OSPF Designated Routers (DR) & Backup Designated Routers (BDR)

Every Ethernet network running OSPF must have a DR and BDR because they are responsible for exchanging the LSDB information to the other routers on the network. The Database Exchange process happens from the DR to each and every router in the OSPF network, which is why the DR is so important to have. For the same reason the BDR is equally important. The BDR is simply the DR’s backup and monitors it status to make sure it isn’t offline. If the DR were ever to fail, the BDR would become DR and elect a new BDR router to replace it and then would be responsible for relaying the LSDB to each router updating their routes.

To determine which routers are the DR and BDR we need to look at the show ip ospf neighbor command. Any neighbor in a FULL state is a DR or BDR because it exchanges database information. All other routers will be in a DROther state because they never exchange database information. DROther shows up as 2-way in the show ip ospf neighbor command.

Neighbor State Adjacency Meaning
2-way adjacent The neighbor router has sent a hello that lists the local router ID. Neighbor verification complete but does not exchange LSDB info.
Full Fully adjacent Either a DR or BDR router. Have completed the exchange of LSDB content.

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