IP Routing

Preface

Ants, single-celled creatures such as the cellular slime mold, plants and animals in ecosystems, and (even) human beings can congregate and display miraculously complex behaviors.^[1] Say a colony of ants in a tree needs to move to another tree, perhaps in search of food. Some ants build a bridge by joining their bodies in a chain stretching from one limb in one tree to another limb in another tree. Other ants cross over this ant structure, walking over their peers. Once all the ants have crossed over, the ants in the bridge begin to gracefully undo the structure, crossing one by one. Each ant only repeats simple actions, over and over again; the net result of thousands of ants working together is the miracle of the bridge, which allows the ant colony to migrate.

In our own human society we hope to emulate this level of cooperation. Each one of us is useful to others in some way: we write, lay bricks, act in the theater, farm, drive buses, repair helicopters, tend to the sick, make coffee... As individuals, we repeat the same acts again and again, whether it be making coffee cup after coffee cup or seeing patient after patient. The net result of all these acts is our complex, wonderful society.

So, what does all this have to do with IP routing? Each router in a network repeats simple processes over and over again, as described in the specification of the routing protocol it is executing. The net result of all the routers in a network repeating these simple processes is IP routing, or the movement of IP packets in a network.

My advice to you, the student of IP routing, is this: study the simple behaviors of each ant, and make sure you understand them in detail. There is no other way to understand how IP networks behave as ordered organisms.

Audience

This text assumes that the reader has knowledge of basic networking concepts -- the ISO/OSI model, bridging versus routing, IP addressing, TCP/IP, etc.

This text is not meant to replace Cisco manuals. Use this text to build concepts. IP routing commands are described as they are used to illustrate concepts; however, this text does not contain an exhaustive list of all IP routing commands. Use Cisco documentation for details on commands, to find out which IOS release supports new features, and for the (inevitable) bug lists.

Most of the time I have used addresses from the private address pool. However, sometimes I have had to dip into the registered address pool, especially when talking about BGP. If you use the examples in this book, be careful that you do not choose addresses that conflict with existing addresses in your environment.

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^[1] The New York Times has reported some intriguing examples of such behavior, quoting research work from the Santa Fe Institute in New Mexico (“Mindless Creatures Acting Mindfully,” The New York Times, March 23, 1999, Science Times).