An OSPF backup designated router (BDR) is the elected standby for the designated router (DR) on a broadcast network segment. If the DR fails, the BDR immediately assumes the DR role, and the routers elect a new BDR. section 18.5
Ethernet interfaces use the broadcast network type by default. Routers discover neighbors dynamically, elect the DR and BDR in the 2-Way state, and form full adjacencies with both. DROthers remain 2-Way neighbors with one another. section 18.3.2 section 18.5
Remember these BDR election and operation rules:
- Select the highest interface priority first, then the highest router ID (RID) as the tiebreaker; the default priority is 1. section 18.5
- Apply the election independently on each network segment, not once for the entire OSPF area. section 18.3.2
- Do not expect a higher priority or RID to preempt an existing DR or BDR; reset the OSPF process to force a new election. section 18.5
- Avoid DR/BDR elections on a two-router link by using the point-to-point network type instead. section 18.3.2
Watching the BDR take over
A neighbor table can identify roles such as FULL/DR and FULL/BDR. After R5's OSPF process is cleared, R4 becomes the new DR and R2 becomes the BDR, demonstrating immediate BDR promotion followed by a new BDR election.
