OSPF DR BDR election and neighbor adjacencies
When OSPF is configured in a multi-access network such as Ethernet, the following guidelines should be considered:
- A DR/BDR election will take place on a per segment basis and not on a per OSPF area basis. A segment is a single broadcast domain, that is, a single IP subnet.
- Neighbor adjacencies will form only between the following types of OSPF routers:
- DR and BDR
- DR and DROTHER
- BDR and DROTHER
- DROTHER to DROTHER adjacencies will not form.
- hello and LSA exchanges take place like so:
- from DROTHER to DR/BDR: 224.0.0.6
- from DR/BDR to DROTHER: 224.0.0.5
- In the OSPF DR BDR election process, the BDR is selected first.
Whether OSPF attempts a DR/BDR election depends on the OSPF network types.
Links:
https://forum.networklessons.com/t/ospf-dr-bdr-election-explained/880/62
https://networklessons.com/ospf/ospf-drbdr-election-explained/