Overview

Like the Routing Information Protocol (RIP) discussed in the previous chapter, the Open Shortest Path First (OSPF) protocol is one of the family of Interior Gateway Protocols (IGPs). OSPF however, represents the latest thinking in this particular area of networking and as such, is poised to become the routing protocol of choice for major vendors of internetworking equipment.

As we saw previously, RIP uses a distance-vector algorithm in order gauge the quality of routing information received. With this type of algorithm, any router within an internet will learn of all reachable networks, but will know of only the next hop router en-route to the final destination, and the relative cost involved. Conversely, OSPF, uses a Link-State protocol. Here, each router within a particular Area of the internet maintains an identical database describing the topology of the enitre Autonomous System ^[1] in terms of reachable networks, the routers that inter-connect them, and the costs involved with the use of each link. OSPF then uses this information to construct what is known as a Shortest Path Tree that describes the topology from its own perspective, and from this determines the routes that should be placed in the routing table.

Since OSPF routers know the exact topology of the Autonomous System, when topology changes occur these routers are able to adapt quickly thus greatly reducing the time taken for convergence. OSPF also allows the Autonomous System to be split into administrative areas. This has the...