Configuring the Designated Router

In the process of implementing OSPF, a large number of multi-access links to routers across the network may cause too many adjacencies to form. To avoid this problem, a Designated Router (DR) is elected per multi-access network to build adjacencies to all other routers on that network. A Backup Designated Router (BDR) is also elected in case the Designated Router (DR) fails, in which case the BDR will become the DR.

Note

A DR is required only for multi-access networks. Point-to-Point links do not need a DR because only a single adjacency is required.

To elect a DR from a host of candidates on the network, each router multicasts a hello packet and examines the priority of hello packets received from other routers. The router with the highest priority is elected the DR, and the router with the next highest priority is elected the BDR. Any router with a priority of 0 will opt out of the DR election process. See the Configuring Router Priority for details on configuring router priority. If DR candidates all share non-zero priorities, OSPF applies the router ID as a tie-breaker where the highest ID is chosen DR and the next highest ID is chosen BDR.