Designated Router (DR) and Backup Designated Router (BDR) are only used on multi-access networks and are elected during the adjacency phase and before they start exchanging database description (DBD). OSPF uses DR/BDR to reduce the amount of unnecessary exchange of LSAs, therefore speeding up convergence. This will be most beneficial on a network with multiple routers on the same LAN segment.

In the example below the 5 OSPF routers are on the same LAN segment. If R1 detects a network change it sends a multicast to, the DR and BDR are the only routers listening on this address. The DR will send an acknowledgement to R1 and then sends a multicast to which all OSPF routers will receive.

If a router is not a DR or BDR then it is a DROTHER (only listens for multicasts on    All OSPF routers listen on this address, only DR sends update traffic to this multicast address    Only DR and BDR receive multicast traffic


DR/BDR Election

The router with the highest priority value will become DR, the default is 1. A priority of 0 means the router will not attempt to take part in the election and can only be a DROTHER. The priority can be manually configure on an interface using the command “ip ospf priority x“. The priority value is between 1–255, with 255 being the highest/most preferred.

  • The router with the highest priority will be elected DR
  • If multiple routers have the same priority then the router with the highest Router ID (RID) wins the election. If no RID then highest loopback IP wins.
  • The router not claiming to be the DR and with the highest priority will become the BDR

A router with a priority that comes online after a DR has been elected will not pre-empt the DR, it will only become the DR if the DR reboots or goes offline.

An OSPF Router will only form FULL adjacency with a DR/BR, an OSPF router (non DR/BDR) would form a 2WAY adjacency with another non DR/BDR router the state in the neighbor table would be 2WAY/DROTHER








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