The modern world is networked in a way that could barely be imagined a few decades ago. Today, the Internet reaches into virtually every business and almost every home. Our children and even our grandparents speak of dot-coms, email, and web sites. The Internet is now part of our culture.
Routers are the glue that holds the Internet together. And Cisco is the most prominent router manufacturer, holding the largest share of the market. Their routers come in all sizes, from inexpensive units for homes and small offices to equipment costing well over $100,000 and capable of routing at gigabit speeds. One of the most impressive facts about their product line is its unified operating system. Almost all of their routers, as well as half of their switches—from the smallest to the largest—run the Internetwork Operating System (IOS). Therefore, they share the same command set, the same user interface, and the same configuration techniques. While an 800-series home router doesn’t have the features or the capacity of a 7500-series router that might be used to connect an ISP to an Internet backbone, you configure them the same way. Both routers use access lists, have similar security mechanisms, support the same set of protocols in the same way, and so on. A home router probably wouldn’t have a Frame Relay interface, but if it did, it would be configured just like a Frame Relay interface on a mid-sized corporate router.
IOS is an extremely powerful and complex operating system with an equally complex configuration language. There are many commands, with many options, and if you get something wrong you can easily take your company offline. That’s why I’ve decided to provide a quick-reference guide to IOS. As large a book as this is, though, it’s impossible to cover all of IOS. Therefore, I’ve limited the discussion to IOS configuration for the TCP/IP protocol family. I’ve included all the commands that you need to work with TCP/IP and the lower-level protocols on which it relies. The trade-off is that I’ve made no attempt to cover other protocols that IOS supports, and there are many: IPX, AppleTalk, SNA, DecNet, and virtually any other protocol suite that is now or ever has been in widespread use.
This book is intended as a quick reference, not as a step-by-step exposition of routing protocols or as an IOS tutorial. I haven’t focused on thorough explanation; instead, I’ve tried to give lots of examples of the things people most frequently need to do when configuring a Cisco router, with just enough explanation to get you by. I’ll start with the user interface, then talk about configuring lines and interfaces (Chapter 4, Chapter 5, and Chapter 6), access lists (Chapter 7), routing protocols (Chapter 8, Chapter 9, and Chapter 10), and finally, dial-on-demand routing, security, and troubleshooting (Chapter 11, Chapter 12, Chapter 13, and Chapter 14). Chapter 15 through Chapter 15 is the quick reference. Chances are, by the time the second edition of this book appears, the quick-reference section will be pretty well thumbed and worn out.
At first, the Cisco user interface appears cryptic. But after learning the interface’s structure, you’ll become much more comfortable with it. Once you have learned some special features, you’ll be able to work with the router’s configuration easily.