Hello Sathish
In a stable OSPF network, a BDR will maintain the same OSPF database and adjacencies as the DR in a network segment. The only difference between how the DR and BDR operate is that the BDR does not generate network LSAs during its operation.
Now having that in mind, how long does it take for a failover to occur? In other words, as soon as the DR fails, how long does it take for the BDR to become the DR?
Well, this is subject to the hello timer and dead interval settings.
If the current DR fails (e.g., due to power failure, link failure, or router shutdown), it stops sending Hello packets and ceases communication with other routers. OSPF routers in the segment detect the absence of Hello packets from the DR, but will only react to this after the dead interval elapses which is typically 40 seconds by default.
Because the DR and BDR maintain an adjacency, as soon as this adjacency fails after the dead timer elapses, only then will the BDR transition to a DR. So, to answer your question, the BDR will take over once the dead timer has expired. Once that has expired, the BDR becomes the DR and begins sending out Type 2 LSAs informing all routers that it is now the DR.
This may seem like a long time, but it’s not. OSPF will continue to route traffic based on established routes. If the DR failure results in a topology change, even that will be subject to the hello and dead interval timers. If this isn’t sufficient for your network, you will need to adjust the OSPF timers accordingly.
I hope this has been helpful!
Laz