Chapter 2. Link State Basics

All current IP routing protocols belong to one of two classes: vector protocols or link state protocols. RIP, RIPv2, RIPng, Cisco Systems' IGRP and EIGRP, and BGP are vector protocols, whereas OSPF and IS-IS are link state. This chapter presents the fundamental concepts of link state protocols. With these concepts firmly understood, the remaining chapters can then demonstrate how OSPF and IS-IS implement each concept similarly or differently.

The objective of a routing protocol is to create, in each router in a network, a database of reachable destination addresses for the purpose of forwarding packets. Each address in the database is associated with the router interface closest to the destination and possibly the address of the next router on the path toward the destination. This database is called the routing information database (RIB), or simply the routing table. Two mechanisms must be defined for a routing protocol to create the RIB:

• A procedure by which destination addresses and their associated information are communicated between routers

• An algorithm that uses shared information to calculate the shortest path to each destination in the database

The fundamental difference between vector and link state is in how the two classes of protocols implement these two mechanisms. To understand the possible advantages of link state protocols, we first briefly examine vector protocols.