Chapter 4. Dynamic Routing Protocols

This chapter covers the following subjects:

• Routing Protocol Basics

• Distance Vector Routing Protocols

• Link State Routing Protocols

• Interior and Exterior Gateway Protocols

• Static or Dynamic Routing?

The previous chapter explains what a router needs to recognize to correctly switch packets to their respective destinations, and how that information is put into the route table manually. This chapter shows how routers can discover this information automatically and share that information with other routers via dynamic routing protocols. A routing protocol is the language a router speaks with other routers to share information about the reachability and status of networks.

Dynamic routing protocols not only perform these path-determination and route-table-update functions, but also determine the next-best path if the best path to a destination becomes unusable. The capability to compensate for topology changes is the most important advantage dynamic routing offers over static routing.

Obviously, for communication to occur, the communicators must speak the same language. Since the advent of IP routing, there have been eight major IP routing protocols from which to choose;^[1] if one router speaks RIP and another speaks OSPF, they cannot share routing information because they are not speaking the same language. Subsequent chapters examine all the IP routing protocols in current use, and even consider how to make a router "bilingual," but first it is necessary to explore some characteristics and issues common to all routing protocolsIP or otherwise.

^[1] Of these eight protocols, BGP has obsoleted EGP, the Cisco Systems EIGRP obsoleted its IGRP, and RIPv2 is quickly replacing RIPv1.