Large local area networks (LANs) that have many routers and therefore many OSPF adjacencies can produce heavy control-packet traffic as LSAs are flooded across the network. To alleviate the potential traffic problem, OSPF uses designated routers (DRs). Rather than broadcasting LSAs to all their OSPF neighbors, the routers send their LSAs to the designated router, which processes the LSAs, generates responses, and multicasts topology updates to all OSPF routers.
In LANs, the election of the designated router takes place when the OSPF network is initially established. When the first OSPF links are active, the router with the highest router identifier (defined by the router-id configuration value or the loopback address) is elected designated router. The router with the second highest router identifier is elected the backup designated router (BDR). If the designated router fails or loses connectivity, the BDR assumes its role and a new BDR election takes place between all the routers in the OSPF network.