CISCO IOS in a Nutshell

Preface

This is a book for everybody who has to deal with Cisco’s routers.

As you well know, Cisco Systems has created an extremely diverse line of routers and other network products. One unifying thread runs through the product line: virtually all of Cisco’s products run the Internetwork Operating System (IOS). This is both a great advantage and a great disadvantage. On the one hand, when you’re familiar with one Cisco router, you’re reasonably familiar with them all. Someone using a small ISDN router in a home office could look at a configuration file for a high-end router at an ISP and not be lost. He might not understand how to configure the more esoteric routing protocols or high-speed network interfaces, but he’d be looking at a language that was recognizably the same.

On the other hand, this uniformity means that just about everything has been crammed into IOS at one time or another. IOS is massive—there’s no other way to say it. And it has evolved over many years. The command-line interface isn’t graceful, and is often non-uniform: many commands don’t do what you think they should, and the same command verbs can mean completely different things in different contexts. This inconsistency is probably a natural result of evolution at an extremely large company with an extremely large number of developers, but it doesn’t make life any easier.

So, where do you find out what commands you need to know? There’s the almost mythical “green wall” of Cisco documentation, but it’s difficult to find what you need in tens of thousands of pages. Of course, even getting to Cisco’s online documentation may be impossible if your router doesn’t work. And the volume of documentation is imposing. A search for ip cef traffic-statistics—not one of the more frequently used commands—yields 163 hits. How do you get to the right one? Beats me. That’s why I wrote this book.

This book is primarily a quick reference to the commands that are most frequently needed to configure Cisco routers for standard IP routing tasks. There are plenty of weasel words in there, and they’re needed. This is far from a complete quick ref to all of IOS—such a quick ref would probably be well over 2000 pages long, clearly too long to be useful. Therefore, I haven’t attempted to cover protocols other than IP (although there is support for everything from AppleTalk to SNA), nor any of the more exotic creatures in the IP space. And even in areas I have covered thoroughly, I was still forced to exclude commands that are useful only in limited cases.

Above all, this is a network administrator’s book: it represents practical experience with IP routing on Cisco routers and covers the commands that you’re likely to need. No doubt some readers will disagree with the choices I’ve made—such disagreement is inevitable. But though you occasionally won’t find information about a command you need to use, you will far more often find precisely what you need to know at your fingertips.

More than anything else, the goal of this book is to give you information quickly. It aspires not to give you in-depth knowledge of how IP routing works, but to help you remember what arguments you need to give to the snmp-server enable traps command, or to help you scan through the many commands that start with ip to jog your memory about which one configures the forwarding of broadcast packets to selected subnets. If I succeed in doing that, I’m happy.

Organization

This book consists primarily of two parts. The first could be considered a tutorial, but that doesn’t quite capture its purpose. I try to teach the basic principles behind configuring the router, but there are many other sources for that information: for example, Scott Ballew’s Managing IP Networks with Cisco Routers, or Jeff Sedayao’s Cisco IOS Access Lists, both from O’Reilly. This part of the book breezes quickly through as many examples of different configuration tasks as possible. I provide explanations, but the focus is on the examples. By studying them, you’ll see how to accomplish many of the tasks involved in setting up a router.

The bulk of the book is the quick reference. There’s nothing fancy here—it’s organized alphabetically, and shows the commands that I felt were most useful to someone using a Cisco router in an IP environment.