Explain OSPF DR/BDR Election?

OSPF is a dynamic routing protocol. OSPF can elect one router as DR (Designated Router) and one router as BDR (Backup Designated Router).If DR fails BDR takes over. Each non-BDR or non-DR router in the network exchange routing information with DR or BDR. To send information to DR or BDR multicast IP of 224.0.0.6 is used DR sends routing information with multicast IP of 224.0.0.5.

DR and BDR rules:

1. Router is highest OSPF priority will become DR and router with second-highest priority will become BDR.
2. If the priority of the routers are same then, the router with the highest Router ID is selected as DR and the router with second-highest Router ID is selected as BDR.

Router ID election:

1. Manually configured Router ID in OSPF.
2. If Router ID is not manually configured, the highest IP address on  any of the loopback interfaces is selected as Router ID.
3. If there are no loopback interfaces the highest IP address given to any active interface is selected as router ID.

Points to noted:

1. In DR/BDR election if priority is 0 then the router can’t be elected as DR or BDR and OSPF
2. OSPF is a non-preemptive activity you must run the command “clear ip ospf process”.
3. DR/BDR election is per multi-access segment not per area.
4. You can check the DR/BDR status in the topology using the command “show ip ospf neighbor”

Topology:

In this topology R3 is the DR and R1 is the BDR because of the non-preemptive nature of OSPF.

When we run the command “clear ip ospf process” on R1.

As we can see now R4 is the BDR and R3 is the DR.