Elections in OSPF

Elections in OSPF

·        When you connects routers configured with OSPF in a LAN segment or Frame-Relay network, they held elections.

·        Elections are done to avoid loops.

·        One Router is elected as DR (Designated Router), one is elected as a BDR(Backup Designated Router) and remaining are known DROther.

·        The router with highest ospf priority is elected as a DR.

·        The router whose priority value is set to 255 will always be elected as a DR and router whose priority value is set to 0 (zero) will not be part of election process and will be elected as a ODR.

·        The default priority of all the OSPF interfaces is set as 1.

·        So the election takes place on the basis of router-id.

·        The router with the highest router-id is elected as a DR.

·        DR/BDR are elected per LAN segment.

·        The Router can be part of multiple LAN segments.

·        Once the DR/BDR are elected, the election will not reoccur until DR goes down.

·        All other routers, “DROther”, establish adjacencies with only the DR and BDR.

·        DRother routers multicast LSAs  to only the DR and BDR

             (224.0.0.6 - all DR routers)

·        DR sends LSA to all adjacent neighbors  (DROthers)

            (224.0.0.5 - all OSPF routers)

Backup Designated Router - BDR

·        Listens, but doesn’t act.

·        If LSA is sent, BDR sets a timer.

·        If timer expires before it sees the reply from the DR, it becomes the DR and takes over the update process.

·        The process for a new BDR begins.