10.1 Introduction

Our discussion in the previous chapter dealt with static routing. The routing table entries were created by default when an interface was configured (for directly connected interfaces), added by the route command (normally from a system bootstrap file), or created by an ICMP redirect (usually when the wrong default was used).

This is fine if the network is small, there is a single connection point to other networks, and there are no redundant routes (where a backup route can be used if a primary route fails). If any of these three conditions is false, dynamic routing is normally used.

This chapter looks at the dynamic routing protocols used by routers to communicate with each other. We concentrate on RIP, the Routing Information Protocol, a widely used protocol that is provided with almost every TCP/IP implementation. We then look at two newer routing protocols, OSPF and BGP. The chapter finishes with an examination of a new routing technique, called classless interdomain routing, that is starting to be implemented across the Internet to conserve class B network numbers .