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 188.8.131.52, 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 184.108.40.206 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 220.127.116.11)
18.104.22.168 All OSPF routers listen on this address, only DR sends update traffic to this multicast address
22.214.171.124 Only DR and BDR receive multicast traffic
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