CCIE Professional Development: Routing TCP/IP, Volume I 
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Copyright© 1998 by Macmillan Technical Publishing 
Cisco Press logo is a trademark of Cisco Systems, Inc. 
All  rights reserved.  No  part  of  this book  m ay  be reproduced  or  transm itted  in  any  form 
or by any means, electronic or mechanical, including photocopying, recording, or by 
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publisher, except for the inclusion of brief quotations in a review. 
Printed in the United States of America 2 3 4 5 6 7 8 9 0 
Library of Congress Cataloging- in- Publication Number 98- 84220 
Warning and Disclaimer 
This book is designed to provide information about TCP/IP. Every effort has been 
made to make this book as complete and as accurate as possible, but no warranty or 
fitness is implied. 
The information is provided on an "as is" basis. The author, Macmillan Technical 
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Dedications 
This book would not have been possible without the concerted efforts of many 
dedicated people. I would like to thank the following people for their contributions: 
First, thanks to Laurie McGuire, development editor, who not only improved the book 
but improved me as a writer. 
Thanks to Jenny DeHaven Carroll and Mike Tibodeau for their careful technical editing. 
I would also like to thank the following people, who provided technical advice or 
reviews on selected sections of the book: Howard Berkowitz, Dave Katz, Burjiz 
Pithawala, Mikel Ravizza, Russ White, and Man- Kit Yueng. 
I would like to thank the following people at Macmillan Technical Publishing: Tracy 
Hughes and Lynette Quinn, who managed the project, and Julie Fairweather, the 
Executive Editor. In addition to being highly competent, they are three of the nicest 
people anyone could hope to work with. Also, thanks to Jim LeValley, Associate 
Publisher, who first approached me about writing this book. 
Thanks to Wandel & Golterman, and to Gary Archuleta, W&G's Regional Sales 
Manager in Denver, for arranging the use of one of their excellent protocol analyzers 
for the length of the project. 
Finally, I want to thank my wife, Sara, and my children: Anna, Carol, James, and 
Katherine. Their patience, encouragement, and support were critical to the completion 
of this book.  
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About the Reviewers 
Jennifer DeHaven Carroll is a principal consultant for International Network Services. She is CCIE 
number 1402. Jennifer has planned, designed and implemented many IP networks over the past 10 
years, utilizing RIP version 2, IGRP, E- IGRP, OSPF and BGP. She has also developed and taught 
theory and Cisco implementation classes on all IP routing protocols. 
Michael Tibodeau is a Systems Engineer for Cisco Systems. Over the past two years, Michael has 
specialized in security technologies for both his own customers and Networkers audiences. He also 
focuses on  the Electronic Com m erce and  Quality  of  Service arenas. Michael holds a Bachelor's 
degree in Systems Engineering from the University of Virginia and holds a Master's degree in 
Systems Engineering and Management, concentrating on telecommunications. 
I nt roduct ion 
Routing is an essential element of all but the smallest data communications networks. At one level, 
routing and the configuration of routers are quite simple. But as internetworks grow in size and 
complexity, routing issues can become at once both large and subtle. Perversely, perhaps, I am 
grateful for the difficult problems large- scale routing can present—as a network systems consultant, 
these problems are my bread and butter. Without them, the phrase "You want fries with that?" could 
be an unfortunate part of my daily vocabulary. 
Cisco Certified Internetwork Experts are widely recognized for their ability to design, troubleshoot, 
and manage large internetworks. This recognition comes from the fact that you cannot become a 
CCIE by attending a few classes and then regurgitating some memorized facts onto a written test. A 
CCIE has proven his or her expertise in an intense, famously difficult hands- on lab exam. 
Objectives 
This book is the first in a series designed to aid you in becoming a Cisco Certified Internetwork 
Expert and the first of two volumes that focuses on TCP/IP routing issues. Early in the project, Kim 
Lew, Cisco Systems program manager, said, "Our objective is to make CCIEs, not to make people 
who can pass the CCIE lab." I entirely agree with that statement and have used it as a guiding 
principle throughout the writing of this book. Although the book includes many case studies and 
exercises to help you prepare for the CCIE lab, my primary objective is to increase your 
understanding of IP routing—both on a generic level and it is implemented on Cisco routers. 
Audience 
The audience for this book is any network designer, administrator, or engineer who needs a full 
understanding of the interior routing protocols of TCP/IP. Although the practical aspects of the book 
focus on Cisco's IOS, the information is applicable to any routing platform. 
The book is not only for readers who plan to become Cisco Certified Internetwork Experts, but for 
anyone who wishes to advance his or her knowledge of TCP/IP routing. These readers will fall into 
one of three categories: 
The "beginner" who has some basic networking knowledge and wishes to begin a deep study 
of internetworking 
The intermediate- level  networking  professional  who  has experience with  routers,  Cisco  or 
otherwise, and plans to advance that experience to the expert level 
The highly experienced networking expert. This individual has extensive hands- on expertise 
with Cisco routers and is ready to take the CCIE lab; however, he or she wants a structured 
review  and  series of  exercises for  verification  and  validation. 
CCI E Professional  Developm ent:  Routing  TCP/ I P,  Volum e I  focuses prim arily  on  the interm ediate-
level  networking  professional  while offering  to  the beginner  a  structured  outline of  fundam ental 
inform ation  and  to  the expert  the required  challenges to hone his or her skills. 
Organization 
The fourteen chapters of the book are divided into three parts. 
Part I examines the basics of networks and routing. Although more advanced readers may wish to 
skip the first two chapters, I recommend that they at least skim Chapter 3, "Static Routing," and 
Chapter 4, "Dynamic Routing Protocols." 
Part II covers the TCP/IP Interior Gateway Protocols. Each protocol- specific chapter begins with a 
discussion of the mechanics and parameters of the protocol. This general overview is followed by 
case studies on configuring and troubleshooting the protocol on Cisco routers in various network 
topologies. 
The Exterior Gateway Protocols, as well as such topics as multicast routing, Quality of Service 
routing, router security and management, and routing IPv6 will be covered in Volume II. 
Part III examines the tools available for creating and managing interoperability with multiple IP 
routing protocols, as well as such tools as default routes and route filtering. These chapters, like the 
ones in Part II, begin with concepts and conclude with case studies. 
Conventions and Features 
Most chapters conclude with a set of review questions, configuration exercises, and troubleshooting 
exercises. The review questions focus on the theoretical aspects of the chapter topic, whereas the 
configuration and troubleshooting exercises address Cisco- specific aspects of the chapter topic. 
Also at the end of each chapter is a table with a brief description of all important Cisco IOS 
commands used in that chapter. The conventions used to present these commands are the same 
conventions used in the IOS Command Reference. The Command Reference describes these 
conventions as follows: 
Vertical bars (|) separate alternative, mutually exclusive, elements. 
Square brackets [] indicate optional elements. 
Braces {} indicate a required choice. 
Braces within square brackets [{}] indicate a required choice within an optional element. 
Boldface indicates commands and keywords that are entered literally as shown. 
I talics indicate arguments for which you supply values. 
I m portant  concepts are called  out  in  m argin  notes for quick reference. 
Figure I.1 shows the conventions used in the illustrations throughout the book.   
 
Figure I.1. Illustration conventions used in this book. 
All protocol analyzer displays shown in the book are taken from a Wandel & Goltermann DA- 320 
DominoLAN Internetwork Analyzer. 
Foreword 
In today's world of networking, mission- critical networks are being designed for data, voice, and 
video. Due to different traffic patterns and the quality of service required by each type of 
inform ation,  solid  hands- on experience is imperative for managing, designing, and troubleshooting 
these networks. 
Attaining a strong degree of hands- on experience translates into in- depth understanding of the 
concepts, scalability, and deployment issues of today's networks. Such experience also builds the 
expertise to analyze traffic patterns and the knowledge of when, where, and how to apply protocol 
and bandwidth features to enhance performance. 
To help further your hands- on experience, Cisco Press is publishing the CCIE Professional 
Development series of books. Books in this series will significantly help your understanding of 
protocol concepts, and they provide real- world examples and case studies to strengthen the 
theoretical  concepts exam ined.  I  highly  recom m end  that  you  use these books as a  hands- on 
learning  tool  by  duplicating  the exam ples and case studies using Cisco products. You can even take 
this further by tweaking the configuration parameters to see which changes each network goes 
through by using the extensive debugging features provided in each Cisco product. 
In the first book of the CCIE Professional Development series, CCI E Professional  Developm ent: 
Routing  TCP/ I P,  Volum e I , Jeff Doyle does a fantastic job of building the TCP/IP concepts, from IP 
address classes to analyzing protocol metrics. Each chapter contains examples, network topologies 
with IP addresses, packet analysis, and Cisco debugging outputs. In my opinion, the best parts are 
the case studies, in which Jeff compares different features of the protocol by using more or less the 
similar topology. This generates a strong understanding of the protocol concepts and features. 
I recommend CCI E Professional  Developm ent:  Routing  TCP/ I P,  Volum e I  for any networking 
certification, and I believe that it also makes an excellent university networking course book. 
Imran Qureshi 
CCIE Program Manager   
Part I: Routing Basics 
Chapter 1 Basic Concepts: Internetworks, Routers, and Addresses 
Chapter 2 TCP/IP Review  
Chapter 3 Static Routing  
Chapter 4 Dynamic Routing Protocols                                               
Chapter 1. Basic Concepts: Internetworks, Routers, and 
Addresses 
Bicycles with Motors  
Data Link Addresses  
Repeaters and Bridges  
Routers  
Network Addresses  
Once upon a time, computing power and data storage were centralized. Mainframes were locked away in 
climate-controlled, highly secure rooms, watched over by a priesthood of IS administrators. Contact with 
a computer was typically accomplished by bringing a stack of Hollerith cards to the priests, who 
interceded on our behalf with the Big Kahuna. 
The advent of the minicomputer took the computers out of the IS temple of corporations and universities 
and brought them to the departmental level. For a mere $100K or two, engineering and accounting and 
any other department with a need for data processing could have their own machines. 
Following on the heels of the minicomputers were microcomputers, bringing data processing right to the 
desktop. Affordability and accessibility dropped from the departmental level to the individual level, 
making the phrase personal computer part of everyone's vocabulary. 
Desktop computing has evolved at a mind-boggling pace, but it was certainly not an immediate 
alternative to centralized, mainframe-based computing. There was a ramping-up period in which both 
software and hardware had to be developed to a level where personal computers could be taken seriously. 
Bicycles with Motors 
One of the difficulties of decentralized computing is that it isolates users from one another and from the 
data and applications they may need to use in common. When a file is created, how is it shared with Tom, 
Dick, and Harriet down the hall? The early solution to this was the storied SneakerNet: Put the file on 
floppy disks and hand carry them to the necessary destinations. But what happens when Tom, Dick, and 
Harriet modify their copies of the file? How does one ensure that all information in all versions are 
synchronized? What if those three coworkers are on different floors or in different buildings or cities? 
What if the file needs to be updated several times a day? What if there are not three coworkers, but 300 
people? What if all 300 people occasionally need to print a hard copy of some modification they have 
made to the file? 
The local-area network, or LAN, is a small step back to centralization. LANs are a means of pooling and 
sharing resources. Servers enable everyone to access a common copy of a file or a common database; no 
more "walkabouts" with floppies, no more worries about inconsistent information. E-mail furnishes a 
compromise between phone calls, which require the presence of the recipient, and physical mail service, 
which is called snail mail for a good reason. The sharing of printers and modem pools eliminates the need 
for expensive, periodically used services on every desk. 
Of course, in their infancy, LANs met with more than a little derision from the mainframe manufacturers. 
A commonly heard jibe during the early years was, "A LAN is like a bike with a motor, and we don't 
make Mopeds!" What a difference a few years and a few billion dollars would make.    
NOTE 
Data link  
Physically, a LAN accomplishes resource pooling among a group of devices by connecting them to a 
common, shared medium, or datalink. This medium may be twisted-pair wires (shielded or unshielded), 
coaxial cable, optical fiber, infrared light, or whatever. What matters is that all devices attach commonly 
to the data link through some sort of network interface. 
A shared physical medium is not enough. Rules must govern how the data link is shared. As in any 
community, a set of rules is necessary to keep life orderly, to ensure that all parties behave themselves, 
and to guarantee that everyone gets a fair share of the available resources. For a local-area network, this 
set of rules, or protocol, is generally called a Media Access Control (MAC). The MAC, as the name 
implies, dictates how each machine will access and share a given medium. 
So far, a LAN has been defined as being a community of devices such as PCs, printers, and servers 
coexisting on a common communications medium and following a common protocol that regulates how 
they access the medium. But there is one last requirement: As in any community, each individual must be 
uniquely identifiable. 
Data Link Addresses 
In a certain community in Colorado, two individuals are named Jeff Doyle. One Jeff Doyle frequently 
receives telephone calls for the person with whom he shares a name—so much so that his clever wife has 
posted the correct number next to the phone to redirect errant callers to their desired destination. In other 
words, because two individuals cannot be uniquely identified, data is occasionally delivered incorrectly 
and a process must be implemented to correct the error. 
Among family, friends, and associates, a given name is usually sufficient for accurately distinguishing 
individuals. However, as this example shows, most names become inaccurate over a larger population. A 
more unique identifier, such as a United States Social Security number, is needed to distinguish one 
person from every other. 
NOTE 
Frame  
Devices on a LAN must also be uniquely and individually identified or they, like humans sharing the 
same name, will receive data not intended for them. When data is to be delivered on aLAN , it is 
encapsulated within an entity called a frame, a kind of binary envelope. Think of data encapsulation as 
being the digital equivalent of placing a letter inside an envelope, as in Figure 1.1[1] . A destination address 
and a return (source) address are written on the outside of the envelope. Without a destination address, the 
postal service would have no idea where to deliver the letter. Likewise, when a frame is placed on a data 
link, all devices attached to the link "see" the frame; therefore, some mechanism must indicate which 
device should pick up the frame and read the enclosed data. 
[1] As will be seen later, creating a data link layer frame is really more like putting an envelope inside a larger envelope.