In an OSPF network, two routers that have an interface to the same network are neighbors. Routers use the Hello protocol to discover their neighbors and to maintain neighbor relationships. On a broadcast or point-to-point network, the Hello protocol dynamically discovers neighbors. On an NBMA network, you must manually configure neighbors for the network.
The Hello protocol provides bidirectional communication between neighbors. Periodically, OSPF routers send hello packets over all interfaces. Included in these hello packets is the following information:
router priority
router hello timer and dead timer values
list of routers that sent the router hello packet on this interface
router choice for DR and backup designated router (BDR)
Bidirectional communication is determined after one router discovers itself listed in the hello packet of its neighbor.
NBMA interfaces whose router priority is a positive, nonzero value are eligible to become DRs for the NBMA network and are configured with a list of all attached routers. The neighbors list includes each neighbor IP address and router priority. In an NBMA network, a router with a priority other than zero is eligible to become the DR for the NBMA network. You must manually configure the IP address, mask, and router priority of neighbors on routers that are eligible to become the DR or BDR for the network.
Log messages indicate when an OSPF neighbor state change occurs. Each log message indicates the previous state and the new state of the OSPF neighbor. The log message generated for system traps also indicates the previous state and the current state of the OSPF neighbor.
Neighbors can form an adjacency to exchange routing information. After two routers form an adjacency, they perform a database exchange process to synchronize their topological databases. After the databases synchronize, the routers are fully adjacent. Adjacency conserves bandwidth because, from this point, the adjacent routers pass only routing change information.
All routers connected by a point-to-point network or a virtual link always form an adjacency. All routers on a broadcast or NBMA network form an adjacency with the DR and the BDR.
In an NBMA network, before the routers elect a DR, the router sends hello packets only to those neighbors eligible to become a DR. The NBMA DR forms adjacencies only with its configured neighbors and drops all packets from other sources. The neighbor configuration also notifies the router of the expected hello behavior for each neighbor.
If a router receives a hello packet from a neighbor with a priority different from that which is already configured for the neighbor, the router can automatically change the configured priority to match the dynamically learned priority.