The fact that the routers are neighbors is not enough to guarantee an exchange of link status updates; they must form adjacencies to exchange link status updates. Adjacency is the next step after the process of establishing neighbors. Adjacent routers are routers that go beyond a simple Greeting exchange and act in the database exchange process. To reduce the amount of information exchange in a given segment, OSPF selects a router as a designated router (DR) and a router as a designated backup router (BDR) in each multiple access segment. The BDR is chosen as the backup mechanism in case the DR fails. The idea behind this is that routers have a central point of contact for the exchange of information. In order to verify if two routers have established an adjacency, you can use the command: show ip ospf neighbor.
