A few years ago, I was preparing to teach my first introductory course on network routing. While seeking courseware material for the class, I examined a number of books on the subject but never found one I felt completely comfortable with. In the end, I chose some standardized courseware, and ended up handing out a series of “white papers” I had authored in order to augment the books used in the course. Those white papers ultimately evolved into this full blown book.

Routing is not rocket science, but it’s a bit of a challenge to explain it in a manner that students don’t find confusing. The basic idea of forwarding packets from one network to another is really not a difficult a concept, but in the maturing, Internet-driven, multi-vendor, multi-protocol, classlessly addressed world of routing we live in today, there are a number of twists and turns when it comes to getting all those millions of packets to their destination. In considering an addition to the various routing primers available to the reader, I saw a need for an up-to-date introduction to the subject that leaves the reader—after making the investment in studying the material—with the reward of having the confidence that they actually understand modern routing enough to go out there and put their knowledge to work. When poorly explained, routing can be a weighty, cumbersome topic. When properly understood, routing is, well . . . fun. It’s a really enjoyable field to work in when you have a handle on how this aspect of networking works.

There is an art to routing as well as a science. In other words, there’s more than one way to get a packet from point Ato point B. As a network engineer with a specialty in routing, you can excel in your field and gain peer recognition by playing a game called "let’s figure out the most efficient way to route packets on this network". We’re here to help you play the game well. A primary goal in the creation of this book was to provide clear and complete information about how modern routing works. A strong emphasis has been placed on giving the reader a broad enough background in each covered topic that the he or she hits "critical mass" if you will, whereby you haven’t just memorized an explanation for how an aspect of routing works, you truly understand why it works the way it does. If, while reading this book, you find yourself saying something like “Hey, I got it!”, then I have done my job.

What Material Is Covered in This Book?
Because routing is an extension of basic networking, the book starts with a review of core networking in Chapter 1. The fundamentals of networking as it relates to routing is presented, including a thorough review of network models, followed by a review of networking equipment. The concept of packet forwarding is explained, and a moderate treatment of the TCP/IP protocol suite is covered. Special attention is paid to classless addressing (subnetting, VLSMs, CIDR, and so on), because it is easily the biggest stumbling block in understanding routing. The Internet runs on CIDR addressed networks now, so it’s not a topic to be brushed aside. The treatment of this subject matter will not only leave you with an understanding of classless addressing, you will be able to subnet with ease.

Chapter 2 provides the basis for understanding how routing works. The explanation starts where routing starts—at the workstation. From there, route tables and how they are populated are explained.

Chapters 3 and 4 explain how static and dynamic routing work, respectively. Chapter 4 is a pivotal chapter. Besides an in-depth primer on routing protocols, the important but elusive topics of route summarization, discontiguous networks, hierarchical addressing, and the longest match principal are covered as well.

Chapters 5 and 6 cover the two legacy routing protocols, RIP and IGRP. IGRP does not support classless addressing and was replaced by EIGRP. Its coverage is somewhat perfunctory, but there is material there that will assist you in understanding EIGRP. RIP was upgraded to support classless networking so it is still in use, but RIP does not support large networks. Regardless, read the treatment of RIP, because the coverage lays a foundation for many topics covered in subsequent chapters.

Chapters 7 and 8 cover the two contemporary routing protocols for large networks: EIGRP and OSPF. EIGRP is Cisco System’s proprietary entry into the realm of routing protocols, whereas OSPF is an open standard protocol, with recognition as the recommended interior routing protocol on the Internet. I have put special effort into the treatment of OSPF, and I think you will feel quite grounded with the protocol after absorbing the material in Chapter 8.

Chapter 9 provides a cursory introduction to the heady topic of the routing protocol that ties the whole Internet together, namely the Border Gateway Protocol.

Chapter 10 covers some particulars of routing that are best served up after spending some time with the routing protocols. Here, the topics of default routing and route redistribution are taken up.

What’s Not Covered?
Any routing primer should give you an idea of what there is to pursue for further study after you have the basics down. Toward that end, the appendix has a list of routing topics not covered in this book. An assumption is made that you know how to access a router and put it into programming mode. If that is not so, the appendix has a Web reference that will help.

Will This book Help Me Pass a Cisco Test?
Glad you asked. This book is not written as a pass-the-test guide. We're proud of that. However, the material in this book will most certainly help you in a testing environment because it is designed to help you truly understand the concepts of routing! Testing these days focuses more on understanding and troubleshooting, and less on raw facts that can be memorized. Because the book tends to give a more in-depth treatment of the topics it covers, it in fact provides a foundation for many of the Cisco certification exams.

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