP.
iBGP
The internal Border Gateway Protocol (iBGP) is used by routers that all
belong to the same autonomous system. These routers may use loopback
interfaces to provide greater reachability in the AS. This is possible because
an IGP can provide multiple routes to any given destination address if the
network has redundant or multiple links to each router. If one interface on
a router goes down, the TCP connection to the loopback address can be
maintained by using redundant interfaces.
Before any BGP route information can be exchanged between two routers, a TCP connection has to be established. And another routing protocol
other than BGP can be used to establish the TCP connection. The TCP connection is made by a three-way handshake using a SYN, ACK, SYN
sequence. Once a TCP connection has been established, route information
can be exchanged.
Routing information from one peer is not advertised from one iBGP to
another iBGP peer. This prevents inconsistent route information and routing
loops in the network. To share route information among all iBGP routers,
you must establish a logical mesh, as shown in Figure 7.5. Routing information is then exchanged only between routers who are members of this mesh.
RouterB can learn BGP networks only from RouterA. When RouterC sends
its BGP information, only its own information is sent. Routing information
learned from RouterA is not included.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
252
Chapter 7
BGP’s Basic Components
FIGURE 7.5
iBGP information exchange
Configuring BGP will be covered in Chapter 8.
eBGP
The external Border Gateway Protocol (eBGP) is used to exchange route
information between different autonomous systems. When only one link
connects two autonomous systems, the IP addresses of the connected interfaces are used to establish a BGP session between the two.
You can use any other IP address on the interfaces, but the address must
be reachable without using an IGP. You can use a static route or a few other
commands, which will be discussed in the Chapter 9. If multiple links are
used to connect to the other autonomous systems, then using a loopback
addresses is your best option.
Outside of each AS, eBGP is used to inject routes owned by one AS
through the enterprise network and into another AS. Two prerequisites need
to be met for internal routes to be propagated via BGP:
In order for a router to advertise routes to BGP, the route must exist
in an IGP’s routing table on the router.
The BGP must be able to learn the route.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Routing Protocols
253
The router can place routes in its routing table by using an IGP to learn
the network topology. It uses its own table and calculates its own routes. A
default (static) route can be configured, or a directly connected network can
advertise the route. BGP has a synchronization option that requires the
BGP’s learned routes and the IGP’s learned routes to synchronize before BGP
will advertise the IGP’s learned network topologies.
BGP can also learn routes through the network from other BGP advertisements, network statements, and redistribution of an IGP into a BGP.
Since redistribution can cause routing loops and route flapping, this method
is not recommended except in a lab scenario.
Routing Protocols
I
n a stub network, which we discussed earlier, there is only one way in
or out of the AS or network—there is no need to use another protocol to find
routes in the network. A static address mapping can be used for any
unknown routes. This means that if the router has determined that the destination in a packet is not on the local network, it merely forwards the packet
to the static default mapping.
As networks grow, however, and there begin to be many routes throughout the network, a static route becomes too difficult to maintain. This occurs
when the network has expanded to the extent at which a routing protocol
will scale well and is the point at which you want to discontinue using static
routes.
The increased network growth imposes a greater number of topology
changes in the network environment. The number of hops between end systems, the number of routes in the routing table, the various ways a route has
to be learned, and route convergence are all seriously affected by network
growth.
To maintain a stable routing environment, it’s absolutely crucial to use a
scalable protocol. When the results of network growth manifest themselves,
whether your network’s routers will be able to meet those challenges is up to
the routing protocol the routers are using. For instance, if you use a protocol
that’s limited by the number of hops it can traverse, how many routes it can
store in its table, or even the inability to communicate with other protocols,
you have a protocol that will likely hinder the growth of your network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
254
Chapter 7
BGP’s Basic Components
BGP keeps its acquired routing table information separate from the IGP’s
routing tables. BGP literally steals information the IGPs have learned of their
local network environments and stored on their routing tables. BGP handles
the translation of information from one routing protocol to another routing
protocol when multiple routing protocols are used in an AS.
Cisco supports two different types of algorithms for IGPs to find and calculate paths through the network. These two types are distance-vector and
link-state routing protocols. In the next two sections, we’ll take a look at
these in greater detail.
Distance-Vector Protocols
BGP is considered an advanced distance-vector protocol. Distance-vector
protocols such as RIP were designed when network topologies were small.
Distance-vector refers to a routing protocol that uses hop counts or vectors
to determine the distance from one device in the network to another. Each
device that a packet must encompass to get to another destination is considered a hop. For example, if you have to go through two routers to get to a
destination node, then there are three devices including the destination node,
making it three hops away from your local workstation or two hops from
your local router.
In small networks (less than 100 routers) where the environment contains
less than 15 hops (the count-to-infinity restriction) between any destination,
the network topology is much more forgiving of routing updates and calculations, and distance-vector protocols perform pretty well. Scaling a distancevector protocol to a larger network creates higher convergence times, high
router overhead CPU utilization, and increased bandwidth utilization, all of
which become factors that hinder scalability.
Other drawbacks to distance-vector protocols include no support for
Variable Length Subnet Masks (VLSM) or for Classless Interdomain Routing (CIDR) and that they don’t take into account the speed of each link. For
example, if the network topology contains two ways to a destination, one
contains an ISDN link (128K) and the other is a Frame Relay link (768K) circuit. The Frame Relay link contains an extra hop. What this means is that a
distance-vector protocol would choose the slow boat, which is the ISDN
link, because it calculates only the hops to a destination and doesn’t care that
there is a faster way to the destination. Your 12MB ZIP file will now take an
additional hour to get to its destination.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Routing Protocols
255
RIPv2 provides support for VLSM and CIDR. RIPv1 does not.
A network’s convergence time is determined by the ability of RIP to propagate changes within the network topology. Distance-vector protocols don’t
use formal neighbor relationships between routers. A router using distancevector algorithms becomes aware of a topology change in two ways and ages
out entries in its routing information base (RIB):
When a router fails to receive a routing update from a directly connected router
When a router receives an update from a neighbor notifying it of a
topology change somewhere in the network
Each routing protocol sends out routing updates at default intervals or at
a manually configured time interval. This means that when a topology
change occurs, using the defaults, it could take 90 seconds or longer before
a neighboring router realizes that there has been a link-state change and
switches to an alternate path. Ninety seconds can be an eternity to the network, causing application timeouts and other problems for network users.
When the router does finally update its RIB with the change, it recalculates
its route table. Then instead of just advertising the change, it advertises its
entire table to all its neighboring routers.
Imagine having 50 routers advertising their entire routing tables and the
impact that this can have on the bandwidth in your network. It compounds
one problem with another. Not only do you lose a link that provides bandwidth, but the more problems you have the worse it gets because a greater
percentage of bandwidth is needed for routing updates.
When the size of the routing table increases, so does the router’s CPU utilization. The reason is that it takes a lot more processing power to calculate
the routing table changes, converge using the new information, and advertise
its new table. This utilization can also be compounded by more routes populating a routing table. The table becomes increasingly complex in order to
determine the best path and next hop for a given destination.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
256
Chapter 7
BGP’s Basic Components
Link-State Routing Protocols
One of the best features of a link-state routing protocol is its ability to count
to infinity. This means that there is no hop count limit. A link-state routing
protocol works on the theory that routers send out a link state, which carries
information about each interface and the nodes attached.
A link-state-type of routing protocol—as well as the Spanning Tree Protocol—uses an algorithm called “graph theory” or the shortest path algorithm, which was developed by Edgar Dijkstra. This theory is used to
construct a loop-free subset of the network topology using bits of information contained in each link-state message to create a directed graph where
each link is represented by vertices and weighted edges, as shown in Figure
7.6. Each link represents a cost.
The weighted edges usually have more hops in the link than the straightthrough points, so these are assigned higher values. When the paths are calculated, each link in the path has a given value, and the total of the values to
a given point or destination is the total weighted value of the path. The lowest total weighted value represents the most efficient path from one point to
another point.
FIGURE 7.6
An example of a directed graph
10
10
10
10
100
10
The graph takes into account that each link’s bandwidth has better convergence times, supports VLSM, and supports CIDR. Link-state routing protocols also differ from distance-vector protocols because of their procedure
for route calculation and advertisement. This procedure enables link-state
routing protocols to scale well with the growth of a network.
Link-state routing protocols also maintain a formal neighbor relationship
with directly connected routers, which allows for faster route convergence.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
When to Use and When Not to Use BGP
257
Link-state routers establish a peering by exchanging Hello packets (also
known as link-state messages) during a session, which cements the neighbor
relationship between two directly connected routers. This relationship expedites network convergence because neighbors are immediately notified of
topology changes.
Hello packets are sent at short intervals, typically every 10 seconds, and
if an interface fails to receive Hello packets from a neighbor within a predetermined hold time, the neighbor is considered down, and the router will
then flood the update out all physical interfaces. This occurs before the new
route table is calculated, so it saves time. Neighbors receive the update, copy
it, flood it out their interfaces, and then calculate the new routing table—this
procedure is followed until the topology change has been propagated
throughout the network.
Unlike distance-vector protocols, which send the entire routing table,
link-state routing protocols advertise only updates or changes, making the
messages much smaller, which saves both bandwidth and CPU utilization.
Plus, if there are no network changes, updates are sent out only at specified,
or default, intervals, which differ among specific routing protocols and can
range from as short as 30 minutes to as long as two hours. EIGRP, which is
a link-state routing protocol, sends updates only when there is a topology
change to a directly connected neighboring router. These updates are called
triggered updates.
IGPs can be used with BGP; however, there are certain instances where
BGP should be used and certain instances where it should not be used. In the
following sections, we will outline these instances.
When to Use and When Not to Use BGP
S
tatic or default routes, which were discussed earlier, may be used in
situations where the complexity of BGP is not required. First let’s take a look
at when you should use BGP. The following scenarios are examples of when
BGP should be used:
When you need to send traffic through one AS to get to another AS
When the flow of data traffic out of your network must be
manipulated
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
258
Chapter 7
BGP’s Basic Components
When you are connecting two or more ISPs, network access points
(NAPs), and exchange points
When you are using multi-homing in an enterprise network that connects to more than one ISP
One of the main reasons not to use BGP is if your router can’t support the
huge routing tables needed to support BGP. Remember that there are about
7,000 AS numbers in use today, and the router learning those huge tables
requires a lot of RAM and processing power.
In a lab scenario, using two Cisco 2500 series routers works fine since the
tables it has to learn are small. But once the table acquires over 70,000 routes
and gets above 35MB, the router begins to develop serious processing problems. This increases the latency dramatically to the point where the throughput through the router is…well…you might as well disconnect the cable to
the router and use sneakernet (placing the data on a disk and walking it to
its destination) because the data will get to its destination faster.
There are some other reasons not to use BGP, including:
When there is a single connection to the Internet. Use a default route
instead. You’ll just be wasting bandwidth, memory, and processing
power.
When your network does not have the bandwidth to support the
amount of data needed to be passed, including BGP’s huge routing
tables.
Ingress Filtering
I
ngress filtering allows you to decide the routes that you will advertise
to other BGP neighbors or peers. When using BGP in your AS, you have the
ability to announce the routes in your AS that you want to be seen by the
Internet. To safeguard this process, many ISPs have policies in place to
accept the announcements of routes that belong to your AS.
RFC 2267 outlines how ISPs should filter ingress routes and traffic.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BGP Update Messages
259
There are some ISPs, however, that do not use any of the outlined techniques from RFC 2267 and that do not filter your announcements. In fact,
some actually announce to the rest of the Internet all the routes that exist in
your network. Ingress traffic filtering is a condition in which an ISP accepts
only packets with a source address in an administrative range that belongs to
one of the ISP’s customers. If all the ISPs on the Internet filtered using ingress
filtering based on source addresses, the Internet as a whole would gain considerable immunity to malicious hackers’ denial-of-service attacks.
The reason is that hackers would not be able to insert a randomly generated or invalid source address in the packets used to attack other networks.
Hackers use these addresses to prevent the attacked network from learning
the true source. Ingress source filtering would block these packets before
they could enter the network.
BGP Update Messages
T
he biggest difference between an IGP and a BGP is the amount of
additional information passed between protocol-running devices because of
the amount of routing information that must be passed. IGPs sometimes use
a prefix, metric, tagging, or a shortest path algorithm such as that found in
the Open Shortest Path First (OSPF) protocol. The updates used by an IGP
can be small compared to the routing updates for BGP, which have the
potential of carrying many path attributes.
RIP is a simple IGP that carries only a few attributes, such as metric information and the next hop. OSPF is a much more complex routing protocol
that has path attributes such as intra-area, inter-area, and external status.
BGP has the ability to attach many attributes to a given route. The minimum
set of path attributes that can be included in an update message is the source
of the update, called the ORIGIN attribute, and the hop information, called
the AS_PATH attribute.
When two routers running BGP begin a communication process to
exchange dynamic routing information, they use a TCP port at Layer 4 of the
OSI Reference Model. Specifically, TCP port 179 is used. The two routers
are called endpoints, BGP peers, or BGP neighbors, and their communications, which are reliable connection-oriented connections, are referred to as
sessions. When a router advertises its prefixes or routes, this router is known
as a BGP speaker. The routes that it advertises are considered valid by the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
260
Chapter 7
BGP’s Basic Components
other endpoints until a specific message is sent that the route is no longer
valid or that the TCP session is lost.
BGP uses TCP so that it does not have to provide a component that controls the orderly delivery of messages, recognizes when data packets have
been lost, detects duplicates, and controls buffering for both ends of the reliable session. Before a session between two or more BGP routers has been initiated, the endpoints are considered to be in the Idle state.
As soon as one endpoint tries to open a TCP session, the endpoint is considered to be in the Connection state. If there is a problem in establishing a
connection between two endpoints, the router trying to initiate the session
will transition to the Active state, where it will periodically try to establish a
TCP session.
When the TCP connection has been established, the endpoints can be
assured that as long as the session is active, there is a reliable connectionoriented path between the endpoints. Messages between the endpoints can
be sent reliably. This connection allows BGP messages to be very simple and
include only the information necessary with little overhead.
BGP must rely on the connection-oriented TCP session to provide the
Connection state as BGP cannot use a keepalive signal but sends a message
with a KEEPALIVE type in a common header to allow routers to verify sessions are active. Standard keepalives are signals sent from one router to
another on a circuit not using a TCP session. Routers use these signals on circuits to verify that there are no failures on the circuit or that the circuit has
not terminated.
Once the TCP connection has been established, BGP sends messages back
and forth in a specific format. The first message is an identification message
from the endpoints. As soon as this message is sent, the router transitions to
the OpenSent state. When the router receives a reply to the identification
message, it transitions to the OpenConfirm state. If a connection is received
and accepted by the endpoints, the Connection state becomes the Established state. From then on, when a message is sent to the endpoint routers,
the routers can respond to the sent message, update their routing table with
new information in the message, or have no reaction to the sent message
whatsoever.
Using the identification message information, the endpoints can accept or
refuse a connection from their BGP neighbor.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BPG Common Header
261
Endpoints typically stay in the Established state until there is a loss of the
session or an error. If this occurs, then the connection returns to the Idle state
and all the information that the BGP endpoints have learned from their
neighboring endpoint will be purged from the BGP routing table.
BPG Common Header
A common header precedes all BGP messages. This header, shown in
Figure 7.7, contains the following fields:
Marker
Length
Type
We will discuss those fields in the following sections.
FIGURE 7.7
The BGP common header
Marker
(16 octets)
Length
Type
(2 octets) (1 octets)
More fields based
on message type
Marker
The Marker field is a field up to two bytes long. It is used for security and
synchronization. The value of this field depends on the type of message
being sent.
Length
The Length field indicates the size of the entire BGP message including the
header.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
262
Chapter 7
BGP’s Basic Components
Type
The Type field indicates the type of message being sent. There are four possible values, as shown in Table 7.1.
TABLE 7.1
Values for the Type Field
Type Value
Message Type
1
OPEN message
2
UPDATE message
3
NOTIFICATION message
4
KEEPALIVE message
Let’s look at the different message types.
OPEN Message
This is the first message sent after a TCP session has been established
between one or more peers. This message is used to identify the AS that the
router is a member of, to agree on protocol parameters, and to determine the
protocol timers the session will use. Figure 7.8 shows the additional fields
included in the BGP header for an OPEN message.
FIGURE 7.8
The additional fields added to the BGP common header for an OPEN message
type
Common
Header
+
Version
(1 octet)
My
Autonomous
System
(2 octets)
Hold
Time
(2 octets)
BGP
Identifier
(4 octets)
Optional
Paramters
Length
(1 octet)
Optional
Paramters
(length
varies)
Let’s take a look at each of the additional field types added to the OPEN
message type.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BPG Common Header
263
Version
The Version field indicates the BGP version being used by the router sending
the OPEN message. This field allows the BGP speakers to informally negotiate the highest common version numbers that each supports. If a BGP version speaker receives a packet indicating a version number of 4, and the BGP
speaker receiving the packet is running a lower version of BGP, then it will
send an error message stating that it does not understand 4 and will terminate the TCP session. The BGPv4 speaker must then reopen the TCP session
using the parameters used in the lower version of BGP.
My Autonomous System
This field indicates the autonomous system number (ASN) membership of
the BGP router sending this OPEN message. Every AS must be identified by
its own unique ASN, which we discussed earlier in the chapter.
Hold Time
This field indicates the amount of time that the sender of the OPEN message
wants to use for its hold-down timer. This hold time indicates the maximum
amount of time that each endpoint will wait for another to send a message
before considering the connection terminated. This means that if an
UPDATE or a KEEPALIVE message is not sent in the indicated amount of
time, the session is considered closed. A value of zero indicates that the
sender does not want to exchange KEEPALIVE messages. This mode is not
recommended since one side will not know if the other has lost communication.
The hold time value is the minimum value set locally on the router or the
advertised hold time value. If the hold time value is not zero, then the hold
time must be at least three seconds. A neighboring endpoint can reject an
OPEN message if the hold time value is unacceptable.
BGP Identifier
This field contains a value that identifies the BGP speaker. This is a random
value chosen by the BGP router when sending an OPEN message. The value
must be unique to all the other BGP speakers communicating with one
another. Although the number can be random, BGP speakers will typically
use the logical IP address assigned to the interface. This number is then used
for every BGP session.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
264
Chapter 7
BGP’s Basic Components
Optional Parameters Length
This field is used to indicate the length of the Optional Parameters field in the
OPEN message. If there are no optional parameters in the field, then this
length is set to zero.
Optional Parameters
This field contains any optional parameters inserted into the OPEN message.
Each optional parameter includes a one-octet parameter type, a one-octet
parameter length, and a variable-length parameter value.
UPDATE Message
This type of message is the actual topology information sent between two
BGP speakers. An UPDATE messages can contain a new route, routes to be
withdrawn, or both. However, only one new route can be advertised by an
UPDATE message.
The UPDATE message adds additional fields to the BGP common header,
as shown in Figure 7.9.
FIGURE 7.9
The additional fields added to the BGP common header when using the
UPDATE message type
Common
Header
+
Withdrawn
Routes
Length
(2 octets)
Withdrawn
Routes
(length
varies)
Total Path
Attributes
Length
(2 octets)
Total Path
Attributes
(length
varies)
Network layer
Reachability
Information
(length varies)
Let’s look at the additional fields added to the BGP common header when
the UPDATE message type is used.
Withdrawn Routes Length
This field is used to indicate the length of the Withdrawn Routes field and
specifies this information in the number of octets. The BGP specification
itself officially calls this field the Unfeasible Routes Length field.
Withdrawn Routes
The Withdrawn Routes field can contain a list of IP prefixes for which the
BGP speaker sending the UPDATE message wants to notify its BGP peer that
a route path either no longer exists or cannot be accessed due to the addition
of a policy. We’ll discuss this topic in more detail in Chapter 9. Each IP prefix
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BPG Common Header
265
being withdrawn adds two fields to the Withdrawn Routes field. An IP Prefix
Length, which is one octet, identifies the length of the second field, called the
IP Prefix field. This field is of variable length and identifies the IP prefix for
the route that needs to be withdrawn. If the prefix does not equal at least
eight bits, then the rest of the field is padded with additional bits to make
each integer a multiple of eight bits.
Total Path Attributes Length
This field is used to indicate how large the Total Path Attributes field is.
Total Path Attributes
There are many path attributes that can be placed in the Total Path
Attributes field. Each attribute has a type code and several bits that describe
each attribute’s usage. These attributes are associated with prefixes found in
the Network Layer Reachability Information (NLRI) field. Each bit indicates a different attribute type. Table 7.2 describes the attribute types (A 0
bit equals OFF and a 1 bit equals ON).
TABLE 7.2
Attribute Types
Bit
Attribute Type
1
ON=optional.
OFF=well-known.
2
ON=transitive.
OFF=non-transitive.
3
ON=partial optional attribute; must be passed on.
OFF=well-known non-transitive; does not need to be
passed on.
4
ON=extended length bit; the total length of the attribute is
more than one octet. (By setting the extended length bit,
attributes can be longer than 255 bytes.)
OFF=the length of the attribute is one octet.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
266
Chapter 7
BGP’s Basic Components
By turning on the first bit, this flag indicates that all well-known attributes
must be passed along to downstream peers after the peers receive and process
the message. BGP does not require every implementation to support every
option. The second bit specifies how implementations handle options they
do not recognize. If the second bit, which is known as the transitive flag, is
on, then if the option is recognized it will pass the information downstream
to its BGP neighbors. If the bit is turned off, then the option is ignored and
not passed downstream to other neighbors.
All well-known attributes are considered transitive.
Some attributes appear only in iBGP or in eBGP. For this book, we consider that iBGP and eBGP are the same protocol but with differences in the
peering points and the types of attributes of each. Remember where each is
used. In iBGP, each peer communicates between speakers in the same AS,
and in eBGP, peers communicate between speakers in different ASes.
Path attributes can be considered as the metrics used by BGP routers that
are passed in UPDATE messages to other BGP peers. These messages can
contain notifications of local routes, foreign routes, or route topology
changes. An attribute can be placed in one of four categories, as listed below:
Well-known mandatory
Well-known discretionary
Optional transitive
Optional non-transitive
Let’s take a look at the characteristics of each attribute in the next sections
and the attributes that can be associated with each type.
WELL-KNOWN MANDATORY
A well-known mandatory attribute is used by a totally compliant BGP implementation to propagate all the network’s BGP neighbors. Well-known mandatory attributes must appear in all BGP update messages. This means that
a well-known mandatory attribute must appear in an advertised route and
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BPG Common Header
267
must be supported by all implementations of BGP. These attributes are as
follows:
Autonomous System Path The AS_PATH (Type Code 2) is a wellknown mandatory attribute. The AS_PATH attribute is composed of a
variable-length series of AS path segments. Each AS path segment contains a path type, a length, and a value. The path segment type is a oneoctet-long field with the values shown in Table 7.3.
The AS_PATH’s fields above are modified only by eBGP speakers that
advertise the route outside the local AS. These eBGP speakers prepend
their own AS numbers to the end of the path vector in each of the fields.
When a BGP speaker originates a route, it should include its own ASN in
UPDATE messages sent to other ASes. The field is empty for an AS_PATH
attribute advertised to iBGP speakers belonging to its own ASN. This
allows iBGP to avoid data loops by implementing a rule that specifies that
each iBGP router must ignore any route learned from an iBGP peer.
The AS_PATH attribute makes BGP a path-vector protocol. BGP messages carry the sequence of AS numbers indicating the complete path a
message has traversed.
Next-hop The NEXT_HOP (Type Code 3) attribute is a well-known
mandatory attribute that indicates the IP address of the next-hop destination router. The next hop for all destinations is listed in the NLRI field of
the UPDATE message. The BGP speaker should never advertise the
address of a peer as the NEXT_HOP of a route the current speaker is originating to that peer. Likewise, the speaker should not install a route that
has itself as the next hop unless the NEXT_HOP_SELF configuration
option is used.
An iBGP speaker can advertise any internal BGP router as the next hop as
long as the IP address of the iBGP border router is on the same subnet as
the local and remote BGP speakers. This means that one router can handle
all the announcements on the same subnet.
A BGP speaker can also advertise any external border router as the next
hop if the IP address of the proposed next-hop router is learned from one
of the advertising router’s peers, and if the connected interface for the
router is on the same subnet as both the local and remote BGP speakers.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
268
Chapter 7
BGP’s Basic Components
Origin The ORIGIN (Type Code 1) attribute is a well-known mandatory attribute used to tell the receiving BGP router the BGP type of the
original source of the NLRI information. The ORIGIN type can be one of
the type codes shown in Table 7.4.
You can override the second option if an eBGP_MULTIHOP configuration is
used. The eBGP_MULTIHOP configuration, which we will discuss in Chapter 8,
can be used when configuring the next hop if two eBGP speakers need to peer
across multiple subnets and the physical connectivity between two eBGP
speakers runs over more than one load-shared link. Do not use this feature if
both iBGP speakers are in the same AS. Another reason to use the eBGP_
MULTIHOP configuration is if you are using a single point-to-multipoint, nonbroadcast multi-access (NBMA) medium, such as Frame Relay.
TABLE 7.3
Path Segment Values
Bit Value
Path Segment
Type
0
Non-defined
1
AS_SET (an unordered list of ASes that the UPDATE
message has traversed)
2
AS_SEQUENCE (an ordered list of ASes that the UPDATE
message has traversed)
3
AS_CONFED_SET (an unordered list of ASes in the local
confederation that the UPDATE message has traversed)
4
AS_CONFED_SEQUENCE (an ordered list of ASes that the
UPDATE message has traversed in the local confederation)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BPG Common Header
TABLE 7.4
269
ORIGIN Type Codes
Code
Value
Type
0
IGP (the originating AS, which has learned about this NLRI
from its own IGP)
1
EGP (the AS sending the NLRI, which was first learned from
an eBGP speaker)
2
INCOMPLETE (the NLRI obtained this route statically, such as
from a configured static route) This code may also be used
when any redistributed route from an IGP to BGP has an
incomplete flag.
WELL-KNOWN DISCRETIONARY
A well-known discretionary attribute might be included in a route description but does not have to be included. These attributes are as follows:
Local Preference The LOCAL_PREF (Type Code 5) attribute is a wellknown discretionary attribute that can contain only a single AS and can
be used only with iBGP.
Atomic Aggregate The ATOMIC_AGGREGATE (Type Code 6)
attribute is a well-known, discretionary attribute that is used to inform
BGP speakers of policy routing decisions that have been made when there
is more than one route, also known as overlapping routes. This attribute
is basically used to indicate that a prefix is or is not to be used. Therefore,
the ATOMIC_AGGREGATE has a path length of 0.
OPTIONAL TRANSITIVE
An optional transitive attribute may not be recognized by some implementations of BGP and is not expected to be. These attributes are used in many
private BGP-enabled networks. If an implementation of BGP does not recognize the optional transitive attribute of a message, it will mark the message
as a partial message but still propagate the message to its neighbors. The
optional transitive attributes are as follows:
Aggregator The AGGREGATOR (Type Code 7) attribute is an
optional transitive attribute of six octets in length: two octets identifying
the ASN and four octets identifying the IP address. This attribute can be
attached to a message that is performing aggregation to identify the AS
and the router that performed the aggregation.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
270
Chapter 7
BGP’s Basic Components
Communities The COMMUNITIES (Type Code 8) attribute is an
optional transitive attribute that allows a given route to belong to one or
more communities. Communities are routes that share some common
property. This attribute was included in BGP to simplify the configuration
of complex BGP routing policies. For example, an academic network that
handles both academic and commercial traffic under an acceptable-use
policy might set a community attribute on the university updates; this
community attribute value would indicate that the route meets the
acceptable-use policy. More than one community can be associated with
a route.
Community attributes are optional, transitive, and variable in length.
Current communities are 32-bits long, structured as two 16-bit fields. By
convention, the first 16 bits are either zero, denoting a “well-known”
community known to the Internet, or the AS number that “owns” the community value. The second 16 bits are meaningful either as defined by the
owning AS or, in the case of well-known communities, by the IETF.
OPTIONAL NON-TRANSITIVE
An optional non-transitive attribute may not be recognized by some implementations. These attributes are used in many private BGP-enabled networks. Even if the implementation of BGP does recognize the optional nontransitive attribute of the message, it is not passed on.
If the network sees the message as an optional non-transitive attribute, say
good-bye to the message. The message is deleted and not sent to other networks. The following are the optional non-transitive attributes:
MED The MULTI_EXIT_DISCRIMINATOR (Type Code 4) attribute
is an optional non-transitive attribute that is used by BGP as an extensive
route-selection component. This component starts to work before the
general route-selection process begins, using a BGP attribute called multiexit discriminator (MED), which was originally called the Inter-AS metric
or the BGP metric. While the previous metrics inform the local AS routers
which path to select when leaving the AS, MEDs inform the neighboring
AS which link to use to receive traffic.
MED routes are used when two autonomous systems are connected by
multiple links or multiple routers. MED values are not propagated to
other autonomous systems and are considered only as part of the BGP
route-selection process. The general route-installation process never sees
these routes.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
BPG Common Header
271
Originator ID and Cluster List Both the ORIGINATOR_ID (Type
Code 9) and CLUSTER_LIST (Type Code 10) optional non-transitive
attributes are used to support the route-reflector feature used to scale
iBGP meshes. These attributes are detailed in BGPv2 and are not covered
in this book. The ORIGINATOR_ID is four octets long, and a
CLUSTER_LIST attribute can vary in length in multiples of four octets.
The ORIGINATOR_ID attribute is used to identify the router that originated a particular route into an iBGP mesh. This way, if an iBGP router
learns of a route again, it will know the source of the original routing
information and not re-advertise this information to those peers that have
already been sent the routing information.
The CLUSTER_LIST attribute is used to detect updates that are looping
inside the cluster. This way, if a route has already been advertised to a
cluster, the advertisement message will be rejected.
Route reflectors and iBGP meshes will be covered in detail in Chapter 9.
Multiprotocol Reachable NLRI The MP_REACH_NLRI (Type
Code 14) attribute is used in the Multiprotocol Extensions for BGP. This
attribute identifies a newly reachable route in a particular address family
other than global IP version 4. This attribute is not covered in this book.
Multiprotocol Unreachable NLRI The MP_UNREACH_NLRI (Type
Code 15) attribute is carried in a BGP UPDATE message for which the
ORIGIN and AS_PATH attributes pertain to the native IPv4 BGP communications that carry the message. The Type Code of 15 identifies a
route that has been withdrawn. This attribute is not covered in this book.
Type Code 11 (Destination Preference) is defined by MCI. Type Code 12
(Advertiser) and Type Code 13 (RCID_PATH) are both defined by Baynet. Type
Code 255 is reserved for development. These type codes will not be covered
in this book.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
272
Chapter 7
BGP’s Basic Components
Network Layer Reachability Information
The Network Layer Reachability Information (NLRI) lists the prefixes that must be updated. One thing to understand is that all the prefixes
listed in this field must match all the attributes listed in the Path Attributes
field.
This means that more than one route can be withdrawn in the same
UPDATE message, but if you want to add a route, you must do so in
another UPDATE message. As opposed to the length of the overall Withdrawn Routes field, prefix lengths apply to specific routes. A length of zero
here implies the default route.
Each prefix in the NLRI field contains a one-octet prefix length and a
variable-length prefix, which does not necessarily contain an IP address.
NOTIFICATION Message
If an error occurs during a BGP session, a BGP NOTIFICATION message is generated. As soon as the BGP speaker sends the NOTIFICATION
message, it immediately terminates its BGP connection. The administrator can
use this message to help troubleshoot why the connection was terminated.
There are two types of error codes in NOTIFICATION message fields to
watch for. These are the Error Code and Error Subcode, which are shown in
Figure 7.10.
FIGURE 7.10
The NOTIFICATION message fields added to the BGP common header
Common
Header
+
Error
Code
(1 octet)
Copyright ©2001 SYBEX , Inc., Alameda, CA
Error
Subcode
(1 octet)
Data
(length varies)
www.sybex.com
NOTIFICATION Message
273
Table 7.5 lists the Error Code field’s error codes.
TABLE 7.5
Error Codes
Code
Number
Type
1
Indicates an error in the common header or a general message error
2
Indicates an OPEN message error
3
Indicates an UPDATE message error
4
Indicates a Hold Time Expired error
5
Indicates an illegal event for the current state
6
Used when no other error codes apply
Table 7.6 lists the Error Subcode field’s error codes for general message
errors.
TABLE 7.6
Error Subcodes for General Message Errors
Code
Number
Type
1
Connection not synchronized or marker field incorrect
2
Bad message length
3
Bad message type
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
274
Chapter 7
BGP’s Basic Components
Table 7.7 lists the Error Subcode field’s error codes for OPEN message
errors.
TABLE 7.7
Error Subcodes for OPEN Message Errors
Code
Number
Type
1
Unsupported version number
2
Bad peer AS information passed
3
Bad BGP Identifier field
4
Unsupported optional parameter
5
Authentication failure
6
Unexcepted Hold Time value
Table 7.8 lists the Error Subcode field’s error codes for UPDATE message
errors.
TABLE 7.8
Error Subcodes for UPDATE Message Errors
Code
Number
Type
1
Error parsing the Path Attributes field
2
Unrecognized well-known path attribute
3
Missing required well-known attribute
4
Attribute flag field not understood
5
Attribute length mismatch or not understood
6
An invalid ORIGIN attribute
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
TABLE 7.8
275
Error Subcodes for UPDATE Message Errors (continued)
Code
Number
Type
7
AS routing loop or looping prefix error
8
Invalid NEXT_HOP prefix
9
Optional attribute error
10
Invalid network field when processing a prefix update
11
Error encountered processing the AS_PATH attribute
KEEPALIVE Message
BGP neighbors use a KEEPALIVE message to confirm that the connection between the neighbors is still active. A BGP speaker sends a KEEPALIVE to each peer, usually at an interval of one-third of the agreed hold
time, which is no more than once per second. If an UPDATE message is not
sent during the established hold time, a KEEPALIVE message is sent in its
place. A KEEPALIVE message consists only of a 19-byte header and can be
turned off by setting the hold time to zero.
Summary
This chapter focused on BGP terminology and the basics components
of BGP. Let’s quickly review what this chapter covered:
Autonomous systems, which are used to identify routers operating in
a common network.
Transit autonomous systems, which are between two autonomous
systems.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
276
Chapter 7
BGP’s Basic Components
BGP peers, which are two routers running BGP and connecting
through a TCP session to exchange messages. BGP peering is a reference to a specific relationship at the policy level, which will be covered
in more detail in Chapter 8.
The differences between Internal BGP and External BGP.
The differences between distance-vector protocols and link-state routing protocols. Link-state routing protocols (or a hybrid of link-state
routing and distance-vector) provide for greater scalability and stability.
When to use BGP and when not to use BGP.
Ingress filtering, which filters BGP messages and announcements
based on the source address and an administrative range.
BGP message types identified in the BGP common header, which are
the OPEN, UPDATE, NOTIFICATION, and KEEPALIVE message
types.
BGP path attributes associated with the UPDATE message type.
BGP should be used only when a network meets a few specific criteria, as
outlined in this chapter. In Chapter 8, we’ll look at how BGP works, how the
BGP metrics are used, how to change the metrics, as well as how to configure BGP.
Key Terms
Before taking the exam, make sure you’re familiar with the following terms:
Active state
AGGREGATOR
ATOMIC_AGGREGATE
autonomous system
AS_PATH
COMMUNITIES
Connection state
Established state
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
exterior routing protocol
external Border Gateway Protocol (eBGP)
ingress filtering
Idle state
internal Border Gateway Protocol (iBGP)
keepalive
LOCAL_PREF
MP_REACH_NLRI
MP_UNREACH_NLRI
Network Layer Reachability Information (NLRI)
NEXT_HOP
OpenConfirm state
OpenSent state
ORIGIN
path-vector protocol
sneakernet
stub AS
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
277
278
Chapter 7
BGP’s Basic Components
Written Lab
1. What BGP autonomous system numbers are reserved for private use?
2. What authority allows the distribution of BGP autonomous system
numbers?
3. BGP uses what TCP port number to establish a session between peers?
4. What type of attribute is required to be in every BGP UPDATE
message?
5. How many entry and exit points can be found in a stub network?
6. Which type of BGP is used inside of an autonomous system?
7. What is the total range of assignable autonomous system numbers?
8. Which type of BGP is used between autonomous systems?
9. An autonomous system in the center of two other autonomous sys-
tems is referred to as what?
10. What RFC defines BGP autonomous systems?
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
279
Review Questions
1. What are the benefits of using a link-state routing protocol? (Choose
all that apply.)
A. It uses the Hello packet to establish adjacencies.
B. It uses several components to calculate the metric of a route.
C. Updates are sent only when changes occur in the network.
D. It is a better protocol than distance-vector is.
2. BGP is used to advertise which of the following?
A. Network hosts
B. Network paths
C. Network switches
D. Network servers
3. Which of the following RFCs explains autonomous systems as a set of
routers under one or more administrations that present a common
routing policy to the Internet?
A. RFC 1930
B. RFC 2047
C. RFC 2047
D. RFC 31
4. Which of the following is not an IGP?
A. RIPv2
B. IGRP
C. IPv4
D. OSPF
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
280
Chapter 7
BGP’s Basic Components
5. Which of the following BGP types runs outside of an AS?
A. oBGP
B. iBGP
C. eBGP
D. xBGP
6. Interior routing protocols operate at what layer of the OSI Reference
Model?
A. Shared layer
B. Network layer
C. Data Link layer
D. Physical layer
E. Routing layer
7. When an AS must traverse another AS to get to its destination, the tra-
versed AS is called which of the following?
A. Complete AS
B. Forwarding AS
C. Transit AS
D. Transistor AS
8. BGP uses which of the following TCP ports to open a session with
another BGP peer?
A. Port 20
B. Port 21
C. Port 80
D. Port 179
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
281
9. Which of the following message types must be sent by a BGP peer dur-
ing the configured hold time to keep a session from terminating?
(Choose the two best answers.)
A. Non-terminate message
B. KEEPALIVE message
C. UPDATE message
D. TIMER message
10. Which of the following authorities is responsible for assigning ASNs?
A. ANSI
B. Internet Police
C. IEEE
D. IANA
11. An autonomous system number is comprised of how many bits?
A. 8
B. 16
C. 32
D. 64
12. How many entry and exit points can be found in a stub network?
A. Five
B. Four
C. Two
D. One
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
282
Chapter 7
BGP’s Basic Components
13. When a BGP peer tries to open a session with another endpoint, the
peer is in which of the following states?
A. Active state
B. Connection state
C. Open state
D. Established state
14. Withdrawn routes are advertised in which of the following message
types?
A. OPEN
B. UPDATE
C. NOTIFICATION
D. KEEPALIVE
15. Which of the following attributes must be included in a BGP UPDATE
message?
A. Well-known mandatory
B. Well-known discretionary
C. Optional transitive
D. Optional non-transitive
E. All of the above
16. Which of the following is not a well-known mandatory attribute?
A. AS_PATH
B. COMMUNITIES
C. ORIGIN
D. NEXT_HOP
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
283
17. Which of the following attributes is considered to be BGP’s extensive
route-selection component?
A. ORIGINATOR_ID
B. MULTI_EXIT_DISCRIMINATOR
C. CLUSTER_LIST
D. AS_PATH
18. The Network Layer Reachability Information (NLRI) field is used to
identify prefixes associated with which of the following fields found in
an UPDATE message?
A. CLUSTER_LIST
B. MED
C. Total Path Attributes
D. ORIGINATOR_ID
19. Which of the following fields is not found in a BGP common header?
A. Marker
B. Length
C. Version
D. Type
20. Which of the following is not a BGP message type?
A. OPEN
B. UPDATE
C. NOTIFICATION
D. KEEPALIVE
E. WAIT
F. All of the above.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
284
Chapter 7
BGP’s Basic Components
Answers to Written Lab
1. 64,512 through 65,535
2. The Internet Assigned Numbers Authority (IANA)
3. Port 179
4. Well-known mandatory
5. One
6. iBGP (Internal BGP)
7. 1 through 65,535
8. eBGP (External BGP)
9. Transit AS
10. RFC 1930
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
285
Answers to Review Questions
1. A, C. Link-state routing protocols use the Hello protocol to establish
adjacencies, and they send updates only when there is a change in the
network topology.
2. B. BGP is a complex external routing protocol used to advertise and
distribute available networks and paths to those
networks.
3. A. RFC 1930 is the RFC that defines an AS.
4. C. An IGP is an interior routing protocol that is used to determine the
network topology inside of an AS. RIP, OSPF, and IGRP are all interior routing protocols. IPv4 is a Layer 3 routed protocol, not a routing
protocol.
5. C. Exterior BGP (eBGP) runs outside of an AS to distribute routing
information and AS policies. Interior BGP (iBGP) runs inside the
AS and uses its own route map separate from that used by the IGPs.
6. B. Interior routing protocols operate at Layer 3 of the OSI Reference
Model, which is called the Network layer.
7. C. A transit AS is an AS through which data from one AS must travel
to get to another AS.
8. D. Port 179 is used by BGP to establish a session with another BGP
peer. Ports 20 and 21 are used by FTP, and port 23 is used by Telnet.
9. B, C. Either a KEEPALIVE message type or an UPDATE message
must be sent in order to keep a session open during the configured
hold time. The Hold Time option can be set to zero in order to disable
it. If it is not disabled, then it can be set to no less than three seconds.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
286
Chapter 7
BGP’s Basic Components
10. D. The Internet Assigned Numbers Authority (IANA) is responsible
for delegating autonomous system numbers. Other organizations
may assign numbers, but only if they are authorized by the IANA.
11. B. An autonomous systems number is 16 bits long and can range
from 1 through 65,535.
12. D. In a stub network, there is one entry and exit point. In this type of
network, it is recommended that you not use BGP but use a static
route instead.
13. B. This peer is in the Connection state until a message is sent to iden-
tify each peer. When the connection is established, it transitions to the
Open state. Once the other peer accepts the connection, the peer transitions to Established state. If the connection is lost, possibly due to a
version mismatch, the peer goes to the Active state and actively tries to
reestablish the connection using the proper version properties.
14. B. An UPDATE message is used to advertise topology updates and
changes, including added or withdrawn routes.
15. A. A well-known mandatory attribute is known to all versions of
BGP and must be included in an UPDATE message.
16. B. The COMMUNITIES attribute is an optional transitive attribute,
which means that it is not required but, if included, can traverse more
than one AS.
17. B. MULTI_EXIT_DISCRIMINATOR (MED) is considered to be
BGP’s extensive route-selection component. The ORIGINATOR_ID
attribute identifies a router that first advertised a route. The
CLUSTER_LIST attribute is used to detect updates that can cause data
loops. The AS_PATH attribute makes BGP a path-vector protocol by
logging all the autonomous systems a message has traveled through.
18. C. The NLRI field shows route prefixes with attributes identified in
the Total Path Attributes field. The other answers listed are all
attributes found in the Total Path Attributes field.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
287
19. C. The Version field is not found in a BGP common header. If the
common header specifies this to be a BGP OPEN message, then the
Version field will be found in the message but not in the BGP common
header.
20. E. There is no WAIT message type. The OPEN message type is used
to establish a connection between BGP peers. The NOTIFICATION
message type is used to advertise errors. The UPDATE message type is
used to advertise topology updates and changes, and the KEEPALIVE message type is sent to keep a session active when no UPDATE
messages are exchanged during the established hold time.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
8
Configuring Basic BGP
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
Configure a basic BGP environment
Identify how to disable BGP synchronization with an IGP
Explain how to configure attributes
Use show commands to verify a BGP configuration
Identify commands used to troubleshoot BGP
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
I
n Chapter 7, “BGP’s Basic Components,” we talked about BGP
and what it was used for, and we maintained a focus on the path attributes
that are used in BGP. In this chapter, we will step into the basic configuration
of BGP. I say basic because Chapter 9, “BGP Scalability and Advanced Features,” will focus on more advanced configurations, scaling, and other
important properties of BGP.
The process of configuring BGP routing is much more complex than setting up any of Cisco’s supported interior routing protocols. Using BGP, we
must configure both interior and exterior protocols. Obviously, this will
involve additional configuration steps to establish a routing process and
indicate which networks can be advertised to other networks.
Minimal Configuration
B
efore the configuration process for BGP can begin, we need to determine some basic information. Table 8.1 contains important BGP router
checklist information. You can refer to this information whenever you need
to configure BGP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
TABLE 8.1
291
BGP Router Checklist Information
Item
Meaning
Identifier
BGP needs a router ID. This can be the
address of the loopback interface or the
IP address of a directly connected interface.
This ID is usually the IP address of the
loopback interface, making the interface
easy to identify.
BGP process number
Our assigned autonomous system number
(ASN) or a private ASN.
Neighbors
We will assign those in our own AS, but the
service provider should provide you the
addresses and ASNs under the provider’s
control.
NLRI (Network Layer
Reachability Information)
to advertise
These are our assigned ASNs that need to
be advertised over the Internet.
Filters/policy mechanisms
Our internal routing policy.
Peers
With BGP, you also need to specify the
peers. Peers are not automatically
discovered. This is a matter of intentional
protocol design, not a limitation. Peers are
other routers running BGP.
Once you have collected this information, you are ready to begin the BGP
configuration process. Since BGP is very complex, it is best to learn it by
starting with a very basic configuration. Our example does not have realworld applicability, but by doing it this way, we will see how to work with
the basic functions of BGP.
Let’s look at Figure 8.1, which shows a very simple configuration of BGP.
If you can get this configuration to work, you should be able to understand
how to create more complex configurations of BGP. The 69.78.0.0 network
is real. I added routers to simulate the 172.16.0.0 and 130.77.0.0 networks.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
292
Chapter 8
Configuring Basic BGP
FIGURE 8.1
A practice BGP topology
Network 63.78.0.0
AS# 34000
AS# 63001
Network 172.16.0.0
Internet
RouterA
63.78.39.174
RouterB
172.16.2.1
Internet
RouterC
Network 130.77.0.0
AS# 59000
Starting the BGP process is similar to configuring an internal routing protocol. BGP is initiated on the router by using the following command and
syntax:
router bgp autonomous-system
Let’s take a look at an example of initiating BGP on RouterA based on
Figure 8.1:
RouterA>enable
RouterB#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#router
bgp
egp
eigrp
igrp
isis
iso-igrp
mobile
odr
ospf
rip
?
Border Gateway Protocol (BGP)
Exterior Gateway Protocol (EGP)
Enhanced Interior Gateway Routing
Protocol (EIGRP)
Interior Gateway Routing Protocol
(IGRP)
ISO IS-IS
IGRP for OSI networks
Mobile routes
On Demand stub Routes
Open Shortest Path First (OSPF)
Routing Information Protocol (RIP)
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
static
traffic-engineering
293
Static routes
Traffic engineered routes
RouterA(config)#router bgp ?
<1-65535> Autonomous system number
RouterA(config)#router bgp 34000
RouterA(config-router)#
So far, we have configured the router with the autonomous system number (ASN) to which it belongs. Next, we must add network statements to
identify the networks that the router must propagate information to in order
for another AS to learn about ours. We are using network statements to
avoid redistribution from an IGP into BGP, which is not recommended. The
network statement establishes those address ranges to be advertised, such as
those learned by the IGPs running in the network. It will not actually advertise anything until peering is established or the route to be advertised is
reachable by the advertising router, meaning that a route makes its way into
the routing table.
When taking the exam, you may need to know what a peer group is. A peer
group is a way of defining a template containing parameters that more than
one peer will use. This becomes useful when many different neighbors use
identical outbound routing policies, which will be discussed in Chapter 9. The
parameters set by a peer group affect only the outbound parameters, and the
inbound parameters can be configured differently. This can simplify the configuration, as updates need to occur only once. BGP peer configurations will
not be discussed in this course.
The next step is to identify peers. In BGP, as opposed to IGPs, with the
exception of certain OSPF configurations, you must explicitly specify the IP
addresses of the routers with which you want to exchange information.
Internal peers will have the same ASN used in the source router’s router
bgp command. External peers are those with a different AS from the AS
defined on the source router. Let’s look at the command and the syntaxes:
neighbor address remote-as autonomous-system-number
The address is the IP address of the neighboring (peer) router. It can be
the loopback address or the directly connected IP address. The autonomoussystem-number is the peer’s ASN. An iBGP peer will have the same ASN as
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
294
Chapter 8
Configuring Basic BGP
the source router. An eBGP peer will have a different ASN than the source
router.
Now let’s look at an example based on Figure 8.1. We’ll add RouterB,
which is at 172.16.2.1, and identify the network in which to advertise to our
neighbor:
RouterB (config-router)#neighbor 172.16.2.1
remote-as 63001
The loopback IP address can be used for both iBGP and eBGP peers. Additional commands must be used when creating a peering session with a loopback interface. For iBGP sessions, the only additional command is the
update-source command. The available syntaxes are as follows:
neighbor [address | peer-group-name] update-source
interface-type interface-number
The IP address of the loopback should be used for the peer address. Since
the loopback interface is being used as the source of the BGP session, the
interface-type should be entered as the loopback. The interfacenumber is the number of the loopback interface that is being used for BGP
peering. This is configured on the router using the loopback address.
The following command adds networks and creates a route in the BGP
table if the route is present in the IP table:
network network-number
Let’s look at an example adding our own network 63.78.0.0:
RouterA(config-router)#network 63.78.0.0 ?
backdoor
Specify a BGP backdoor route
mask
Network mask
route-map Route-map to modify the attributes
weight
Set BGP weight for network
<cr>
RouterA(config-router)#network 63.78.0.0 mask
255.255.255.0 ?
backdoor
Specify a BGP backdoor route
route-map Route-map to modify the attributes
weight
Set BGP weight for network
<cr>
RouterA(config-router)#network 63.78.0.0 mask 255.255.0.0
RouterA(config-router)#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
295
Again, network-number represents the network that is to be advertised
using the BGP process. The IP network specified in the BGP network statement does not have to be directly connected to the router. Network statements within the BGP protocol session allow BGP to advertise routes learned
by an IGP that are contained in the route table. The network mask is applied
because BGPv4 can support subnetting and supernetting. When a logical
BGP mesh is in place, each IGP session should have network statements configured for only those routes learned from the IGP. Network statements
should not be duplicated among internal BGP routers.
BGP configuration can be very complicated. Several different options may be
configured to optimize BGP routing. When only one link is used to peer with
another AS or ISP, the configuration can be straightforward. As more links are
used, or multiple ISPs or autonomous systems are connected to a router, the
configuration becomes increasingly complex.
Verifying BGP Configurations
After BGP is configured, several commands will allow us to verify the BGP
configuration and troubleshoot the operation of BGP. We can also use these
commands to monitor the BGP process and its operations.
Table 8.2 summarizes all of the commands that can be used to verify BGP.
TABLE 8.2
BGP Monitoring Command Summary
Command
Description
show ip bgp
Shows all BGP configuration information for
the selected interface.
show ip bgp neighbors
Shows all configured BGP neighbors. It provides detailed statistics and information
about each neighbor.
show ip bgp community
Displays routes belonging to the specified
community.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
296
Chapter 8
Configuring Basic BGP
TABLE 8.2
BGP Monitoring Command Summary (continued)
Command
Description
show ip bgp cidr-only
Displays classless routes.
show ip bgp filter-list
Displays AS path lists.
show ip bgp paths
Displays all path information for the local
router.
show ip bgp peer-group
Provides information on the members of the
specified peer group.
show ip bgp summary
Shows the status of all BGP connections.
The detailed use of some of these commands will be explained in the section “Troubleshooting BGP.”
In earlier versions of the Cisco IOS, in particular versions 11.1 and 11.3, some
of the show commands listed above can cause the router to reload. Cisco
became aware of the problem and has resolved it in later versions.
Cisco has a configurable proprietary attribute that allows us to use
weights as a metric in deciding the best route. Let’s take a look at this
attribute in the next section. We’ll also see how to configure the
MED attribute discussed in Chapter 7.
Configuring BGP Route-Selection Attributes
BGP uses several metrics as criteria when selecting the best possible route to
a destination. Each metric can be configured manually. Other criteria that
influence BGP route selection may also be configured.
To quickly understand how BGP selects a route, review Figure 8.2. This
figure summarizes the steps that the BGP process takes to choose the best
route. Ten different criteria are used in path selection, several of which are
configurable.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
FIGURE 8.2
BGP path-selection diagram
Is Next-hop
reachable?
No
Ignore route.
Yes
Choose the path
with the highest
Weight.
No
Are Weights equal?
Yes
Choose the biggest
Local Preference.
Are Local
Preferences equal?
Was route originated
by local router?
No
Yes
Yes
No
Choose the
shortest AS Path.
Are AS
Paths equal?
Choose lowest
Origin Code.
Select route.
Yes
Are Origin
Codes equal?
Yes
No
Choose the
lowest MED.
Are MEDs equal?
Choose External
over Internal.
No
Yes
No
Are path
types equal?
Yes
No
Choose closest
IGP neighbor.
Are IGP
neighbors equal?
No
Choose the path
with the lowest
Router ID.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
297
298
Chapter 8
Configuring Basic BGP
Now let’s discuss some of these criteria separately. We will also learn how
to configure them.
Configuring the Atomic Aggregate Attribute
When using classless interdomain routing (CIDR), you can create aggregate
routes to minimize the size of routing tables. You can configure aggregate routes
in BGP by redistributing an aggregate route into BGP. The Atomic Aggregate
attribute can be configured using the aggregate-address command. This
command allows you to configure an aggregate or summary entry in the BGP
table. The command has several syntaxes. Let’s look at the command and the
possible syntaxes:
aggregate-address ip-address mask [summary-only] [as-set]
The ip-address and mask indicate the aggregate address to be created.
By default, BGP advertises both aggregate routes and more specific routes.
By using the summary-only syntax, the BGP router will advertise only the
aggregate route. If you use the as-set syntax, the BGP router will advertise
the route as coming from your AS and will set the Atomic Aggregate
attribute to show that information regarding the route may be missing.
Configuring the Weight Attribute
The Weight attribute is a Cisco proprietary attribute used for path selection.
This attribute, which is also considered a metric, allows a system administrator to manually assign a value to all paths learned from other BGP peers.
The larger the weight value, the more desirable the path.
This metric is particularly helpful when a router is connected to multiple
autonomous systems. The weight assigned stays local to the router on which
it is configured. When paths are learned from multiple sources, the Weight
metric can be used to force BGP to select a specified interface over the others.
This metric is configured using the following command from within the
BGP routing session:
neighbor [ip-address | peer-group-name] weight weight
The ip-address is the IP address of the neighbor. The peer-group-name
may be used when assigning weight to all routes learned via the BGP peer
group. The weight value has a range from 0 to 65,535. The default value is
32,768.
Configuring the Local Preference Attribute
The Local Preference attribute is used to assign metric values that are used
among IBGP peers. We learned that the Weight metric remains local to a
router. The Local Preference attribute is useful when multiple iBGP peers
have their own eBGP peers.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
299
When a path is learned via two different border routers, both paths are
advertised to other iBGP peers. Either path is valid and can be used. However, if one path is to be used only as a backup route, you can set local preference values on both routers.
The Local Preference attribute is configured by using the following
command:
bgp default local-preference value
The command must be issued within the BGP session Configuration
mode. The configured values for the Local Preference range from 0 to
4,294,967,295. Higher values are preferred over lower values.
Configuring in an NBMA Network
When you have a non-broadcast multi-access (NBMA) network in which the
router you are configuring needs to advertise itself as the next hop to a destination, use the next-hop-self syntax for the neighbor command. This
allows the normal BGP process to override what it’s learned and forces
updates to advertise this router as the next hop, even if there is another way
to the destination. The command is as follows:
neighbor ip address | peer-group-name next-hop-self
Configuring MED
While the previous metrics inform local AS routers which path to select when
leaving the AS, Multi-Exit Discriminators (MEDs) inform the neighboring
AS which link to use to receive traffic.
MEDs are used when two autonomous systems are connected via multiple
links or multiple routers. MED values are not propagated to other autonomous systems.
Configuring MEDs is more complicated than configuring Weight or Local
Preference values. Because of the complexity of the configuration, more CPU
resources are needed. MEDs are set using route maps. Route maps are a form
of access list. Here is an example of a BGP configuration using MEDs:
Router1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#router bgp 63001
Router1(config-router)#neighbor 172.16.2.1 route-map
ANEXAMPLE out
Router1(config-router)#exit
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
300
Chapter 8
Configuring Basic BGP
Router1(config)#route-map ANEXAMPLE permit 10
Router1(config-rou)#match ip address 1
Router1(config-rou)#set metric 25
Router1(config-rou)#exit
Router1(config)#route-map ANEXAMPLE permit 20
Router1(config-rou)#exit
Router1(config)#access-list 1 permit 172.16.0.0
0.0.255.255
Router1(config)#^Z
Router1#
router bgp 63001
network 172.16.0.0
neighbor 172.16.1.1 remote-as 59000
neighbor 172.16.2.1 route-map ANEXAMPLE out
!
ip classless
access-list 1 permit 172.16.0.0 0.0.255.255
route-map ANEXAMPLE permit 10
match ip address 1
set metric 25
!
route-map ANEXAMPLE permit 20
!
This configuration sets a MED of 25 for all networks belonging to
172.16.0.0. ASN 59000 will use this value. Lower MED values are preferred. The second permit statement of the route-map ANEXAMPLE permits
all other networks to be advertised but does not assign a MED value. We’ll
discuss route maps in more detail in Chapter 9.
Clearing BGP Routes
The BGP configurations can easily be removed from the router using the
clear ip bgp command. Let’s look at the command and the available syntaxes that are used in Privileged EXEC mode, and then we’ll explain each
syntax:
clear ip bgp *|address [soft[in|out]
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
301
Using the * means that you wish to clear the entire BGP routing table. You
can use the soft syntax so that the router advertises all its routing updates
again and the configuration is not cleared. Using the address syntax instead
of the asterisk, only the network address identified is removed from the BGP
table. The in and out syntaxes are used with the soft syntax to identify that
the triggered updates are to occur either on triggered inbound updates or
outbound updates.
Disabling BGP Synchronization
If all of the routers in your AS are running BGP, then there is no need to have
synchronization turned on between BGP and your IGPs that are running.
When BGP Synchronization is turned on, the router will wait to learn about
internal routes from an IGP instead of advertising routes learned by BGP.
With BGP Synchronization turned off, you can carry fewer IGP learned
routes in the topology table and BGP can converge much more quickly. To
turn off BGP Synchronization, use the following command in BGP Configuration mode:
Router1(config-router)# no synchronization
Troubleshooting BGP
The most important part of troubleshooting is verifying the status of the
peering router. When you issue the show ip bgp neighbors command, the
basic troubleshooting information is displayed on the screen. Let’s first take
a look at the command syntaxes and then view a problem configuration
where the BGP peers have not synchronized.
2514#show ip bgp ?
A.B.C.D
IP prefix <network>/<length>, e.g.,
35.0.0.0/8
A.B.C.D
Network in the BGP routing table to
display
cidr-only
Display only routes with non-natural
netmasks
community
Display routes matching the communities
community-list
Display routes matching the communitylist
dampened-paths
Display paths suppressed due to
dampening
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
302
Chapter 8
Configuring Basic BGP
filter-list
flap-statistics
inconsistent-as
neighbors
paths
peer-group
regexp
Display routes conforming to the
filter-list
Display flap statistics of routes
Display only routes with inconsistent
origin ASes
Detailed information on TCP and BGP
neighbor connections
Path information
Display information on peer-groups
Display routes matching the AS path
regular expression
Summary of BGP neighbor status
summary
<cr>
Notice in the output below that no connections are established, as
indicated by the bottom line. This means that the peer has not synchronized.
If the number of connections established keeps incrementing, there could be
a problem with the link between the two neighbors. This output is from IOS
version 12.0(5):
2514#show ip bgp neighbors
BGP neighbor is 172.16.2.1, remote AS 63001, external link
Index 1, Offset 0, Mask 0x2
BGP version 4, remote router ID 0.0.0.0
BGP state = Idle, table version = 0
Last read 00:00:07, hold time is 180, keepalive interval
is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 0; dropped 0
Last reset never
0 accepted prefixes consume 0 bytes
0 history paths consume 0 bytes
External BGP neighbor not directly connected.
No active TCP connection
2514#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
303
Now let’s look at the same router with the connection established:
2514#show ip bgp neighbors
BGP neighbor is 172.16.2.1, remote AS 63001, external
link
Index 1, Offset 0, Mask 0x2
BGP version 4, remote router ID 172.16.2.1
BGP state = Idle, table version = 0
Last read 00:00:07, hold time is 180, keepalive interval
is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 4582 messages, 0 notifications, 0 in queue
Sent 3552 messages, 0 notifications, 0 in queue
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 1; dropped 0
Last reset never
0 accepted prefixes consume 0 bytes
0 history paths consume 0 bytes
External BGP neighbor not directly connected.
2514#
On an older IOS, the output will look similar to this output from version 11.1:
Router#show ip bgp neighbors
BGP neighbor is 172.16.2.1, remote AS 63001, external link
Index 1, Offset 0, Mask 0x2
BGP version 4, remote router ID 172.16.2.1
BGP state = Established, table version = 508, up for
3d20h
Last read 00:00:45, hold time is 180, keepalive interval
is 60 seconds
Minimum time between advertisement runs is 30 seconds
Received 5579 messages, 0 notifications, 0 in queue
Sent 5703 messages, 0 notifications, 0 in queue
Inbound path policy configured
Outbound path policy configured
Incoming update AS path filter list is 10
Outgoing update AS path filter list is 1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
304
Chapter 8
Configuring Basic BGP
Connections established 1; dropped 0
Last reset never
No. of prefix received 10
Connection state is ESTAB, I/O status: 1, unread input
bytes: 0
Local host: 172.16.65.1, Local port: 179
Foreign host: 172.16.65.10, Foreign port: 29768
Enqueued packets for retransmit: 0, input: 0
0 (0 bytes)
Event Timers (current time is 0x14322791):
Timer
Starts
Wakeups
Retrans
5677
1
TimeWait
0
0
AckHold
5578
4246
SendWnd
0
0
KeepAlive
0
0
GiveUp
0
0
PmtuAger
0
0
DeadWait
0
0
mis-ordered:
iss: 1337567913 snduna: 1337679159
sndwnd: 15066
irs: 4270375806 rcvnxt: 4270482004
delrcvwnd:
836
Next
0x0
0x0
0x0
0x0
0x0
0x0
0x0
0x0
sndnxt: 1337679159
rcvwnd:
15548
SRTT: 309 ms, RTTO: 708 ms, RTV: 45 ms, KRTT: 0 ms
minRTT: 4 ms, maxRTT: 453 ms, ACK hold: 300 ms
Flags: passive open, nagle, gen tcbs
Datagrams (max data segment is 1460 bytes):
Rcvd: 11252 (out of order: 0), with data: 5579, total data
bytes: 106216
Sent: 9996 (retransmit: 1), with data: 5675, total data
bytes: 111245
Router#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Minimal Configuration
305
A great deal of information is provided by the show ip bgp neighbor
command. When a peering relationship has trouble getting established, use
this command to see if the TCP connection has failed. This will give you a
starting point for troubleshooting.
When the problem seems to be route information-oriented, you can use
the following command:
show ip bgp regexp regular-expression
Use this command to see which routes are being learned from the neighboring AS. If the neighboring AS is not receiving given routes from your AS,
you can use the following command to see what you are advertising to the AS:
show ip bgp neighbor address advertised-routes
A quick summary command can be used to verify connectivity via BGP:
show ip bgp summary
These are just a few of the commands that you can use when troubleshooting BGP. Many other commands and procedures can be used to accomplish this task, but they are beyond the scope of this book.
Using Debug with BGP
The debug ip bgp command can be used to display events as they occur. The
only drawback to this command is that not only does the BGP process being
used to advertise ASNs across the Internet use considerable processing
power, but the debug command is assigned a high priority on the router and
can kill your processing power. To stop all debugging on a router, use the
undebug all command or the no debug all command. Let’s look at a
short summary of the debug commands in Table 8.3.
TABLE 8.3
The Debug Commands Related to BGP
Command
Description
debug ip bgp dampening
Displays BGP dampening events as they
occur.
debug ip bgp events
Displays all BGP events as they occur.
debug ip bgp keepalives
Displays all events related to BGP keepalive
packets.
debug ip bgp updates
Displays information on all BGP update
packets.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
306
Chapter 8
Configuring Basic BGP
Summary
In Chapter 7 we looked at how BGP is used, when to use BGP, when not
to use BGP, and the type of protocol BGP is, and we focused on the attributes
sent in BGP update messages. In this chapter, we looked at how to enable
BGP, identify the network number the router belongs to so it can be advertised to its neighbors, identify the BGP neighbors, and assign the Weight
attribute.
In addition, we covered how to configure several other attributes, such as
the Next-hop, MED, Atomic Aggregate, and Local Preference attributes.
Then we covered the show commands that can be used to verify the configuration and troubleshoot problems that might arise in the configuration
of BGP.
As you may have noticed while looking at some of the output that
appeared throughout the chapter, BGP has many command syntaxes that
make configuring BGP very complex—much more complex than we will
cover in the following Hands-on Lab section below. In Chapter 9, we will cover
many more aspects of BGP, including the addition of filters and policies,
route flapping, and using BGP in large-scale networks.
Key Terms
Before taking the exam, make sure you are familiar with the following term:
peer group
Commands Used in This Chapter
aggregate-address
Allows you to configure aggregate routes
in BGP and CIDR addressing.
bgp default localpreference
Allows you to assign a Local Preference
attribute value in the range of 0 to
4,294,967,295. Higher values are
preferred over lower values.
clear ip bgp
Allows you to clear all or an identified set
of routes from the BGP table.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
debug ip bgp
dampening
Displays BGP dampening events as they
occur.
debug ip bgp events
Displays all BGP events as they occur.
debug ip bgp
keepalives
Displays all events related to BGP
keepalive packets.
debug ip bgp updates
Displays information on all BGP update
packets.
neighbors
This command has many syntaxes that
allow you to identify the internal and
external neighbors and assign different
metrics to each.
network
Identifies the networks and masks
associated with the local router.
no synchronization
Allows you to turn off synchronization
between the IGPs and BGP for faster
convergence.
router bgp
Begins the BGP process and identifies the
local ASN.
show ip bgp community Used to display routes belonging to the
specified community.
show ip bgp cidr-only Displays classless routes.
show ip bgp
filter-list
Displays AS path lists.
show ip bgp paths
Displays all path information for the local
router.
show ip bgp
peer-group
Provides information on the members of
the specified peer group.
Show ip bgp summary
Shows the status of all BGP connections.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
307
308
Chapter 8
Configuring Basic BGP
Written Lab
1. What command in BGP Configuration mode allows you to identify a
BGP peer with the IP address 172.16.1.1 in the 55009 AS?
2. What command would you use to clear the entire BGP routing table
on a router?
3. What command allows you to display all of the BGP events as they
occur?
4. What command would you use to identify a CIDR aggregate address?
5. What command displays all the BGP path information learned by the
router?
6. What command shows all the BGP peers?
7. If your AS resides in the network 63.78.0.0 and you use a 24-bit sub-
net mask, how do you identify this in the BGP configuration?
8. What command allows you to view all events related to BGP updates?
9. What command can be used to view routes belonging to a specific
community?
10. What command can be used to view the status of all connections?
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
309
Hands-on Lab
In this lab, we will demonstrate a very simple configuration of BGP over
three routers in two autonomous systems using information found in the
graphic below:
AS 3
10.1.25.254
AS 2
192.16.1.254
RouterA
RouterB
AS 1
172.16.3.254
RouterC
Let’s look at the following objectives:
On each router, enable BGP with its autonomous system number.
Identify the network each resides in.
Identify both of each router’s neighbors and the AS they reside in.
Assign the Weight attribute to each neighbor, giving any router in AS 2 a
Weight of 10 and the rest a Weight of 1.
Let’s now look at an example of the configuration of all three routers,
which are identified by the hostname on each router.
RouterA#conf terminal
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#router bgp 3
RouterA(config-router)#network 10.0.0.0 mask 255.255.255.0
RouterA(config-router)#neighbor 192.16.1.254 remote-as 2
RouterA(config-router)#neighbor 172.16.3.254 remote-as 1
RouterA(config-router)#neighbor 192.16.1.254 weight 10
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
310
Chapter 8
Configuring Basic BGP
RouterA(config-router)#neighbor 172.16.3.254 weight 1
RouterB#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
RouterB(config)#router bgp 2
RouterB(config-router)#network 192.16.1.0 mask
255.255.255.0
RouterB(config-router)#neighbor 172.16.3.254 remote-as 1
RouterB(config-router)#neighbor 10.1.25.254 remote-as 3
RouterB(config-router)#neighbor 172.16.3.254 weight 1
RouterB(config-router)#neighbor 10.1.25.254 weight 1
RouterB(config-router)#
RouterC#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
RouterC(config)#router bgp 1
RouterC(config-router)#network 172.16.0.0 mask
255.255.255.0
RouterC(config-router)#neighbor 10.1.25.254 remote-as 3
RouterC(config-router)#neighbor 192.16.1.254 remote-as 2
RouterC(config-router)#neighbor 192.16.1.254 weight 10
RouterC(config-router)#neighbor 10.1.25.254 weight 1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
311
Review Questions
1. Which of the following are needed when configuring BGP?
A. Router ID
B. BGP process number
C. External neighbors
D. Internal peers
E. All of the above
2. Which command shows the BGP routes?
A. show ip bgp routes
B. show ip bgp all
C. show ip route bgp
D. show ip bgp paths
3. Which of the following is an example of properly enabling BGP on a
router and identifying an internal network?
A. router ip bgp 10
network 10.1.1.1 100
B. router bgp 10
network 10.1.1.1 remote-as 10
C. router bgp 100
neighbor 10.1.1.1 remote-as 100
D. router bgp 100
neighbor 10.1.1.1 remote-as 200
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
312
Chapter 8
Configuring Basic BGP
4. Which of the following is an example of how to configure BGP with
an external BGP neighbor?
A. router ip bgp 10
network 10.1.1.1 100
B. router bgp 10
network 10.1.1.1 remote-as 10
C. router bgp 100
neighbor 10.1.1.1 remote-as 100
D. router bgp 100
neighbor 10.1.1.1 remote-as 200
5. Which of the following are valid BGP show commands?
A. show ip bgp
B. show ip bgp paths
C. show ip bgp summary
D. show bgp all
E. show bgp ip debug
6. In the command neighbor 172.16.2.2 remote-as 500, what is the
function of the remote-as syntax?
A. The syntax identifies the AS of the remote router that the local
router will initiate a session with.
B. It identifies the local remote ASN.
C. It tells the local router to find the remote autonomous system.
D. It allows BGP to bypass the TCP connection process because you
have identified the ASN.
7. Which of the following commands creates a route in the BGP table
and identifies the networks that are part of the internal AS?
A. aggregate-paths
B. show ip bgp
C. neighbors summary-only
D. network
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
313
8. Which of the following identifies the function of the clear ip bgp *
command?
A. It clears an identified entry in the BGP routing.
B. It clears all entries in the BGP table.
C. It clears all entries in the IGP’s topology table.
D. It resets IP sessions.
9. Which of the following BGP attributes informs neighboring AS rout-
ers as to which link to use to receive traffic?
A. Atomic Aggregate
B. Next-hop
C. MED
D. Weight
10. In the following list of attributes, which of the following is a Cisco
proprietary attribute?
A. Weight attribute
B. Next-hop attribute
C. MED attribute
D. Atomic Aggregate attribute
11. Which of the following commands displays the status of all BGP
connections?
A. show ip bgp
B. show ip bgp status
C. show ip bgp all
D. show ip bgp summary
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
314
Chapter 8
Configuring Basic BGP
12. Using the clear ip bgp command, which syntax is used to identify
that the command is to affect inbound or outbound triggered updates?
A. inbound
B. soft
C. triggered
D. outbound
13. When using debug commands with BGP, which of the following dis-
plays all the BGP events as they occur?
A. debug ip bgp dampening
B. debug ip bgp summary
C. debug ip bgp events
D. debug ip bgp all
14. The Weight attribute can be set to a value in which of the following
ranges?
A. 1 through 65,535
B. 1 through 32,768
C. 1 through 512
D. 0 through 65,535
15. Using the BGP aggregate-address command, which of the follow-
ing syntaxes can be used to advertise routes as coming from your AS?
A. as-local
B. as-set
C. as-summary
D. as-well-known
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
315
16. Which of the following commands would be used to troubleshoot a
problem with an external AS not receiving updates from your AS?
(Choose all that apply.)
A. show ip bgp events
B. show ip bgp neighbors
C. show ip bgp all
D. show ip bgp
17. To disable synchronization when all of the routers in an AS are run-
ning BGP, which of the commands would be used?
A. disable synchronization
B. no ip bgp synchronization
C. synchronization disable
D. no synchronization
18. Which of the following attributes can be used to help identify the cor-
rect networks for CIDR implementations?
A. Atomic Aggregate
B. Next-hop
C. MED
D. Weight
E. Local Preference
19. When using the neighbor command with the weight syntax, in
which range of values can the weight syntax be set?
A. No limit
B. 1 to 255
C. 0 to 65,535
D. 1 to 1000
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
316
Chapter 8
Configuring Basic BGP
20. Which of the following commands will begin a BGP process on a
router and place you in BGP Configuration mode?
A. router enable bgp
B. router ip bgp 45323
C. router bgp 32455
D. router enable bgp 34657
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Written Lab
Answers to Written Lab
1. neighbor 172.16.1.1, remote-as 55009
2. clear ip bgp *
3. debug ip bgp events
4. aggregate-address ip-address mask [summary-only] [as-
set]
5. show ip bgp paths
6. show ip bgp neighbors
7. (config-router)#network 63.78.0.0 mask 255.255.255.0
8. debug ip bgp updates
9. show ip bgp community
10. show ip bgp summary
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
317
318
Chapter 8
Configuring Basic BGP
Answers to Review Questions
1. E. The Router ID, BGP process number, external neighbors, and
internal peers are all needed to configure BGP.
2. A. The show ip bgp routes command is used to view all the
learned BGP routes.
3. D. The only valid command statements shown above for configuring
BGP are router bgp 100 and neighbor 10.1.1.1 remote-as 200.
4. D. The only commands that are shown above that are valid to enable
BGP and identify an external BGP neighbor are router bgp 100 and
neighbor 10.1.1.1 remote-as 200.
5. A, B, C. The valid BGP show commands listed above
are show ip bgp, show ip bgp paths, and show ip bgp summary.
The show ip bgp command displays the BGP routing table. The show
ip bgp paths command displays all the router’s known BGP paths.
The show ip bgp summary command tells you the status on every
BGP connection. The other two commands are not valid.
6. A. The remote-as syntax identifies the peer router that the local
router will enable a session with. The IP address identifies the interface
attached to the peer router. If the ASN is the same number as the internal ASN, it identifies an internal AS; if it is different, it identifies an
external AS.
7. D. The network command creates a route in the BGP table if the route
is present in the IP table.
8. B. The clear ip bgp * command is used to clear all the entries in
the BGP table.
9. C. The MED attribute is used to inform other external AS routers as
to which route to use in order to receive traffic.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
319
10. A. The Weight attribute is a Cisco proprietary BGP attribute used as
a metric to find the best routes through the networks.
11. B. The show ip bgp status command displays the status of all BGP
connections. The show ip bgp summary command displays the BGP
configuration. The other two commands are not valid.
12. B. Using the soft syntax followed by the in and out syntaxes, you
can identify whether the clear ip bgp command is to affect inbound
or outbound triggered updates.
13. C. The debug ip bgp events command is used to display all BGP
events as they occur. The only other valid command listed is debug ip
bgp dampening.
14. D. The Weight attribute can be set between 0 and 65,535 using the
neighbors command. The default value is 32,768.
15. B. If you use the as-set syntax, the BGP router will advertise the
route as coming from your AS. This syntax will also set the Atomic
Aggregate attribute to show that information regarding the route may
be missing.
16. B, D. The show ip bgp neighbors and the show ip bgp commands
are both troubleshooting commands that would be used to solve this
problem. The show ip bgp neighbors command displays all the
advertised routes, and the show ip bgp command looks at all the connections.
17. D. The no synchronization command disables synchronization,
allowing much faster convergence.
18. A. The Atomic Aggregate attribute is used to help identify the correct
networks for networks that implement CIDR.
19. C. The weight value can be set to a range from 0 to 65,535. The
default value is 32,768.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
320
Chapter 8
Configuring Basic BGP
20. C. The router bgp 32455 command is the only valid command
listed to place the router in BGP Configuration mode, which is identified by the (config-router) prompt.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
9
BGP Scalability and
Advanced Features
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
Understanding BGP management problems
Configuring BGP route reflectors
Configuring BGP confederations
Configuring AS_PATH attribute filters
Understanding BGP policies
Understanding prefix lists and distribute lists
Understanding BGP communities and peer groups
Configuring prefix lists
Configuring distribute lists
Configuring BGP communities
Configuring BGP peer groups
Understanding multi-homing classifications
Configuring default routes for more than one ISP
Understanding and configuring route maps
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
L
arge BGP networks have several methods for managing large
BGP networks, such as using filters, configuring private AS numbers (ASNs),
creating peer groups, creating confederations, and using route reflectors. All
of these methods are quite complex. In this chapter, we will learn more about
all of these problems.
When a router reaches more than 100 BGP sessions running concurrently,
most network administrators fluent in BGP recommend that you configure
route reflectors. When route reflectors are used, a router needs to become a
peer only with a route reflector instead of with each individual router. The
route reflector’s responsibility is to maintain a routing table for all internal
peers connected to the reflector. The route reflector can collect the same
number of routes that a router can learn from a full mesh.
You can use confederations to control the size of interior BGP meshes and
allow an AS to be broken up. Several sub-AS numbers can be configured
inside of a real AS. Sub-ASes must use the private reserved ASNs. All of the
routers within a sub-AS must be fully meshed, and one of the routers in a
sub-AS creates a peer with one of the routers from the other sub-AS. This
enables the overhead on each router to be reduced and a complete BGP table
to be shared to the entire AS.
Before we look at the more advanced features just described, let’s first
take a look at different methods of applying policies to the routes advertised
by BGP, such as using distribute lists, route maps, and prefix lists.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Filters
323
Filters
When configuring BGP, the AS path length is considered in selecting
a route. With the use of route maps, the AS path may be lengthened by adding false AS numbers. This is called AS path pre-pending. It is another way
to influence route selection.
In addition to manipulating route selection, BGP has features that allow
network advertisements to be aggregated before they are advertised to neighboring autonomous systems. There are many reasons to influence the routes
that are advertised. You will mainly want to control the route-selection process to stop unnecessary advertisements in order to eliminate router confusion and the high CPU utilization that can occur when routes flap. A route
flap is defined as a change in the state of the route. Once a route is established
and then removed from the BGP table, one flap has occurred. You can prevent routing problems by using the bgp dampening command. The bgp
dampening command maintains a threshold for route flaps. This means that
when the threshold is exceeded, the route is put into a hold-down. Holddowns implement a timing mechanism, and during the hold-down time, BGP
uses internal processes to monitor the route’s status to see if the route comes
back up. If the route stops flapping for a given period of time, the route is
allowed back into the BGP table and can be advertised.
One of the most important items to define is the type of AS you are administering. When multiple autonomous systems interconnect, one or all of the
ASes can become a transit AS, which we discussed in Chapter 7, “BGP’s
Basic Components.” Depending on your network policy, this can be a good
thing or a bad thing.
One of the biggest problems occurs when you connect to another ISP and
the ISP uses your circuits, equipment, and bandwidth to connect to a neighboring AS instead of using their own resources. You can eliminate this situation by using AS path filters. Using regular expressions, you can compare
AS path information and then either permit or deny it. Let’s look at a sample
configuration detailing how to implement AS filters:
router bgp 200
no synchronization
bgp dampening
neighbor 172.16.65.10 remote-as 100
neighbor 172.16.65.10 filter-list 10 in
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
324
Chapter 9
BGP Scalability and Advanced Features
neighbor
neighbor
neighbor
neighbor
172.16.65.10
172.16.65.11
172.16.65.11
172.16.65.11
filter-list 1 out
remote-as 300
filter-list 11 in
filter-list 1 out
!
!
ip as-path access-list 1 permit ^200$
ip as-path access-list 10 permit ^100$
ip as-path access-list 11 permit ^300$
!
!
To implement filters, use the neighbor command. Using the AS path syntax, you can configure filters to block routes that contain the AS path information that does not match the regular expression. The output above shows
access list 1 allowing only routes that originate from AS 200 to be sent to the
respective neighbors. Access lists 10 and 11 above allow only routes that do
not originate within AS 100 and AS 300 to be sent.
Creating BGP Policies
W
e use policies with BGP to tell other BGP neighbors the paths
through our own network. By not advertising certain routes through our network, we keep other networks from learning about them; it is difficult to
route a packet through a network you don’t know about. We can modify
routes that we wish to advertise using both prefix lists and distribute lists.
Distribute lists use access lists to control the routes advertised by a routing
protocol. A prefix list is similar to an access list but is more flexible and less
complicated to configure than an access list.
Distribute Lists
Distribute lists are standard or extended access lists applied to a router’s BGP
session to permit or deny advertised routes through the network. Distribute
lists can be applied to filter BGP advertisements either coming in or going out
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Creating BGP Policies
325
of the router. Let’s look at an example of an access list that allows routes
from network 172.16.0.0.
RouterA(config)#access-list 105 permit ip 172.16.0.0
0.0.255.255 host 255.255.0.0
There is always an implicit deny all at the end of the access list that can’t
be seen. We’re permitting only network 172.16.0.0 in this access list. However, although the access list has been created, we need to filter all of the BGP
traffic coming in. Let’s take a look at how to do this:
RouterA(config)#router bgp 31400
RouterA(config-router)#neighbor 172.16.11.254 remote-as
31400
RouterA(config-router)#neighbor 172.16.12.254 remote-as
31400
RouterA(config-router)#neighbor 172.16.11.254 distributelist 105 in
Prefix Lists
Prefix lists are actually new and have been added to version 12.0 and later
of the Cisco IOS. You can use a prefix list as an alternative to the access lists
used in many of the BGP route-filtering commands. There are many advantages to using prefix lists. Prefix lists don’t tax the processor as much as
access lists, which can improve the router’s performance.
With a prefix list, you need to make configuration modifications to each
router, but you can do this incrementally just as you can with route reflectors. This means that you can implement prefix lists on just a few routers in
your network at a time instead of all at once.
The biggest advantage of prefix lists over distribute lists is that prefix lists
have much greater flexibility and are considerably easier to configure. If you
make a mistake with an access list, you must start over because access lists
are read in the order you type them in, making them hard to modify. Prefix
lists allow you to add and delete lines without starting over.
Prefix lists use the same line-by-line read rule as access lists, which says
that as soon as I have a match in my list to the data I receive, I start processing. You need to also remember that, just as in access lists, the same implicit
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
326
Chapter 9
BGP Scalability and Advanced Features
deny all still exists at the bottom of the list for the data that does not have
a match in our prefix list. However, if there are no lines in our prefix list,
instead of an implicit deny all, there is an implicit permit any.
The rule to remember when using prefix lists is that if a prefix is permitted, the
route is advertised; if a prefix is denied, the route is not advertised.
One improvement from access lists is the use of sequence numbers for
each statement in the prefix list. The statement with the smallest sequence
numbers is read first. This also allows us to modify a sequence statement
without starting over when there is a change in the network that must be
applied to our prefix list.
Configuring Prefix Lists
We create a prefix list using the prefix-list command followed by a list
name, which we will call list1. We can then optionally identify the
sequence value using the seq syntax followed by the sequence number we
wish to use. The sequence number can be any number. The lowest number
gets read first. This means that if our first sequence number is 15 and our second is 18, then we can add 16 and 17 later if we need to modify the prefix
list with a new statement.
If we now create this prefix list, our prefix list is called ip prefix-list
list1 seq 15; if no sequence number is identified, the number is automatically assigned in increments of 5, meaning that the first would be 10 and
then 15 and so on. We now need to permit a network using the permit syntax. If we do not have at least one permit statement, then we effectively deny
all the routes. It is best to start with permit statements and then move on to
selective deny statements.
If you wish to stop the incremental sequence numbers, you can use the no ip
prefix-list sequence-number command. To re-enable the sequence numbering, use the ip prefix-list sequence-number command.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Creating BGP Policies
327
We now must identify the network in which we wish to permit. In this
case, I would like to advertise the 172.16.0.0 network. To do this, I must also
identify the 32-bit subnet mask either as the number of bits or a decimal
value. So the statement would read
prefix-list list1 seq 15 172.16.0.0/24 permit
Let’s now walk through the whole process step by step, looking at all the
options available:
Cisco3640(config)#ip prefix-list ?
WORD
Name of a prefix list
WORD
Name of a prefix list
Cisco3640(config)#ip prefix-list list1 ?
deny
Specify packets to reject
description Prefix-list specific description
permit
Specify packets to forward
seq
sequence number of an entry
Cisco3640(config)#ip prefix-list list1 seq ?
<1-4294967294> Sequence number
Cisco3640(config)#ip prefix-list list1 seq 15 ?
deny
Specify packets to reject
permit Specify packets to forward
Cisco3640(config)#ip prefix-list list1 seq 15 permit ?
A.B.C.D IP prefix <network>/<length>, e.g., 35.0.0.0/8
Cisco3640(config)#ip prefix-list list1 seq 15 permit
172.16.0.0/24 ?
ge Minimum prefix length to be matched
le Maximum prefix length to be matched
<cr>
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
328
Chapter 9
BGP Scalability and Advanced Features
The ge-value syntax is used to specify the range of the prefix length that is to
be matched for prefixes that are more than the subnet mask identified in the
network/len syntax. If the range runs from the /len value to 32, then only the
ge syntax needs to be specified. The le-value syntax is used to specify the
range of the prefix length to be matched, for prefixes that are of higher value
specified in the specific network/len syntax. The le syntax identifies the values from the len to le-value specified, indicating a range of networks. Both
ge and le are optional syntaxes and are used only when you need to specify
a range of the prefix that is more specific than that identified in the network/
len syntax. Just remember this rule: len < ge-value < le-value <= 32.
An exact match is assumed when neither ge nor le is specified, as shown
below:
Cisco3640(config)#ip prefix-list list1 seq 15 permit
172.16.0.0/24
Cisco3640(config)#
The available syntaxes for the ip prefix-list command are shown
below:
ip prefix-list list-name [seq seq-value] {deny | permit}
network/len [ge ge-value] [le le-value]
Now that we have created the prefix list, we need to apply it to BGP using
the neighbor command. Let’s look at the syntaxes and then apply the small
prefix list we created above:
neighbor {ip-address | peer-group-name} prefix-list
prefix-listname {in | out}
Now let’s apply the access list created above:
RouterA(config)#router bgp 31400
RouterA(config-router)#neighbor 172.16.11.254 remote-as
31400
RouterA(config-router)#neighbor 172.16.12.254 remote-as
31400
RouterA(config-router)#neighbor 172.16.12.254 prefix-list
list1 in
RouterA(config-router)#exit
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Creating BGP Policies
329
The no ip prefix-list command followed by the list name is used to delete
a prefix list.
Monitoring Prefix Lists
To view a prefix list, use the show ip prefix-list command. Let’s take a
look at the available syntaxes and the output:
RouterA#show ip prefix-list ?
WORD
Name of a prefix list
detail
Detail of prefix lists
summary Summary of prefix lists
<cr>
RouterA#show ip prefix-list
ip prefix-list list1: 1 entries
seq 15 permit 172.16.0.0/24
RouterA#show ip prefix-list list1
ip prefix-list list1: 1 entries
seq 15 permit 172.16.0.0/24
RouterA#
You can use the clear ip prefix-list command to clear the hit count
of the prefix list entries. You can also select the prefix list name to clear, as
shown below:
RouterA#clear ip prefix ?
WORD Name of a prefix list
<cr>
RouterA#clear ip prefix-list ?
WORD Name of a prefix list
<cr>
RouterA#clear ip prefix-list list1
RouterA#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
330
Chapter 9
BGP Scalability and Advanced Features
Route Maps
Route maps are used with BGP to control as well as modify routing table
information and to define when routes are redistributed between ASes.
Route maps can be defined as very complex access lists that allow some conditions to be applied to identified routes. If the conditions find a match, then
an action you identify as the administrator using the set command takes
place.
Route maps are used in communities, which we will discuss later in this chapter.
Unlike standard and extended access lists for filtering incoming and outgoing data on interfaces, the statements in route maps are sequentially numbered, allowing statements to be edited, inserted, and deleted. A collection of
route-map statements using an identical route-map name is considered a
single route map. One way that route maps are similar to access lists is that
you must specify the source and destination address as well as the subnet mask.
To configure route maps, you begin in the Global Configuration mode.
You then issue the route-map command followed by the name of the route
map. You must then identify a condition you would like to set for the routing
information. You have two choices: either deny or permit the routing information. You can then optionally identify a sequence number. You then press
the Enter key, which will take you into a new command-line interface
mode called Route Map Configuration mode, which is indicated by the
Router(config-route-map)# prompt. This is a new mode that you probably have never seen on a router. Let’s look at the command and the syntaxes, and then we’ll demonstrate how to use this command on a router:
route-map map-tag [permit|deny] [sequence-number]
Now we will create a route map using 10 as the first sequence number
(which is the default first sequence number):
RouterA(config)#route-map routemap1 permit 10
RouterA(config-route-map)#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Creating BGP Policies
331
We must now match conditions. Let’s assume that previously we set an
access list for the AS_PATH attribute numbered 6, permitting only the IP
network 172, as shown below:
RouterA(config-router)# ip as-path access-list 6 permit
172
We then need to add a match statement to allow us to use this in our route
map, as shown below:
RouterA(config)#route-map routemap1 permit 10
RouterA(config-route-map)#match as-path 6
We now can use a set statement to add a local preference of 50 to all the
matching routes:
RouterA(config)#route-map routemap1 permit 10
RouterA(config-route-map)#match as-path 6
RouterA(config-route-map)#set local-preference 50
Upon creating this list, we have effectively denied all the other routing
updates including all the non-route updates. In order to keep those nonupdate packets going through our router, we need to create a permit route
map, which we will number 25, as shown below:
RouterA(config)#route-map routemap1 permit 25
Statements in a route list are processed from the top down just like a standard
or extended access list. If a match is found for a route, the set conditions are
applied and the match is no longer looked for. The sequence number is used
only for inserting or deleting specific route-map statements.
Just like in ACLs, there is an implicit deny any at the end of a route map.
If all the statements in the route map are checked and there are no matches,
then there is an automatic denial of the route. The following are the match
and set commands that can be used for route maps:
match as-path
match community
match clns
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
332
Chapter 9
BGP Scalability and Advanced Features
match interface
match ip address
match ip next-hop
match ip route-source
match length
match metric
match route-type
match tag
set as-path
set clns
set automatic-tag
set community
set interface
set default interface
set ip default next-hop
set ip next-hop
set ip precedence
set level
set local-preference
set metric
set metric-type
set next-hop
set origin
set tag
set weight
Route Reflectors
B
efore you can truly understand the beauty of route reflectors, you
need to understand some things about BGP split-horizon rules and the reason for a full mesh of routers. A full mesh means that every router has a
direct connection to all the other routers in the network. This is easy to maintain in a network where there are only three routers, but what happens when
you have 20 or 1000 routers? There is an easy method of calculating how
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Reflectors
333
many circuits or connections you will need in a full-mesh network by using
the formula n(n-1)/2. This means that for 20 routers, there are 190 circuits
or connections between the routers. Let’s look at Figure 9.1 to see a fullmesh network.
FIGURE 9.1
A small full-mesh network
RouterA
RouterB
RouterE
RouterC
Full mesh
RouterD
In a normal network, split-horizon rules mean that if we have two routers,
one named RouterA and another named RouterB, when RouterA sends
RouterB an update, RouterB will never send that information back to
RouterA. BGP split-horizon rules mean that if an iBGP peer in AS 100 sends
an update to a peer in AS 200, it will never send another router in AS 200 the
same update. This is the reason for the full mesh in the internal network—
so that all the routers in the network can share information they have learned
with one another.
Having 190 connections or peers in a network can be a problem, however—not just the severe cost, but the overhead on the routers that are sending updates to one another. You can configure one router as a concentration
router to handle all of the BGP updates. Making a router a route reflector
places the main concentration of your configuration on only one router and
eliminates the need for a full mesh.
The route reflector is allowed to propagate iBGP routes to other iBGP
peers. Route reflectors can be very beneficial when ISPs use a considerable
number of internal neighbor statements. The concentration router needs to
be the only router configured with neighbor statements and becomes the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
334
Chapter 9
BGP Scalability and Advanced Features
concentration router for the entire network. All the other BGP routers in the
AS need only peer with the concentration router; they become known as clients.
Route reflectors reduce the number of BGP neighbor peering relationships in
an AS by maintaining a single central source for updates to their route reflector clients.
Some of the main things to remember when using route reflectors are
listed below:
Route reflectors are used when the internal neighbor statements
become excessive.
Route reflectors do not affect the paths that IP packets take through
the network; they only identify how the routing information is distributed through the network.
Route reflectors relieve iBGP of a full-mesh requirement.
An IGP is still used in order to carry local routes and next-hop
addresses.
Route reflectors receive updates from their configured peers whether
they are clients or non-clients.
Non-client refers to any iBGP peer that is not participating in the route reflector cluster as a client.
Non-client updates are sent to route reflector clients in the cluster only.
Updates from eBGP peers are sent to all clients and non-clients.
Updates that the route reflector receives from a route reflector client
are sent to all non-client peers and all route reflector clients, with the
exception of the client listed in the ORIGINATOR_ID attribute field.
You can configure multiple route reflectors for redundancy purposes.
Other iBGP and eBGP peers can co-exist.
Route reflectors modify the BGP split-horizon rule by allowing the
router configured as the route reflector (the concentration router) to
be the only router that propagates routes learned by iBGP to other
iBGP peers.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Reflectors
335
The router being used as the concentration router needs to have its normal
BGP configuration, neighbor statements, and routers configured as clients.
On the concentration router (the route reflector), we need to identify the clients using the neighbor <peer clients IP address> routereflector-client command. You need not migrate all the routers that
you need to migrate to using route reflectors at once since non-routereflector BGP routers can co-exist with route reflectors within an AS.
The concentration router needs to have a peer connection with the other
BGP routers that are non-clients of route reflectors and the other route
reflectors in the AS. The route-reflector clients need only have a neighbor
statement to peer with the route reflector. Let’s look at some of the terms you
need to know when configuring route reflectors:
Route reflector A router configured to be able to advertise the routes it
learns from iBGP peers.
Client A router that is not configured as a route reflector but will share
information with the routers configured as route reflectors. Two route
reflectors can be configured as clients.
Cluster The combination of the routers configured as route reflectors
and the clients.
Cluster ID Used when a cluster has more than one route reflector, the
cluster ID allows route reflectors to recognize updates from other route
reflectors in the same cluster. A cluster that has a single route reflector is
identified by the Router ID of the route reflector.
The route reflector creates the ORIGINATOR_ID BGP attribute. This attribute is
used to carry the Router ID of the router that originated route information in
the local AS. This allows the originator to know if it receives back any information it sent out. For more information on the ORIGINATOR_ID attribute, see
Chapter 7, “BGP’s Basic Components.”
Configuring a Route Reflector
When you configure route reflectors, you must configure them one at a time
and then configure the clients. Make sure that the routes propagate correctly. Always remember that when you are trying to control BGP routes,
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
336
Chapter 9
BGP Scalability and Advanced Features
you are controlling all the routes in the organization. You can affect routes
and information not only in your organization but in others’ as well. If you
have a hub and spoke network, you should first configure the hub as the
route reflector and then configure the routers in the spokes as clients.
You should be especially careful that you know BGP thoroughly if your system is a transit AS. A transit AS means that another AS or organization uses
your AS to connect to another AS. Transit ASes are discussed in Chapter 7.
Let’s look at an example of configuring RouterA as the route reflector for
the other two routers in autonomous system 100, shown in Figure 9.2.
FIGURE 9.2
A small route reflector configuration
eBGP
172.16.12.1
s0
Concentration Router
s1
iBGP
iBGP
AS
100
172.16.12.254
s0
eBGP
RouterA
172.16.11.1
172.16.11.254
s0
RouterC
RouterB
eBGP
RouterA: 172.16.10.254
RouterB: 172.16.11.254
RouterC: 172.16.12.254
RouterA(config)#router bgp 100
RouterA(config-router)#neighbor
100
RouterA(config-router)#neighbor
reflector-client
RouterA(config-router)#neighbor
100
RouterA(config-router)#neighbor
reflector-client
Copyright ©2001 SYBEX , Inc., Alameda, CA
172.16.11.254 remote-as
172.16.11.254 route172.16.12.254 remote-as
172.16.12.254 route-
www.sybex.com
Route Reflectors
337
If you have more than one route reflector, do not forget to peer them with one
another.
Both RouterB and RouterC merely need to become peers with the concentration router and do not require neighbor statements identifying each
other. To verify the configuration, we would use the show ip bgp neighbor
command. Let’s look at an example of the output:
RouterA# show ip bgp neighbor
BGP neighbor is 172.16.11.254, remote AS 100, internal
link
Index 1, Offset 0, Mask 0x2
Route-Reflector Client
BGP version 4, remote router ID 10.16.1.1
BGP state = Established, table version = 1, up for
12:10:16
Last read 00:00:06, hold time is 180,
keepalive interval is 60 seconds
Minimum time between advertisement runs is 5 seconds
Received 143 messages, 0 notifications, 0 in queue
Sent 52 messages, 0 notifications, 0 in queue
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 2; dropped 1
Last reset 12:10:16, due to User reset
53 accepted prefixes consume 32 bytes
0 history paths consume 0 bytes
--More—
BGP neighbor is 172.16.12.254, remote AS 100, internal
link
Index 1, Offset 0, Mask 0x2
Route-Reflector Client
BGP version 4, remote router ID 10.16.1.1
BGP state = Established, table version = 1, up for
12:10:16
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
338
Chapter 9
BGP Scalability and Advanced Features
Last read 00:00:05, hold time is 180,
keepalive interval is 60 seconds
Minimum time between advertisement runs is 5 seconds
Received 14 messages, 0 notifications, 0 in queue
Sent 12 messages, 0 notifications, 0 in queue
Prefix advertised 0, suppressed 0, withdrawn 0
Connections established 2; dropped 1
Last reset 12:10:16, due to User reset
53 accepted prefixes consume 32 bytes
0 history paths consume 0 bytes
Confederations
A
confederation is another extension of using route reflectors. The
difference is that instead of looking from the iBGP standpoint, BGP now
looks at the entire autonomous system. Using confederations allows you to
divide an AS into sub-ASes running eBGP between them. A confederation is
a conglomerate of all the sub-ASes being advertised to the outside world as
one giant AS. To the outside world, a confederation is invisible.
Confederations are a little more difficult to configure than route reflectors, unfortunately. The reason I say this is that when using confederations,
you must perform a reconfiguration on each of the routers in the AS and also
let non-optimal routes seep in to the BGP table, creating routing problems in
your network. The way around learning the non-optimal routes is to reconfigure your BGP policies on each individual router participating in the
confederation.
When you use confederations, you use the router bgp command to configure all of the sub-AS routers with their own BGP ASN. You then configure
the confederation’s ASN on each of the routers using the bgp confederation
identifier command. Next, you configure each of the peer sub-ASNs using
the bgp confederation peers command.
To get a better idea of how to configure confederations, let’s examine Figure 9.3. You see that all of the routers are part of confederation AS 31,400,
but it been broken down into three sub-ASes.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Confederations
FIGURE 9.3
339
A small confederation
Confederation
AS 31,400
RouterA
RouterB
Router
RouterA
RouterB
RouterC
RouterC
AS
1
1
1
IP Address
172.16.10.254
172.16.11.254
172.16.12.254
To make this easy to understand, we’ll do the configuration on all three
routers displayed above. First, let’s take a look at the configuration of
RouterA in our confederation.
RouterA(config)#router bgp 1
RouterA(config-router)#bgp confederation identifier 31400
RouterA(config-router)#bgp confederation peers 2 3
RouterA(config-router)#neighbor 172.16.11.254 remote-as 2
RouterA(config-router)#neighbor 172.16.12.254 remote-as 3
Now let’s look at RouterB’s configuration.
RouterB(config)#router bgp 2
RouterB(config-router)#bgp confederation identifier 31400
RouterB(config-router)#bgp confederation peers 1 3
RouterB(config-router)#neighbor 172.16.10.254 remote-as 1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
340
Chapter 9
BGP Scalability and Advanced Features
Lastly, let’s look at RouterC’s configuration, which is very similar to that
of RouterB.
RouterC(config)#router bgp 3
RouterC(config-router)#bgp confederation identifier 31400
RouterC(config-router)#bgp confederation peers 1 2
RouterC(config-router)#neighbor 172.16.10.254 remote-as 1
Peer Groups and Communities
Peer groups and communities are used to eliminate some of the overhead
associated with BGP. BGP, as you can see, is a very complex protocol with
many configuration options. Communities are a way of tagging routes to
make sure that a consistent filtering or route-selection policy exists when
using route maps.
All the BGP routers can tag routes coming into or going out of their interfaces when doing routing updates. The COMMUNITIES attribute (Type
Code 8) is used to carry the communities information in the BGP update
packets. BGP routers can then filter routes in incoming or outgoing updates
or use preferred routes based on the COMMUNITIES attribute. By default,
the communities information is stripped from any outgoing BGP update.
Without any communities configured, each individual BGP neighbor would
require either a statement in an access list for a distribute list or a statement
in a prefix list.
Some implementations do not understand the concept of communities.
When such is the case, the router will still send the information onto the next
router. When the implementation does understand the concept of communities, then the router must be configured to propagate the COMMUNITIES
attribute—otherwise, the communities information will be dropped.
The COMMUNITIES attribute can contain a value in the range of 0 to
4,294,967,200. Remember that a network can be a member of multiple
communities and that you can use route maps to set the community
attributes. The COMMUNITIES attribute can be 32 bits long, with the
upper 16 bits indicating the AS number that was defined in the community.
The lower 16 bits have only local significance and are the community number. Enter the value as a single decimal number in the format AS:nn, where
AS is the AS number and nn is the lower 16-bit local community number. The
total community value is displayed as one long decimal number by default.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Confederations
341
There are a few well-known communities, shown here:
Internet All routers by default belong to this community and can be used
to advertise routes to all other routers.
No-export Indicates that the route will not be passed outside the AS
using eBGP.
No-advertise Keeps the route secret from every other router.
Local-as Used in confederations and was introduced in version 12.0 of
the Cisco IOS. We do not cover this topic in this book. You can visit
Cisco’s Web site for more information on this community.
The community name is set in the Route Map Configuration mode after
the route map is created. Let’s look at the syntaxes shown below:
set community { community-number [additive]}|none
Here’s an example of using the command:
RouterA(config-route-map)#route-map COM1 permit 10
RouterA(config-route-map)#match ip address 1
RouterA(config-route-map)#set community 1 additive
The additive syntax is used to add the router to an existing community.
You must then instruct BGP to perform community propagations. To do so,
you need to use the send community syntax with the neighbor command.
Let’s look at the neighbor command and the syntaxes used with it:
neighbor { ip-address|peer-group-name} send-community
This command tells BGP that the BGP COMMUNITIES attribute should
be sent to a BGP neighbor. Let’s look at an example of using the command:
RouterA(config)#router bgp 31400
Router(config-router)#network 172.16.0.0
Router(config-router)#neighbor 10.1.1.254 remote-as 500
Router(config-router)#neighbor 10.1.1.254 send-community
Router(config-router)#neighbor 10.1.1.254 route-map
Routemap1 out
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
342
Chapter 9
BGP Scalability and Advanced Features
Peer Groups
When you maintain a large BGP network, there tend to be many small configuration changes that need to be made to a number of BGP routers. To
avoid making an individual change to each and every router, peer groups
were created. This allows you to place those routers that share common policies into a group. You then make policy changes to the peer group instead
of to each individual router. Policies in your peer group can be overridden,
but only for incoming updates. The outgoing policies must always be identical for all of the members in your peer group. Peer group policies include
outbound route maps, distribute lists, filter lists, and prefix lists.
All members of the peer group are internal members of an AS and always
share the same ASN. You can assign a peer group name, but the name is local
only to the router it is configured on; it is not passed to any other router.
To configure a peer group, use the neighbor command followed by the
peer group name and then the peer-group syntax, as shown below:
neighbor peer-group-name peer-group
Let’s look at an example of configuring a peer group using the command
and assigning the peer group name of group1 to AS 31,400:
RouterA(config)#router bgp 31400
RouterA(config-router)#neighbor group1 peer-group
We now have to identify the neighboring routers in our peer group using
the neighbor command followed by our BGP peer’s IP address, the peergroup syntax, and the peer group’s name. Let’s take a look at the command
and the syntaxes:
neighbor ip-address peer-group peer-group-name
Let’s now use the command to add the two neighbors to RouterA that we
have used in most of the demonstrations in this chapter, those being RouterB
using IP address 172.16.11.254 and RouterC using IP address
172.16.12.254.
RouterA(config)#router bgp 31400
RouterA(config-router)#neighbor group1 peer-group
RouterA(config-router)#neighbor 172.16.11.254 peer-group
group1
RouterA(config-router)#neighbor 172.16.12.254 peer-group
group1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Confederations
343
You can clear the BGP connection of a peer on any BGP router using the clear
ip bgp peer-group peer-group-name command in Privilege mode.
Multi-homing
Multi-homing is the process of connecting two or more service providers to
one network in an effort to provide redundancy to the outside world. Multihoming can be used with or without BGP. If you are not using BGP, you can
use default routes. Default routes must be manually configured on a router.
Remember, a manually configured route is a static route. BGP finds its own
routes, and the routes do not need to be manually configured. These routes
are called dynamic routes when the administrator does not need to manually
configure them. Static routes can be configured with BGP; regardless of
whether BGP knows a better route through the network, it will use the static
route. BGP does this by trusting a static route more than a route that it has
learned itself.
Static routes give only the interface of the destination for the next hop.
BGP learns the entire route from one point to another.
Using default static routes relieves the processor on the router from handling
BGP processes that tax the processor heavily. It also frees RAM in the router
for other uses.
In the example below, BGP is not configured. We use the ip route command followed by 0.0.0.0, which means any destination IP address; the
second 0.0.0.0 indicates any mask the router doesn’t know about from the
IGP. In this case, we use OSPF as the IGP. We then use the defaultinformation orginate always command to instruct all the other OSPF
routers to learn this default route. Figure 9.4 shows our network using a
single static route.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
344
Chapter 9
BGP Scalability and Advanced Features
FIGURE 9.4
A single static (default) route to an ISP
AS 10
RouterA
S0
S0
ISP Router
RouterA(config)#ip route 0.0.0.0 0.0.0.0 serial 0
RouterA(config)#router ospf 10
RouterA(config-router)#network 172.16.0.0 0.0.255.255 area 1
RouterA(config-router)#default-information ?
originate Distribute a default route
RouterA(config-router)#default-information originate ?
always
Always advertise default route
metric
OSPF default metric
metric-type OSPF metric type for default routes
route-map
Route-map reference
<cr>
RouterA(config-router)#default-information originate
always
RouterA(config-router)#
You can configure static routes for multiple ISPs as well. In Figure 9.5, we
see that there are two ISP routers from two different ISPs. The primary link
uses a 1.544Mbps link; the other uses a 512Kbps link as a backup. We can
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Confederations
345
tell the router to use one link over another by assigning an administrative distance. In this case, we assign 200 to one link and 201 to the other in the following code. The link used will be the link with the lowest administrative
distance.
FIGURE 9.5
Configuring static routes to multiple ISPs
AS 10
RouterA
S0
RouterA
Port
Link
Bandwidth
S0
S1
1.544Mbps
512Kbps
S1
ISP #1
Router
ISP #2
Router
RouterA(config)#ip route 0.0.0.0 0.0.0.0 serial 0 ?
<1-255>
Distance metric for this route
A.B.C.D
Forwarding router's address
permanent permanent route
tag
Set tag for this route
<cr>
RouterA(config)#ip route 0.0.0.0 0.0.0.0 s0 200
RouterA(config)#ip route 0.0.0.0 0.0.0.0 s1 201
RouterA(config)#router ospf 10
RouterA(config-router)#network 172.16.0.0 0.0.255.255
area 0
RouterA(config-router)#default-information originate
always
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
346
Chapter 9
BGP Scalability and Advanced Features
When multi-homing two networks, three classifications are possible:
Basic
Medium
Full
Basic multi-homing is the simplest and requires only that the AS attach to
the ISP. The ISP will offer only default routes to the Internet. The internal AS
(your AS) decides the ISP connection to use, similar to the one portrayed in
Figure 9.5. This method uses the least amount of CPU time and the least
amount of RAM on the router.
Medium multi-homing uses default routes and BGP. In this case, the internal AS gets more routing information and can select the best ISP to use based
on that information.
Full multi-homing uses only BGP, and all the routes are learned using the
AS-PATH attribute information to make routing decisions. This is the most
processor-intensive type and requires a lot of RAM.
Summary
T
his was one of the most complex chapters that we have ever written.
Not only was it the most complex, it was the longest to write and we used
more equipment and cabling than we have ever used writing a chapter. If you
implement BGP incorrectly, you can not only screw up advertising your own
internal network, but you can also wreak havoc on other organizations’ networks as well, and this includes your own ISP’s network. If you do not know
how to configure or maintain BGP in a network, you should not implement it.
For the exam, you need a firm understanding of route maps, prefix lists,
communities, peer groups, multi-homing, static routes, access lists, distribute lists, and confederations. We covered all of these in detail in this chapter.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Summary
347
Key Terms
Before taking the exam, make sure you’re familiar with the following terms:
AS path pre-pending
confederation
distribute list
full mesh
multi-homing
prefix list
route flap
route map
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
348
Chapter 9
BGP Scalability and Advanced Features
Written Lab
1. What command is used to instruct a route map what to do when it
finds a match?
2. In which command mode would you use the show ip bgp neighbors
command?
3. A grouping of BGP routers that share the same common policies is
called what?
4. What command would you use to assign peer group group6 to a
router?
5. What are the three classifications for multi-homing BGP networks?
6. What command will assign a static default route to Serial 0 on a
router?
7. What command would you use if you needed to assign an administra-
tive distance of 200 to a default route on serial 0?
8. What command would you use to clear a peer group named group3
from a router?
9. What command can be used to place a hold-down time on a link that
has flapped before it is re-advertised?
10. When using the set command, what mode should the router be in?
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
349
Hands-on Lab
In this lab, we will take what we learned about configuring basic BGP
and add to it. We will configure RouterA as a route reflector, and we need
to configure the router’s basic configuration as well. If you have forgotten
how to do this, refer to the Hands-on Lab section in Chapter 8, “Configuring Basic BGP.” Let’s look at the graphic below to see the three routers in our
network. This graphic seems somewhat complex but is very easy to configure step by step since it is only a small configuration with five routers. While
there is more than one way to configure the routers in the diagram below, I
will give a suggested configuration example based on the information from
Chapter 8 and this one.
S0 10.11.15.254
Local Network
e0
AS
14,140
S0 172.16.12.1
S1 172.16.11.1
e0 172.16.10.1
ISP Router #1
S1
S0
S0 10.1.1.254
RouterA
AS
7,501
Local Network
S1
S2
S0
S1
e0
S0
ISP Router #2
e0
S0 RouterC
AS
62,141
RouterB
Local Network
S1 172.16.12.254
e0 172.16.15.1
S0
S1
e0
S2
172.16.11.254
10.1.1.1
172.16.16.1
10.11.15.1
Let’s look at the lab objectives in the following list:
On each router, enable BGP with its autonomous system number
(ASN).
Identify the network each resides in.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
350
Chapter 9
BGP Scalability and Advanced Features
Identify both of the route reflector’s neighboring routers and the AS
they reside in.
Assign an IGP. (We will use OSPF with the ASN of 10 and assign all
the routers in the internal AS to Area 1.)
Configure multi-homing on RouterB using the links to the two-ISP
number router and configure ISP Router #1 as the primary link.
Assign an administrative distance of 100 to one link to the ISP and 150
to the other.
Configure RouterA as the route reflector, making RouterB and
RouterC clients.
Let’s now look at an example of the configuration of all three routers,
which are identified by the hostname on each router. We’ll start with
RouterA:
RouterA(config)#router bgp 7501
RouterA(config-router)#network 172.16.0.0 mask 255.255.0
RouterA(config-router)#network 10.0.0.0 mask 255.0.0
RouterA(config-router)#neighbor 172.16.11.254 remote-as
7501
RouterA(config-router)#neighbor 172.16.11.254 routereflector-client
RouterA(config-router)#neighbor 172.16.12.254 remote-as
7501
RouterA(config-router)#neighbor 172.16.11.254 routereflector-client
RouterA(config-router)#exit
RouterA(config)#router ospf 10
RouterA(config-router)#network 172.16.0.0 0.0.255.255 area 0
RouterB(config-router)#default-information originate
always
RouterA(config-router)#exit
RouterA(config)#interface ethernet 0
RouterA(config-if)#ip address 172.16.10.1 255.255.255.0
RouterA(config-if)#no shut
RouterA(config-if)#interface serial 0
RouterA(config-if)#ip address 172.16.12.1 255.255.255.0
RouterA(config-if)#no shut
RouterA(config-if)#exit
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
351
RouterA(config)#interface serial 1
RouterA(config-if)#ip address 172.16.11.1 255.255.255.0
RouterA(config-if)#no shut
RouterA(config-if)#
Now let’s look at the suggested configuration for RouterB:
RouterB#conf t
Enter configuration commands, one per line. End with
CNTL/Z.
RouterB(config)#router bgp 7501
RouterB(config-router)#network 172.16.0.0 mask
255.255.255.0
RouterB(config-router)#network 10.0.0.0 mask 255.0.0
RouterB(config-router)#neighbor 172.16.12.1 remote-as 7501
RouterB(config-router)#neighbor 10.11.15.254 remote-as
14140
RouterB(config-router)#neighbor 10.1.1.254 remote-as 62141
RouterB(config-router)#redist static
RouterB(config-router)#exit
RouterB(config)#router ospf 10
RouterB(config-router)#network 172.16.0.0 0.0.255.255 area 0
RouterB(config-router)#default-information originate
always
RouterB(config)#ip route 0.0.0.0 0.0.0.0 serial 1 100
RouterB(config)#ip route 0.0.0.0 0.0.0.0 serial 2 150
RouterB(config)# interface ethernet 0
RouterB(config-if)#ip address 172.16.16.1 255.255.255.0
RouterB(config-if)#no shut
RouterB(config-if)#exit
RouterB(config)#interface serial 0
RouterB(config-if)#ip address 172.16.11.254 255.255.255.0
RouterB(config-if)#no shut
RouterB(config-if)#exit
RouterB(config)#interface serial 1
RouterB(config-if)#ip address 10.1.1.1 255.0.0
RouterB(config-if)#no shut
RouterB(config-if)#exit
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
352
Chapter 9
BGP Scalability and Advanced Features
RouterB(config)#interface serial 2
RouterB(config-if)#ip address 10.11.15.1 255.0.0
RouterB(config-if)#no shut
RouterB(config-if)#
Connecting to two or more simulated ISPs might be a problem because it
requires the use of more than two serial ports on the router. I used a 3600
series. If you do not have a router available with more than two serial links,
simulate only one ISP. BGP can load balance over as many as six external
links to ISPs.
Here’s the suggested configuration for RouterC:
RouterC(config)#router bgp 7501
RouterC(config-router)#network 172.16.0.0 mask
255.255.255.0
RouterC(config-router)#neighbor 172.16.11.1 remote-as 7501
RouterC(config-router)#exit
RouterC(config)#router ospf 10
RouterC(config-router)#network 172.16.0.0 0.0.255.255 area 0
RouterC(config-router)#default-information originate
always
RouterC(config-router)#exit
RouterC(config)#interface ethernet 0
RouterC(config-if)#ip address 172.16.15.1 255.255.255.0
RouterC(config-if)#no shut
RouterC(config-if)#exit
RouterC(config)#interface serial 1
RouterC(config-if)#ip address 172.16.12.254 255.255.255.0
RouterC(config-if)#no shut
RouterC(config-if)#exit
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
353
Review Questions
1. Access lists are read from top to bottom. Therefore, which of the fol-
lowing should be near the top?
A. Least restrictive
B. Most restrictive
C. Permit or deny
D. Deny only
2. Which of the following should be used to reduce the number of routes
in a routing table?
A. EIGRP
B. DDR
C. Route carving
D. Route summarization
3. Which of the following are true of a prefix list? (Choose all that
apply.)
A. Statements can be added at any time.
B. Statements can be assigned a sequence number.
C. Statements use the set command to tell the prefix list what to do.
D. Any statements can be deleted at any time.
4. A router acting as a route reflector should have a peer connection with
which of the following routers?
A. Other route reflectors
B. The route reflector’s clients
C. Non-clients
D. All of the above
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
354
Chapter 9
BGP Scalability and Advanced Features
5. If multiple ISPs are connected to your network, BGP can load balance
over up to how many links?
A. Eight
B. Thirty-two
C. Six
D. One
6. You can define communities using which type of filters?
A. Standard access lists
B. Route maps
C. Prefix lists
D. Extended access lists
7. Which of the following can be used to avoid creating a full-mesh net-
work? (Choose all that apply.)
A. Confederations
B. Route maps
C. Prefix lists
D. Route reflectors
8. Which of the following commands shows the configured peer BGP
routers and the current connection state?
A. show ip bgp all
B. show cdp bgp neighbors
C. show running-config
D. show ip bgp neighbors
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
355
9. What router command mode is used to start BGP using the router
bgp 100 command?
A. User mode
B. Privilege mode
C. Global Configuration mode
D. Interface Configuration mode
E. Route Map Configuration mode
10. What are two advantages of prefix lists over distribute lists?
A. Less CPU usage
B. Easy to configure
C. Affect advertised routes and data coming into an interface
D. Can be configured on individual interfaces
11. Which of the following is not a way of managing routes advertised by
BGP routers?
A. Using route maps
B. Using prefix lists
C. Using distribute lists
D. Using path filters
E. Using redistribution lists
12. You can lengthen the AS-PATH length by doing which of the following?
A. Add a new value using the ip bgp as-path value command.
B. Add false AS numbers
C. Add a new value using the set as-path extended command.
D. Use the bgp dampening command.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
356
Chapter 9
BGP Scalability and Advanced Features
13. Statements in distribute lists are processed in which order? (Choose all
that apply.)
A. The order in which they were entered
B. From the top down
C. The order given by the sequence number
D. All of the above
14. When configuring a prefix list, if the seq syntax is not used, in what
sequence are numbers assigned and in what increment?
A. 1 (1,2,3…)
B. 5 (10,15,20…)
C. 10 (10,20,30…)
D. 25 (25,50,75…)
15. A BGP router not participating in a route reflector cluster is called
which of the following?
A. Non-cluster client
B. Non-BGP router
C. Non-client
D. Non-iBGP client
16. The COMMUNITIES attribute can contain a value in what range of
numbers?
A. 1 to 1012
B. 1 to 255
C. 0 to 512
D. 1 to 4,294,967,200
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
357
17. Which of the following is not used in confederations?
A. iBGP
B. eBGP
C. Sub-ASes
D. Sequence numbers
E. Confederation identifier
18. Which command can be used to disable sequence numbering when
creating prefix lists?
A. ip bgp prefix-list sequence-number disable
B. no ip prefix-list sequence-number
C. disable ip bgp prefix-list sequence-number
D. no ip prefix-list
19. Which of the following ranges of numbers can be assigned to a BGP
distribute list?
A. 299 to 399
B. 1 to 200
C. 1 to 199
D. 1 to 2,000
20. When creating prefix lists, which of the following are optional
syntaxes?
A. list-name
B. ge
C. le
D. seq
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
358
Chapter 9
BGP Scalability and Advanced Features
Answers to Written Lab
1. The set command
2. Privileged mode
3. Peer group
4. neighbor group6 peer-group
5. Basic, Medium, and Full
6. ip route 0.0.0.0 0.0.0.0 serial 0
7. ip route 0.0.0.0 0.0.0.0 serial 0 200
8. clear ip peer-group group3
9. bgp dampening
10. Route Map Configuration mode displayed on the router prompt as
Router(config-route-map)#
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
359
Answers to Review Questions
1. A. The least restrictive statements should be placed at the top of an
access list. This means that if the last statement is the implicit deny
all, then the permit statements should be first unless you want to
deny a subset of what was permitted. A good rule to remember is that
the most specific statements should be at the top.
2. D. Route summarization reduces the number of entries found in the
routing table, creating a single summarized route for all the entries in
the routing table for networks residing out a single interface.
3. A, B, D. A prefix list can be reconfigured with new statements, or you
can delete statements at any time as long as they are numbered with
sequence numbers. The set command is used to tell the router what
to do when a match is made in a route map.
4. D. A route reflector is used to manage larger networks. A route reflec-
tor should be peered with other route reflectors, its own route reflector
clients, and those routers not participating in a route reflector cluster.
5. C. You can have up to six physical links to ISPs and use those links to
send data traffic back and forth from your network to your ISP’s network. This effectively allows you to not only have redundant links,
but to use those redundant links to load balance your traffic.
6. B. The COMMUNITIES attribute can be used in route maps. The
COMMUNITIES attribute identifies a common set of BGP routers
participating in a community.
7. A, D. Confederations and route reflectors can both be configured to
avoid creating a full-mesh network where the neighbors command
is used excessively.
8. D. The show bgp neighbors command shows the configured BGP
peers and the current connection status.
9. C. The router bgp command is used in the Global Configuration mode.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
360
Chapter 9
BGP Scalability and Advanced Features
10. A, B. Prefix lists use considerably less CPU space and are much easier
to configure than access lists. They cannot affect advertised routes
coming into an interface and are configured globally on a router, not
on each interface.
11. E. There is no such thing as a redistribution list using BGP. The other
ways listed are all valid ways of manipulating routes advertised by
BGP.
12. B. You can increase the AS-PATH length by adding false AS numbers.
Although the ip bgp as-path value command and the set aspath extended command appear convincing enough, they are not
real commands. The bgp dampening command is used by BGP to set
a hold time before a route can be re-advertised after route
flapping.
13. A, B. Statements are entered in a distribute list by configuring an
access list. The statements are processed in the order in which they
were entered and from the top down. Sequence numbers are not used
in distribute lists.
14. B. Sequence numbers are assigned in increments of five when no
sequence number was assigned when the prefix list statements were
configured.
15. C. BGP routers not participating in a route reflector client are called
non-client routers.
16. D. The COMMUNITIES attribute value can be any number between
1 and 4,294,967,200.
17. D. The sequence number is used in prefix lists. Confederations use
iBGP on routers in sub-ASes and then use eBGP to connect the subASes.
18. B. The no ip prefix-list sequence-number command is used to
disable sequence numbering for prefix lists. The only other real command is the no ip prefix-list command, which is used to delete
a prefix list.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
361
19. C. This is sort of a trick question. The reason is that distribute lists are
created using access lists. IP standard access lists are numbered 1 to
99, and extended access lists are numbered 100 to 199.
20. B, C, D. The prefix-list command is followed by the list-name
syntax. The ge, le, and seq syntaxes are all optional and not required.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Chapter
Route Optimization
10
THE CCNP ROUTING EXAM TOPICS COVERED
IN THIS CHAPTER ARE AS FOLLOWS:
Show the need for route redistribution
Review the metrics of commonly used routing protocols
Illustrate how to redistribute routing protocols, including RIP,
OSPF, IGRP, and EIGRP
Learn how to verify and troubleshoot route redistribution
Explore how to fine-tune route redistribution through the use of
access lists and route maps
Recognize the benefits of policy routing
Detail how to direct traffic flows through the use of policy
routing
Configure route maps to control traffic flows
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
I
n this chapter, we will discuss how to take networks running different routing protocols and allow them to exchange routing information,
through a process called route redistribution. One of the challenges of route
redistribution is that many routing protocols use different metrics. To overcome this challenge, we will show you how to set default metrics for various
routing protocols. After examining several redistribution examples, we will
review commands for verifying and troubleshooting route redistribution.
We will discuss many advanced route-manipulation techniques, including
setting metrics on a protocol-by-protocol basis and setting metrics for specific routes. We’ll introduce the distribute-list feature as a tool for filtering the receiving or advertising of routes, and we’ll show the virtual
interface Null0 to be an efficient way of discarding packets destined for specified networks. We will also detail how to redistribute static and connected
routes. In addition, we’ll introduce the powerful features of route maps.
Route Redistribution
W
e have previously discussed various routing protocols available on
Cisco routers. Some of the more common routing protocols are RIP, IGRP,
EIGRP, and OSPF. However, we have not considered what happens when
we interconnect networks that are running differing routing protocols. To
illustrate this situation, let’s consider the implications of when two businesses (or divisions within the same business) merge. Let’s say that Company
A had a network infrastructure that used the Cisco proprietary EIGRP protocol, as shown in Figure 10.1.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Redistribution
FIGURE 10.1
365
Company A’s EIGRP configuration
Router eigrp 10
network 1.0.0.0
network 2.0.0.0
network 3.0.0.0
network 4.0.0.0
2.2.2.0/24
Internet
1.1.1.0/24
3.3.3.0/24
4.4.4.0/24
Network A
EIGRP - Process ID 10
Company B ran RIP as its interior routing protocol, as shown in Figure 10.2,
because Company B’s network had mixture of routing vendors. One day,
Company A and Company B merged.
FIGURE 10.2
Company B’s RIP configuration
Router rip
network 5.0.0.0
network 6.0.0.0
network 7.0.0.0
network 8.0.0.0
5.5.5.0/24
Internet
8.8.8.0/24
6.6.6.0/24
7.7.7.0/24
Network B
RIP
When the backbone routers of each company were interconnected, as
illustrated in Figure 10.3, the Company A routers did not automatically
learn the routes from the Company B routers, nor vice versa. A common misconception is that if the router joining two networks runs both routing protocols, then route redistribution will just happen—this is not so.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
366
Chapter 10
Route Optimization
FIGURE 10.3
Improper redistribution
I can see routes
both from RouterA
and RouterC.
I cannot see
routes
from RouterC.
RouterA
1.1.1.0/24
RouterB
I cannot see
routes
from RouterA.
8.8.8.0/24
Network A
EIGRP - Process ID 10
RouterC
Network B
RIP
router eigrp 10
network 1.0.0.0
router rip
network 8.0.0.0
The solution to this problem of mixed routing protocols is route redistribution. The reason that route redistribution does not happen automatically
between diverse routing protocols is that the protocols have different methods of representing the desirability of a route. This desirability is called a
metric. Also, some routing protocols include subnet information (prefix
information) within routing updates (e.g., classless routing protocols), and
some routing protocols do not include subnet information (e.g., classful
routing protocols). Therefore, to better understand how we redistribute one
routing protocol into another, let’s first review some characteristics of various routing protocols.
Routing Protocol Metrics
In this section, we will discuss the various routing protocols and metrics used
to calculate the best path to all remote networks. It is important to remember
that a router first used the administrative distance as a tool to find the best
path to a remote network. For example, if you have a network route being
advertised to a router with both RIP and IGRP, the IGRP route will be used
and the RIP route will be ignored. If two or more routes are being advertised
as available routes to the remote network, then the metric of a routing protocol is used to determine the best path. If the metrics are the same, the routing
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Redistribution
367
protocol will perform load balancing over the available routes. It is important that you understand the default administrative distance of each routing
protocol and the metrics used so that you can effectively troubleshoot and
maintain an internetwork.
IP RIP
RIP (Routing Information Protocol) uses a simple metric called the hop
count. The hop count for a network is simply the number of routers that a
packet must pass through to reach that network. The hop-count metric does
not take into account such things as the speed or reliability of a link, just the
number of hops. In this way, RIP is similar to the AppleTalk routing protocol of RTMP (Routing Table Maintenance Protocol). Novell’s NetWare IPX
RIP uses ticks to determine the best path to a remote network. Ticks are calculated as approximately 1/18 of a second and are Novell’s way of using load
and delay of the line as metrics. If the IPX RIP finds multiple paths to the
same location with the same tick count, then hops are used as a tiebreaker.
The other important characteristic of RIP that we are considering is that
RIP is a classful routing protocol. This means that the subnet mask (prefix
information) is not sent with the route updates as it is with classless routing
protocols. RIP cannot effectively work with classless routing protocols like
EIGRP and OSPF because of this reason. However, RIP version 2 sends prefix information with the router updates and its routes can be redistributed
with OSPF and EIGRP, for example. To configure RIP version 2, you just
add the command version 2 under the router rip process command,
as shown below:
Router#config t
Router(config)#router rip
Router(config-router)#network 172.16.0.0
Router(config-router)#network 10.0.0.0
Router(config-router)#version 2
It is important to remember to advertise your directly attached networks
as classful addresses. However, if you have a router attached to network
172.16.0.0/24 but are using subnets 172.16.30.0 and 172.16.40.0, you
would advertise 172.16.0.0, and the routing process would find and advertise your subnets. However, we see many students type the network
172.16.30.0 as the network number under RIP; this command works
because the router will change it to 172.16.0.0 (the classful boundary) for
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
368
Chapter 10
Route Optimization
you. You need to remember that even though the router will fix it for you,
the Cisco certification exam will not, and you will get a wrong answer. Just
remember that a classful routing protocol is always configured with all subnet and host bits off.
Another thought to keep in mind regarding RIP version 1 is that it doesn’t
work with VLSM because subnet mask information is not sent with the route
updates. Since RIP version 2 does send prefix information, you absolutely
must use RIP version 2 if you are trying to perform any type of VLSM
networking.
OSPF
O
SPF (Open Shortest Path First) uses an algorithm to determine a
composite metric. Specifically, the algorithm used is based on the Dijkstra
Algorithm, named after its inventor, Edsger Dijkstra. This algorithm uses
only the bandwidth of a link to determine the cost to a remote network.
Remember that OSPF does not summarize by default like IGRP, EIGRP, and
RIP. However, unlike RIP, OSPF is a classless routing protocol, which means
that it includes subnet information (prefix information) in its routing
updates. OSPF is typically the fastest converging routing protocol for IP.
However, we have found that EIGRP can give it a run for the money in
smaller networks in regards to convergence times.
IGRP
IGRP (Internet Gateway Routing Protocol) is a Cisco proprietary protocol and therefore cannot run on routers from other vendors. Similar to
RIP, IGRP is a classful, distance-vector protocol. However, IGRP uses a
much more complex metric than RIP. Specifically, the metric for IGRP is
made up of the following five components:
Bandwidth The bandwidth value is represented by the number of Kbps
that a particular interface is capable of. For example, a 10Mbps Ethernet
port would, by default, have a bandwidth value of 10,000 (10,000Kbps
= 10Mbps). Similarly, a 56Kbps serial interface would have a bandwidth
value of 56. All Cisco routers have a default bandwidth of 1.544Mbps on
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
IGRP
369
the router’s serial interfaces. It is important to change the bandwidth of an
interface if you are using a routing protocol that uses the bandwidth of a
link to calculate the best path to a remote network, for example, IGRP,
EIGRP, and OSPF. However, it is also important to understand that the
bandwidth command has absolutely nothing to do with the speed of the
link. Yes, it would be nice to type in a command on a serial interface and
boost your bandwidth. Unfortunately, the only thing the bandwidth command is used for on an interface is to help routing protocols make smart
decisions.
Delay The delay value is calculated by adding up the delay (in 10microsecond increments) along the path to the next router.
Reliability The reliability component of the metric is determined by how
many errors are occurring on the interface. The best possible reliability
value is 255. So, if we had an interface that was experiencing multiple
errors, and its reliability value was 128, then we would know that its reliability was approximately 50 percent.
Load The load value, like the reliability value, has a maximum value of
255. However, in the case of load, lower values are better. If a particular
serial link were being used at approximately 25 percent of capacity, its
load value would be 63 (255 x .25 = 63.25). A value of 1 is the best.
MTU MTU is the Maximum Transmit Unit size, in bytes, allowed over
an interface. An Ethernet and serial interface, for example, has a default
MTU size of 1500 bytes. Traffic over an interface is more efficient at
larger MTU sizes (assuming the link is not experiencing multiple errors,
requiring retransmission), because with a larger MTU size, a message
does not have to be broken up into as many packets. Therefore, with
fewer packets, there is lower overhead (header information that is contained in each packet). With lower overhead, there is a higher rate of data
throughput.
An easy way to remember the metric components of IGRP is to recall the
acrostic “Big Dogs Really Like Me,” where B is bandwidth, D is delay, R is reliability, L is load, and M is MTU size.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
370
Chapter 10
Route Optimization
If you were to look at a network analyzer, you would see that IGRP sends
route updates with all the metric information described above. However, by
default, IGRP routing protocols use only bandwidth and delay of the line to
determine the best route. MTU, reliability, and load have to be configured by
the administrator. I don’t recommend this unless it is a rainy Saturday and
you have absolutely nothing else to do in the world and you want to amaze
your friends at work on Monday morning.
EIGRP
E
IGRP (Enhanced IGRP) is a Cisco proprietary protocol, like IGRP.
The good news is that EIGRP uses the same metric components as IGRP
(bandwidth, delay, reliability, load, and MTU size) but also uses only bandwidth and delay of the line by default as IGRP does. The news gets even better. Since EIGRP and IGRP use the same metrics, they can automatically be
redistributed into each other, provided that they are using the same autonomous system number. Later we’ll present an example that will clarify this
automatic redistribution. Unlike IGRP, however, EIGRP is a classless routing protocol. Therefore, EIGRP is capable of sending subnet information in
its routing updates.
Configuring Route Redistribution
N
ow that we have an understanding of the issues involved in route
redistribution (metrics and classless versus classful), we will examine some
basic scenarios of route redistribution. Let’s first consider the situation presented at the beginning of the chapter—two companies merging and needing
to redistribute RIP and EIGRP into each other.
As you already know, we have two networks that are merging together.
RouterB’s routing table knows the routes from both RouterA and RouterC,
because it is configured to run both the RIP and EIGRP routing processes.
However, RouterA and RouterC cannot see the routes from each other, as
shown in Figure 10.4.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring Route Redistribution
FIGURE 10.4
371
Improper redistribution
I can see routes
from both RouterA
and RouterC.
I cannot see
routes
from RouterC.
RouterA
1.1.1.0/24
RouterB
I cannot see
routes
from RouterA.
8.8.8.0/24
Network A
EIGRP - Process ID 10
RouterC
Network B
RIP
router eigrp 10
network 1.0.0.0
router rip
network 8.0.0.0
Since RouterB connects to both the EIGRP and RIP networks, RouterB is
the router where we can redistribute EIGRP and RIP into each other. The
syntax on router RouterB to redistribute EIGRP and RIP into one another is
as follows:
router eigrp 10
where 10 is the AS number.
redistribute rip
which tells the router to take routes learned via RIP and re-advertise them via
EIGRP.
network 1.0.0.0
where network is the network that is part of the EIGRP routing process.
default-metric 56 10 255 1 1500
where default-metric is the metric to be used when redistributing routes
from other routing protocols, 56 is the bandwidth (56Kbps), 10 is the delay
(in 10-microsecond increments), 255 is the reliability (100 percent reliable),
l is the load (no load), and 1500 is the MTU size (1,500 bytes).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
372
Chapter 10
Route Optimization
!
router rip
which enables the RIP routing process.
redistribute eigrp 10
which tells the router to take routes learned via EIGRP AS 10 and readvertise them via RIP.
network 8.0.0.0
where network is the network that is part of the RIP routing process.
default-metric 3
where 3 is to be used as the default metric (hop count) when other routing
protocols are injected into RIP.
At this point, both RouterA and RouterC can see each other’s routes. Also
note that it would have been possible to do a one-way redistribution. A oneway redistribution is where one routing protocol is redistributed into
another, but not vice versa. For example, if we had omitted the RouterB configuration command redistribute rip under the router eigrp 10 section of the configuration, then routes learned via RIP would not have been
re-advertised by EIGRP. In some situations, it is good design practice to use
one-way redistribution, to avoid routing loops. This is of particular importance when a router’s split-horizon function (which prevents routing loops)
has been disabled.
EIGRP and IGRP Route Redistribution
We mentioned earlier that EIGRP and IGRP use the same metrics and can
therefore automatically redistribute their routes into each other without the
need for manual redistribution, which was required in the previous example.
The one caveat is that the EIGRP and IGRP AS must be the same.
For example, consider a variation on our original scenario. This time,
Company B’s network uses IGRP, as shown in Figure 10.5.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Configuring Route Redistribution
FIGURE 10.5
373
EIGRP and IGRP redistribution
I can see routes
from both RouterA
and RouterC.
I cannot see
routes
from RouterC.
RouterA
1.1.1.0/24
RouterB
I cannot see
routes
from RouterA.
8.8.8.0/24
Network A
EIGRP - Process ID 10
RouterC
Network B
IGRP - Process ID 20
router eigrp 10
network 1.0.0.0
router igrp 20
network 8.0.0.0
However, with the syntax shown, route redistribution still doesn’t work.
The reason is that Network A is using a process ID of 10 for EIGRP, and
Network B is using an AS of 20 for IGRP. If we change the IGRP AS of Network B from 20 to 10, route redistribution functions correctly. The final syntax of RouterB would be as follows:
router eigrp 10
network 1.0.0.0
!
router igrp 10
network 8.0.0.0
Notice that no redistribute or default-metric commands are necessary. Since the AS for both EIGRP and IGRP is now set to 10, redistribution occurs automatically.
Before considering more advanced concepts of route redistribution, we
will examine one final scenario of route summarization. This time, we will
be redistributing EIGRP and OSPF.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
374
Chapter 10
Route Optimization
EIGRP and OSPF Route Redistribution
In this variation of our original example, let us say that Company B ran
OSPF as its internal routing process. Figure 10.6 shows the appropriate syntax for redistributing EIGRP and OSPF into one another.
FIGURE 10.6
EIGRP and OSPF redistribution
I can see routes
from both RouterA
and RouterC.
I can see
routes
from RouterC.
RouterA
1.1.1.0/24
RouterB
Network A
EIGRP - Process ID 10
I can see
routes
from RouterA.
RouterC
8.8.8.0/24
Network B
OSPF
Process ID 1 - Area 0
router eigrp 10
redistribute ospf 1
network 1.0.0.0
default-metric 56 10 255 1 1500
!
router ospf 1
redistribute eigrp 1
network 0.0.0.0 0.0.0.255 area 0
default-metric 128
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Verifying and Troubleshooting Route Redistribution
375
Verifying and Troubleshooting Route
Redistribution
Y
ou can use various commands to verify and troubleshoot route redistribution. The best commands to use on your router are trace, show ip
route, and show ip protocol. Let’s take a closer look at each of these
commands:
trace This command is useful for verifying and troubleshooting route
redistribution. Using the trace command, you can determine over which
path your traffic is flowing and decide if the optimal path is being used.
show ip route This command displays what routes the router has
learned and by which routing protocol it learned them. You can verify
that all the networks are in the routing table.
show ip protocols This command displays the IP routing protocols
configured on the router and shows what each routing process is redistributing. Let’s look at an example:
Routing Protocol is “ospf 1”
Sending updates every 0 seconds
Invalid after 0 seconds, hold down 0, flushed after 0
Outgoing update filter list for all interfaces is not
set
Incoming update filter list for all interfaces is not
set
Default redistribution metric is 128
Redistributing: ospf 1, eigrp 10
In addition to the above listed commands, protocol-specific debug commands may be useful. For example, if you are viewing RIP updates (by using
the debug ip rip command), you may see a network advertised as unreachable. An unreachable network will not show up in the routing table. This situation can occur when you forget to set a protocol’s default metric.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
376
Chapter 10
Route Optimization
Advanced Redistribution
Our redistribution examples up to this point have set the default metric for a particular routing protocol. That metric is then used every time
another routing protocol is redistributed into that particular routing protocol. Also, in our examples, all the routes have been redistributed; that is, we
have not been filtering the content of our routing updates.
We will now examine how to control redistribution with a higher degree
of granularity. For example, we may want RIP to apply one metric to OSPF
and another metric to IGRP, as these products are redistributed into the RIP
routing process.
Also, in some cases, we may want only a subset of routes redistributed.
When, in our example, the two companies merge, perhaps we don’t want the
users on Network B to be able to access the services on network 2.0.0.0 in
Network A, for security reasons. In other instances, we may not want to
advertise or accept advertisements from all available routes, due to the sheer
volume of routes. For example, if our router is connected to an Internet Service Provider and is accepting full routes from the Internet, we would have
over 65,000 routing entries in our routing table. Such a large number of
routing entries consumes a significant amount of RAM and processor
overhead.
Protocol-Specific Metrics
Let’s examine how to set up redistribution such that we set the metric for a
redistributed protocol as part of the redistribute command. To illustrate,
let’s consider the first example given in the chapter, where Network A is running EIGRP and Network B is running RIP. Figure 10.7 shows how we could
configure RouterB to accomplish route redistribution. This syntax gives us
the flexibility to specify alternate metrics for EIGRP and RIP to use if other
routing protocols, such as OSPF, were being redistributed into them.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Advanced Redistribution
FIGURE 10.7
377
Protocol-specific metrics
I can see routes
from both RouterA
and RouterC.
I can see
routes
from RouterC.
RouterA
1.1.1.0/24
RouterB
I can see
routes
from RouterA.
8.8.8.0/24
Network A
EIGRP - Process ID 10
RouterC
Network B
RIP
router eigrp 10
network 1.0.0.0
redistribute rip metric 56 100 255 1 1500
!
router rip
network 8.0.0.0
redistribute eigrp 10 metric 3
Route-Specific Metrics
To get even greater control over how metrics are assigned, we can use access
lists and route maps to set metrics for specific routes. As an example, let’s
examine how to set a specific metric for network 5.0.0.0 as it is redistributed
from RIP in Network B into EIGRP in Network A.
First, we create an access list that permits only network 5.0.0.0. Since
we’re not concerned with destination addresses, a standard IP access list (an
access list numbered from 1 through 99) will be fine for our purposes. Following is the syntax to create an access list that permits only network
5.0.0.0:
access-list 1 permit 5.0.0.0
Next, we need to create a route map. We will examine route maps in more
detail later in the chapter. However, for now we can think of a route map as
an IF-THEN-ELSE statement. IF the advertised route is permitted by our
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
378
Chapter 10
Route Optimization
access list, THEN apply a specific metric, or ELSE apply a different metric.
Following is the syntax to create our route map:
route-map test 1
assigns the name test to our route-map, and designates this section
as sequence number 1.
match ip address 1
checks access-list 1 to see if a permit condition is met.
set metric 56 100 255 100 1500
If the permit condition was met in the previous statement, then assign
the given metric. Notice that we have set this metric to indicate that
the link is partially loaded (load is set to 100, indicating that the link
is approximately 39 percent [100/255] loaded).
route-map test 2
designates this section as sequence number 2 of the route-map named
test.
set metric 56 100 255 1 1500
If the match condition in sequence number 1 was not met, then apply
the given metric. Notice that this metric specifies an unloaded link
(load is set to 1).
Finally, we need to apply the route map to the EIGRP routing process with
the following syntax:
router eigrp 10
redistribute rip route-map test
Here we redistribute routes learned via RIP into EIGRP, with the parameters specified in the route-map named test.
Now, when network 5.0.0.0 is being redistributed into EIGRP, it will
have a different metric than other networks, as shown in Figure 10.8.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Advanced Redistribution
FIGURE 10.8
379
Route-specific metrics
I can see routes from RouterC.
network 5.0.0.0 has a metric of 56 100 255 100 1500
network 6.0.0.0 has a metric of 56 100 255 1 1500
network 7.0.0.0 has a metric of 56 100 255 1 1500
network 8.0.0.0 has a metric of 56 100 255 1 1500
I can see routes
from both RouterA
and RouterC.
RouterA
1.1.1.0/24
RouterB
I can see
routes
from RouterA.
8.8.8.0/24
Network A
EIGRP - Process ID 10
RouterC
Network B
RIP
router eigrp 10
network 1.0.0.0
redistribute rip route-map test
!
router rip
network 8.0.0.0
redistribute eigrp 10 metric 3
!
access-list 1 permit 5.0.0.0
!
route-map test 1
match ip address 1
set metric 56 100 255 100 1500
!
route-map test 2
set metric 56 100 255 1 1500
Filtering Routes
In the previous section, we explored how we could specify different metrics
for different routes. However, as we mentioned earlier, there are many
instances where we want only a subset of routes advertised or received. We
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
380
Chapter 10
Route Optimization
can use the distribute-list command to accomplish this goal. To illustrate how to use the distribute-list command, consider a variation of
our original example. This time, Network A is running EIGRP, Network B
is running RIP, and we want only the 1.0.0.0 network and the 2.0.0.0 network to be advertised to Network B.
First, we create an access list that specifies which networks we want to be
learned by RouterB:
access-list 2 permit 1.0.0.0 0.255.255.255
access-list 2 permit 2.0.0.0 0.255.255.255
Next, we use the distribute-list command to permit only the networks specified in the access list:
router eigrp 10
distribute-list 2 in
Notice the in argument in the distribute-list command. The in argument tells the EIGRP routing process to learn only routes that are permitted
by the access list, as shown in Figure 10.9.
FIGURE 10.9
Distribute list
I can only see
networks 1.0.0.0 and
2.0.0.0 from
Network A.
RouterA
1.1.1.0/24
RouterB
8.8.8.0/24
Network A
EIGRP - Process ID 10
RouterC
Network B
RIP
router eigrp 10
network 1.0.0.0
redistribute rip metric 56 100 255 1 1500
distribute-list 2 in
!
router rip
network 8.0.0.0
redistribute eigrp 10 metric 3
!
access-list 2 permit 1.0.0.0 0.255.255.255
access-list 2 permit 2.0.0.0 0.255.255.255
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Advanced Redistribution
381
Alternatively, we could have used the out argument with the RIP routing
process. The out argument tells the RIP routing process to advertise only
routes that are permitted by the access list. If we had used the out argument,
the syntax would be
router rip
distribute-list 1 out
Using the Null0 Interface
Like the distribute-list feature, the Null0 interface can be used to creatively control routes. Specifically, Null0 is a logical interface, and when
traffic is directed (routed) to this interface, the traffic is discarded. In some
of the literature, you may see Null0 referred to as a “bit bucket” or a “black
hole,” because whatever goes in does not come out. Traffic can be directed
into the Null0 interface using a static route. Consider the following example.
In order to conserve IP numbers, some networks use private IP addresses
(e.g., 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16). If we were using private address space inside our network, then we would certainly not want to
advertise these networks outside our network, because we might accidentally advertise these routes to someone else who was using private addressing. Therefore, we could use our distribute-list command to prevent the
advertisement of private address space. Alternatively, we could create a
static route that directed all traffic destined for this private address space to
be discarded, as shown in Figure 10.10.
FIGURE 10.10
The bit bucket
ip route 10.0.0.0 255.0.0.0 null0
Destination 10.4.4.4
Destination 10.2.2.2
10.3.3.0/24
Internet
10.2.2.0/24
10.1.1.0/24
Bit
Bucket
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
382
Chapter 10
Route Optimization
Let’s examine the syntax used in Figure 10.10:
ip route 10.0.0.0 255.0.0.0 null0
The standard syntax of the ip route command is
ip route network network_mask {address | interface}
where address is the IP number of the next-hop address (the next router on
the way to the destination network) and interface is the interface on the
local router to which traffic should be forwarded.
In our example, interface is the logical interface Null0. Therefore, all
traffic destined for network 10.0.0.0/8 is discarded.
Static, Connected, and Default Routes
So far in this chapter, we have examined how to take routes from one routing
protocol and re-advertise them into a different routing protocol. We’ll now
take a look at some special redistribution cases.
If we want to redistribute static routes that we have created or routes that
are part of directly connected interfaces on a router, we can use the following
commands:
redistribute static
which redistributes routes that have been manually configured.
redistribute connected
which redistributes routes of networks that are connected directly to the
local router. These routes may or may not be a part of a routing process.
The Null0 interface example we considered earlier would be an example
of a static route, as would a default route. A default route tells a router where
to send packets if the destination network is not in its routing table. For
example, if a client were trying to reach an IP address on the Internet, and the
local router did not have an explicit route to the destination, then the packet
would be sent along the default route to either a particular router interface
or to the IP address of the next-hop router. When creating a static default
route, the syntax is as follows:
ip route 0.0.0.0 0.0.0.0 {address | interface}
where 0.0.0.0 0.0.0.0 is the network number for a default route.
Redistributing a default route can be particularly useful for systems such
as older Unix hosts that run RIP. In the case of these hosts, some do not have
a default gateway setting. Rather, these hosts rely on RIP advertisements to
find the default gateway, as shown in Figure 10.11.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Maps
FIGURE 10.11
383
Redistributing static default routes
Company A
I don’t have an entry in my
routing table for network 12.0.0.0.
So, I’ll send the packet to my
default route, which points to 5.5.5.5.
Data
Data
(Destination 12.1.2.3) (Destination 12.1.2.3)
1.1.1.1
5.5.5.4
Data
(Destination 12.1.2.3)
5.5.5.5
Internet
Unix Host
RIP Advertisement
Default Gateway = 1.1.1.1
router rip
network 1.0.0.0
redistribute static
!
ip route 0.0.0.0 0.0.0.0 5.5.5.5
Route Maps
E
arlier in the chapter, we saw how route maps can be used to fine-tune
route redistribution. Other examples of policy-based routing benefits
include
Load sharing, based on the type of traffic
Sending low-priority or non-interactive traffic (e.g., routine batch processes) over slower, low-cost links
Traffic prioritization, based on the type of traffic
Route maps have two primary components: a match clause and a set
clause. The function of the match clause is to specify the traffic that is to be
policy routed. The traffic may be specified by using an access list (either standard or extended), packet length, metric value, route type, or tag value. The
route map processes the match statements sequentially until a match is
found. As with access lists, there is an implicit deny statement at the end of
the match list.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
384
Chapter 10
Route Optimization
Once the match clause is satisfied, the set clause can manipulate the routing of the traffic by setting the next-hop or default interface or IP address,
setting the IP TOS (Type of Service), or setting the IP precedence of the traffic. Note that route maps are set on the source router (the first router to
receive the packet), not the destination router. To better understand how
policy routing works, let’s consider a couple of examples.
Policy-Routing TCP Services
As shown in Figure 10.12, Company A has two connections to the Internet:
one to ISP A and the other to ISP B. The connection to ISP A is much faster
than the connection to ISP B. Therefore, Company A wants to send all
latency-sensitive traffic over the link to ISP A and all non-interactive traffic
over the link to ISP B. So, for our example, we will configure RouterA to
direct SMTP (Simple Mail Transfer Protocol) traffic over the link to ISP B, and
all other traffic will be sent over the link to ISP A.
FIGURE 10.12
Policy routing example 1
ISP A
RouterB
172.16.1.2/24
1.544Mbps link
Company A
RouterA
s0
172.16.1.1/24
s1
Internet
172.16.2.1/24
ISP B
56Kbps link
RouterC
172.16.2.2/24
First we need to create an access list that will define the traffic we are
interested in, specifically SMTP traffic:
access-list 101 permit tcp any any eq 25
where 25 is the tcp port number for SMTP.
Next, we need to create a route map. In this example, we will name it
“routemail”:
route-map routemail permit 10
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Maps
385
where routemail is an arbitrary name chosen for the route map, 10 is the
route map sequence number, and the sequence number is 10 by default,
unless another number is specified.
We will now set the criteria for the first match clause:
match ip address 101
where 101 is the access list that the match clause is checking traffic against.
We will now use the set clause to specify the next-hop address of traffic
satisfying the match clause:
set ip next-hop 172.16.2.2
where 172.16.2.2 is the IP address of the next-hop router.
Finally, we need to specify where to send traffic if the match condition is
not met:
route-map routemail permit 20
set ip next-hop 172.16.1.2
where 20 is the sequence number and 172.16.1.2 is the next-hop router.
RouterA will now send all SMTP traffic over the 56Kbps link to RouterC and
all other traffic over the 1.544Mbps link to RouterB, as shown in Figure 10.13.
FIGURE 10.13
Policy routing configuration for example 1
ISP A
RouterB
172.16.1.2/24
WWW, FTP, etc.
Company A
RouterA
1.544Mbps link
s0
172.16.1.1/24
s1
172.16.2.1/24
56Kbps link
Internet
ISP B
RouterC
SMTP
172.16.2.2/24
access-list 101 permit tcp any any eq 25
!
route-map routemail permit 10
match ip address 101
set ip next-hop 172.16.2.2
!
route-map routemail permit 20
set ip next-hop 172.16.1.2
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
386
Chapter 10
Route Optimization
Policy Routing Subnets
As another example, consider the following scenario. Company A has the
same ISP connections as in the previous example. Inside Company A, we
have two VLANs, the Engineering VLAN (172.16.3.0/24) and the Accounting VLAN (172.16.4.0/24), as illustrated in Figure 10.14. Our goal is to
direct traffic from the Engineering VLAN (172.16.3.0/24) out the 1.544Mbps
link (interface s0) and to direct traffic from the Accounting VLAN
(172.16.4.0/24) out the 56Kbps link (interface s1).
FIGURE 10.14
Policy routing example 2
Engineering VLAN
ISP A
Company A
RouterB
172.16.1.2/24
RouterA
172.16.3.1/24
1.544Mbps link
e0
s0
e1
172.16.1.1/24
172.16.2.1/24
s1
56Kbps link
172.16.4.1/24
Internet
ISP B
RouterC
Accounting VLAN
172.16.2.2/24
First, we need to create our access lists, specifying the conditions we’re
looking for, that is, whether a packet was sourced from the Engineering or
Accounting VLAN:
access-list 1 permit 172.16.3.0 0.0.0.255
access-list 2 permit 172.16.4.0 0.0.0.255
Next, we need to create our route map and specify the match and set
parameters. In this example, we will use the name “routevlan”:
route-map routevlan permit 10
match ip address 1
set interface serial0
!
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Maps
387
route-map routevlan permit 20
match ip address 2
set interface serial1
The Accounting users will now have their Internet traffic directed to ISP B,
while the Engineering users will have their Internet traffic directed to ISP A, as
shown in Figure 10.15.
FIGURE 10.15
Policy routing configuration for example 2
Engineering VLAN
ISP A
Company A
RouterB
172.16.1.2/24
RouterA
172.16.3.1/24
1.544Mbps link
e0
s0
e1
172.16.1.1/24
172.16.2.1/24
s1
56Kbps link
172.16.4.1/24
Internet
ISP B
RouterC
Accounting VLAN
172.16.2.2/24
access-list 1 permit 172.16.3.0 0.0.0.255
access-list 2 permit 172.16.4.0 0.0.0.255
!
route-map routevlan permit 10
match ip address 1
set interface serial0
!
route-map routevlan permit 20
match ip address 2
set interface serial1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
388
Chapter 10
Route Optimization
Summary
In this chapter, we discussed how to take networks running different
routing protocols and allow them to exchange routing information, through
a process called route redistribution. One of the challenges of route redistribution is that many routing protocols use different metrics. To overcome this
challenge, we set default metrics for various routing protocols. After examining several redistribution examples, we reviewed commands for verifying
and troubleshooting route redistribution.
We explored various advanced route-manipulation techniques including
setting metrics on a protocol-by-protocol basis and setting metrics for specific routes. We introduced the distribute-list feature as a tool for filtering the receiving or advertising of routes, and we showed the virtual
interface Null0 to be an efficient way of discarding packets destined for specified networks. We also detailed how to redistribute static and connected
routes.
Finally, we introduced the powerful features of route maps. We used the
route map components, match and set clauses, in examples where we
routed traffic based on the source network and Layer 4 information (TCP
port numbers).
Key Terms
Before taking the exam, make sure you are familiar with the following terms:
hop count
metric
Null0
route redistribution
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Written Lab
389
Written Lab
Write the configuration for the RouterA, such that HTTP traffic is
directed to RouterC, and all other traffic is directed to RouterB, as shown in
the following graphic:
RouterB
172.16.2.1
Telnet, FTP, etc.
RouterA
T1
Internet
T3
HTTP
RouterC
172.16.1.1
Solution
The first step is to create an access list that specifies the traffic we are interested in, which in this case is HTTP traffic. By default, HTTP uses TCP port
80. Therefore, our access list would be
access-list 101 permit tcp any any eq 80
Next, we need to create a route map. In this exercise, we will name it
“routeweb”:
route-map routeweb permit 10
We’ll now set the criteria for the first match clause:
match ip address 101
Next, we will use the set clause to specify the next-hop address of traffic
satisfying the match clause:
set ip next-hop 172.16.1.1
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
390
Chapter 10
Route Optimization
Finally, we need to specify where to send traffic if the match condition is
not met:
route-map routeweb permit 20
set ip next-hop 172.16.2.1
RouterA will now send all HTTP traffic over the T3 link to RouterC and
all other traffic over the T1 link to RouterB.
Hands-on Lab
A
dd a distribute-list command to RouterA, such that RouterB
can see network 1.1.1.0/24 in its routing table, but not network 2.2.2.0/24,
as shown in the following graphic:
Lo0
1.1.1.1/24
RouterA
RIP
s0
RouterB
s0
3.3.3.1/24
3.3.3.2/24
Lo1
2.2.2.2/24
Solution
RouterA and RouterB should have their s0 interfaces interconnected with a
serial crossover cable. We will arbitrarily designate RouterB as the DCE side
and RouterA as the DTE side.
First, we create an access list on RouterA that specifies which networks
we want to be advertised:
access-list 1 permit 1.0.0.0 0.255.255.255
Next, we use the distribute-list command to permit only the network specified in the access list:
router rip
distribute–list 1 out
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Hands-on Lab
Following are possible configurations for RouterA and RouterB:
RouterA
!
version 11.2
!
hostname RouterA
!
interface Loopback0
ip address 1.1.1.1 255.255.255.0
!
interface Loopback1
ip address 2.2.2.2 255.255.255.0
!
interface Serial0
ip address 3.3.3.1 255.255.255.0
!
router rip
distribute-list 1 out
network 1.0.0.0
network 2.0.0.0
network 3.0.0.0
!
access-list 1 permit 1.0.0.0 0.255.255.255
!
no ip classless
!
line con 0
line aux 0
line vtp 0 4
login
!
end
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
391
392
Chapter 10
Route Optimization
RouterB
!
version 11.2
!
hostname RouterB
!
interface Serial0
ip address 3.3.3.1 255.255.255.0
clockrate 56000
!
router rip
network 3.0.0.0
!
no ip classless
!
line con 0
line aux 0
line vtp 0 4
login
!
end
Issuing the show ip route command on RouterB reveals that RouterB
has entries for networks 1.1.1.0 and 3.3.3.0 but not for network 2.2.2.0.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
393
Review Questions
1. Which of the following is the metric used by RIP?
A. Ticks
B. Bandwidth
C. Delay
D. Hop count
2. Which of the following is the network address and subnet mask of the
default route?
A. 127.0.0.1 255.255.255.255
B. 0.0.0.0 0.0.0.0
C. 255.255.255.255 0.0.0.0
D. 255.255.255.255 255.255.255.255
3. What are the two clauses used by route maps?
A. metric
B. match
C. access
D. set
4. Which of the following are Cisco proprietary protocols? (Choose all
that apply.)
A. RIP
B. IGRP
C. OSPF
D. EIGRP
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
394
Chapter 10
Route Optimization
5. Which of the following are metric components of IGRP? (Choose all
that apply.)
A. Delay
B. Hops
C. Bandwidth
D. Ticks
6. What number would we assign to the load component of an EIGRP
metric to indicate that a link was approximately 10 percent loaded?
A. 10
B. 1
C. 2.5
D. 25
7. What is the default MTU size metric component for an Ethernet
interface?
A. 1500 bytes
B. 1518 bytes
C. 64 bytes
D. 4000 bytes
8. What is the most desirable value for the reliability metric component?
A. 255
B. 0
C. 1
D. 100
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
395
9. Which of the following protocols share similar metrics?
A. RIP and OSPF
B. IGRP and EIGRP
C. BGP and RTMP
D. NLSP and RIP
10. What command, under the router eigrp 10 command, would you
enter to set the default metric for EIGRP to use, regardless of which
protocol was being distributed into the EIGRP process?
A. redistribute eigrp metric 56 10 255 1 1500
B. default-metric 56 10 255 1 1500
C. metric 10
D. default metric 10
11. Given the access list access-list 1 permit 10.0.0.0 0.255.255.255, what
command would allow only network 10.0.0.0/8 to be added to a
router’s routing table?
A. access-list 1 in
B. distribute-list 1 out
C. distribute-list 1 in
D. distribute 1 out
12. What happens to packets that are forwarded to the Null0 interface?
A. The packets are sent to the gateway of last resort.
B. The packets are policy-routed.
C. The packets are marked as discard eligible.
D. The packets are discarded.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
396
Chapter 10
Route Optimization
13. Which of the following commands will display the default redistribu-
tion metric for a protocol?
A. show ip route
B. trace
C. show ip protocols
D. debug ip route
14. Which of the following commands will redistribute manually config-
ured routes into a specified routing process?
A. redistribute connected
B. redistribute static
C. redistribute default
D. redistribute local
15. If a local interface is not part of a routing process, what command may
be used to inject its route into a routing process?
A. redistribute connected
B. redistribute static
C. redistribute default
D. redistribute local
16. Which of the following are possible applications of route maps?
(Choose all that apply.)
A. Load sharing
B. Directing different traffic types over different links
C. Route redistribution
D. Traffic prioritization
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Review Questions
397
17. Given the command route-map routemail permit 20, what does
20 represent?
A. The sequence number of the route map
B. The access list number being used
C. The TCP port number being routed
D. The percent of bandwidth to be allocated
18. In order for IGRP and EIGRP to automatically redistribute routes into
each other, what must be true?
A. They must be advertising subnets of the same major network.
B. They must have the same process ID (the same Autonomous
System).
C. They must be in different areas.
D. They must both be in totally stubby areas.
19. What command could you use to view RIP updates as they occur?
A. show ip protocols
B. show ip route
C. show ip rip
D. debug ip rip
20. What does the command distribute-list 2 out do?
A. It prevents or permits the routes specified in access-list 2 from
being added to the local routing table.
B. It prevents or permits the routes specified in access-list 2 from
being advertised.
C. It redirects the routes specified in access-list 2 to the Null0
interface.
D. It redistributes routes specified in access-list 2 to all routing
protocols.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
398
Chapter 10
Route Optimization
Answers to Review Questions
1. D. RIP considers only the number of routers (hops) to be traversed en
route to a destination network.
2. B. Cisco uses 0.0.0.0 0.0.0.0 to indicate a default route in a static
routing statement.
3. B and D. Route maps have two primary components, a match clause
and a set clause. The function of the match clause is to specify the
traffic that is to be policy routed. The purpose of the set clause is to
manipulate the routing of traffic by adjusting such parameters as nexthop address.
4. B and D. IGRP and EIGRP are similar in that they use the same met-
ric components, and they are both Cisco proprietary.
5. A and C. The metric components of IGRP are bandwidth, delay, reli-
ability, load, and MTU size.
6. D. The values for load range from 1 through 255, where 1 is
unloaded and 255 is completely loaded. A 10 percent load may be calculated by multiplying 255 by .1, which equals 25.5. Since we need to
specify load in terms of an integer, we choose 25.
7. A. Even though the maximum MTU size for Ethernet is 1518 bytes,
the default MTU value used in the metric calculation for an Ethernet
interface is 1500 bytes.
8. A. Reliability values range from 1 through 255, where 1 is com-
pletely unreliable and 255 is completely reliable.
9. B. Both IGRP and EIGRP use bandwidth, delay, reliability, load, and
MTU size as their metric components.
10. B. The command default-metric 56 10 255 1 1500 can be used
to specify the metric to be used for any route being redistributed into
EIGRP. This may not be optimal if you want to specify different metrics for different protocols.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Review Questions
399
11. C. The in parameter on a distribute-list command specifies
routes to be added to a router’s routing table. The out parameter
specifies routes to be advertised from a router.
12. D. The Null0 interface is a virtual interface, which is sometimes
referred to as a bit bucket. Packets sent to the Null0 interface are discarded. Since a static route requires less processing than an access list,
a route to Null0 is sometimes preferable to an access list denying a particular host or network.
13. C. The show ip protocols command will display such information
as redistribution parameters.
14. B. The redistribute static command redistributes routes that
have been manually configured.
15. A. The redistribute connected command redistributes routes of
networks that are connected directly to the local router. These routes
may or may not be a part of a routing process.
16. A, B, C, and D. Route maps, which use a combination of match and
set clauses, can be used for all of the listed applications.
17. A. The 20 is the sequence number of the route map. The sequence
numbers of the route map determine in what order the match and set
clauses will be evaluated.
18. B. If IGRP and EIGRP processes are both running on the same
router, and they both have the same process ID, then their routes will
automatically be redistributed into each other.
19. D. The debug ip rip command can be used to view the contents of
RIP updates as they occur.
20. B. The out parameter of the distribute-list command deals with
advertising routes, while the in parameter deals with adding routes to
the local routing table.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Appendix
Practice Exam
A
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
1. How is a BGP session established between two routers?
A. Telnet
B. TCP (SYN, ACK, SYN)
C. UDP (SYN, ACK, SYN)
D. IPX SAP
2. Which of the following is true concerning a stub area?
A. It does not receive summary Link State Advertisements.
B. It does not receive Type 5 LSAs.
C. It is configured with the IOS command area stub area-id.
D. Only the ABR needs to be configured as stubby.
3. Which two of the following would you use to avoid creating a full-
mesh BGP network?
A. Confederations
B. Route maps
C. Prefix lists
D. Route reflectors
4. Which of the following are used by EIGRP to send queries to other
EIGRP neighbors?
A. Broadcasts
B. Multicasts
C. ACKs
D. Unicasts
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
403
5. Which of the following is the IOS command to clear all dynamic IP
routes from a router's routing table?
A. clear ip route *
B. clear ip route 0.0.0.0
C. clear ip route all
D. clear ip route 127.0.0.1
6. When should BGP be used? (Choose all that apply.)
A. When multi-homing
B. When connecting multiple ISPs
C. When connecting routers within the same AS
D. When configuring backup links
7. What route/subnet mask combination indicates a default route?
A. 127.0.0.1 255.255.255.255
B. 255.255.255.255 255.255.255.255
C. 0.0.0.0 255.255.255.255
D. 0.0.0.0 0.0.0.0
8. Which Cisco layer is responsible for breaking up collision domains?
A. Core
B. Backbone
C. Distribution
D. Access
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
404
Appendix A
Practice Exam
9. What does the IOS command show ip ospf virtual-links do?
A. It shows the router link states.
B. It shows the network link states.
C. It shows the status of a router’s virtual links.
D. It shows the virtual memory that a router is using to maintain its
link state database.
10. What does the command redistribute static do?
A. It makes dynamically learned routes permanent.
B. It takes manually configured routes and redistributes them into a
specified routing protocol.
C. It takes routes from directly connected interfaces and redistributes
them into a specified routing protocol.
D. It causes the same metric to be used for a routing protocol, regard-
less of which routing protocol it is being redistributed into.
11. Which syntax used with the clear ip bgp command is used to iden-
tify that the command is to affect inbound or outbound triggered
updates?
A. open in
B. soft
C. triggered inbound
D. triggered outbound
12. What is the default administrative distance of EIGRP?
A. 0
B. 1
C. 90
D. 100
E. 110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
405
13. If an external AS is not receiving updates from your AS, which of the
following show commands can be used to troubleshoot this? (Choose
all that apply.)
A. show ip bgp events
B. show ip bgp neighbor
C. show ip bgp all
D. show ip bgp
14. If you wanted to summarize networks 172.16.100.0/24 and
172.16.106.0/24, which network and mask would you use?
A. 172.16.0.0/24
B. 172.16.100.0/20
C. 172.16.106.0/20
D. 172.16.96.0/20
15. Which of the following statements are true? (Choose all that apply.)
A. Every OSPF network must have an Area 0.
B. A router with one or more interfaces in Area 0 is said to be a back-
bone router.
C. If an IGRP routing process connects to a multi-area OSPF net-
work, the router through which it enters the OSPF network is
called an ABR.
D. An ASBR separates two or more OSPF areas.
16. If you wanted to reduce bandwidth usage, which Cisco IOS features
could you use? (Choose all that apply.)
A. Access lists
B. Snapshot routing
C. Compression of WANs
D. TTL
E. DDR
F. Incremental updates
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
406
Appendix A
Practice Exam
17. Which IOS command will display redistribution parameters?
A. show ip protocols
B. debug ip protocols
C. debug ip redistribution
D. show ip redistribution
18. If you wanted to manually summarize an EIGRP, which of the follow-
ing commands must you use under the EIGRP configuration?
A. no summary
B. no auto-summary
C. no summary stub
D. no route-summary
19. Which is true regarding routing protocols?
A. Classless routing protocols send periodic subnet mask
information.
B. Classless routing protocols send incremental subnet mask
information.
C. Classless routing protocols send prefix mask information.
D. All devices on a network running classless routing protocols must
use the same mask.
20. Which of the following commands is used to specify the NBMA net-
work type?
A. ip ospf nmba network
B. ip ospf network
C. ip ospf nmba-network
D. ip ospf network-nmba
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
407
21. The following output is an example of using which command?
BGP neighbor is 172.16.11.254, remote AS 100, internal
link
Index 1, Offset 0, Mask 0x2
Route-Reflector Client
BGP version 4, remote router ID 10.16.1.1
BGP state = Established, table version = 1, up for
12:10:16
Last read 00:00:06, hold time is 180,
keepalive interval is 60 seconds
Minimum time between advertisement runs is 5 seconds
Received 143 messages, 0 notifications, 0 in queue
Sent 52 messages, 0 notifications, 0 in queue
Prefix advertised , suppressed 0, withdrawn 0
Connections established 2; dropped 1
Last reset 12:10:16, due to User reset
53 accepted prefixes consume 32 bytes
0 history paths consume 0 bytes
A. show ip bgp all
B. show cdp bgp neighbors
C. show running-config
D. show ip bgp neighbors
22. When an AS must traverse another AS to get to its destination, the tra-
versed AS is called which of the following?
A. Transfer AS
B. Forwarding AS
C. Transit AS
D. Transmitting AS
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
408
Appendix A
Practice Exam
23. How many steps (neighbor states) are involved in establishing an
OSPF adjacency?
A. Five
B. Six
C. Seven
D. Four
24. Which of the following is true regarding route summarization?
A. It’s used primarily with discontiguous networks.
B. It’s used primarily with contiguous networks.
C. Do not use with VLSM.
D. It’s used with non-hierarchical addressing.
25. The OSPF ____ state is simply the state of receiving Hello packets on
the interface.
A. Down
B. Up
C. Init
D. Active
26. What happens to traffic that is sent to the Null0 interface?
A. It is set to a full CIR.
B. It is discarded.
C. It is sent to the default interface.
D. It is multiplexed with other traffic leaving the router on interface
Null0.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
409
27. Which of the following are true regarding IP unnumbered? (Choose all
that apply.)
A. It does not work over HDLC networks.
B. It is not compatible with SNMP.
C. It does not work over X.25 networks.
D. You cannot ping an unnumbered interface.
28. Which of the following is true regarding routing protocols?
A. Classful routing protocols send periodic subnet mask information.
B. Classful routing protocols send incremental subnet mask
information.
C. Classful routing protocols send prefix mask information.
D. All devices on a network running classful routing protocols must
use the same mask.
29. In the IOS command used to create an OSPF virtual link, area area-
id virtual-link router-id, what is the area-id?
A. The transit area’s ID
B. The IP address of the highest loopback interface configured on the
router
C. The ID of the area that is not physically adjacent to the
backbone area
D. The highest IP address configured on the router
30. If you wanted to see the configured peer BGP routers and the current
connection state, which Cisco IOS command would you use?
A. show ip bgp all
B. show cdp bgp neighbors
C. show running-config
D. show ip bgp neighbors
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
410
Appendix A
Practice Exam
31. If you wanted to provide stability and availability, which Cisco IOS
features could you use? (Choose all that apply.)
A. Reachability
B. Convergence
C. Alternative path routing
D. Snapshot routing
E. Tunneling
F. Dial backup
G. Load balancing
32. Which of the following is a disadvantage to segmenting a network
with bridges?
A. Bridges do not segment the network.
B. Bridges only create internetworks.
C. Bridges forward all broadcasts.
D. Bridges only filter frames.
33. Which of the following indicates EIGRP process numbers?
A. Link-state value
B. Autonomous system number
C. Path cost
D. Number of ACKs
34. What is the default administrative distance of static routes?
A. 0
B. 1
C. 90
D. 100
E. 110
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
411
35. The IOS commands router bgp 100 and neighbor 10.1.1.1
remote-as 200 provide what function?
A. They configure BGP with an internal iBGP neighbor.
B. They configure BGP with an external iBGP neighbor.
C. They configure BGP with an internal BGP neighbor.
D. They configure BGP with an external BGP neighbor.
36. Which of the following are problems that may occur if route redistri-
bution occurs across multiple EIGRP processes? (Choose all that
apply.)
A. Non-optimal route choices
B. Slow convergence
C. Routing loops
D. Broadcast storms
37. If you want to allow routes to enter a route interface, but deny any
route information from exiting the specified interface, which IOS
command must you use?
A. no routing
B. access filters
C. access lists
D. passive interface
38. In the IOS command match ip address 1, what does the 1 indicate?
A. Sequence number
B. Access list number
C. Process ID
D. Apply to incoming traffic
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
412
Appendix A
Practice Exam
39. If you have a subnet mask of 255.255.255.224, what is the CIDR?
A. /17
B. /23
C. /24
D. /27
40. Which of the following are timers used by the EIGRP neighbor table?
(Choose all that apply.)
A. SRTT
B. RTO
C. Hold timer
D. Time To Live timer
E. Stop timer
41. If you wanted to connect to your company network and load balance
with connections to four different ISPs, which routing protocol must
you use?
A. RIP
B. OSPF
C. BGP
D. EIGRP
42. What is port 179 used for in a BGP session?
A. Route updates
B. To set up FTP between BGP peers
C. To close a session with another BGP peer
D. To open a session with another BGP peer
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
413
43. Which of the following commands can be used on a router when
routes continuously flap to make sure that a link is up before it is
advertised?
A. ip bgp as-path
B. ip bgp hold time
C. set as-path extended
D. bgp dampening
44. Which of the following is a Cisco proprietary BGP attribute?
A. Atomic Aggregate
B. Next-hop
C. MED
D. Weight
45. What is the default administrative distance of OSPF?
A. 0
B. 1
C. 90
D. 100
E. 110
46. If OSPF Hello packets are not received from the neighbor, what state
will the neighbor interface be in?
A. Down
B. Up
C. Init
D. Active
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
414
Appendix A
Practice Exam
47. The BGP hold timer is established in which of the following BGP mes-
sage types?
A. OPEN
B. UPDATE
C. NOTIFICATION
D. KEEPALIVE
48. Which is a solution for a large internetwork that has thousands of
route entries in the routing tables?
A. Route summarization
B. Incremental updates
C. IP filtering
D. VLANs
49. What is the router configuration IOS command to configure a router
as totally stubby?
A. area area-id stub
B. area area-id totally-stubby
C. area area-id stub no-summary
D. area area-id nssa
50. To verify BGP on your Cisco router, which of the following com-
mands should you use?
A. show ip bgp
B. show ip bgp paths
C. show ip bgp summary
D. show bgp all
E. show bgp ip debug
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Practice Exam
1. B. Using TCP port 179, a BGP session is established.
2. B. A stub area does not receive AS External Link Advertisements
(Type 5 LSAs).
3. A, D. Confederations and route reflectors can both be configured to
avoid creating a full-mesh network where the neighbors command
is used excessively.
4. B. EIGRP relies on multicasts to send queries and updates to other
EIGRP peers.
5. A. The IOS command clear ip route * will clear dynamically
established routes from a router’s routing table. When troubleshooting routing problems, this command can be useful in clearing route
tables immediately, instead of waiting for convergence processes to
complete. The other options are invalid syntax.
6. A, B. BGP should be used when multi-homing is used or when you
are connecting more than one ISP to your network.
7. D. Cisco uses 0.0.0.0 0.0.0.0 to represent a default route.
8. D. The Access layer is responsible for breaking up collision domains.
9. C. To show the status of a router’s virtual links (links that span a
transit area in order to reach the backbone area), you can use the IOS
command show ip ospf virtual-links.
10. B. The redistribute static command takes routes that have been
manually configured (static routes) and redistributes them into a specified dynamic routing protocol.
11. B. Using the soft syntax followed by the in and out syntaxes, you
can identify whether the clear ip bgp command is to affect inbound
or outbound triggered updates.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
416
Appendix A
Practice Exam
12. C. EIGRP routes have a default administrative distance
of 90.
13. B, D. The show ip bgp neighbor command displays all the adver-
tised routes, and the show ip bgp command looks at all the
connections.
14. D. If you write out the networks 172.16.100.0/24 and 172.16.106.0/
24 in binary and see how many leading bits that they have in common,
you will find that the first 20 bits are the same for both networks. If
you then convert these 20 bits back into decimal, you will have the
address of the summarized route.
15. A and B. In an OSPF network, there must always be a backbone
area, which is numbered as Area 0. If a router has any of its interfaces
connected to Area 0, that router is said to be a backbone router.
16. A, B, C, E, F. Access lists, snapshot routing, compression tech-
niques, Dial-on-Demand Routing (DDR), and incremental updates all
can help reduce bandwidth usage.
17. A. The show ip protocols command will display such information
as redistribution parameters.
18. B. The command no auto-summary is a router-
configuration command that disables the automatic summarization
of routes.
19. C. Classless routing protocols send prefix routing information with
each update.
20. B. The other options are invalid syntax.
21. D. The show bgp neighbors command shows the configured BGP
peers and the current connection status as shown above.
22. C. A transit AS is an AS through which data from one AS must travel
to get to another AS.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Answers to Practice Exam
417
23. B. The answer here can be tricky. There are six initialization steps,
but not all of them are always seen. Many times the Loading phase is
not necessary between the Exchange state and the Full state.
24. B. Route summarization is most effective when used with contiguous
address space, because contiguous address space tends to have the
most higher-order bits in common.
25. C. The init state is simply the state of receiving Hello packets on the
interface; no adjacencies or other information have been exchanged at
this point.
26. B. Traffic sent to the virtual interface Null0 is discarded.
27. C, D. IP unnumbered is not supported on X.25 or SMDS networks.
Since the serial interface has no IP number, you will not be able to ping
the interface to see if it is up. However, you can determine the interface
status with SNMP. Also, IP security options are not supported on an
IP unnumbered interface.
28. D. Classful routing protocols send no subnet mask information with
the routing updates, so all devices on the network must use the same
subnet mask.
29. A. The area-id parameter in the area area-id virtual-link
router-id command refers to the ID of the transit area. The transit
area connects the backbone area to the area requiring the virtual link.
30. D. The show bgp neighbors command shows the configured BGP
peers and the current connection status.
31. C, D, E, F. Alternate path routing, which provides redundancy and
load balancing, along with snapshot routing, tunneling, and dial
backup all provide stability and availability in an
internetwork.
32. C. Both switches and bridges break up collision domains but are one
large broadcast domain by default. All broadcasts are forwarded to all
network segments with a bridge or switch.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
418
Appendix A
Practice Exam
33. B. The EIGRP process number is always the number assigned to an
autonomous system.
34. B. Static routes have a default administrative distance of 1.
35. D. The commands router bgp 100 and neighbor 10.1.1.1
remote-as 200 configure BGP with an external BGP neighbor.
36. A, B, C. Slow convergence, non-optimal routes, and routing loops
are all problems that can occur by using route redistribution.
37. D. Passive interfaces are used for such interfaces as BRI where you do
not want to have routing updates sent out the interface. If routing
updates were sent out of a BRI interface, then the interface would
never disconnect.
38. B. The 1 refers to the access list against which the match command is
testing traffic.
39. D. If you write out 255.255.255.224 in binary, you will find that the
first 27 bits are ones, and the remaining five bits are zeros. Therefore,
we say that it is a /27.
40. A, B, C. The smooth round-trip timer (SRTT), the retransmission
timer (RTO), and the hold timer are all used by the neighbor table to
track its neighboring routers. The Time To Live and Stop timers are
not used by the EIGRP neighbor table.
41. C. Border Gateway Protocol can load balance connections with as
many as six different ISPs.
42. D. Port 179 is used by BGP to establish a session with another BGP
peer. Ports 20 and 21 are used by FTP, and port 23 is used by Telnet.
43. D. The bgp dampening command is used by BGP to set a hold time
before a route can be re-advertised after route flapping.
44. D. The Weight attribute is a Cisco proprietary attribute used as a
metric only in Cisco implementations of BGPv4.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Practice Exam
419
45. E. OSPF has an administrative distance of 110.
46. A. This status could result from an interface being down, but the spe-
cific OSPF definition is the lack of Hello packets received from the
neighbor.
47. A. The OPEN message type is used to establish a connection between
BGP peers and to negotiate the hold time. The UPDATE message type
is used to advertise topology updates and changes. The NOTIFICATION message type is used to advertise errors. The KEEPALIVE message type is sent to keep a session active when no UPDATE
messages are exchanged during the established hold time.
48. A. Route summarization is used to send fewer route entries in an
update. This can reduce the routing table entries.
49. C. The area area-id stub no-summary IOS router configuration
command is used to configure a router as totally stubby for the specified area. Remember that by becoming totally stubby, a router stops
receiving summary Link State Advertisements.
50. A, B, C. The valid BGP show commands listed above are show ip
bgp, show ip bgp paths, and show ip bgp summary. The show ip
bgp command displays the BGP routing table. The show ip bgp
paths command displays all the router’s known BGP paths. The show
ip bgp summary command tells you the status on every BGP connection. The other two commands are not valid.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Appendix
B
Commands in This Study
Guide
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
T
he following is a compilation of the commands listed in the
“Commands Used in This Chapter” sections, along with an explanation for
each command:
Command
Description
Chapter
aggregate-address
Allows you to configure
aggregate routes in BGP and
CIDR addressing.
8
bgp default localpreference
Allows you to assign a Local
Preference attribute value in
the range of 0 to
4,294,967,295. Higher values
are preferred over lower
values.
8
clear ip bgp
Allows you to clear all or an
identified set of routes from
the BGP table.
8
debug ip bgp
dampening
Displays BGP dampening
events as they occur.
8
debug ip bgp events
Displays all BGP events as
they occur.
8
debug ip bgp
keepalives
Displays all events related to
BGP keepalive packets.
8
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Commands in This Study Guide
Command
Description
Chapter
debug ip bgp updates
Displays information on all
BGP update packets.
8
distance
When multiple protocols are
running, this command allows
a distance value from 1 to 255
to decide which path is the
best. The lowest value wins.
6
ip unnumbered
Allows serial interfaces to
borrow an IP number from
another router interface
(which may or may not be
specified), so that it can joint
two contiguous address
spaces.
3
neighbors
This command has many
syntaxes that allow you to
identify the internal and
external neighbors and assign
different metrics to each.
8
network
Identifies the networks and
masks associated with the
local router.
8
no auto-summary
Used to disable the automatic
route summarization
performed by various classless
routing protocols, such as
RIPv2 and EIGRP.
3
no synchronization
Allows you to turn off
synchronization between the
IGPs and BGP for faster
convergence.
8
passive-interface
interface-type
interface-number
Identifies interfaces that do
not participate in EIGRP
updates.
6
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
423
424
Appendix B
Commands in This Study Guide
Command
Description
Chapter
router bgp
Begins the BGP process and
identifies the local ASN.
8
router eigrp
Starts EIGRP processes on a
router.
6
show ip bgp cidr-only Displays classless routes.
8
show ip bgp community Used to display routes
belonging to the specified
community.
8
show ip bgp
filter-list
Displays AS path lists.
8
show ip bgp paths
Displays all path information
for the local router.
8
show ip bgp
peer-group
Provides information on the
members of the specified peer
group.
8
show ip bgp summary
Shows the status of all BGP
connections.
8
show ip eigrp events
Shows a log of EIGRP events.
These are routes being added
to or removed from the
routing table.
6
show ip eigrp
neighbors
Shows directly connected
EIGRP-enabled routers.
6
show ip eigrp
topology
Shows entries in the EIGRP
topology table.
6
show ip eigrp traffic Shows the packet count for
EIGRP packets sent and
received.
6
show ip protocols
Shows information about the
active protocol sessions.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
6
Commands in This Study Guide
Command
Description
Chapter
show ip route eigrp
Shows all EIGRP neighbors.
6
variance
Assigns a weight to each
feasible successor.
6
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
425
Appendix
Route Summarization
C
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
A
s the size of your networks grows, so does the size of your
route topology and routing tables. Unfortunately, this large amount of information causes more CPU processes to occur and requires more physical
memory on the router. If something isn’t done about the size of the collection
of information contained in the routing tables, your router will be advertising routing information that may be unnecessary to advertise.
Summarization provides an excellent way to reduce the size of the topology and routing tables and significantly reduce the load on the router. Summarization provides a way to aggregate routing information, summarize the
known routes, and reduce the lines in the IGP tables. If summarization
doesn’t occur, every route—including those the router doesn’t need to know
about—will be learned by the router and stored in the tables. This appendix
covers route summarization related to Open Shortest Path First (OSPF),
Enhanced Interior Gateway Routing Protocol (EIGRP), and Border Gateway
Protocol (BGP).
Route Summarization for OSPF
B
y implementing route summarization for OSPF, you help to eliminate the number of Link State Advertisements that are sent when there is a
change in the topology of the network. When route summarization is applied
to OSPF, and when there are frequent changes in the router’s topology, you
can eliminate the advertising of those changes, particularly in the backbone
(Area 0).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Summarization for EIGRP
429
Three individual route types can be found with OSPF and its various configured areas. The route types are as follows:
Intra-area routes (Type O) Routes that are explicit network or subnet
routes. These must be carried inside a configured area, and all area member routers must know about them.
Intra-area routes (Type IA) Routes that exist in the internal autonomous system but not in the router’s configured area.
External routes (Types E1 and E2) Routes that exchange routing
information between autonomous systems.
Configured areas help divide shared routing information. Area Border
Routers (ABR) advertise IA routers from one area to another area.
Route Summarization for EIGRP
E
IGRP does not build the same hierarchy tables that OSPF does but is
capable of reducing the learned routes. By default, EIGRP automatically
summarizes its routes when Variable-Length Subnet Masking (VLSM) is not
used. This means that if your addressing scheme uses an 8-, 16-, or 24-bit
mask with a class A, B, or C network, EIGRP will handle route summarization just fine. If you do use VLSM, then you need to disable the default summarization by using the no auto summary command in the Router
Configuration mode. Then you can manually configure a summarized route
using the ip summary eigrp command on each interface. The command
and syntaxes are as follows:
ip summary-address eigrp <AS number> <network> <mask>
Let’s look at an example of the command where the IP address of
172.16.5.254 is connected to another a router that is connected to two other
routers with a network range of 172.16.16.0 to 172.16.24.0. If we write out
the network numbers in bits, we see that the first 20 bits are identical in each
network address. These first 20 bits are referred to as a CIDR Block. This
block allows the network to be advertised as a single route to the outside
world. Instead of keeping a giant routing table of all the networks individually, the tables have only one entry for all the networks contained in the
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
430
Appendix C
Route Summarization
CIDR Block. This does mean that all those network numbers must be well
planned, and they must reside only out Serial 0 on RouterA in Figure C.1.
FIGURE C.1
Summarizing Routes EIGRP
172.16.5.0/22
RouterA
S0
172.16.16.0/30
S0
RouterB
172.16.20.0/30
172.16.24.0/30
RouterD
RouterC
Let’s take a look at an example of using the ip summary-address eigrp
command:
Cisco3640(config)#interface serial 0
Cisco3640(config-if)#ip address 172.16.16.254
255.255.255.0
Cisco3640(config-if)#ip summary-address eigrp 10
172.16.0.0 255.255.240.0
Cisco3640(config-if)#bandwidth 64
Cisco3640(config-if)#no shut
Route Summarization for BGP
W
e employ route summarization with BGP to limit the number of
routes in the routing table by using the aggregate-address command in
the BGP router configuration mode. This command creates an atomic aggregate, or summarized, entry in the BGP table. The syntax summary-only tells
BGP to advertise only the summary and not the specific routes to each destination. You can use the as-set syntax to include a list of all of the AS numbers
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Route Summarization for BGP
431
that the more specific routes have passed through. The command and the
syntaxes are as follows:
aggregate-address ip-address mask [summary-only]
[as-set]
Let’s take a look at a sample configuration using this command:
RouterA(config)#router bgp 65000
RouterA(config-router)#network 172.16.0.0 mask 255.255.0.0
RouterA(config-router)#neighbor 10.1.1.2 remote-as 64500
RouterA(config-router)#neighbor 172.16.1.50 remote-as
65000
RouterA(config-router)#network 172.16.10.0 mask
255.255.255.0
RouterA(config-router)#network 172.16.1.0 mask
255.255.255.0
RouterA(config-router)#no synchronization
RouterA(config-router)#neighbor 172.16.1.50 next-hop-self
RouterA(config-router)#aggregate-address 172.16.0.0
255.255.0.0 summary-only
Advertising Networks into BGP
Redistribution of routing information occurs in a number of ways. The primary way is the network command, which was discussed in Chapters 8, “Configuring Basic BGP” and 9, “BGP Scalability and Advanced Features.” The
network command allows BGP to advertise a network that is already in the
IP table. When using the network command, you must identify all the networks in the AS that you want to advertise.
You can also use the ip route command to create a static route. The
static route is then redistributed into BGP. Redistribution occurs when a
router uses different protocols to advertise routing information received
between the protocols. BGP considers a static route to be a protocol. Static
route information is advertised to BGP.
The third way to create a static route is to redistribute dynamically learned
routes (routes learned through an IGP) into BGP. In Chapter 8 we learned the
commands to enable this; however, Cisco does not recommend this
approach because of convergence issues and the possibility of introducing
routing loops into the network. Convergence is the time it takes for the network to recover from a change in the network’s topology.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
A&B bit signaling Used in T1 transmission facilities and sometimes called
“24th channel signaling.” Each of the 24 T1 subchannels in this procedure
uses one bit of every sixth frame to send supervisory signaling information.
AAA Authentication, authorization, and accounting: A Cisco description
of the processes that are required to provide a remote access security solution. Each is implemented separately, but each can rely on the others for
functionality.
AAL ATM Adaptation Layer: A service-dependent sublayer of the Data
Link layer, which accepts data from other applications and brings it to the
ATM layer in 48-byte ATM payload segments. CS and SAR are the two sublayers that form AALs. Currently, the four types of AAL recommended by
the ITU-T are AAL1, AAL2, AAL3/4, and AAL5. AALs are differentiated by
the source-destination timing they use, whether they are CBR or VBR, and
whether they are used for connection-oriented or connectionless mode data
transmission. See also: AAL1, AAL2, AAL3/4, AAL5, ATM, and ATM
layer.
AAL1 ATM Adaptation Layer 1: One of four AALs recommended by the
ITU-T, it is used for connection-oriented, time-sensitive services that need
constant bit rates, such as isochronous traffic and uncompressed video.
See also: AAL.
AAL2 ATM Adaptation Layer 2: One of four AALs recommended by the
ITU-T, it is used for connection-oriented services that support a variable bit
rate, such as voice traffic. See also: AAL.
AAL3/4 ATM Adaptation Layer 3/4: One of four AALs (a product of two
initially distinct layers) recommended by the ITU-T, supporting both connectionless and connection-oriented links. Its primary use is in sending
SMDS packets over ATM networks. See also: AAL.
AAL5 ATM Adaptation Layer 5: One of four AALs recommended by the
ITU-T, it is used to support connection-oriented VBR services primarily to
transfer classical IP over ATM and LANE traffic. This least complex of the
AAL recommendations uses SEAL, offering lower bandwidth costs and simpler processing requirements but also providing reduced bandwidth and
error-recovery capacities. See also: AAL.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
435
AARP AppleTalk Address Resolution Protocol: The protocol in an AppleTalk stack that maps data-link addresses to network addresses.
AARP probe packets Packets sent by the AARP to determine whether a
given node ID is being used by another node in a nonextended AppleTalk
network. If the node ID is not in use, the sending node appropriates that
node’s ID. If the node ID is in use, the sending node will select a different ID
and then send out more AARP probe packets. See also: AARP.
ABM Asynchronous Balanced Mode: When two stations can initiate a
transmission, ABM is an HDLC (or one of its derived protocols) communication technology that supports peer-oriented, point-to-point communications between both stations.
ABR area border router: An OSPF router that is located on the border of
one or more OSPF areas. ABRs are used to connect OSPF areas to the OSPF
backbone area.
access control Used by Cisco routers to control packets as they pass
through a router. Access lists are created and then applied to router interfaces to accomplish this.
Access layer One of the layers in Cisco’s three-layer hierarchical model.
The Access layer provides users with access to the internetwork.
access link Is a link used with switches and is only part of one Virtual
LAN (VLAN). Trunk links carry information from multiple VLANs.
access list A set of test conditions kept by routers that determines “interesting traffic” to and from the router for various services on the network.
access method The manner in which network devices approach gaining
access to the network itself.
access rate Defines the bandwidth rate of the circuit. For example, the
access rate of a T1 circuit is 1.544Mbps. In Frame Relay and other technologies, there may be a fractional T1 connection—256Kbps, for example—
however, the access rate and clock rate is still 1.544Mbps.
access server Also known as a “network access server,” it is a communications process connecting asynchronous devices to a LAN or WAN through
network and terminal emulation software, providing synchronous or asynchronous routing of supported protocols.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
436
Glossary
acknowledgment Verification sent from one network device to another
signifying that an event has occurred. May be abbreviated as ACK or Ack.
Contrast with: NAK.
accounting One of the three components in AAA. Accounting provides
auditing and logging functionalities to the security model.
ACR allowed cell rate: A designation defined by the ATM Forum for managing ATM traffic. Dynamically controlled using congestion control measures, the ACR varies between the minimum cell rate (MCR) and the peak
cell rate (PCR). See also: MCR and PCR.
active monitor The mechanism used to manage a Token Ring. The network node with the highest MAC address on the ring becomes the active
monitor and is responsible for management tasks such as preventing loops
and ensuring that tokens are not lost.
address learning Used with transparent bridges to learn the hardware
addresses of all devices on an internetwork. The switch then filters the network with the known hardware (MAC) addresses.
address mapping By translating network addresses from one format to
another, this methodology permits different protocols to operate
interchangeably.
address mask A bit combination descriptor identifying which portion of
an address refers to the network or subnet and which part refers to the host.
Sometimes simply called the mask. See also: subnet mask.
address resolution The process used for resolving differences between
computer addressing schemes. Address resolution typically defines a method
for tracing Network layer (Layer 3) addresses to Data Link layer (Layer 2)
addresses. See also: address mapping.
adjacency The relationship made between defined neighboring routers
and end nodes, using a common media segment, to exchange routing
information.
administrative distance A number between 0 and 225 that expresses the
value of trustworthiness of a routing information source. The lower the
number, the higher the integrity rating.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
437
administrative weight A value designated by a network administrator to
rate the preference given to a network link. It is one of four link metrics
exchanged by PTSPs to test ATM network resource availability.
ADSU ATM Data Service Unit: The terminal adapter used to connect to an
ATM network through an HSSI-compatible mechanism. See also: DSU.
advertising The process whereby routing or service updates are transmitted at given intervals, allowing other routers on the network to maintain
a record of viable routes.
AEP AppleTalk Echo Protocol: A test for connectivity between two AppleTalk nodes where one node sends a packet to another and receives an echo,
or copy, in response.
AFI Authority and Format Identifier: The part of an NSAP ATM address
that delineates the type and format of the IDI section of an ATM address.
AFP AppleTalk Filing Protocol: A Presentation layer protocol, supporting
AppleShare and Mac OS File Sharing, that permits users to share files and
applications on a server.
AIP ATM Interface Processor: Supporting AAL3/4 and AAL5, this interface for Cisco 7000 series routers minimizes performance bottlenecks at the
UNI. See also: AAL3/4 and AAL5.
algorithm A set of rules or processes used to solve a problem. In networking, algorithms are typically used for finding the best route for traffic
from a source to its destination.
alignment error An error occurring in Ethernet networks in which a
received frame has extra bits; that is, a number not divisible by eight. Alignment errors are generally the result of frame damage caused by collisions.
all-routes explorer packet An explorer packet that can move across an
entire SRB network, tracing all possible paths to a given destination. Also
known as an all-rings explorer packet. See also: explorer packet, local
explorer packet, and spanning explorer packet.
AM Amplitude Modulation: A modulation method that represents information by varying the amplitude of the carrier signal. See also: modulation.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
438
Glossary
AMI Alternate Mark Inversion: A line-code type on T1 and E1 circuits that
shows zeros as “01” during each bit cell, and ones as “11” or “00,” alternately, during each bit cell. The sending device must maintain ones density
in AMI but not independently of the data stream. Also known as binarycoded, alternate mark inversion. Contrast with: B8ZS. See also: ones density.
amplitude
An analog or digital waveform’s highest value.
analog Analog signaling is a technique to carry voice and data over copper
and wireless media. When analog signals are transmitted over wires or
through the air, the transmission conveys information through a variation of
some type of signal amplitude, frequency, and phase.
analog connection Provides signaling via an infinitely variable waveform. This differs from a digital connection, in which a definite waveform is
used to define values. Traditional phone service is an analog connection.
analog transmission Signal messaging whereby information is represented by various combinations of signal amplitude, frequency, and phase.
ANSI American National Standards Institute: The organization of corporate, government, and other volunteer members that coordinates standardsrelated activities, approves U.S. national standards, and develops U.S. positions in international standards organizations. ANSI assists in the creation of
international and U.S. standards in disciplines such as communications, networking, and a variety of technical fields. It publishes over 13,000 standards
for engineered products and technologies ranging from screw threads to networking protocols. ANSI is a member of the IEC and ISO.
anycast An ATM address that can be shared by more than one end system,
allowing requests to be routed to a node that provides a particular service.
AppleTalk Currently in two versions, the group of communication protocols designed by Apple Computer for use in Macintosh environments. The
earlier Phase 1 protocols support one physical network with only one network number that resides in one zone. The later Phase 2 protocols support
more than one logical network on a single physical network, allowing networks to exist in more than one zone. See also: zone.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
439
Application layer Layer 7 of the OSI Reference Model, supplying services
to application procedures (such as electronic mail or file transfer) that are
outside the OSI model. This layer chooses and determines the availability of
communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery.
ARA AppleTalk Remote Access: A protocol for Macintosh users establishing their access to resources and data from a remote AppleTalk location.
area A logical, rather than physical, set of segments (based on either
CLNS, DECnet, or OSPF) along with their attached devices. Areas are commonly connected to others using routers to create a single autonomous
system. See also: autonomous system.
ARM Asynchronous Response Mode: An HDLC communication mode
using one primary station and at least one additional station, in which transmission can be initiated from either the primary or one of the secondary
units.
ARP Address Resolution Protocol: Defined in RFC 826, the protocol that
traces IP addresses to MAC addresses. See also: RARP.
ASBR Autonomous System Boundary Router: An area border router
placed between an OSPF autonomous system and a non-OSPF network that
operates both OSPF and an additional routing protocol, such as RIP. ASBRs
must be located in a non-stub OSPF area. See also: ABR, non-stub area,
and OSPF.
ASCII American Standard Code for Information Interchange: An 8-bit
code for representing characters, consisting of seven data bits plus one parity bit.
ASICs Application-Specific Integrated Circuits: Used in Layer 2 switches
to make filtering decisions. The ASIC looks in the filter table of MAC
addresses and determines which port the destination hardware address of a
received hardware address is destined for. The frame will be allowed to
traverse only that one segment. If the hardware address is unknown, the
frame is forwarded out all ports.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
440
Glossary
ASN.1 Abstract Syntax Notation One: An OSI language used to describe
types of data that is independent of computer structures and depicting
methods. Described by ISO International Standard 8824.
ASP AppleTalk Session Protocol: A protocol employing ATP to establish,
maintain, and tear down sessions, as well as sequence requests. See also: ATP.
AST Automatic Spanning Tree: A function that supplies one path for spanning explorer frames traveling from one node in the network to another, supporting the automatic resolution of spanning trees in SRB networks. AST is
based on the IEEE 802.1 standard. See also: IEEE 802.1 and SRB.
asynchronous connection Defines the start and stop of each octet. As a
result, each byte in asynchronous connections requires two bytes of overhead. Synchronous connections use a synchronous clock to mark the start
and stop of each character.
asynchronous dial-up Asynchronous dial-up is interchangeable with
analog dial-up. Both terms refer to traditional modem-based connections.
asynchronous transmission Digital signals sent without precise timing,
usually with different frequencies and phase relationships. Asynchronous
transmissions generally enclose individual characters in control bits (called
start and stop bits) that show the beginning and end of each character. Contrast with: isochronous transmission and synchronous transmission.
ATCP AppleTalk Control Program: The protocol for establishing and configuring AppleTalk over PPP, defined in RFC 1378. See also: PPP.
ATDM Asynchronous Time-Division Multiplexing: A technique for
sending information, it differs from normal TDM in that the time slots are
assigned when necessary rather than preassigned to certain transmitters.
Contrast with: FDM, statistical multiplexing, and TDM.
ATG Address Translation Gateway: The mechanism within Cisco DECnet
routing software that enables routers to route multiple, independent DECnet
networks and to establish a user-designated address translation for chosen
nodes between networks.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
441
ATM Asynchronous Transfer Mode: The international standard, identified
by fixed-length 53-byte cells, for transmitting cells in multiple service systems, such as voice, video, or data. Transit delays are reduced because the
fixed-length cells permit processing to occur in the hardware. ATM is
designed to maximize the benefits of high-speed transmission media, such as
SONET, E3, and T3.
ATM ARP server A device that supplies logical subnets running classical
IP over ATM with address-resolution services.
ATM endpoint The initiating or terminating connection in an ATM network. ATM endpoints include servers, workstations, ATM-to-LAN
switches, and ATM routers.
ATM Forum The international organization founded jointly by Northern
Telecom, Sprint, Cisco Systems, and NET/ADAPTIVE in 1991 to develop
and promote standards-based implementation agreements for ATM technology. The ATM Forum broadens official standards developed by ANSI
and ITU-T and creates implementation agreements before official standards
are published.
ATM layer A sublayer of the Data Link layer in an ATM network that is
service-independent. To create standard 53-byte ATM cells, the ATM layer
receives 48-byte segments from the AAL and attaches a 5-byte header to
each. These cells are then sent to the Physical layer for transmission across
the physical medium. See also: AAL.
ATMM ATM Management: A procedure that runs on ATM switches,
managing rate enforcement and VCI translation. See also: ATM.
ATM user-user connection A connection made by the ATM layer to
supply communication between at least two ATM service users, such as
ATMM processes. These communications can be uni- or bidirectional, using
one or two VCCs, respectively. See also: ATM layer and ATMM.
ATP AppleTalk Transaction Protocol: A Transport-level protocol that
enables reliable transactions between two sockets, where one requests the
other to perform a given task and to report the results. ATP fastens the
request and response together, assuring a loss-free exchange of requestresponse pairs.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
442
Glossary
attenuation In communication, weakening or loss of signal energy, typically caused by distance.
AURP AppleTalk Update-based Routing Protocol: A technique for encapsulating AppleTalk traffic in the header of a foreign protocol that allows the
connection of at least two noncontiguous AppleTalk internetworks through
a foreign network (such as TCP/IP) to create an AppleTalk WAN. The connection made is called an AURP tunnel. By exchanging routing information
between exterior routers, the AURP maintains routing tables for the complete AppleTalk WAN. See also: AURP tunnel.
AURP tunnel A connection made in an AURP WAN that acts as a single,
virtual link between AppleTalk internetworks separated physically by a foreign network such as a TCP/IP network. See also: AURP.
authentication The first component in the AAA model. Users are typically
authenticated via a username and password, which are used to uniquely
identify them.
authority zone A portion of the domain-name tree associated with DNS
for which one name server is the authority. See also: DNS.
authorization The act of permitting access to a resource based on authentication information in the AAA model.
auto duplex A setting on Layer 1 and 2 devices that sets the duplex of a
switch or hub port automatically.
automatic call reconnect A function that enables automatic call
rerouting away from a failed trunk line.
autonomous confederation A collection of self-governed systems that
depend more on their own network accessibility and routing information
than on information received from other systems or groups.
autonomous switching The ability of Cisco routers to process packets
more quickly by using the ciscoBus to switch packets independently of the
system processor.
autonomous system (AS) A group of networks under mutual administration that share the same routing methodology. Autonomous systems are
subdivided by areas and must be assigned an individual 16-bit number by the
IANA. See also: area.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
443
autoreconfiguration A procedure executed by nodes within the failure
domain of a Token Ring, wherein nodes automatically perform diagnostics,
trying to reconfigure the network around failed areas.
Auto-RP An IOS feature that allows multicast-enabled routers to detect RP
and forward the summary information to other routers and hosts.
auxiliary port The console port on the back of Cisco routers that allows
you to dial the router and make console configuration settings.
AVVID Architecture for Voice, Video and Integrated Data: This is a Cisco
marketing term to group their convergence efforts. Convergence is the integration of historically distinct services into a single service.
B8ZS Binary 8-Zero Substitution: A line-code type, interpreted at the
remote end of the connection, that uses a special code substitution whenever
eight consecutive zeros are transmitted over the link on T1 and E1 circuits.
This technique assures ones density independent of the data stream. Also
known as bipolar 8-zero substitution. Contrast with: AMI. See also: ones
density.
backbone The basic portion of the network that provides the primary
path for traffic sent to and initiated from other networks.
back end A node or software program supplying services to a front end.
See also: server.
backup designated router (BDR) Used in OSPF routing to make sure
area information is still advertised if the designated router goes down.
bandwidth The gap between the highest and lowest frequencies employed
by network signals. More commonly, it refers to the rated throughput
capacity of a network protocol or medium.
BoD Bandwidth on Demand: This function allows an additional B channel
to be used to increase the amount of bandwidth available for a particular
connection.
baseband A feature of a network technology that uses only one carrier frequency. For example, Ethernet. Also named “narrowband.” Compare with:
broadband.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
444
Glossary
Basic Management Setup Used with Cisco routers when in setup mode.
Provides only enough management and configuration to get the router
working so someone can telnet into the router and configure it.
baud Synonymous with bits per second (bps), if each signal element represents one bit. It is a unit of signaling speed equivalent to the number of separate signal elements transmitted per second.
B channel Bearer channel: A full-duplex, 64Kbps channel in ISDN that
transmits user data. Compare with: D channel, E channel, and H channel.
beacon An FDDI device or Token Ring frame that points to a serious
problem with the ring, such as a broken cable. The beacon frame carries the
address of the station thought to be down. See also: failure domain.
bearer service Used by service providers to provide DS0 service to ISDN
customers. A DS0 is one 64K channel. An ISDN bearer service provides
either two DS0s, called two bearer channels, for a Basic Rate Interface (BRI),
or 24 DS0s, called a Primary Rate Interface (PRI).
BECN Backward Explicit Congestion Notification: BECN is the bit set by
a Frame Relay network in frames moving away from frames headed into a
congested path. A DTE that receives frames with the BECN may ask higherlevel protocols to take necessary flow control measures. Compare with: FECN.
BGP4 BGP Version 4: Version 4 of the interdomain routing protocol most
commonly used on the Internet. BGP4 supports CIDR and uses routecounting mechanisms to decrease the size of routing tables. See also: CIDR.
bidirectional shared tree A method of shared tree multicast forwarding.
This method allows group members to receive data from the source or the
RP, whichever is closer. See also: RP (rendezvous point).
binary A two-character numbering method that uses ones and zeros. The
binary numbering system underlies all digital representation of information.
BIP Bit Interleaved Parity: A method used in ATM to monitor errors on a
link, sending a check bit or word in the link overhead for the previous block
or frame. This allows bit errors in transmissions to be found and delivered as
maintenance information.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
445
BISDN Broadband ISDN: ITU-T standards created to manage highbandwidth technologies such as video. BISDN presently employs ATM technology along SONET-based transmission circuits, supplying data rates
between 155Mbps and 622Mbps and beyond. See also: BRI, ISDN,
and PRI.
bit-oriented protocol Regardless of frame content, the class of Data Link
layer communication protocols that transmits frames. Bit-oriented protocols, as compared with byte-oriented, supply more efficient and trustworthy
full-duplex operation. Compare with: byte-oriented protocol.
Boot ROM Used in routers to put the router into bootstrap mode. Bootstrap mode then boots the device with an operating system. The ROM can
also hold a small Cisco IOS.
border gateway A router that facilitates communication with routers in
different autonomous systems.
border router Typically defined within Open Shortest Path First (OSPF)
as a router that connected an area to the backbone area. However, a border
router can be a router that connects a company to the Internet as well.
See also: OSPF.
BPDU Bridge Protocol Data Unit: A Spanning Tree Protocol initializing
packet that is sent at definable intervals for the purpose of exchanging information among bridges in networks.
BRI Basic Rate Interface: The ISDN interface that facilitates circuitswitched communication between video, data, and voice; it is made up of
two B channels (64Kbps each) and one D channel (16Kbps). Compare with:
PRI. See also: BISDN.
bridge A device for connecting two segments of a network and transmitting packets between them. Both segments must use identical protocols to
communicate. Bridges function at the Data Link layer, Layer 2 of the OSI
Reference Model. The purpose of a bridge is to filter, send, or flood any
incoming frame, based on the MAC address of that particular frame.
bridge ID Used to find and elect the root bridge in a Layer 2 switched internetwork. The bridge ID is a combination of the bridge priority and base
MAC address.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
446
Glossary
bridging A Layer 2 process to block or forward frames based on MAC
layer addresses. Bridges are lower speed, lower port density switches.
broadband A transmission methodology for multiplexing several independent signals onto one cable. In telecommunications, broadband is classified as any channel with bandwidth greater than 4kHz (typical voice grade).
In LAN terminology, it is classified as a coaxial cable on which analog signaling is employed. Also known as “wideband.” Contrast with: baseband.
broadcast A data frame or packet that is transmitted to every node on the
local network segment (as defined by the broadcast domain). Broadcasts are
known by their broadcast address, which is a destination network and host
address with all the bits turned on. Also called “local broadcast.” Compare
with: directed broadcast.
broadcast domain A group of devices receiving broadcast frames initiating from any device within the group. Because they do not forward broadcast frames, broadcast domains are generally surrounded by routers.
broadcast storm An undesired event on the network caused by the simultaneous transmission of any number of broadcasts across the network segment. Such an occurrence can overwhelm network bandwidth, resulting in
time-outs.
brute force attack A brute force attack bombards the resource with
attempted connections until successful. In the most common brute force
attack, different passwords are repeatedly tried until a match is found that
is then used to compromise the network.
buffer A storage area dedicated to handling data while in transit. Buffers
are used to receive/store sporadic deliveries of data bursts, usually received
from faster devices, compensating for the variations in processing speed.
Incoming information is stored until everything is received prior to sending
data on. Also known as an “information buffer.”
bursting Some technologies, including ATM and Frame Relay, are considered burstable. This means that user data can exceed the bandwidth normally reserved for the connection; however, this cannot exceed the port
speed. An example of this would be a 128Kbps Frame Relay CIR on a T1—
depending on the vendor, it may be possible to send more than 128Kbps for
a short time.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
447
bus topology A linear LAN architecture in which transmissions from various stations on the network are reproduced over the length of the medium
and are accepted by all other stations. Compare with: ring and star.
bus Any physical path, typically wires or copper, through which a digital
signal can be used to send data from one part of a computer to another.
BUS broadcast and unknown servers: In LAN emulation, the hardware or
software responsible for resolving all broadcasts and packets with unknown
(unregistered) addresses into the point-to-point virtual circuits required by
ATM. See also: LANE, LEC, LECS, and LES.
BX.25
AT&T’s use of X.25. See also: X.25.
bypass mode
an interface.
An FDDI and Token Ring network operation that deletes
bypass relay A device that enables a particular interface in the Token
Ring to be closed down and effectively taken off the ring.
byte
Eight bits of binary.
byte-oriented protocol Any type of data-link communication protocol
that, in order to mark the boundaries of frames, uses a specific character
from the user character set. These protocols have generally been superseded
by bit-oriented protocols. Compare with: bit-oriented protocol.
cable modem A cable modem is not actually an analog device, like an
asynchronous modem, but rather a customer access device for linking to a
broadband cable network. These devices are typically bridges that have a
COAX connection to link to the cable network and a 10BaseT Ethernet connection to link to the user’s PC.
cable range In an extended AppleTalk network, the range of numbers
allotted for use by existing nodes on the network. The value of the cable
range can be anywhere from a single to a sequence of several touching network numbers. Node addresses are determined by their cable range value.
CAC Connection Admission Control: The sequence of actions executed by
every ATM switch while connection setup is performed in order to determine if a request for connection is violating the guarantees of QoS for established connections. Also, CAC is used to route a connection request through
an ATM network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
448
Glossary
call admission control A device for managing traffic in ATM networks,
determining the possibility of a path containing adequate bandwidth for a
requested VCC.
call priority In circuit-switched systems, the defining priority given to each
originating port; it specifies in which order calls will be reconnected. Additionally, call priority identifies which calls are allowed during a bandwidth
reservation.
call set-up time The length of time necessary to effect a switched call
between DTE devices.
candidate packets Packets identified by the MLS-SE as having the potential for establishing a flow cache. This determination is made based on the
destination MAC (DMAC) address. The DMAC address must be a MAC
address associated with a known MLS-RP. See also: MLS-SE, MLS-SE, and
MLS-RP.
CBR Constant Bit Rate: An ATM Forum QoS class created for use in ATM
networks. CBR is used for connections that rely on precision clocking to
guarantee trustworthy delivery. Compare with: ABR and VBR.
CD Carrier Detect: A signal indicating that an interface is active or that a
connection generated by a modem has been established.
CDP Cisco Discovery Protocol: Cisco’s proprietary protocol that is used to
tell a neighboring Cisco device about the type of hardware, software version,
and active interfaces that the Cisco device is using. It uses a SNAP frame
between devices and is not routable.
CDVT Cell Delay Variation Tolerance: A QoS parameter for traffic management in ATM networks specified when a connection is established. The
allowable fluctuation levels for data samples taken by the PCR in CBR transmissions are determined by the CDVT. See also: CBR and PCR.
cell In ATM networking, the basic unit of data for switching and multiplexing. Cells have a defined length of 53 bytes, including a 5-byte header
that identifies the cell’s data stream and 48 bytes of payload. See also: cell
relay.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
449
cell payload scrambling The method by which an ATM switch maintains
framing on some medium-speed edge and trunk interfaces (T3 or E3 circuits). Cell payload scrambling rearranges the data portion of a cell to maintain the line synchronization with certain common bit patterns.
cell relay A technology that uses small packets of fixed size, known as
cells. Their fixed length enables cells to be processed and switched in hardware at high speeds, making this technology the foundation for ATM and
other high-speed network protocols. See also: cell.
Centrex A local exchange carrier service, providing local switching that
resembles that of an on-site PBX. Centrex has no on-site switching capability. Therefore, all customer connections return to the CO. See also: CO.
CER Cell Error Ratio: In ATM the ratio of the number of transmitted cells
having errors to the total number of cells sent in a transmission within a certain span of time.
CGMP Cisco Group Management Protocol: A proprietary protocol developed by Cisco. The router uses CGMP to send multicast membership commands to Catalyst switches.
Challenge Used to provide authentication in Challenge Handshake
Authentication Protocol (CHAP) as part of the handshake process. This
numerically unique query is sent to authenticate the user without sending the
password unencrypted across the wire. See also: CHAP.
channelized E1 Operating at 2.048Mpbs, an access link that is sectioned
into 29 B channels and one D channel, supporting DDR, Frame Relay, and
X.25. Compare with: channelized T1.
channelized T1 Operating at 1.544Mbps, an access link that is sectioned
into 23 B channels and one D channel of 64Kbps each, where individual
channels or groups of channels connect to various destinations, supporting
DDR, Frame Relay, and X.25. Compare with: channelized E1.
CHAP Challenge Handshake Authentication Protocol: Supported on lines
using PPP encapsulation, it is a security feature that identifies the remote end,
helping keep out unauthorized users. After CHAP is performed, the router or
access server determines whether a given user is permitted access. It is a
newer, more secure protocol than PAP. Compare with: PAP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
450
Glossary
character mode connections Character mode connections are typically
terminated at the access server and include Telnet and console connections.
checksum A test for ensuring the integrity of sent data. It is a number calculated from a series of values taken through a sequence of mathematical
functions, typically placed at the end of the data from which it is calculated,
and then recalculated at the receiving end for verification. Compare with: CRC.
choke packet When congestion exists, it is a packet sent to inform a transmitter that it should decrease its sending rate.
CIDR Classless Interdomain Routing: A method supported by classless
routing protocols, such as OSPF and BGP4, based on the concept of ignoring
the IP class of address, permitting route aggregation and VLSM that enable
routers to combine routes in order to minimize the routing information that
needs to be conveyed by the primary routers. It allows a group of IP networks to appear to other networks as a unified, larger entity. In CIDR, IP
addresses and their subnet masks are written as four dotted octets, followed
by a forward slash and the numbering of masking bits (a form of subnet
notation shorthand). See also: BGP4.
CIP Channel Interface Processor: A channel attachment interface for use in
Cisco 7000 series routers that connects a host mainframe to a control unit.
This device eliminates the need for an FBP to attach channels.
CIR Committed Information Rate: Averaged over a minimum span of time
and measured in bps, a Frame Relay network’s agreed-upon minimum rate
of transferring information.
circuit switching Used with dial-up networks such as PPP and ISDN.
Passes data, but needs to set up the connection first—just like making a
phone call.
Cisco FRAD Cisco Frame-Relay Access Device: A Cisco product that supports Cisco IPS Frame Relay SNA services, connecting SDLC devices to
Frame Relay without requiring an existing LAN. May be upgraded to a fully
functioning multiprotocol router. Can activate conversion from SDLC to
Ethernet and Token Ring, but does not support attached LANs.
See also: FRAD.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
451
CiscoFusion Cisco’s name for the internetworking architecture under
which its Cisco IOS operates. It is designed to “fuse” together the capabilities
of its disparate collection of acquired routers and switches.
Cisco IOS software Cisco Internet Operating System software. The
kernel of the Cisco line of routers and switches that supplies shared functionality, scalability, and security for all products under its CiscoFusion
architecture. See also: CiscoFusion.
CiscoView GUI-based management software for Cisco networking
devices, enabling dynamic status, statistics, and comprehensive configuration information. Displays a physical view of the Cisco device chassis and
provides device-monitoring functions and fundamental troubleshooting
capabilities. May be integrated with a number of SNMP-based network
management platforms.
Class A network Part of the Internet Protocol hierarchical addressing
scheme. Class A networks have only 8 bits for defining networks and 24 bits
for defining hosts on each network.
Class B network Part of the Internet Protocol hierarchical addressing
scheme. Class B networks have 16 bits for defining networks and 16 bits for
defining hosts on each network.
Class C network Part of the Internet Protocol hierarchical addressing
scheme. Class C networks have 24 bits for defining networks and only 8 bits
for defining hosts on each network.
classical IP over ATM Defined in RFC 1577, the specification for running
IP over ATM that maximizes ATM features. Also known as “CIA.”
classless routing Routing that sends subnet mask information in the
routing updates. Classless routing allows Variable-Length Subnet Mask
(VLSM) and supernetting. Routing protocols that support classless routing
are RIP version 2, EIGRP, and OSPF.
CLI Command Line Interface: Allows you to configure Cisco routers and
switches with maximum flexibility.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
452
Glossary
clocking Used in synchronous connections to provide a marker for the
start and end of data bytes. This is similar to the beat of a drum with a
speaker talking only when the drum is silent.
CLP Cell Loss Priority: The area in the ATM cell header that determines
the likelihood of a cell being dropped during network congestion. Cells with
CLP = 0 are considered insured traffic and are not apt to be dropped. Cells
with CLP = 1 are considered best-effort traffic that may be dropped during
congested episodes, delivering more resources to handle insured traffic.
CLR Cell Loss Ratio: The ratio of discarded cells to successfully delivered
cells in ATM. CLR can be designated a QoS parameter when establishing a
connection.
CO Central Office: The local telephone company office where all loops in
a certain area connect and where circuit switching of subscriber lines occurs.
collapsed backbone A nondistributed backbone where all network segments are connected to each other through an internetworking device. A collapsed backbone can be a virtual network segment at work in a device such
as a router, hub, or switch.
collapsed core A collapsed core is defined as one switch performing both
Core and Distribution layer functions. Typically found in a small network,
the functions of the Core and Distribution layers are still distinct.
collision The effect of two nodes sending transmissions simultaneously in
Ethernet. When they meet on the physical media, the frames from each node
collide and are damaged. See also: collision domain.
collision domain The network area in Ethernet over which frames that
have collided will spread. Collisions are propagated by hubs and repeaters,
but not by LAN switches, routers, or bridges. See also: collision.
composite metric Used with routing protocols, such as IGRP and EIGRP,
that use more than one metric to find the best path to a remote network.
IGRP and EIGRP both use bandwidth and delay of the line by default. However, maximum transmission unit (MTU), load, and reliability of a link can
be used as well.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
453
compression A technique to send more data across a link than would be
normally permitted by representing repetitious strings of data with a single
marker.
configuration register A 16-bit configurable value stored in hardware or
software that determines how Cisco routers function during initialization. In
hardware, the bit position is set using a jumper. In software, it is set by specifying specific bit patterns used to set startup options, configured using a
hexadecimal value with configuration commands.
congestion
Traffic that exceeds the network’s ability to handle it.
congestion avoidance To minimize delays, the method an ATM network
uses to control traffic entering the system. Lower-priority traffic is discarded
at the edge of the network when indicators signal it cannot be delivered, thus
using resources efficiently.
congestion collapse The situation that results from the retransmission of
packets in ATM networks where little or no traffic successfully arrives at
destination points. It usually happens in networks made of switches with
ineffective or inadequate buffering capabilities combined with poor packet
discard or ABR congestion feedback mechanisms.
connection ID Identifications given to each Telnet session into a router.
The show sessions command will give you the connections a local router
will have to a remote router. The show users command will show the connection IDs of users telnetted into your local router.
connectionless Data transfer that occurs without the creating of a virtual
circuit. No overhead, best-effort delivery, not reliable. Contrast with: connection-oriented. See also: virtual circuit.
connection-oriented Data transfer method that sets up a virtual circuit
before any data is transferred. Uses acknowledgments and flow control for
reliable data transfer. Contrast with: connectionless. See also: virtual circuit.
console port Typically an RJ-45 port on a Cisco router and switch that
allows command line interface capability.
contention media Media access method that is a baseband media; that is,
first come, first served. Ethernet is an example of a contention media access.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
454
Glossary
control direct VCC One of three control connections defined by Phase I
LAN Emulation; a bidirectional virtual control connection (VCC) established in ATM by an LEC to an LES. See also: control distribute VCC.
control distribute VCC One of three control connections defined by
Phase 1 LAN Emulation; a unidirectional virtual control connection (VCC)
set up in ATM from an LES to an LEC. Usually, the VCC is a point-tomultipoint connection. See also: control direct VCC.
convergence The process required for all routers in an internetwork to
update their routing tables and create a consistent view of the network, using
the best possible paths. No user data is passed during a convergence time.
core block If you have two or more switch blocks, the Cisco rule of thumb
states that you need a core block. No routing is performed at the core, only
transferring of data. It is a pass-through for the switch block, the server
block, and the Internet. The core is responsible for transferring data to and
from the switch blocks as quickly as possible. You can build a fast core with
a frame, packet, or cell (ATM) network technology.
Core layer Top layer in the Cisco three-layer hierarchical model, which
helps you design, build, and maintain Cisco hierarchical networks. The Core
layer passes packets quickly to Distribution layer devices only. No packet filtering should take place at this layer.
cost An arbitrary value, based on hop count, bandwidth, or other calculation, that is typically assigned by a network administrator and used by the
routing protocol to compare different routes through an internetwork.
Routing protocols use cost values to select the best path to a certain destination: The lowest cost identifies the best path. Also known as “path cost.”
See also: routing metric.
count to infinity A problem occurring in routing algorithms that are slow
to converge where routers keep increasing the hop count to particular networks. To avoid this problem, various solutions have been implemented into
each of the different routing protocols. Some of those solutions include
defining a maximum hop count (defining infinity), route poisoning, poison
reverse, and split horizon.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
455
CPCS Common Part Convergence Sublayer: One of two AAL sublayers
that are service-dependent, it is further segmented into the CS and SAR sublayers. The CPCS prepares data for transmission across the ATM network;
it creates the 48-byte payload cells that are sent to the ATM layer. See also:
AAL and ATM layer.
CPE Customer Premises Equipment: Items such as telephones, modems,
and terminals installed at customer locations and connected to the telephone
company network.
crankback In ATM, a correction technique used when a node somewhere
on a chosen path cannot accept a connection setup request, blocking the
request. The path is rolled back to an intermediate node, which then uses
GCAC to attempt to find an alternate path to the final destination.
CRC Cyclic Redundancy Check: A methodology that detects errors,
whereby the frame recipient makes a calculation by dividing frame contents
with a prime binary divisor and compares the remainder to a value stored in
the frame by the sending node. Contrast with: checksum.
CSMA/CD Carrier Sense Multiple Access Collision Detect: A technology
defined by the Ethernet IEEE 802.3 committee. Each device senses the cable
for a digital signal before transmitting. Also, CSMA/CD allows all devices on
the network to share the same cable, but one at a time. If two devices
transmit at the same time, a frame collision will occur and a jamming pattern
will be sent; the devices will stop transmitting, wait a predetermined amount
of time, and then try to transmit again.
CST Common Spanning Tree: The IEEE uses what is called Common
Spanning Tree (CST), which is defined with IEEE 802.1q. The IEEE 802.1q
defines one spanning tree instance for all VLANs.
CSU Channel Service Unit: A digital mechanism that connects end-user
equipment to the local digital telephone loop. Frequently referred to along
with the Data Service Unit as CSU/DSU. See also: DSU.
CTD Cell Transfer Delay: For a given connection in ATM, the time period
between a cell exit event at the source user-network interface (UNI) and the
corresponding cell entry event at the destination. The CTD between these
points is the sum of the total inter-ATM transmission delay and the total
ATM processing delay.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
456
Glossary
custom queuing Used by Cisco router IOS to provide a queuing method
to slower serial links. Custom queuing allows an administrator to configure
the type of traffic that will have priority over the link.
cut-through frame switching A frame-switching technique that flows
data through a switch so that the leading edge exits the switch at the output
port before the packet finishes entering the input port. Frames will be read,
processed, and forwarded by devices that use cut-through switching as soon
as the destination address of the frame is confirmed and the outgoing port is
identified.
data compression
See: compression.
data direct VCC A bidirectional point-to-point virtual control connection
(VCC) set up between two LECs in ATM and one of three data connections
defined by Phase 1 LAN Emulation. Because data direct VCCs do not guarantee QoS, they are generally reserved for UBR and ABR connections.
Compare with: control distribute VCC and control direct VCC.
data encapsulation The process in which the information in a protocol is
wrapped, or contained, in the data section of another protocol. In the OSI
Reference Model, each layer encapsulates the layer immediately above it as
the data flows down the protocol stack.
data frame Protocol Data Unit encapsulation at the Data Link layer of the
OSI Reference Model. Encapsulates packets from the Network layer and
prepares the data for transmission on a network medium.
datagram A logical collection of information transmitted as a Network
layer unit over a medium without a previously established virtual circuit. IP
datagrams have become the primary information unit of the Internet. At various layers of the OSI Reference Model, the terms cell, frame, message,
packet, and segment also define these logical information groupings.
data link control layer Layer 2 of the SNA architectural model, it is
responsible for the transmission of data over a given physical link and compares somewhat to the Data Link layer of the OSI model.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
457
Data Link layer Layer 2 of the OSI reference model, it ensures the trustworthy transmission of data across a physical link and is primarily concerned with physical addressing, line discipline, network topology, error
notification, ordered delivery of frames, and flow control. The IEEE has further segmented this layer into the MAC sublayer and the LLC sublayer. Also
known as the Link layer. Can be compared somewhat 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.
DCC Data Country Code: Developed by the ATM Forum, one of two ATM
address formats designed for use by private networks. Compare with: ICD.
DCE data communications equipment (as defined by the EIA) or data circuit-terminating equipment (as defined by the ITU-T): The mechanisms and
links of a communications network that make up the network portion of the
user-to-network interface, such as modems. The DCE supplies the physical
connection to the network, forwards traffic, and provides a clocking signal
to synchronize data transmission between DTE and DCE devices. Compare
with: DTE.
D channel 1) Data channel: A full-duplex, 16Kbps (BRI) or 64Kbps (PRI)
ISDN channel. Compare with: B channel, E channel, and H channel. 2) In
SNA, anything that provides a connection between the processor and main
storage with any peripherals.
DDP Datagram Delivery Protocol: Used in the AppleTalk suite of protocols
as a connectionless protocol that is responsible for sending datagrams
through an internetwork.
DDR dial-on-demand routing: A technique that allows a router to automatically initiate and end a circuit-switched session per the requirements of
the sending station. By mimicking keepalives, the router fools the end station
into treating the session as active. DDR permits routing over ISDN or telephone lines via a modem or external ISDN terminal adapter.
DE Discard Eligibility: Used in Frame Relay networks to tell a switch that
a frame can be discarded if the switch is too busy. The DE is a field in the
frame that is turned on by transmitting routers if the Committed Information Rate (CIR) is oversubscribed or set to 0.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
458
Glossary
DE bit The DE bit marks a frame as discard eligible on a Frame Relay network. If a serial link is congested and the Frame Relay network has passed
the Committed Information Rate (CIR), then the DE bit will always be on.
default route The static routing table entry used to direct frames whose
next hop is not spelled out in the dynamic routing table.
delay The time elapsed between a sender’s initiation of a transaction and
the first response they receive. Also, the time needed to move a packet from
its source to its destination over a path. See also: latency.
demarc The demarcation point between the customer premises equipment
(CPE) and the telco’s carrier equipment.
demodulation A series of steps that return a modulated signal to its original form. When receiving, a modem demodulates an analog signal to its
original digital form (and, conversely, modulates the digital data it sends into
an analog signal). See also: modulation.
demultiplexing The process of converting a single multiplex signal, comprising more than one input stream, back into separate output streams. See
also: multiplexing.
denial-of-service attack A denial-of-service attack, or DoS, blocks access
to a network resource by saturating the device with attacking data. Typically, this is targeted against the link (particularly lower bandwidth links) or
the server. DDoS attacks, or distributed denial-of-service attacks, make use
of multiple originating attacking resources to saturate a more capable
resource.
designated bridge In the process of forwarding a frame from a segment
to the route bridge, the bridge with the lowest path cost.
designated port Used with the Spanning Tree Protocol (STP) to designate
forwarding ports. If there are multiple links to the same network, STP will
shut a port down to stop network loops.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
459
designated router An OSPF router that creates LSAs for a multi-access
network and is required to perform other special tasks in OSPF operations.
Multi-access OSPF networks that maintain a minimum of two attached
routers identify one router that is chosen by the OSPF Hello protocol, which
makes possible a decrease in the number of adjacencies necessary on a multiaccess network. This in turn reduces the quantity of routing protocol traffic
and the physical size of the database.
destination address The address for the network devices that will receive
a packet.
dial backup Dial backup connections are typically used to provide redundancy to Frame Relay connections. The backup link is activated over an
analog modem.
digital A digital waveform is one where distinct ones and zeros provide the
data representation. See also: analog.
directed broadcast A data frame or packet that is transmitted to a specific
group of nodes on a remote network segment. Directed broadcasts are
known by their broadcast address, which is a destination subnet address
with all the bits turned on.
discovery mode Also known as dynamic configuration, this technique is
used by an AppleTalk interface to gain information from a working node
about an attached network. The information is subsequently used by the
interface for self-configuration.
distance-vector protocol Type of routing protocol that sends complete
routing table on periodic intervals to neighbor routers.
distance-vector routing algorithm In order to find the shortest path, this
group of routing algorithms repeats on the number of hops in a given route,
requiring each router to send its complete routing table with each update,
but only to its neighbors. Routing algorithms of this type tend to generate
loops, but they are fundamentally simpler than their link-state counterparts.
See also: link-state routing algorithm and SPF.
Distribution layer Middle layer of the Cisco three-layer hierarchical
model, which helps you design, install, and maintain Cisco hierarchical networks. The Distribution layer is the point where Access layer devices connect. Routing is performed at this layer.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
460
Glossary
distribution lists Access lists used to filter incoming and outgoing route
table entries on a router.
DLCI Data-Link Connection Identifier: Used to identify virtual circuits in a
Frame Relay network.
DNS
Domain Name System: Used to resolve host names to IP addresses.
DSAP Destination Service Access Point: The service access point of a network node, specified in the destination field of a packet. See also: SSAP and SAP.
DSL Digital Subscriber Line: DSL technologies are used to provide broadband services over a single copper pair, typically to residential customers.
Most vendors are providing DSL services at up to 6Mbps downstream, but
the technology can support 52Mbps service.
DSR Data Set Ready: When a DCE is powered up and ready to run, this
EIA/TIA-232 interface circuit is also engaged.
DSU Data Service Unit: This device is used to adapt the physical interface
on a data terminal equipment (DTE) mechanism to a transmission facility
such as T1 or E1 and is also responsible for signal timing. It is commonly
grouped with the Channel Service Unit and referred to as the CSU/DSU.
See also: CSU.
DTE data terminal equipment: Any device located at the user end of a usernetwork interface serving as a destination, a source, or both. DTE includes
devices such as multiplexers, protocol translators, and computers. The connection to a data network is made through data communications equipment
(DCE) such as a modem, using the clocking signals generated by that device.
See also: DCE.
DTR data terminal ready: An activated EIA/TIA-232 circuit communicating to the DCE the state of preparedness of the DTE to transmit or
receive data.
DUAL Diffusing Update Algorithm: Used in Enhanced IGRP, this convergence algorithm provides loop-free operation throughout an entire route’s
computation. DUAL grants routers involved in a topology revision the
ability to synchronize simultaneously, while routers unaffected by this
change are not involved. See also: Enhanced IGRP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
461
DVMRP Distance Vector Multicast Routing Protocol: Based primarily on
the Routing Information Protocol (RIP), this Internet gateway protocol
implements a common, condensed-mode IP multicast scheme, using IGMP
to transfer routing datagrams between its neighbors. See also: IGMP.
DXI Data Exchange Interface: Described in RFC 1482, DXI defines the
effectiveness of a network device such as a router, bridge, or hub to act as an
FEP to an ATM network by using a special DSU that accomplishes packet
encapsulation.
dynamic entries Used in Layer 2 and 3 devices to create a table of either
hardware addresses or logical addresses dynamically.
dynamic routing Also known as adaptive routing, this technique automatically adapts to traffic or physical network revisions.
dynamic VLAN An administrator will create an entry in a special server
with the hardware addresses of all devices on the internetwork. The server
will then assign dynamically used VLANs.
E1 Generally used in Europe, a wide-area digital transmission scheme carrying data at 2.048Mbps. E1 transmission lines are available for lease from
common carriers for private use.
E.164 1) Evolved from standard telephone numbering system, the standard
recommended by ITU-T for international telecommunication numbering,
particularly in ISDN, SMDS, and BISDN. 2) Label of field in an ATM
address containing numbers in E.164 format.
E channel Echo channel: A 64Kbps ISDN control channel used for circuit
switching. Specific description of this channel can be found in the 1984 ITUT ISDN specification, but was dropped from the 1988 version. See also:
B channel, D channel, and H channel.
edge device A device that enables packets to be forwarded between legacy
interfaces (such as Ethernet and Token Ring) and ATM interfaces based on
information in the Data Link and Network layers. An edge device does not
take part in the running of any Network layer routing protocol; it merely
uses the route description protocol in order to get the forwarding information required.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
462
Glossary
EEPROM Electronically Erasable Programmable Read-Only Memory:
Programmed after their manufacture, these nonvolatile memory chips can be
erased if necessary using electric power and reprogrammed. See also: EPRO
and PROM.
EFCI Explicit Forward Congestion Indication: A congestion feedback
mode permitted by ABR service in an ATM network. The EFCI may be set
by any network element that is in a state of immediate or certain congestion.
The destination end system is able to carry out a protocol that adjusts and
lowers the cell rate of the connection based on the value of the EFCI.
See also: ABR.
80/20 rule The 80/20 rule means that 80 percent of the users’ traffic
should remain on the local network segment and only 20 percent or less
should cross the routers or bridges to the other network segments
EIGRP
See: Enhanced IGRP.
EIP Ethernet Interface Processor: A Cisco 7000 series router interface processor card, supplying 10Mbps AUI ports to support Ethernet Version 1 and
Ethernet Version 2 or IEEE 802.3 interfaces with a high-speed data path to
other interface processors.
ELAN Emulated LAN: An ATM network configured using a client/server
model in order to emulate either an Ethernet or Token Ring LAN. Multiple
ELANs can exist at the same time on a single ATM network and are made
up of a LAN Emulation Client (LEC), a LAN Emulation Server (LES), a
Broadcast and Unknown Server (BUS), and a LAN Emulation Configuration
Server (LECS). ELANs are defined by the LANE specification. See also:
LANE, LEC, LECS, and LES.
ELAP EtherTalk Link Access Protocol: In an EtherTalk network, the linkaccess protocol constructed above the standard Ethernet Data Link layer.
enable packets Packets that complete the flow cache. Once the MLS-SE
determines that the packet meets enable criteria, such as source MAC
(SMAC) address and destination IP, the flow cache is established and subsequent packets are Layer 3 switched. See also: MLS-SE and MLS-RP.
encapsulation The technique used by layered protocols in which a layer
adds header information to the protocol data unit (PDU) from the layer
above. As an example, in Internet terminology, a packet would contain a
header from the Physical layer, followed by a header from the Network layer
(IP), followed by a header from the Transport layer (TCP), followed by the
application protocol data.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
463
encryption The conversion of information into a scrambled form that
effectively disguises it to prevent unauthorized access. Every encryption
scheme uses some well-defined algorithm, which is reversed at the receiving
end by an opposite algorithm in a process known as decryption.
end-to-end VLANs VLANs that span the switch-fabric from end to end;
all switches in end-to-end VLANs understand about all configured VLANs.
End-to-end VLANs are configured to allow membership based on function,
project, department, and so on.
Enhanced IGRP Enhanced Interior Gateway Routing Protocol: An
advanced routing protocol created by Cisco, combining the advantages of
link-state and distance-vector protocols. Enhanced IGRP has superior convergence attributes, including high operating efficiency. See also: IGP,
OSPF, and RIP.
enterprise network A privately owned and operated network that joins
most major locations in a large company or organization.
enterprise services Defined as services provided to all users on the internetwork. Layer 3 switches or routers are required in this scenario because the
services must be close to the core and would probably be based in their own
subnet. Examples of these services include Internet access, e-mail, and possibly videoconferencing. If the servers that host these enterprise services were
placed close to the backbone, all users would have the same distance to them,
but this also means that all users’ data would have to cross the backbone to
get to these services.
EPROM Erasable Programmable Read-Only Memory: Programmed after
their manufacture, these nonvolatile memory chips can be erased if necessary
using high-power light and reprogrammed. See also: EEPROM and PROM.
error correction
the data stream.
Error correction uses a checksum to detect bit errors in
ESF Extended Superframe: Made up of 24 frames with 192 bits each, with
the 193rd bit providing other functions including timing. This is an
enhanced version of SF. See also: SF.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
464
Glossary
Ethernet A baseband LAN specification created by the Xerox Corporation and then improved through joint efforts of Xerox, Digital Equipment
Corporation, and Intel. Ethernet is similar to the IEEE 802.3 series standard
and, using CSMA/CD, operates over various types of cables at 10Mbps. Also
called DIX (Digital/Intel/Xerox) Ethernet. See also: 10BaseT, FastEthernet,
and IEEE.
EtherTalk A data-link product from Apple Computer that permits AppleTalk networks to be connected by Ethernet.
excess rate In ATM networking, traffic exceeding a connection’s insured
rate. The excess rate is the maximum rate less the insured rate. Depending on
the availability of network resources, excess traffic can be discarded during
congestion episodes. Compare with: maximum rate.
expansion The procedure of directing compressed data through an algorithm, restoring information to its original size.
expedited delivery An option that can be specified by one protocol layer,
communicating either with other layers or with the identical protocol layer
in a different network device, requiring that identified data be processed
faster.
explorer packet An SNA packet transmitted by a source Token Ring
device to find the path through a source-route-bridged network.
extended IP access list IP access list that filters the network by logical
address, protocol field in the Network layer header, and even the port field
in the Transport layer header.
extended IPX access list IPX access list that filters the network by logical
IPX address, protocol field in the Network layer header, or even socket
number in the Transport layer header.
Extended Setup Used in setup mode to configure the router with more
detail than Basic Setup mode. Allows multiple-protocol support and interface configuration.
external route processor A router that is external to the switch. An
external Layer-3 routing device can be used to provide routing between
VLANs.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
465
exterior routing protocol Routing protocol that connects and advertises
autonomous systems.
failure domain The region in which a failure has occurred in a Token
Ring. When a station gains information that a serious problem, such as a
cable break, has occurred with the network, it sends a beacon frame that
includes the station reporting the failure, its NAUN, and everything
between. This defines the failure domain. Beaconing then initiates the procedure known as autoreconfiguration. See also: autoreconfiguration and
beacon.
fallback In ATM networks, this mechanism is used for scouting a path if
it isn’t possible to locate one using customary methods. The device relaxes
requirements for certain characteristics, such as delay, in an attempt to find
a path that meets a certain set of the most important requirements.
Fast EtherChannel Fast EtherChannel uses load distribution to share the
links called a bundle, which is a group of links managed by the Fast EtherChannel process. Should one link in the bundle fail, the Ethernet Bundle
Controller (EBC) informs the Enhanced Address Recognition Logic (EARL)
ASIC of the failure, and the EARL in turn ages out all addresses learned on
that link. The EBC and the EARL use hardware to recalculate the source and
destination address pair on a different link.
Fast Ethernet Any Ethernet specification with a speed of 100Mbps. Fast
Ethernet is 10 times faster than 10BaseT, while retaining qualities like MAC
mechanisms, MTU, and frame format. These similarities make it possible for
existing 10BaseT applications and management tools to be used on Fast
Ethernet networks. Fast Ethernet is based on an extension of IEEE 802.3
specification (IEEE 802.3u). Compare with: Ethernet. See also: 100BaseT,
100BaseTX, and IEEE.
fast switching A Cisco feature that uses a route cache to speed packet
switching through a router. Contrast with: process switching.
FDM Frequency-Division Multiplexing: A technique that permits information from several channels to be assigned bandwidth on one wire based on
frequency. See also: TDM, ATDM, and statistical multiplexing.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
466
Glossary
FDDI Fiber Distributed Data Interface: A LAN standard, defined by ANSI
X3T9.5 that can run at speeds up to 200Mbps and uses token-passing media
access on fiber-optic cable. For redundancy, FDDI can use a dual-ring
architecture.
feasible successor A route that is kept in a topology table and will be
placed in the routing table if the current successor goes down.
FECN Forward Explicit Congestion Notification: A bit set by a Frame
Relay network that informs the DTE receptor that congestion was encountered along the path from source to destination. A device receiving frames
with the FECN bit set can ask higher-priority protocols to take flow-control
action as needed. See also: BECN.
FEIP Fast Ethernet Interface Processor: An interface processor employed
on Cisco 7000 series routers, supporting up to two 100Mbps 100BaseT
ports.
firewall A barrier purposefully erected between any connected public networks and a private network, made up of a router or access server or several
routers or access servers, that uses access lists and other methods to ensure
the security of the private network.
Flash Electronically Erasable Programmable Read-Only Memory
(EEPROM). Used to hold the Cisco IOS in a router by default.
flash memory Developed by Intel and licensed to other semiconductor
manufacturers, it is nonvolatile storage that can be erased electronically and
reprogrammed, physically located on an EEPROM chip. Flash memory permits software images to be stored, booted, and rewritten as needed. Cisco
routers and switches use flash memory to hold the IOS by default. See also:
EPROM and EEPROM.
flat network Network that is one large collision domain and one large
broadcast domain.
flooding When traffic is received on an interface, it is then transmitted to
every interface connected to that device with the exception of the interface
from which the traffic originated. This technique can be used for traffic
transfer by bridges and switches throughout the network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
467
flow A shortcut or MLS cache entry that is defined by the packet properties. Packets with identical properties belong to the same flow. See also: MLS.
flow control A methodology used to ensure that receiving units are not
overwhelmed with data from sending devices. Pacing, as it is called in IBM
networks, means that when buffers at a receiving unit are full, a message is
transmitted to the sending unit to temporarily halt transmissions until all the
data in the receiving buffer has been processed and the buffer is again ready
for action.
FRAD Frame Relay Access Device: Any device affording a connection
between a LAN and a Frame Relay WAN. See also: Cisco FRAD and FRAS.
fragment Any portion of a larger packet that has been intentionally segmented into smaller pieces. A packet fragment does not necessarily indicate
an error and can be intentional. See also: fragmentation.
fragmentation The process of intentionally segmenting a packet into
smaller pieces when sending data over an intermediate network medium that
cannot support the larger packet size.
FragmentFree LAN switch type that reads into the data section of a frame
to make sure fragmentation did not occur. Sometimes called modified cutthrough.
frame A logical unit of information sent by the Data Link layer over a
transmission medium. The term often refers to the header and trailer,
employed for synchronization and error control, that surround the data contained in the unit.
Frame Relay A more efficient replacement of the X.25 protocol (an unrelated packet relay technology that guarantees data delivery). Frame Relay is
an industry-standard, shared-access, best-effort, switched Data Link layer
encapsulation that services multiple virtual circuits and protocols between
connected mechanisms.
Frame Relay bridging Defined in RFC 1490, this bridging method uses
the identical spanning-tree algorithm as other bridging operations but permits packets to be encapsulated for transmission across a Frame Relay
network.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
468
Glossary
Frame Relay switching When a router at a service provider provides
packet switching for Frame Relay packets.
frame tagging VLANs can span multiple connected switches, which
Cisco calls a switch-fabric. Switches within this switch-fabric must keep
track of frames as they are received on the switch ports, and they must keep
track of the VLAN they belong to as the frames traverse this switch-fabric.
Frame tagging performs this function. Switches can then direct frames to the
appropriate port.
framing Encapsulation at the Data Link layer of the OSI model. It is called
framing because the packet is encapsulated with both a header and a trailer.
FRAS Frame Relay Access Support: A feature of Cisco IOS software that
enables SDLC, Ethernet, Token Ring, and Frame Relay-attached IBM
devices to be linked with other IBM mechanisms on a Frame Relay network.
See also: FRAD.
frequency The number of cycles of an alternating current signal per time
unit, measured in Hertz (cycles per second).
FSIP Fast Serial Interface Processor: The Cisco 7000 routers’ default serial
interface processor, it provides four or eight high-speed serial ports.
FTP File Transfer Protocol: The TCP/IP protocol used for transmitting files
between network nodes, it supports a broad range of file types and is defined
in RFC 959. See also: TFTP.
full duplex The capacity to transmit information between a sending station and a receiving unit at the same time. See also: half duplex.
full mesh A type of network topology where every node has either a physical or a virtual circuit linking it to every other network node. A full mesh
supplies a great deal of redundancy but is typically reserved for network
backbones because of its expense. See also: partial mesh.
gateway of last resort
Term used when the default route is set.
Gigabit EtherChannel
See: Fast EtherChannel.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
469
Gigabit Ethernet 1000Mbps version of the IEEE 802.3. Fast Ethernet
offers a speed increase of 10 times that of the 10BaseT Ethernet specification
while preserving qualities such as frame format, MAC, mechanisms, and MTU.
GNS Get Nearest Server: On an IPX network, a request packet sent by a
customer for determining the location of the nearest active server of a given
type. An IPX network client launches a GNS request to get either a direct
answer from a connected server or a response from a router disclosing the
location of the service on the internetwork to the GNS. GNS is part of IPX
and SAP. See also: IPX and SAP.
grafting A process that activates an interface that has been deactivated by
the pruning process. It is initiated by an IGMP membership report sent to the
router.
GRE Generic Routing Encapsulation: A tunneling protocol created by
Cisco with the capacity for encapsulating a wide variety of protocol packet
types inside IP tunnels, thereby generating a virtual point-to-point connection to Cisco routers across an IP network at remote points. IP tunneling
using GRE permits network expansion across a single-protocol backbone
environment by linking multiprotocol subnetworks in a single-protocol
backbone environment.
Group of Four Used by Cisco Local Management Interface on Frame
Relay networks to manage the permanent virtual circuits (PVCs). See also: PVC.
guard band The unused frequency area found between two communications channels, furnishing the space necessary to avoid interference between
the two.
half duplex The capacity to transfer data in only one direction at a time
between a sending unit and receiving unit. See also: full duplex.
handshake Any series of transmissions exchanged between two or more
devices on a network to ensure synchronized operations.
H channel High-speed channel: A full-duplex, ISDN primary rate channel
operating at a speed of 384Kbps. See also: B channel, D channel, and
E channel.
HDLC High-Level Data Link Control: Using frame characters, including
checksums, HDLC designates a method for data encapsulation on synchronous serial links and is the default encapsulation for Cisco routers. HDLC is
a bit-oriented synchronous Data Link layer protocol created by ISO and
derived from SDLC. However, most HDLC vendor implementations
(including Cisco’s) are proprietary. See also: SDLC.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
470
Glossary
helper address The unicast address specified, which instructs the Cisco
router to change the client’s local broadcast request for a service into a
directed unicast to the server.
hierarchical addressing Any addressing plan employing a logical chain of
commands to determine location. IP addresses are made up of a hierarchy of
network numbers, subnet numbers, and host numbers to direct packets to
the appropriate destination.
hierarchical network A multi-segment network configuration providing
only one path through intermediate segments between source segments and
destination segments.
hierarchy
See: hierarchical network.
HIP HSSI Interface Processor: An interface processor used on Cisco 7000
series routers, providing one HSSI port that supports connections to ATM,
SMDS, Frame Relay, or private lines at speeds up to T3 or E3.
hold-down The state a route is placed in so that routers can neither advertise the route nor accept advertisements about it for a defined time period.
Hold-down is used to surface bad information about a route from all routers
in the network. A route is generally placed in hold-down when one of its
links fails.
hop The movement of a packet between any two network nodes. See also:
hop count.
hop count A routing metric that calculates the distance between a
source and a destination. RIP employs hop count as its sole metric.
See also: hop and RIP.
host address Logical address configured by an administrator or server on
a device. Logically identifies this device on an internetwork.
HSCI High-Speed Communication Interface: Developed by Cisco, a singleport interface that provides full-duplex synchronous serial communications
capability at speeds up to 52Mbps.
HSRP Hot Standby Router Protocol: A protocol that provides high network availability and provides nearly instantaneous hardware fail-over
without administrator intervention. It generates a Hot Standby router
group, including a lead router that lends its services to any packet being
transferred to the Hot Standby address. If the lead router fails, it will be
replaced by any of the other routers—the standby routers—that monitor it.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
471
HSSI High-Speed Serial Interface: A network standard physical connector
for high-speed serial linking over a WAN at speeds of up to 52Mbps.
hubs Physical layer devices that are really just multiple port repeaters.
When an electronic digital signal is received on a port, the signal is reamplified or regenerated and forwarded out all segments except the segment from
which the signal was received.
ICD International Code Designator: Adapted from the subnetwork model
of addressing, this assigns the mapping of Network layer addresses to ATM
addresses. HSSI is one of two ATM formats for addressing created by the
ATM Forum to be utilized with private networks. See also: DCC.
ICMP Internet Control Message Protocol: Documented in RFC 792, it is a
Network layer Internet protocol for the purpose of reporting errors and providing information pertinent to IP packet procedures.
IEEE Institute of Electrical and Electronics Engineers: A professional organization that, among other activities, defines standards in a number of fields
within computing and electronics, including networking and communications. IEEE standards are the predominant LAN standards used today
throughout the industry. Many protocols are commonly known by the reference number of the corresponding IEEE standard.
IEEE 802.1 The IEEE committee specification that defines the bridging
group. The specification for STP (Spanning Tree Protocol) is IEEE 802.1d.
The STP uses SPA (spanning-tree algorithm) to find and prevent network
loops in bridged networks. The specification for VLAN trunking is IEEE
802.1q.
IEEE 802.3 The IEEE committee specification that defines the Ethernet
group, specifically the original 10Mbps standard. Ethernet is a LAN protocol that specifies Physical layer and MAC sublayer media access. IEEE
802.3 uses CSMA/CD to provide access for many devices on the same network. Fast Ethernet is defined as 802.3u, and Gigabit Ethernet is defined as
802.3q. See also: CSMA/CD.
IEEE 802.5
IEEE committee that defines Token Ring media access.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
472
Glossary
IGMP Internet Group Management Protocol: Employed by IP hosts, the
protocol that reports their multicast group memberships to an adjacent
multicast router. The first version, IGMPv1, allows hosts to subscribe to or
join specified multicast groups. Enhancements were made to IGMPv2 to
facilitate a host-initiated leave process.
IGMP Join process The process by which hosts may join a multicast session outside of the Membership Query interval.
IGMP Leave process IGMPv1 does not have a formal leave process; a
period of three query intervals must pass with no host confirmation before
the interface is deactivated. IGMPv2 does allow the host to initiate the leave
process immediately.
IGMP Query process The router uses IGMP to query hosts for Membership Reports, thus managing multicast on its interfaces.
IGP Interior Gateway Protocol: Any protocol used by the Internet to
exchange routing data within an independent system. Examples include RIP,
IGRP, and OSPF.
ILMI Integrated (or Interim) Local Management Interface. A specification
created by the ATM Forum, designated for the incorporation of networkmanagement capability into the ATM UNI. Integrated Local Management
Interface cells provide for automatic configuration between ATM systems.
In LAN emulation, ILMI can provide sufficient information for the ATM
end station to find an LECS. In addition, ILMI provides the ATM NSAP
(Network Service Access Point) prefix information to the end station.
in-band management In-band management is the management of a network device “through” the network. Examples include using Simple Network Management Protocol (SNMP) or Telnet directly via the local LAN.
Compare with: out-of-band management.
in-band signaling Configuration of a router from within the network.
Examples are Telnet, Simple Network Management Protocol (SNMP), or a
Network Management Station (NMS).
insured burst In an ATM network, it is the largest, temporarily permitted
data burst exceeding the insured rate on a PVC and not tagged by the traffic
policing function for being dropped if network congestion occurs. This
insured burst is designated in bytes or cells.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
473
inter-area routing Routing between two or more logical areas. Contrast
with: intra-area routing. See also: area.
interface processor Any of several processor modules used with Cisco 7000
series routers. See also: AIP, CIP, EIP, FEIP, HIP, MIP, and TRIP.
Interior Gateway Routing Protocol (IGRP)
vector protocol.
Cisco proprietary distance-
internal route processors Route Switch Modules (RSM) and Route
Switch Feature Cards (RSFC) are called internal route processors because the
processing of Layer 3 packets is internal to a switch.
Internet The global “network of networks,” whose popularity has
exploded in the last few years. Originally a tool for collaborative academic
research, it has become a medium for exchanging and distributing information of all kinds. The Internet’s need to link disparate computer platforms
and technologies has led to the development of uniform protocols and standards that have also found widespread use within corporate LANs. See also:
TCP/IP and MBONE.
internet Before the rise in the use of the Internet, this lowercase form was
shorthand for “internetwork” in the generic sense. Now rarely used. See
also: internetwork.
Internet Protocol Any protocol belonging to the TCP/IP protocol stack.
See also: TCP/IP.
internetwork Any group of private networks interconnected by routers
and other mechanisms, typically operating as a single entity.
internetworking Broadly, anything associated with the general task of
linking networks to each other. The term encompasses technologies, procedures, and products. When you connect networks to a router, you are creating an internetwork.
inter-VLAN routing Cisco has created the proprietary protocol InterSwitch Link (ISL) to allow routing between VLANs with only one Ethernet
interface. To run ISL, you need to have two VLAN-capable Fast Ethernet or
Gigabit Ethernet devices like a Cisco 5000 switch and a 7000 series router.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
474
Glossary
intra-area routing Routing that occurs within a logical area. Contrast
with: inter-area routing.
intruder detection Intruder detection systems operate by monitoring the
data flow for characteristics consistent with security threats. In this manner,
an intruder can be monitored or blocked from access. One trigger for an
intruder detection system is multiple ping packets from a single resource in
a brief period of time.
Inverse ARP Inverse Address Resolution Protocol: A technique by which
dynamic mappings are constructed in a network, allowing a device such as
a router to locate the logical network address and associate it with a permanent virtual circuit (PVC). Commonly used in Frame Relay to determine the
far-end node’s TCP/IP address by sending the Inverse ARP request to the
local DLCI.
IP Internet Protocol: Defined in RFC 791, it is a Network layer protocol
that is part of the TCP/IP stack and allows connectionless service. IP furnishes an array of features for addressing, type-of-service specification, fragmentation and reassembly, and security.
IP address Often called an Internet address, this is an address uniquely
identifying any device (host) on the Internet (or any TCP/IP network). Each
address consists of four octets (32 bits), represented as decimal numbers separated by periods (a format known as “dotted-decimal”). Every address is
made up of a network number, an optional subnetwork number, and a host
number. The network and subnetwork numbers together are used for
routing, while the host number addresses an individual host within the network or subnetwork. The network and subnetwork information is extracted
from the IP address using the subnet mask. There are five classes of IP
addresses (A–E), which allocate different numbers of bits to the network,
subnetwork, and host portions of the address. See also: CIDR, IP, and
subnet mask.
IPCP IP Control Program: The protocol used to establish and configure IP
over PPP. See also: IP and PPP.
IP multicast A technique for routing that enables IP traffic to be reproduced from one source to several endpoints or from multiple sources to
many destinations. Instead of transmitting only one packet to each individual point of destination, one packet is sent to a multicast group specified
by only one IP endpoint address for the group.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
475
IP unnumbered Cisco proprietary protocol that allows two point-topoint links to communicate without an IP address.
IPX Internetwork Packet Exchange: Network layer protocol (Layer 3) used
in Novell NetWare networks for transferring information from servers to
workstations. Similar to IP and XNS.
IPXCP IPX Control Program: The protocol used to establish and configure
IPX over PPP. See also: IPX and PPP.
IPX spoofing Provides IPX RIP/SAP traffic without requiring a connection to the opposing network. This allows a per-minute tariffed link, such as
ISDN or analog phone, to support IPX without requiring the link to remain
active.
IPXWAN Protocol used for new WAN links to provide and negotiate line
options on the link using IPX. After the link is up and the options have been
agreed upon by the two end-to-end links, normal IPX transmission begins.
IRDP ICMP Router Discovery Protocol: Allows hosts to use the Internet
Control Message Protocol (ICMP) to find a new path when the primary
router becomes unavailable. IRDP is an extension to the ICMP protocol and
not a dynamic routing protocol. This ICMP extension allows routers to
advertise default routes to end stations.
ISDN Integrated Services Digital Network: Offered as a service by telephone companies, a communication protocol that allows telephone networks to carry data, voice, and other digital traffic. See also: BISDN, BRI,
and PRI.
ISL routing Inter-Switch Link routing is a Cisco proprietary method of
frame tagging in a switched internetwork. Frame tagging is a way to identify
the VLAN membership of a frame as it traverses a switched internetwork.
isochronous transmission Asynchronous data transfer over a synchronous data link, requiring a constant bit rate for reliable transport. Compare
with: asynchronous transmission and synchronous transmission.
ITU-T International Telecommunication Union Telecommunication Standardization Sector: This is a group of engineers that develops worldwide
standards for telecommunications technologies.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
476
Glossary
keepalive Used to tell directly connected neighbors that a router is still up
and functioning.
LAN local area network: Broadly, any network linking two or more computers and related devices within a limited geographical area (up to a few
kilometers). LANs are typically high-speed, low-error networks within a
company. Cabling and signaling at the Physical and Data Link layers of the
OSI are dictated by LAN standards. Ethernet, FDDI, and Token Ring are
among the most popular LAN technologies. Compare with: MAN.
LANE LAN emulation: The technology that allows an ATM network to
operate as a LAN backbone. To do so, the ATM network is required to provide multicast and broadcast support, address mapping (MAC-to-ATM),
SVC management, in addition to an operable packet format. Additionally,
LANE defines Ethernet and Token Ring ELANs. See also: ELAN.
LAN switch A high-speed, multiple-interface transparent bridging mechanism, transmitting packets between segments of data links, usually referred
to specifically as an Ethernet switch. LAN switches transfer traffic based on
MAC addresses. Multilayer switches are a type of high-speed, specialpurpose, hardware-based router. See also: multilayer switch and store-andforward packet switching.
LAPB Link Accessed Procedure, Balanced: A bit-oriented Data Link layer
protocol that is part of the X.25 stack and has its origin in SDLC. See also:
SDLC and X.25.
LAPD Link Access Procedure on the D channel: The ISDN Data Link layer
protocol used specifically for the D channel and defined by ITU-T Recommendations Q.920 and Q.921. LAPD evolved from LAPB and is created to
comply with the signaling requirements of ISDN basic access.
latency Broadly, the time it takes a data packet to get from one location to
another. In specific networking contexts, it can mean either 1) the time
elapsed (delay) between the execution of a request for access to a network by
a device and the time the mechanism actually is permitted transmission, or 2) the
time elapsed between when a mechanism receives a frame and the time that
frame is forwarded out of the destination port.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
477
Layer 2 switching Layer 2 switching is hardware based, which means it
uses the MAC address from the hosts’ NIC cards to filter the network.
Switches use Application-Specific Integrated Circuits (ASICs) to build and
maintain filter tables. It is okay to think of a Layer 2 switch as a multiport
bridge.
Layer 3 switch
See: multilayer switch.
layered architecture Industry standard way of creating applications to
work on a network. Layered architecture allows the application developer to
make changes in only one layer instead of the whole program.
LCP Link Control Protocol: The protocol designed to establish, configure,
and test data link connections for use by PPP. See also: PPP.
leaky bucket An analogy for the basic cell rate algorithm (GCRA) used in
ATM networks for checking the conformance of cell flows from a user or
network. The bucket’s “hole” is understood to be the prolonged rate at
which cells can be accommodated, and the “depth” is the tolerance for cell
bursts over a certain time period.
learning bridge A bridge that transparently builds a dynamic database of
MAC addresses and the interfaces associated with each address. Transparent
bridges help to reduce traffic congestion on the network.
LE ARP LAN Emulation Address Resolution Protocol: The protocol providing the ATM address that corresponds to a MAC address.
leased lines Permanent connections between two points leased from the
telephone companies.
LEC LAN Emulation Client: Software providing the emulation of the Data
Link layer interface that allows the operation and communication of all
higher-level protocols and applications to continue. The LEC client runs in
all ATM devices, which include hosts, servers, bridges, and routers. The LEC
is responsible for address resolution, data transfer, address caching, interfacing to the emulated LAN, and driver support for higher-level services. See
also: ELAN and LES.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
478
Glossary
LECS LAN Emulation Configuration Server: An important part of emulated LAN services, providing the configuration data that is furnished upon
request from the LES. These services include address registration for Integrated Local Management Interface (ILMI) support, configuration support
for the LES addresses and their corresponding emulated LAN identifiers, and
an interface to the emulated LAN. See also: LES and ELAN.
LES LAN Emulation Server: The central LANE component that provides
the initial configuration data for each connecting LEC. The LES typically is
located on either an ATM-integrated router or a switch. Responsibilities of
the LES include configuration and support for the LEC, address registration
for the LEC, database storage and response concerning ATM addresses, and
interfacing to the emulated LAN. See also: ELAN, LEC, and LECS.
link compression
See: compression.
link-state router Type of routing protocol run on a router that sends partial route updates incrementally.
link-state routing algorithm A routing algorithm that allows each router
to broadcast or multicast information regarding the cost of reaching all its
neighbors to every node in the internetwork. Link-state algorithms provide
a consistent view of the network and are therefore not vulnerable to routing
loops. However, this is achieved at the cost of somewhat greater difficulty in
computation and more widespread traffic (compared with distance-vector
routing algorithms). See also: distance-vector routing algorithm.
LLAP LocalTalk Link Access Protocol: In a LocalTalk environment, the
Data Link–level protocol that manages node-to-node delivery of data. This
protocol provides node addressing and management of bus access, and it also
controls data sending and receiving to assure packet length and integrity.
LLC Logical Link Control: Defined by the IEEE, the higher of two Data
Link layer sublayers. LLC is responsible for error detection (but not correction), flow control, framing, and software-sublayer addressing. The predominant LLC protocol, IEEE 802.2, defines both connectionless and
connection-oriented operations. See also: Data Link layer and MAC.
LMI An enhancement to the original Frame Relay specification. Among the
features it provides are a keepalive mechanism, a multicast mechanism,
global addressing, and a status mechanism.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
479
LNNI LAN Emulation Network-to-Network Interface: In the Phase 2
LANE specification, an interface that supports communication between the
server components within one ELAN.
load balancing
The sharing of paths to a remote network.
local explorer packet In a Token Ring SRB network, a packet generated
by an end system to find a host linked to the local ring. If no local host can
be found, the end system will produce one of two solutions: a spanning
explorer packet or an all-routes explorer packet.
local loop Connection from a demarcation point to the closest switching
office.
local services Users trying to get to network services that are located on
the same subnet or network are defined as local services. Users do not cross
Layer 3 devices, and the network services are in the same broadcast domain
as the users. This type of traffic never crosses the backbone.
LocalTalk Utilizing CSMA/CD, in addition to supporting data transmission at speeds of 230.4Kbps, LocalTalk is Apple Computer’s proprietary
baseband protocol, operating at the Data Link and Physical layers of the OSI
Reference Model.
local VLANs Local VLANs are configured by geographic location; these
locations can be a building or just a closet in a building, depending on switch
size. Geographically configured VLANs are designed around the fact that the
business or corporation is using centralized resources, like a server farm.
loop avoidance If multiple connections between switches are created for
redundancy, network loops can occur. STP is used to stop network loops and
allow redundancy.
LSA link-state advertisement: Contained inside of link-state packets
(LSPs), these advertisements are usually multicast packets, containing information about neighbors and path costs, that are employed by link-state protocols. Receiving routers use LSAs to maintain their link-state databases and,
ultimately, routing tables.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
480
Glossary
LSA acknowledgement A Link State Advertisement acknowledgement is
sent from a router back to an originating router to acknowledge receipt of an
LSA from an OSPF router.
LSA flooding OSPF floods the network with Link State Advertisements if
a change in the network occurs, permitting rapid convergence.
LUNI LAN Emulation User-to-Network Interface: Defining the interface
between the LAN Emulation Client (LEC) and the LAN Emulation Server
(LES), LUNI is the ATM Forum’s standard for LAN Emulation on ATM networks. See also: LES and LECS.
LZW algorithm A data-compression process named for its inventors,
Lempel, Ziv, and Welch. The algorithm works by finding longer and longer
strings of data to compress with shorter representations.
MAC Media Access Control: The lower sublayer in the Data Link layer, it
is responsible for hardware addressing, media access, and error detection of
frames. See also: Data Link layer and LLC.
MAC address A Data Link layer hardware address that every port or
device needs in order to connect to a LAN segment. These addresses are used
by various devices in the network for accurate location of logical addresses.
MAC addresses are defined by the IEEE standard and their length is six characters, typically using the burned-in address (BIA) of the local LAN interface. Variously called “hardware address,” “physical address,” “burned-in
address,” or “MAC layer address.”
MacIP In AppleTalk, the Network layer protocol encapsulating IP packets
in Datagram Delivery Protocol (DDP) packets. MacIP also supplies substitute ARP services.
MAN metropolitan area network: Any network that encompasses a metropolitan area; that is, an area typically larger than a LAN but smaller than
a WAN. See also: LAN.
Manchester encoding A method for digital coding in which a mid-bit–
time transition is employed for clocking, and a 1 (one) is denoted by a high
voltage level during the first half of the bit time. This scheme is used by
Ethernet and IEEE 802.3.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
481
maximum burst Specified in bytes or cells, the largest burst of information exceeding the insured rate that will be permitted on an ATM permanent
virtual connection for a short time and will not be dropped even if it goes
over the specified maximum rate. Compare with: insured burst. See also:
maximum rate.
maximum rate The maximum permitted data throughput on a particular
virtual circuit, equal to the total of insured and uninsured traffic from the
traffic source. Should traffic congestion occur, uninsured information may
be deleted from the path. Measured in bits or cells per second, the maximum
rate represents the highest throughput of data the virtual circuit is ever able
to deliver and cannot exceed the media rate. Compare with: excess rate. See
also: maximum burst.
MBS Maximum Burst Size: In an ATM signaling message, this metric,
coded as a number of cells, is used to convey the burst tolerance.
MBONE multicast backbone: The multicast backbone of the Internet, it is
a virtual multicast network made up of multicast LANs, including point-topoint tunnels interconnecting them.
MCDV Maximum Cell Delay Variation: The maximum two-point CDV
objective across a link or node for the identified service category in an ATM
network. The MCDV is one of four link metrics that are exchanged using
PTSPs to verify the available resources of an ATM network. Only one
MCDV value is assigned to each traffic class.
MCLR Maximum Cell Loss Ratio: The maximum ratio of cells in an ATM
network that fail to transit a link or node compared with the total number
of cells that arrive at the link or node. MCDV is one of four link metrics that
are exchanged using PTSPs to verify the available resources of an ATM network. The MCLR applies to cells in VBR and CBR traffic classes whose CLP
bit is set to zero. See also: CBR, CLP, and VBR.
MCR Minimum Cell Rate: A parameter determined by the ATM Forum for
traffic management of the ATM networks. MCR is specifically defined for
ABR transmissions and specifies the minimum value for the allowed cell rate
(ACR). See also: ACR and PCR.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
482
Glossary
MCTD Maximum Cell Transfer Delay: In an ATM network, the total of
the maximum cell delay variation and the fixed delay across the link or node.
MCTD is one of four link metrics that are exchanged using PNNI topology
state packets to verify the available resources of an ATM network. There is
one MCTD value assigned to each traffic class. See also: MCDV.
metric
See: routing metric.
MIB Management Information Base: Used with SNMP management software to gather information from remote devices. The management station
can poll the remote device for information, or the MIB running on the
remote station can be programmed to send information on a regular basis.
microsegmentation:
Layer 2 switching.
Term used to describe LAN segmentation using
MIP Multichannel Interface Processor: The resident interface processor on
Cisco 7000 series routers, providing up to two channelized T1 or E1 connections by serial cables connected to a CSU. The two controllers are capable
of providing 24 T1 or 30 E1 channel groups, with each group being introduced to the system as a serial interface that can be configured individually.
mips
millions of instructions per second: A measure of processor speed.
MLP Multilink PPP: A technique used to split, recombine, and sequence
datagrams across numerous logical data links.
MLS Multi-Layer Switching: Switching normally takes place at Layer 2.
When Layer 3 information is allowed to be cached, Layer 2 devices have the
capability of rewriting and forwarding frames based on the Layer 3
information.
MLSP Multilayer Switching Protocol: A protocol that runs on the router
and allows it to communicate to the MLS-SE regarding topology or security
changes.
MLS-RP Multilayer Switching Route Processor: An MLS-capable router or
an RSM (Route Switch Module) installed in the switch. See also: RSM and
MLS.
MLS-SE Multilayer Switching Switching Engine: An MLS-capable switch
(a 5000 with an NFFC or a 6000 with an MSFC and PFC). See also: MLS,
NFFC, MSFC and PFC.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
483
MMP Multichassis Multilink PPP: A protocol that supplies MLP support
across multiple routers and access servers. MMP enables several routers and
access servers to work as a single, large dial-up pool with one network
address and ISDN access number. MMP successfully supports packet fragmenting and reassembly when the user connection is split between two physical access devices.
modem modulator-demodulator: A device that converts digital signals to
analog and vice versa so that digital information can be transmitted over analog
communication facilities, such as voice-grade telephone lines. This is
achieved by converting digital signals at the source to analog for transmission and reconverting the analog signals back into digital form at the destination. See also: modulation and demodulation.
modemcap database Stores modem initialization strings on the router
for use in auto-detection and configuration.
modem eliminator A mechanism that makes possible a connection
between two DTE devices without modems by simulating the commands
and physical signaling required.
modulation The process of modifying some characteristic of an electrical
signal, such as amplitude (AM) or frequency (FM), in order to represent digital or analog information. See also: AM.
MOSPF Multicast OSPF: An extension of the OSPF unicast protocol that
enables IP multicast routing within the domain. See also: OSPF.
MP bonding MultiPoint bonding: A process of linking two or more physical connections into a single logical channel. This may use two or more
analog lines and two or more modems, for example.
MPOA Multiprotocol over ATM: An effort by the ATM Forum to standardize how existing and future Network layer protocols such as IP, Ipv6,
AppleTalk, and IPX run over an ATM network with directly attached hosts,
routers, and multilayer LAN switches.
MSFC Multilayer Switch Feature Card: A route processor (parallel to an
RSM, or Route Switch Module) that is installed as a daughter card on Cisco
Catalyst 6000 series switches. See also: RSM.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
484
Glossary
mtrace (multicast traceroute) Used to establish the SPT for a specified
multicast group.
MTU maximum transmission unit: The largest packet size, measured in
bytes, that an interface can handle.
multicast Broadly, any communication between a single sender and multiple receivers. Unlike broadcast messages, which are sent to all addresses on
a network, multicast messages are sent to a defined subset of the network
addresses; this subset has a group multicast address, which is specified in the
packet’s destination address field. See also: broadcast and directed broadcast.
multicast address A single address that points to more than one device on
the network by specifying a special non-existent MAC address specified in
that particular multicast protocol. Identical to group address. See also:
multicast.
multicast group A group set up to receive messages from a source. These
groups can be established based on Frame Relay or IP in the TCP/IP protocol
suite, as well as other networks.
multicast send VCC A two-directional point-to-point virtual control connection (VCC) arranged by an LEC to a BUS, it is one of the three types of
informational links specified by phase 1 LANE. See also: control distribute
VCC and control direct VCC.
multilayer switch A highly specialized, high-speed, hardware-based type
of LAN router, the device filters and forwards packets based on their Layer
2 MAC addresses and Layer 3 network addresses. It’s possible that even
Layer 4 can be read. Sometimes called a Layer 3 switch. See also: LAN
switch.
multilayer switching Multilayer switching combines Layer 2, 3, and 4
switching technology and provides very high-speed scalability with low
latency. This is provided by huge filter tables based on the criteria designed
by the network administrator.
multiplexing The process of converting several logical signals into a single
physical signal for transmission across one physical channel. Contrast with:
demultiplexing.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
485
NAK negative acknowledgment: A response sent from a receiver, telling the
sender that the information was not received or contained errors. Compare
with: acknowledgment.
NAT Network Address Translation: An algorithm instrumental in minimizing the requirement for globally unique IP addresses, permitting an organization whose addresses are not all globally unique to connect to the
Internet, regardless, by translating those addresses into globally routable
address space.
NBP Name Binding Protocol: In AppleTalk, the Transport-level protocol
that interprets a socket client’s name, entered as a character string, into the
corresponding DDP address. NBP gives AppleTalk protocols the capacity to
discern user-defined zones and names of mechanisms by showing and
keeping translation tables that map names to their corresponding socket
addresses.
NCP Network Control Protocol: A protocol at the Logical Link Control
sublayer of the Data Link layer used in the PPP stack. It is used to allow multiple Network layer protocols to run over a nonproprietary HDLC serial
encapsulation.
neighboring routers Two routers in OSPF that have interfaces to a
common network. On networks with multi-access, these neighboring
routers are dynamically discovered using the Hello protocol of OSPF.
NetBEUI NetBIOS Extended User Interface: An improved version of the
NetBIOS protocol used in a number of network operating systems including
LAN Manager, Windows NT, LAN Server, and Windows for Workgroups,
implementing the OSI LLC2 protocol. NetBEUI formalizes the transport
frame not standardized in NetBIOS and adds more functions. See also: OSI.
NetBIOS Network Basic Input/Output System: The API employed by
applications residing on an IBM LAN to ask for services, such as session termination or information transfer, from lower-level network processes.
NetView A mainframe network product from IBM, used for monitoring
SNA (Systems Network Architecture) networks. It runs as a VTAM (Virtual
Telecommunications Access Method) application.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
486
Glossary
NetWare A widely used NOS created by Novell, providing a number of
distributed network services and remote file access.
network address Used with the logical network addresses to identify the
network segment in an internetwork. Logical addresses are hierarchical in
nature and have at least two parts: network and host. An example of a hierarchical address is 172.16.10.5, where 172.16 is the network and 10.5 is the
host address.
Network layer In the OSI reference model, it is Layer 3—the layer in
which routing is implemented, enabling connections and path selection
between two end systems. See also: Application layer, Data Link layer, Physical layer, Presentation layer, Session layer, and Transport layer.
NFFC NetFlow Feature Card: A module installed on Cisco Catalyst 5000
series switches. It is capable of examining each frame’s IP header as well as
the Ethernet header. This in turn allows the NFFC to create flows.
NFS Network File System: One of the protocols in Sun Microsystems’
widely used file system protocol suite, allowing remote file access across a
network. The name is loosely used to refer to the entire Sun protocol suite,
which also includes RPC, XDR (External Data Representation), and other
protocols.
NHRP Next Hop Resolution Protocol: In a nonbroadcast multi-access
(NBMA) network, the protocol employed by routers in order to dynamically
locate MAC addresses of various hosts and routers. It enables systems to
communicate directly without requiring an intermediate hop, thus facilitating increased performance in ATM, Frame Relay, X.25, and SMDS
systems.
NHS Next Hop Server: Defined by the NHRP protocol, this server maintains the next-hop resolution cache tables, listing IP-to-ATM address maps
of related nodes and nodes that can be reached through routers served by
the NHS.
NIC network interface card: An electronic circuit board placed in a computer. The NIC provides network communication to a LAN.
NLSP NetWare Link Services Protocol: Novell’s link-state routing protocol, based on the IS-IS model.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
487
NMP Network Management Processor: A Catalyst 5000 switch processor
module used to control and monitor the switch.
node address Used to identify a specific device in an internetwork. Can be
a hardware address, which is burned into the network interface card, or a
logical network address, which an administrator or server assigns to the node.
Non-Broadcast Multi-Access (NBMA) A type of network that does not,
by default, allow LAN broadcasts to be transmitted on the network. An
example of an NBMA is Frame Relay.
nondesignated port The Spanning Tree Protocol tells a port on a Layer 2
switch to stop transmitting and creating a network loop. Only designated
ports can send frames.
non-stub area In OSPF, a resource-consuming area carrying a default
route, intra-area routes, inter-area routes, static routes, and external routes.
Non-stub areas are the only areas that can have virtual links configured
across them and exclusively contain an anonymous system boundary router
(ASBR). Compare with: stub area. See also: ASBR and OSPF.
NRZ Nonreturn to Zero: One of several encoding schemes for transmitting
digital data. NRZ signals sustain constant levels of voltage with no signal
shifting (no return to zero-voltage level) during a bit interval. If there is a
series of bits with the same value (1 or 0), there will be no state change. The
signal is not self-clocking. See also: NRZI.
NRZI Nonreturn to Zero Inverted: One of several encoding schemes for
transmitting digital data. A transition in voltage level (either from high to
low or vice versa) at the beginning of a bit interval is interpreted as a value
of 1; the absence of a transition is interpreted as a 0. Thus, the voltage
assigned to each value is continually inverted. NRZI signals are not selfclocking. See also: NRZ.
NT1 network termination 1: An ISDN designation to devices that understand ISDN standards.
NT2 network termination 2: An ISDN designation to devices that do not
understand ISDN standards. To use a NT2, you must use a terminal
adapter (TA).
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
488
Glossary
NVRAM Non-Volatile RAM: Random-access memory that keeps its contents intact while power is turned off.
OC Optical Carrier: A series of physical protocols, designated as OC-1,
OC-2, OC-3, and so on, for SONET optical signal transmissions. OC signal
levels place STS frames on a multimode fiber-optic line at various speeds, of
which 51.84Mbps is the lowest (OC-1). Each subsequent protocol runs at a
speed divisible by 51.84. See also: SONET.
octet Base-8 numbering system used to identify a section of a dotted decimal IP address. Also referred to as a byte.
100BaseT Based on the IEEE 802.3u standard, 100BaseT is the Fast
Ethernet specification of 100Mbps baseband that uses UTP wiring. 100BaseT
sends link pulses (containing more information than those used in
10BaseT) over the network when no traffic is present. See also: 10BaseT,
FastEthernet, and IEEE 802.3.
100BaseTX Based on the IEEE 802.3u standard, 100BaseTX is the
100Mbps baseband FastEthernet specification that uses two pairs of UTP or
STP wiring. The first pair of wires receives data; the second pair sends data.
To ensure correct signal timing, a 100BaseTX segment cannot be longer than
100 meters.
ones density Also known as pulse density, this is a method of signal
clocking. The CSU/DSU retrieves the clocking information from data that
passes through it. For this scheme to work, the data needs to be encoded to
contain at least one binary 1 for each eight bits transmitted. See also: CSU
and DSU.
one-time challenge tokens Used to provide a single-use password. This
prevents replay attacks and snooping; however, it also requires the user to
have a device that provides the token. This physical component of the security model works to prevent hackers from guessing or obtaining the user’s
password.
OSI Open Systems Interconnection: International standardization program designed by ISO and ITU-T for the development of data networking
standards that make multivendor equipment interoperability a reality.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
489
OSI reference model Open Systems Interconnection reference model: A
conceptual model defined by the International Organization for Standardization (ISO), describing how any combination of devices can be connected
for the purpose of communication. The OSI model divides the task into
seven functional layers, forming a hierarchy with the applications at the top
and the physical medium at the bottom, and it defines the functions each
layer must provide. See also: Application layer, Data Link layer, Network layer,
Physical layer, Presentation layer, Session layer, and Transport layer.
OSPF Open Shortest Path First: A link-state, hierarchical IGP routing algorithm derived from an earlier version of the IS-IS protocol, whose features
include multipath routing, load balancing, and least-cost routing. OSPF is
the suggested successor to RIP in the Internet environment. See also:
Enhanced IGRP, IGP, and IP.
OSPF areas Small areas within an autonomous system that share routing
information.
OUI Organizationally Unique Identifier: Assigned by the IEEE to an organization that makes network interface cards. The organization then puts this
OUI on each and every card they manufacture. The OUI is 3 bytes (24 bits)
long. The manufacturer then adds a 3-byte identifier to uniquely identify the
host on an internetwork. The total length of the address is 48 bits (6 bytes)
and is called a hardware address or MAC address.
out-of-band management Management “outside” of the network’s
physical channels. For example, using a console connection not directly
interfaced through the local LAN or WAN or a dial-in modem. Compare to:
in-band management.
out-of-band signaling Within a network, any transmission that uses
physical channels or frequencies separate from those ordinarily used for data
transfer. For example, the initial configuration of a Cisco Catalyst switch
requires an out-of-band connection via a console port.
packet In data communications, the basic logical unit of information
transferred. A packet consists of a certain number of data bytes, wrapped or
encapsulated in headers and/or trailers that contain information about
where the packet came from, where it’s going, and so on. The various protocols involved in sending a transmission add their own layers of header
information, which the corresponding protocols in receiving devices then
interpret.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
490
Glossary
packet mode connections Packet mode connections are typically passed
through the router or remote access device. This includes Point-to-Point Protocol (PPP) sessions.
packet switch A physical device that makes it possible for a communication channel to share several connections, its functions include finding the
most efficient transmission path for packets.
packet switching A networking technology based on the transmission of
data in packets. Dividing a continuous stream of data into small units—
packets—enables data from multiple devices on a network to share the same
communication channel simultaneously but also requires the use of precise
routing information.
PAD Packet assembler and disassembler: Used to buffer incoming data that
is coming in faster than the receiving device can handle it. Typically, only
used in X.25 networks.
PAP Password Authentication Protocol: In Point-to-Point Protocol (PPP)
networks, a method of validating connection requests. The requesting
(remote) device must send an authentication request, containing a password
and ID, to the local router when attempting to connect. Unlike the more
secure CHAP (Challenge Handshake Authentication Protocol), PAP sends
the password unencrypted and does not attempt to verify whether the user
is authorized to access the requested resource; it merely identifies the remote
end. See also: CHAP.
parity checking A method of error-checking in data transmissions. An
extra bit (the parity bit) is added to each character or data word so that the
sum of the bits will be either an odd number (in odd parity) or an even
number (even parity).
partial mesh A type of network topology in which some network nodes
form a full mesh (where every node has either a physical or a virtual circuit
linking it to every other network node), but others are attached to only one
or two nodes in the network. A typical use of partial-mesh topology is in
peripheral networks linked to a fully meshed backbone. See also: full mesh.
PAT Port Address Translation: This process allows a single IP address to
represent multiple resources by altering the source TCP or UDP port
number.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
491
payload compression Reduces the number of bytes required to accurately represent the original data stream. Header compression is also possible. See also: compression.
PCR Peak Cell Rate: As defined by the ATM Forum, the parameter specifying, in cells per second, the maximum rate at which a source may transmit.
PDN Public Data Network: Generally for a fee, a PDN offers the public
access to computer communication network operated by private concerns or
government agencies. Small organizations can take advantage of PDNs,
aiding them creating WANs without investing in long-distance equipment
and circuitry.
PDU Protocol Data Unit: The name of the processes at each layer of the
OSI model. PDUs at the Transport layer are called segments; PDUs at the
Network layer are called packets or datagrams; and PDUs at the Data Link
layer are called frames. The Physical layer uses bits.
PFC Policy Feature Card: The PFC can be paralleled with the NFFC used
in Catalyst 5000 switches. It is a device that is capable of examining IP and
Ethernet headers in order to establish flow caches.
PGP Pretty Good Privacy: A popular public-key/private-key encryption
application offering protected transfer of files and messages.
Physical layer The lowest layer—Layer 1—in the OSI reference model, it
is responsible for converting data packets from the Data Link layer (Layer 2)
into electrical signals. Physical layer protocols and standards define, for
example, the type of cable and connectors to be used, including their pin
assignments and the encoding scheme for signaling 0 and 1 values. See also:
Application layer, Data Link layer, Network layer, Presentation layer, Session layer, and Transport layer.
PIM Protocol Independent Multicast: A multicast protocol that handles the
IGMP requests as well as requests for multicast data forwarding.
PIM DM Protocol Independent Multicast dense mode: PIM DM utilizes the
unicast route table and relies on the source root distribution architecture for
multicast data forwarding.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
492
Glossary
PIM SM Protocol Independent Multicast sparse mode: PIM SM utilizes the
unicast route table and relies on the shared root distribution architecture for
multicast data forwarding.
PIM sparse-dense mode An interface configuration that allows the interface to choose the method of PIM operation.
ping packet Internet groper: A Unix-based Internet diagnostic tool, consisting of a message sent to test the accessibility of a particular device on the
IP network. The acronym (from which the “full name” was formed) reflects
the underlying metaphor of submarine sonar. Just as the sonar operator
sends out a signal and waits to hear it echo (“ping”) back from a submerged
object, the network user can ping another node on the network and wait to
see if it responds.
pinhole congestion Two links to the same remote network with equal
hops but with different bandwidths. Distance vector will try to load balance
and waste bandwidth.
pleisochronous Nearly synchronous, except that clocking comes from an
outside source instead of being embedded within the signal as in synchronous transmissions.
PLP Packet Level Protocol: Occasionally called X.25 Level 3 or X.25 Protocol, a Network layer protocol that is part of the X.25 stack.
PNNI Private Network-Network Interface: An ATM Forum specification
for offering topology data used for the calculation of paths through the network, among switches and groups of switches. It is based on well-known
link-state routing procedures and allows for automatic configuration in networks whose addressing scheme is determined by the topology.
point-to-multipoint connection In ATM, a communication path going
only one way, connecting a single system at the starting point, called the
“root node,” to systems at multiple points of destination, called “leaves.”
See also: point-to-point connection.
point-to-point connection In ATM, a channel of communication that
can be directed either one way or two ways between two ATM end systems.
See also: point-to-multipoint connection.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
493
poison reverse updates These update messages are transmitted by a
router back to the originator (thus ignoring the split-horizon rule) after route
poisoning has occurred. Typically used with DV routing protocols in order
to overcome large routing loops and offer explicit information when a
subnet or network is not accessible (instead of merely suggesting that the network is unreachable by not including it in updates). See also: route poisoning.
polling The procedure of orderly inquiry, used by a primary network
mechanism, to determine if secondary devices have data to transmit. A message is sent to each secondary, granting the secondary the right to transmit.
POP 1) Point Of Presence: The physical location where an interexchange
carrier has placed equipment to interconnect with a local exchange carrier.
2) Post Office Protocol (currently at version 3): A protocol used by client
e-mail applications for recovery of mail from a mail server.
port density Port density reflects the capacity of the remote access device
regarding the termination of interfaces. For example, the port density of an
access server that serves four T1 circuits is 96 analog lines (non ISDN PRI).
port security Used with Layer 2 switches to provide some security. Not
typically used in production because it is difficult to manage. Allows only
certain frames to traverse administrator-assigned segments.
POTS Plain Old Telephone Service: This refers to the traditional analog
phone service that is found in most installations.
PPP Point-to-Point Protocol: The protocol most commonly used for dialup Internet access, superseding the earlier SLIP. Its features include address
notification, authentication via CHAP or PAP, support for multiple protocols, and link monitoring. PPP has two layers: the Link Control Protocol
(LCP) establishes, configures, and tests a link; and then any of various Network Control Programs (NCPs) transport traffic for a specific protocol suite,
such as IPX. See also: CHAP, PAP, and SLIP.
PPP callback The point-to-point protocol supports callback to a predetermined number to augment security.
Predictor A compression technique supported by Cisco. See also:
compression.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
494
Glossary
prefix routing A routing protocol that sends subnet mask information
along with route updates. Used in classless routing.
Presentation layer Layer 6 of the OSI reference model, it defines how
data is formatted, presented, encoded, and converted for use by software at
the application layer. See also: Application layer, Data Link layer, Network
layer, Physical layer, Session layer, and Transport layer.
PRI Primary Rate Interface: A type of ISDN connection between a PBX and
a long-distance carrier, which is made up of a single 64Kbps D channel in
addition to 23 (T1) or 30 (E1) B channels. See also: ISDN.
priority queuing A routing function in which frames temporarily placed
in an interface output queue are assigned priorities based on traits such as
packet size or type of interface.
process switching As a packet arrives on a router to be forwarded, it’s
copied to the router’s process buffer, and the router performs a lookup on
the Layer 3 address. Using the route table, an exit interface is associated with
the destination address. The processor forwards the packet with the added
new information to the exit interface, while the router initializes the fastswitching cache. Subsequent packets bound for the same destination address
follow the same path as the first packet.
PROM programmable read-only memory: ROM that is programmable
only once, using special equipment. Compare with: EPROM.
propagation delay The time it takes data to traverse a network from its
source to its destination.
protocol In networking, the specification of a set of rules for a particular
type of communication. The term is also used to refer to the software that
implements a protocol.
protocol stack
A collection of related protocols.
Proxy ARP Proxy Address Resolution Protocol: Used to allow redundancy
in case of a failure with the configured default gateway on a host. Proxy ARP
is a variation of the ARP protocol in which an intermediate device, such as
a router, sends an ARP response on behalf of an end node to the
requesting host.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
495
pruning The act of trimming down the Shortest Path Tree. This deactivates interfaces that do not have group participants.
PSE
Packet Switch Exchange: The X.25 term for a switch.
PSN packet-switched network: Any network that uses packet-switching
technology. Also known as packet-switched data network (PSDN). See also:
packet switching.
PSTN Public Switched Telephone Network: Colloquially referred to as
“plain old telephone service” (POTS). A term that describes the assortment
of telephone networks and services available globally.
PVC permanent virtual circuit: In a Frame-Relay network, a logical connection, defined in software, that is maintained permanently. Compare with:
SVC. See also: virtual circuit.
PVP
permanent virtual path: A virtual path made up of PVCs. See also: PVC.
PVP tunneling permanent virtual path tunneling: A technique that links
two private ATM networks across a public network using a virtual path;
wherein the public network transparently trunks the complete collection of
virtual channels in the virtual path between the two private networks.
PVST Per-VLAN Spanning Tree: A Cisco proprietary implementation of
STP. PVST uses ISL and runs a separate instance of STP for each and
every VLAN.
PVST+ Per-VLAN Spanning Tree+: Allows CST information to be passed
into PVST.
QoS Quality of Service: A set of metrics used to measure the quality of
transmission and service availability of any given transmission system.
queue Broadly, any list of elements arranged in an orderly fashion and
ready for processing, such as a line of people waiting to enter a movie theater. In routing, it refers to a backlog of information packets waiting in line
to be transmitted over a router interface.
queuing A quality of service process that allows packets to be forwarded
from the router based on administratively defined parameters. This may be
used for time-sensitive protocols, such as SNA.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
496
Glossary
R reference point Used with ISDN networks to identify the connection
between an NT1 and an S/T device. The S/T device converts the four-wire
network to the two-wire ISDN standard network.
RADIUS Remote Access Dial-in User Service: A protocol that is used to
communicate between the remote access device and an authentication
server. Sometimes an authentication server running RADIUS will be called a
RADIUS server.
RAM random access memory: Used by all computers to store information.
Cisco routers use RAM to store packet buffers and routing tables, along with
the hardware addresses cache.
RARP Reverse Address Resolution Protocol: The protocol within the TCP/
IP stack that maps MAC addresses to IP addresses. See also: ARP.
rate queue A value, assigned to one or more virtual circuits, that specifies
the speed at which an individual virtual circuit will transmit data to the
remote end. Every rate queue identifies a segment of the total bandwidth
available on an ATM link. The sum of all rate queues should not exceed the
total available bandwidth.
RCP Remote Copy Protocol: A protocol for copying files to or from a file
system that resides on a remote server on a network, using TCP to guarantee
reliable data delivery.
redistribution Command used in Cisco routers to inject the paths found
from one type of routing protocol into another type of routing protocol. For
example, networks found by RIP can be inserted into an IGRP network.
redundancy In internetworking, the duplication of connections, devices,
or services that can be used as a backup in the event that the primary connections, devices, or services fail.
reference point Used to define an area in an ISDN network. Providers
used these reference points to find problems in the ISDN network.
relay system
Another term for a router.
reliability The measure of the quality of a connection. It is one of the metrics that can be used to make routing decisions.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
reload
497
An event or command that causes Cisco routers to reboot.
remote access A generic term that defines connectivity to distant
resources using one of many technologies, as appropriate.
remote services Network services close to users but not on the same network or subnet as the users. The users would have to cross a Layer 3 device
to communicate with the network services, but they might not have to cross
the backbone.
reverse Telnet Maps a Telnet port to a physical port on the router or
access device. This allows the administrator to connect to a modem or other
device attached to the port.
RFC Request for Comments: RFCs are used to present and define standards in the networking industry.
RIF Routing Information Field: In source-route bridging, a header field
that defines the path direction of the frame or token. If the Route Information Indicator (RII) bit is not set, the RIF is read from source to destination
(left to right). If the RII bit is set, the RIF is read from the destination back
to the source, so the RIF is read right to left. It is defined as part of the Token
Ring frame header for source-routed frames, which contains path information.
ring Two or more stations connected in a logical circular topology. In this
topology, which is the basis for Token Ring, FDDI, and CDDI, information
is transferred from station to station in sequence.
ring topology A network logical topology comprising a series of repeaters
that form one closed loop by connecting unidirectional transmission links.
Individual stations on the network are connected to the network at a
repeater. Physically, ring topologies are generally organized in a closed-loop
star. Compare with: bus topology and star topology.
RIP Routing Information Protocol: The most commonly used interior
gateway protocol in the Internet. RIP employs hop count as a routing metric.
See also: Enhanced IGRP, IGP, OSPF, and hop count.
RIP version 2 Newer, updated version of Routing Information Protocol
(RIP). Allows VLSM. See also: VLSM.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
498
Glossary
RJ connector registered jack connector: Is used with twisted-pair wiring
to connect the copper wire to network interface cards, switches, and hubs.
robbed bit signaling
mechanisms.
Used in Primary Rate Interface clocking
ROM read-only memory: Chip used in computers to help boot the device.
Cisco routers use a ROM chip to load the bootstrap, which runs a power-on
self test, and then find and load the IOS in flash memory by default.
root bridge Used with the Spanning Tree Protocol to stop network loops
from occurring. The root bridge is elected by having the lowest bridge ID.
The bridge ID is determined by the priority (32,768 by default on all bridges
and switches) and the main hardware address of the device. The root bridge
determines which of the neighboring Layer 2 devices’ interfaces become the
designated and nondesignated ports.
routed protocol Routed protocols (such as IP and IPX) are used to
transmit user data through an internetwork. By contrast, routing protocols
(such as RIP, IGRP, and OSPF) are used to update routing tables between
routers.
route redistribution Translation of routing information from one type of
routing protocol to another. See also: redistribution.
route poisoning Used by various DV routing protocols in order to overcome large routing loops and offer explicit information about when a subnet
or network is not accessible (instead of merely suggesting that the network
is unreachable by not including it in updates). Typically, this is accomplished
by setting the hop count to one more than maximum. See also: poison
reverse updates.
route summarization In various routing protocols, such as OSPF,
EIGRP, and IS-IS, the consolidation of publicized subnetwork addresses so
that a single summary route is advertised to other areas by an area border
router.
router A Network layer mechanism, either software or hardware, using
one or more metrics to decide on the best path to use for transmission of network traffic. Sending packets between networks by routers is based on the
information provided on Network layers. Historically, this device has sometimes been called a gateway.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
499
router on a stick A term that identifies a single router interface connected
to a single Distribution-layer switch port. The router is an external router
that provides trunking protocol capabilities for routing between multiple
VLANs. See also: RSM, MSFC.
routing The process of forwarding logically addressed packets from their
local subnetwork toward their ultimate destination. In large networks, the
numerous intermediary destinations a packet might travel before reaching its
destination can make routing very complex.
routing by rumor Term used by a distance-vector protocol to explain
how neighbor routers learn about remote networks.
routing domain Any collection of end systems and intermediate systems
that operate under an identical set of administrative rules. Every routing
domain contains one or several areas, all individually given a certain area
address.
routing metric Any value that is used by routing algorithms to determine
whether one route is superior to another. Metrics include such information
as bandwidth, delay, hop count, path cost, load, MTU, reliability, and communication cost. Only the best possible routes are stored in the routing table,
while all other information may be stored in link-state or topological databases. See also: cost.
routing protocol Any protocol that defines algorithms to be used for
updating routing tables between routers. Examples include IGRP, RIP,
and OSPF.
routing table A table kept in a router or other internetworking mechanism that maintains a record of only the best possible routes to certain network destinations and the metrics associated with those routes.
RP 1) rendezvous point: A router that acts as the multicast source in a
multicast network. Primarily in a shared tree distribution. 2) Route Processor: Also known as a supervisory processor, a module on Cisco 7000
series routers that holds the CPU, system software, and most of the memory
components used in the router.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
500
Glossary
RSFC Route Switch Feature Card: Used to provide routing between
VLANs. The RSFC is a daughter card for the Supervisor engine II G and
Supervisor III G cards. The RSFC is a fully functioning router running the
Cisco IOS.
RSM Route Switch Module: A route processor that is inserted into the
chassis of a Cisco Catalyst 5000 series switch. The RSM is configured
exactly like an external router.
RSP Route/Switch Processor: A processor module combining the functions
of RP and SP used in Cisco 7500 series routers. See also: RP and SP.
RTS Request To Send: An EIA/TIA-232 control signal requesting permission to transmit data on a communication line.
S reference point ISDN reference point that works with a T reference
point to convert a four-wire ISDN network to the two-wire ISDN network
needed to communicate with the ISDN switches at the network provider.
sampling rate The rate at which samples of a specific waveform amplitude are collected within a specified period of time.
SAP 1) Service Access Point: A field specified by IEEE 802.2 that is part of
an address specification. 2) Service Advertising Protocol: The Novell NetWare protocol that supplies a way to inform network clients of resources and
services availability on network, using routers and servers. See also: IPX.
SCR Sustainable Cell Rate: An ATM Forum parameter used for traffic
management, it is the long-term average cell rate for VBR connections that
can be transmitted.
scripts A script predefines commands that should be issued in sequence,
typically to complete a connection or accomplish a repetitive task.
SDLC Synchronous Data Link Control: A protocol used in SNA Data Link
layer communications. SDLC is a bit-oriented, full-duplex serial protocol
that is the basis for several similar protocols, including HDLC and LAPB. See
also: HDLC and LAPB.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
501
security policy Document that defines the business requirements and processes that are to be used to protect corporate data. A security policy might
be as generic as “no file transfers allowed” to very specific, such as “FTP puts
allowed only to server X.”
security server A centralized device that authenticates access requests,
typically via a protocol such as TACACS+ or RADIUS. See also: TACACS+,
RADIUS.
seed router In an AppleTalk network, the router that is equipped with the
network number or cable range in its port descriptor. The seed router specifies the network number or cable range for other routers in that network
section and answers to configuration requests from nonseed routers on its
connected AppleTalk network, permitting those routers to affirm or modify
their configurations accordingly. Every AppleTalk network needs at least
one seed router physically connected to each network segment.
server
Hardware and software that provide network services to clients.
Session layer Layer 5 of the OSI reference model, responsible for creating, managing, and terminating sessions between applications and overseeing data exchange between Presentation layer entities. See also:
Application layer, Data Link layer, Network layer, Physical layer, Presentation layer, and Transport layer.
set-based Set-based routers and switches use the set command to configure devices. Cisco is moving away from set-based commands and is using
the Command-Line Interface (CLI) on all new devices.
setup mode Mode that a router will enter if no configuration is found in
nonvolatile RAM when the router boots. Allows the administrator to configure a router step-by-step. Not as robust or flexible as the Command-Line
Interface.
SF super frame: A super frame (also called a D4 frame) consists of 12
frames with 192 bits each, and the 193rd bit providing other functions
including error checking. SF is frequently used on T1 circuits. A newer version of the technology is Extended Super Frame (ESF), which uses 24 frames.
See also: ESF.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
502
Glossary
shared trees A method of multicast data forwarding. Shared trees use an
architecture in which multiple sources share a common rendezvous point.
signaling packet An informational packet created by an ATM-connected
mechanism that wants to establish a connection with another such mechanism. The packet contains the QoS parameters needed for connection and
the ATM NSAP address of the endpoint. The endpoint responds with a message of acceptance if it is able to support the desired QoS, and the connection
is established. See also: QoS.
silicon switching A type of high-speed switching used in Cisco 7000
series routers, based on the use of a separate processor (the Silicon Switch
Processor, or SSP). See also: SSE.
simplex The mode at which data or a digital signal is transmitted. Simplex
is a way of transmitting in only one direction. Half duplex transmits in two
directions but only one direction at a time. Full duplex transmits in both
directions simultaneously.
sliding window The method of flow control used by TCP, as well as several Data Link layer protocols. This method places a buffer between the
receiving application and the network data flow. The “window” available
for accepting data is the size of the buffer minus the amount of data already
there. This window increases in size as the application reads data from it and
decreases as new data is sent. The receiver sends the transmitter announcements of the current window size, and it may stop accepting data until the
window increases above a certain threshold.
SLIP Serial Line Internet Protocol: An industry standard serial encapsulation for point-to-point connections that supports only a single routed protocol, TCP/IP. SLIP is the predecessor to PPP. See also: PPP.
SMDS Switched Multimegabit Data Service: A packet-switched,
datagram-based WAN networking technology offered by telephone companies that provides high speed.
SMTP Simple Mail Transfer Protocol: A protocol used on the Internet to
provide electronic mail services.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
503
SNA System Network Architecture: A complex, feature-rich, network
architecture similar to the OSI reference model but with several variations;
created by IBM in the 1970s and essentially composed of seven layers.
SNAP Subnetwork Access Protocol: SNAP is a frame used in Ethernet,
Token Ring, and FDDI LANs. Data transfer, connection management, and
QoS selection are three primary functions executed by the SNAP frame.
snapshot routing Snapshot routing takes a point-in-time capture of a
dynamic routing table and maintains it even when the remote connection
goes down. This allows the use of a dynamic routing protocol without
requiring the link to remain active, which might incur per-minute usage
charges.
socket 1) A software structure that operates within a network device as a
destination point for communications. 2) In AppleTalk networks, an entity
at a specific location within a node; AppleTalk sockets are conceptually similar to TCP/IP ports.
SOHO
small office, home office: A contemporary term for remote users.
SONET Synchronous Optical Network: The ANSI standard for synchronous transmission on fiber-optic media, developed at Bell Labs. It specifies a
base signal rate of 51.84Mbps and a set of multiples of that rate, known as
Optical Carrier levels, up to 2.5Gbps.
source trees A method of multicast data forwarding. Source trees use the
architecture of the source of the multicast traffic as the root of the tree.
SP Switch Processor: Also known as a ciscoBus controller, it is a Cisco 7000
series processor module acting as governing agent for all CxBus activities.
span
A full-duplex digital transmission line connecting two facilities.
SPAN Switched Port Analyzer: A feature of the Catalyst 5000 switch,
offering freedom to manipulate within a switched Ethernet environment by
extending the monitoring ability of the existing network analyzers into the
environment. At one switched segment, the SPAN mirrors traffic onto a predetermined SPAN port, while a network analyzer connected to the SPAN
port is able to monitor traffic from any other Catalyst switched port.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
504
Glossary
spanning explorer packet Sometimes called limited-route or single-route
explorer packet, it pursues a statically configured spanning tree when
searching for paths in a source-route bridging network. See also: all-routes
explorer packet, explorer packet, and local explorer packet.
spanning tree A subset of a network topology, within which no loops
exist. When bridges are interconnected into a loop, the bridge, or switch,
cannot identify a frame that has been forwarded previously, so there is no
mechanism for removing a frame as it passes the interface numerous times.
Without a method of removing these frames, the bridges continuously forward them—consuming bandwidth and adding overhead to the network.
Spanning trees prune the network to provide only one path for any packet.
See also: Spanning Tree Protocol and spanning tree algorithm.
spanning-tree algorithm (STA) An algorithm that creates a spanning
tree using the Spanning Tree Protocol (STP). See also: spanning tree and
Spanning Tree Protocol.
Spanning Tree Protocol (STP) The bridge protocol (IEEE 802.1d) that
enables a learning bridge to dynamically avoid loops in the network
topology by creating a spanning tree using the spanning-tree algorithm.
Spanning-tree frames called bridge protocol data units (BPDUs) are sent and
received by all switches in the network at regular intervals. The switches participating in the spanning tree don’t forward the frames; instead, they’re processed to determine the spanning-tree topology itself. Cisco Catalyst series
switches use STP 802.1d to perform this function. See also: BPDU, learning
bridge, MAC address, spanning tree, and spanning-tree algorithm.
SPF Shortest Path First algorithm: A routing algorithm used to decide on
the shortest-path spanning tree. Sometimes called Dijkstra’s algorithm and
frequently used in link-state routing algorithms. See also: link-state routing
algorithm.
SPID Service Profile Identifier: A number assigned by service providers or
local telephone companies and assigned by administrators to a BRI port.
SPIDs are used to determine subscription services of a device connected via
ISDN. ISDN devices use SPID when accessing the telephone company switch
that initializes the link to a service provider.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
505
split horizon Useful for preventing routing loops, a type of distance-vector
routing rule where information about routes is prevented from leaving the
router interface through which that information was received.
spoofing 1) In dial-on-demand routing (DDR), where a circuit-switched
link is taken down to save toll charges when there is no traffic to be sent,
spoofing is a scheme used by routers that causes a host to treat an interface
as if it were functioning and supporting a session. The router pretends to
send “spoof” replies to keepalive messages from the host in an effort to convince the host that the session is up and running. See also: DDR. 2) The
illegal act of sending a packet labeled with a false address, in order to deceive
network security mechanisms such as filters and access lists.
spooler A management application that processes requests submitted to it
for execution in a sequential fashion from a queue. A good example is a print
spooler.
SPX Sequenced Packet Exchange: A Novell NetWare transport protocol
that augments the datagram service provided by Network layer (Layer 3)
protocols, it was derived from the Switch-to-Switch Protocol of the XNS
protocol suite.
SQE Signal Quality Error: In an Ethernet network, a message sent from a
transceiver to an attached machine that the collision-detection circuitry is
working.
SRB Source-Route Bridging: Created by IBM, the bridging method used in
Token-Ring networks. The source determines the entire route to a destination before sending the data and includes that information in route information fields (RIF) within each packet. Contrast with: transparent bridging.
SRT source-route transparent bridging: A bridging scheme developed by
IBM, merging source-route and transparent bridging. SRT takes advantage
of both technologies in one device, fulfilling the needs of all end nodes.
Translation between bridging protocols is not necessary. Compare with:
SR/TLB.
SR/TLB source-route translational bridging: A bridging method that
allows source-route stations to communicate with transparent bridge stations aided by an intermediate bridge that translates between the two bridge
protocols. Used for bridging between Token Ring and Ethernet. Compare
with: SRT.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
506
Glossary
SSAP Source Service Access Point: The SAP of the network node identified
in the Source field of the packet. See also: DSAP and SAP.
SSE Silicon Switching Engine: The software component of Cisco’s silicon
switching technology, hard-coded into the Silicon Switch Processor (SSP).
Silicon switching is available only on the Cisco 7000 with an SSP. Siliconswitched packets are compared to the silicon-switching cache on the SSE.
The SSP is a dedicated switch processor that offloads the switching process
from the route processor, providing a fast-switching solution, but packets
must still traverse the backplane of the router to get to the SSP and then back
to the exit interface.
SS-7 signaling Signaling System 7: The current standard for telecommunications switching control signaling. This is out-of-band signaling
that establishes circuits and provides billing information.
Stac A compression method developed by Stacker Corporation for use
over serial links.
standard IP access list IP access list that uses only the source IP addresses
to filter a network.
standard IPX access list IPX access list that uses only the source and destination IPX address to filter a network.
star topology A LAN physical topology with endpoints on the network
converging at a common central switch (known as a hub) using point-topoint links. A logical ring topology can be configured as a physical star
topology using a unidirectional closed-loop star rather than point-to-point
links. That is, connections within the hub are arranged in an internal ring.
See also: bus topology and ring topology.
startup range If an AppleTalk node does not have a number saved from
the last time it was booted, then the node selects from the range of values
from 65280 to 65534.
state transitions Digital signaling scheme that reads the “state” of the
digital signal in the middle of the bit cell. If it is five volts, the cell is read as
a one. If the state of the digital signal is zero volts, the bit cell is read as a zero.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
507
static route A route whose information is purposefully entered into the
routing table and takes priority over those chosen by dynamic routing
protocols.
static VLANs Static VLANs are manually configured port-by-port. This is
the method typically used in production networks.
statistical multiplexing Multiplexing in general is a technique that allows
data from multiple logical channels to be sent across a single physical
channel. Statistical multiplexing dynamically assigns bandwidth only to
input channels that are active, optimizing available bandwidth so that more
devices can be connected than with other multiplexing techniques. Also
known as statistical time-division multiplexing, or stat mux.
STM-1 Synchronous Transport Module Level 1. In the European SDH
standard, one of many formats identifying the frame structure for the
155.52Mbps lines that are used to carry ATM cells.
store-and-forward packet switching A technique in which the switch
first copies each packet into its buffer and performs a cyclical redundancy
check (CRC). If the packet is error-free, the switch then looks up the destination address in its filter table, determines the appropriate exit port, and
sends the packet.
STP 1) Shielded Twisted Pair: A two-pair wiring scheme, used in many network implementations, that has a layer of shielded insulation to reduce EMI.
2) Spanning Tree Protocol.
stub area An OSPF area carrying a default route, intra-area routes, and
inter-area routes, but no external routes. Configuration of virtual links
cannot be achieved across a stub area, and stub areas are not allowed to contain an ASBR. See also: non-stub area, ASBR, and OSPF.
stub AS
An area that accepts only default routes.
stub network
A network having only one connection to a router.
STUN Serial Tunnel: A technology used to connect an HDLC link to an
SDLC link over a serial link.
subarea A portion of an SNA network made up of a subarea node and its
attached links and peripheral nodes.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
508
Glossary
subarea node An SNA communications host or controller that handles
entire network addresses.
subchannel A frequency-based subdivision that creates a separate broadband communications channel.
subinterface
interface.
subnet
One of many virtual interfaces available on a single physical
See: subnetwork.
subnet address The portion of an IP address that is specifically identified
by the subnet mask as the subnetwork. See also: IP address, subnetwork, and
subnet mask.
subnet mask Also simply known as mask, a 32-bit address mask used in
IP to identify the bits of an IP address that are used for the subnet address.
Using a mask, the router does not need to examine all 32 bits, only those
selected by the mask. See also: address mask and IP address.
subnetwork 1) Any network that is part of a larger IP network and is identified by a subnet address. A network administrator segments a network into
subnetworks in order to provide a hierarchical, multilevel routing structure,
and at the same time protect the subnetwork from the addressing complexity
of networks that are attached. Also known as a subnet. See also: IP address,
subnet mask, and subnet address. 2) In OSI networks, the term specifically
refers to a collection of ESs and ISs controlled by only one administrative
domain, using a solitary network connection protocol.
SVC switched virtual circuit: A dynamically established virtual circuit, created on demand and dissolved as soon as transmission is over and the circuit
is no longer needed. In ATM terminology, it is referred to as a switched virtual connection. See also: PVC.
switch 1) In networking, a device responsible for multiple functions such
as filtering, flooding, and sending frames. It works using the destination
address of individual frames. Switches operate at the Data Link layer of the
OSI model. 2) Broadly, any electronic/mechanical device allowing connections to be established as needed and terminated if no longer necessary.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
509
switch block The switch block is a combination of Layer 3 switches and
Layer 3 routers. The Layer 2 switches connect users in the wiring closet into
the Access layer and provide 10 or 100Mbps dedicated connections.
1900/2820 and 2900 Catalyst switches can be used in the switch block.
switched Ethernet Device that switches Ethernet frames between segments by filtering on hardware addresses.
switched LAN
LAN switch.
Any LAN implemented using LAN switches. See also:
switch-fabric The central functional block of any switch design; responsible for buffering and routing the incoming data to the appropriate output
ports.
synchronous transmission Signals transmitted digitally with precision
clocking. These signals have identical frequencies and contain individual
characters encapsulated in control bits (called start/stop bits) that designate
the beginning and ending of each character. See also: asynchronous transmission and isochronous transmission.
T reference point Used with an S reference point to change a 4-wire ISDN
network to a 2-wire ISDN network.
T1 Digital WAN that uses 24 DS0s at 64K each to create a bandwidth of
1.536Mbps, minus clocking overhead, providing 1.544Mbps of usable
bandwidth.
T3
Digital WAN that can provide bandwidth of 44.763Mbps.
TACACS+ Terminal Access Control Access Control System: An enhanced
version of TACACS, this protocol is similar to RADIUS. See also: RADIUS.
tag switching Based on the concept of label swapping, where packets or
cells are designated to defined-length labels that control the manner in which
data is to be sent, tag switching is a high-performance technology used for
forwarding packets. It incorporates Data Link layer (Layer 2) switching and
Network layer (Layer 3) routing and supplies scalable, high-speed switching
in the network core.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
510
Glossary
tagged traffic ATM cells with their cell loss priority (CLP) bit set to 1.
Also referred to as discard-eligible (DE) traffic. Tagged traffic can be eliminated in order to ensure trouble-free delivery of higher priority traffic, if the
network is congested. See also: CLP.
TCP Transmission Control Protocol: A connection-oriented protocol that
is defined at the Transport layer of the OSI reference model. Provides reliable
delivery of data.
TCP header compression A compression process that compresses only
the TCP header information, which is typically repetitive. This would not
compress the user data. See also: compression.
TCP/IP Transmission Control Protocol/Internet Protocol. The suite of protocols underlying the Internet. TCP and IP are the most widely known protocols in that suite. See also: IP and TCP.
TDM time division multiplexing: A technique for assigning bandwidth on
a single wire, based on preassigned time slots, to data from several channels.
Bandwidth is allotted to each channel regardless of a station’s ability to send
data. See also: ATDM, FDM, and multiplexing.
TE terminal equipment: Any peripheral device that is ISDN-compatible
and attached to a network, such as a telephone or computer. TE1s are
devices that are ISDN-ready and understand ISDN signaling techniques.
TE2s are devices that are not ISDN-ready and do not understand ISDN signaling techniques. A terminal adapter must be used with a TE2.
TE1 A device with a four-wire, twisted-pair digital interface is referred to
as terminal equipment type 1. Most modern ISDN devices are of this type.
TE2 Devices known as terminal equipment type 2 do not understand
ISDN signaling techniques, and a terminal adapter must be used to convert the signaling.
telco
A common abbreviation for the telephone company.
Telnet The standard terminal emulation protocol within the TCP/IP protocol stack. Method of remote terminal connection, enabling users to log in
on remote networks and use those resources as if they were locally connected. Telnet is defined in RFC 854.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
511
10BaseT Part of the original IEEE 802.3 standard, 10BaseT is the Ethernet
specification of 10Mbps baseband that uses two pairs of twisted-pair, Category 3, 4, or 5 cabling—using one pair to send data and the other to receive.
10BaseT has a distance limit of about 100 meters per segment. See also:
Ethernet and IEEE 802.3.
terminal adapter A hardware interface between a computer without a
native ISDN interface and an ISDN line. In effect, a device to connect a standard async interface to a non-native ISDN device, emulating a modem.
terminal emulation The use of software, installed on a PC or LAN server,
that allows the PC to function as if it were a “dumb” terminal directly
attached to a particular type of mainframe.
TFTP Conceptually, a stripped-down version of FTP, it’s the protocol of
choice if you know exactly what you want and where it’s to be found. TFTP
doesn’t provide the abundance of functions that FTP does. In particular, it
has no directory-browsing abilities; it can do nothing but send and receive
files.
Thicknet Also called 10Base5. Bus network that uses a thick cable and
runs Ethernet up to 500 meters.
Thinnet Also called 10Base2. Bus network that uses a thin coax cable and
runs Ethernet media access up to 185 meters.
token A frame containing only control information. Possessing this control information gives a network device permission to transmit data onto the
network. See also: token passing.
token bus LAN architecture that is the basis for the IEEE 802.4 LAN specification and employs token passing access over a bus topology. See also: IEEE.
token passing A method used by network devices to access the physical
medium in a systematic way based on possession of a small frame called a
token. See also: token.
Token Ring IBM’s token-passing LAN technology. It runs at 4Mbps or
16Mbps over a ring topology. Defined formally by IEEE 802.5. See also: ring
topology and token passing.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
512
Glossary
toll network WAN network that uses the Public Switched Telephone Network (PSTN) to send packets.
trace IP command used to trace the path a packet takes through an
internetwork.
traffic shaping Used on Frame Relay networks to provide priorities of data.
transparent bridging The bridging scheme used in Ethernet and IEEE
802.3 networks, it passes frames along one hop at a time, using bridging
information stored in tables that associate end-node MAC addresses within
bridge ports. This type of bridging is considered transparent because the
source node does not know it has been bridged, because the destination
frames are sent directly to the end node. Contrast with: SRB.
Transport layer Layer 4 of the OSI reference model, used for reliable communication between end nodes over the network. The Transport layer provides mechanisms used for establishing, maintaining, and terminating virtual
circuits, transport fault detection and recovery, and controlling the flow of
information. See also: Application layer, Data Link layer, Network layer,
Physical layer, Presentation layer, and Session layer.
TRIP Token Ring Interface Processor: A high-speed interface processor
used on Cisco 7000 series routers. The TRIP provides two or four ports for
interconnection with IEEE 802.5 and IBM media with ports set to speeds of
either 4Mbps or 16Mbps set independently of each other.
trunk link Link used between switches and from some servers to the
switches. Trunk links carry information about many VLANs. Access links
are used to connect host devices to a switch and carry only VLAN information that the device is a member of.
TTL Time To Live: A field in an IP header, indicating the length of time a
packet is valid.
TUD Trunk Up-Down: A protocol used in ATM networks for the monitoring of trunks. Should a trunk miss a given number of test messages being
sent by ATM switches to ensure trunk line quality, TUD declares the trunk
down. When a trunk reverses direction and comes back up, TUD recognizes
that the trunk is up and returns the trunk to service.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
513
tunneling A method of avoiding protocol restrictions by wrapping
packets from one protocol in another protocol’s packet and transmitting this
encapsulated packet over a network that supports the wrapper protocol. See
also: encapsulation.
20/80 rule This rule means that 20 percent of what the user performs on
the network is local, whereas up to 80 percent crosses the network segmentation points to get to network services.
UART The Universal Asynchronous Receiver/Transmitter: A chip that
governs asynchronous communications. Its primary function is to buffer
incoming data, but it also buffers outbound bits.
U reference point Reference point between a TE1 and an ISDN network.
The U reference point understands ISDN signaling techniques and uses a
two-wire connection.
UDP User Datagram Protocol: A connectionless Transport layer protocol
in the TCP/IP protocol stack that simply allows datagrams to be exchanged
without acknowledgements or delivery guarantees, requiring other protocols
to handle error processing and retransmission. UDP is defined in RFC 768.
unicast Used for direct host-to-host communication. Communication is
directed to only one destination and is originated only from one source.
unidirectional shared tree A method of shared tree multicast forwarding.
This method allows only multicast data to be forwarded from the RP.
unnumbered frames HDLC frames used for control-management purposes, such as link startup and shutdown or mode specification.
UTP unshielded twisted-pair: Copper wiring used in small-to-large networks to connect host devices to hubs and switches. Also used to connect
switch to switch or hub to hub.
VBR Variable Bit Rate: A QoS class, as defined by the ATM Forum, for use
in ATM networks that is subdivided into real time (RT) class and non-real
time (NRT) class. RT is employed when connections have a fixed-time relationship between samples. Conversely, NRT is employed when connections
do not have a fixed-time relationship between samples, but still need an
assured QoS.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
514
Glossary
VCC Virtual Channel Connection: A logical circuit that is created by
VCLs. VCCs carry data between two endpoints in an ATM network. Sometimes called a virtual circuit connection.
VIP 1) Versatile Interface Processor: An interface card for Cisco 7000 and
7500 series routers, providing multilayer switching and running the Cisco
IOS software. The most recent version of VIP is VIP2. 2) Virtual IP: A function making it possible for logically separated switched IP workgroups to
run Virtual Networking Services across the switch ports of a Catalyst 5000.
virtual circuit Abbreviated VC, a logical circuit devised to assure reliable
communication between two devices on a network. Defined by a virtual path
connection (VPC)/virtual path identifier (VCI) pair, a virtual circuit can be
permanent (PVC) or switched (SVC). Virtual circuits are used in Frame
Relay and X.25. Known as virtual channel in ATM. See also: PVC and SVC.
virtual link Used to attach an OSPF area to Area 0 across an area other
than Area 0.
virtual ring In an SRB network, a logical connection between physical
rings, either local or remote.
VLAN Virtual LAN: A group of devices on one or more logically segmented LANs (configured by use of management software), enabling devices
to communicate as if attached to the same physical medium, when they are
actually located on numerous different LAN segments. VLANs are based on
logical instead of physical connections and thus are tremendously flexible.
VLSM Variable-Length Subnet Mask: Helps optimize available address
space and specify a different subnet mask for the same network number on
various subnets. Also commonly referred to as “subnetting a subnet.”
VPN virtual private network: A method of encrypting point-to-point logical connections across a public network, such as the Internet. This allows
secure communications across a public network.
VTP VLAN Trunk Protocol: Used to update switches in a switch-fabric
about VLANs configured on a VTP server. VTP devices can be a VTP server,
client, or transparent device. Servers update clients. Transparent devices are
only local devices and do not share information with VTP clients. VTPs send
VLAN information down trunked links only.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Glossary
515
VTP pruning VLAN Trunk Protocol is used to communicate VLAN information between switches in the same VTP domain. VTP pruning stops
VLAN update information from being sent down trunked links if the
updates are not needed.
WAN wide area network: A designation used to connect LANs together
across a DCE (data communications equipment) network. Typically, a
WAN is a leased line or dial-up connection across a PSTN network. Examples
of WAN protocols include Frame Relay, PPP, ISDN, and HDLC.
weighted fair queuing
Cisco routers.
Default queuing method on serial links on all
wildcard Used with access-list, supernetting, and OSPF configurations.
Wildcards are designations used to identify a range of subnets.
windowing Flow-control method used with TCP at the Transport layer of
the OSI model.
WinSock Windows Socket Interface: A software interface that makes it
possible for an assortment of applications to use and share an Internet connection. The WinSock software consists of a Dynamic Link Library (DLL)
with supporting programs such as a dialer program that initiates the connection.
workgroup switching A switching method that supplies high-speed
(100Mbps) transparent bridging between Ethernet networks as well as highspeed translational bridging between Ethernet and CDDI or FDDI.
X.25 An ITU-T packet-relay standard that defines communication
between DTE and DCE network devices. X.25 uses a reliable Data Link
layer protocol called LAPB. X.25 also uses PLP at the Network layer. X.25
has mostly been replaced by Frame Relay.
X.25 protocol First packet-switching network, but now mostly used in
Europe. Replaced in the U.S. by Frame Relay.
XTAG A locally significant numerical value assigned by the MLS-SE to
each MLS-RP in the Layer 2 network. See also: MLS-SE, MLS-RP.
ZIP Zone Information Protocol: A Session layer protocol used by AppleTalk to map network numbers to zone names. NBP uses ZIP in the determination of networks containing nodes that belong to a zone. See also: ZIP
storm and zone.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
516
Glossary
ZIP storm A broadcast storm occurring when a router running AppleTalk
reproduces or transmits a route for which there is no corresponding zone
name at the time of execution. The route is then forwarded by other routers
downstream, thus causing a ZIP storm. See also: broadcast storm and ZIP.
zone
A logical grouping of network devices in AppleTalk. See also: ZIP.
Copyright ©2001 SYBEX , Inc., Alameda, CA
www.sybex.com
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement