What is BDR?

A BDR (Backup Designated Router) is the OSPF router on a shared segment, such as an Ethernet LAN, that takes over if the Designated Router fails.

On a broadcast segment, OSPF avoids a full mesh of adjacencies by electing a Designated Router (DR) to act as the hub: every other router forms a full adjacency with the DR and exchanges updates through it. If the DR were the only hub, its failure would force every router to re-elect and rebuild its database before routing could recover. The BDR prevents that. It forms full adjacencies with every router, just like the DR, and listens to the same updates on the AllDRouters multicast address, 224.0.0.6, so it always has an up-to-date copy of the database. While the DR is alive the BDR stays quiet: it does not originate the segment's Network (Type 2) LSA or flood updates back onto the segment. If the DR's dead timer expires, the BDR is promoted straight away and a new BDR is elected.

The election uses interface priority (default 1; 0 means the router never stands) with the highest router ID as the tie-breaker. It is not pre-emptive: a router with a higher priority that joins later does not take over, so the routers that boot first usually win, which surprises people in labs. On a link with only two routers, the DR and BDR roles add nothing, which is why router-to-router Ethernet links are usually set to the point-to-point network type. Routers that are neither DR nor BDR are called DROthers. The OSPF Fundamentals lesson walks through network types and the DR/BDR election.