Dell EMC SmartFabric OS10 User Guide Release 10.5.0

OSPF elects a designated router (DR) and a backup designated router (BDR). The DR generates LSAs for the entire multiaccess network. Designated routers allow a reduction in network traffic and in the size of the topological database.

Designated router

Maintains a complete topology table of the network and sends updates to the other routers via multicast. All routers in an area form a slave/master relationship with the DR. Every time a router sends an update, the router sends it to the DR and BDR. The DR sends the update to all other routers in the area.

Backup designated router

Router that takes over if the DR fails.

Each router exchanges information with the DR and BDR. The DR and BDR relay information to other routers. On broadcast network segments, the number of OSPF packets reduces by the DR sending OSPF updates to a multicast IP address that all OSPF routers on the network segment are listening on.

DRs and BDRs are configurable. If you do not define the DR or BDR, OS10 assigns them per the protocol. To determine which routers are the DR and BDR, OSPF looks at the priority of the routers on the segment. The default router priority is 1. The router with the highest priority is elected DR. If there is a tie, the router with the higher router ID takes precedence. After the DR is elected, the BDR is elected the same way. A router with a router priority set to zero cannot become a DR or BDR.