Link-State Protocols
Like distance vector protocols, link-state protocols use routing tables that are populated with the currentlybest routes. Link-state protocols, however, differ from Distance vector protocols in the methods they use to build their routing tables. The biggest difference between the two is that distance vector protocols advertise little information.
Unlike distance vector protocols, link-state protocols do not receive metrics in the routing table updates. Instead they must calculate the metric form the topology information learned by a router, which includes a cost associated with each link in the network. A router totals the cost associated with each link in each route to find the metric associated with that route. Link-state protocols use the Shortest Path First (SPF) algorithm, which is also called the Dijkstra SPF algorithm, to calculate route metrics.
When a new router that is configured with a link-state protocol is booted for the first time, it does not start broadcasting topology information out every interface. Instead, the router uses the Hello protocol to send and receive a small Hello packet to discover neighbors, i.e., other routers that use the same link-state protocol and share a common subnet. It has a source address of the router and a multicast destination address set to AllSPFRouters (224.0.0.5). All routers running OSPF or the SPF algorithm listen for the hello packet and send their own hello packets periodically. Once a router identifies a neighbor, the two routers exchange routing information, which is called the topology database, and then run the SPF algorithm to calculate new routes. When their topology databases are synchronized, the neighbors are said to be fully adjacent. The Hello protocol continues to transmit the Hello packets periodically.