How OSPF Works

In a typical distance vector protocol (such as IGRP), each router advertises its table of reachable destinations (vectors) and the distances to them (distance) on each of its interfaces on a regular basis (periodic updates). OSPF routers advertise the state of their directly connected links to all routers on the network (through flooding). Although OSPF uses periodic updates to the entire network, there are long periods of time between them, reducing network traffic to a minimum. Each router receives these link-state advertisements LSAs) from its neighbors and floods them out each of its other interfaces, making certain that all routers on the network receive all LSAs.

Once all routers have received all advertisements, they perform the shortest path first calculation to find the best path to each destination on the network. OSPF uses neighbor relationships to reliably flood LSAs and enforces hierarchy in a network through areas.