OSPF - Neighbor vs Adjacency
When implementing OSPF in a multi-access network such as in a broadcast network type, an OSPF DR/BDR election takes place. Within such a multi access network all OSPF routers will see each other as neighbors, but not all routers will form adjacencies.
Adjacencies will only take place between the DR and BDR, between the DR and the DROTHERs, and between the BDR and DROTHERs. Even so, all OSPF routers appear in the output of the show ip ospf neighbors command.
In this context, neighbors and adjacencies differ in the following way:
- Neighbors: When two OSPF routers on the same network segment recognize each other (through OSPF Hello packets), they become OSPF neighbors.
- Adjacencies: Neighbors may then form adjacencies, which are deeper relationships that involve the exchange of routing information (LSAs. Not all neighbors will form adjacencies.
Links:
https://networklessons.com/ospf/troubleshooting-ospf-neighbor-adjacency
https://networklessons.com/ospf/ospf-hello-and-dead-interval
https://networklessons.com/ospf/ospf-drbdr-election-explained