OSPF - Adjacencies in a Multi-Access network segment
In OSPF networks, adjacencies are formed to optimize routing efficiency. For a multi-access network segment, OSPF creates adjacencies only between:
- The Designated Router (DR) and Backup Designated Router (BDR)
- The DR and DROTHER routers
- The BDR and DROTHER routers
The DR and BDR are elected using particular election criteria.
No adjacency is formed between two DROTHER routers. This mechanism reduces the number of necessary adjacencies in a network segment with multiple OSPF routers.
However, DROTHER routers do appear in the output of the show ip ospf neighbors command. This is because the routers are detected (i.e. they are neighbors), and their existence is indicated in the table, but adjacencies are not formed. This is because there is a difference between being OSPF neighbors and being adjacent.
Links:
https://networklessons.com/ospf/ospf-drbdr-election-explained