OSPF DR election

[vpalania]

Hi,

consider there are three routers R1( prio 4 , DR) , R2(prio 3, BDR), R3 (Prio 2, DROther).Consider another router R4(Prio 5) has joined this network.Since DR/BDR is alive, R4's higher priority won't affect the network.What happens if DR goes down, in that case the DR/BDR election resumes, in that case how it chooses DR ? since R4 is of higher priority than R2(prio 3, BDR).

Thanks,

Vijay

[pkhatri]

Hi Vijay,

The BDR will be promoted to DR. A new BDR will be appointed based on the highest priority - in your example, it would be router R4.

Hope that helps - pls rate the post if it does.

Paresh

[vpalania]

Hi,

Thats what I am asking, how the BDR is promoted as new DR, what is the procedure ?

What happens if another router other BDR identifies DR down ?

Thanks,

Vijay

[pkhatri]

Hi,

All routers will eventually discover that the DR is down - during the transition, the choice of DR/BDR by a router could change a few times. All routers will eventually get a common understanding of the DR/BDR.

When a router stops hearing Hellos from the DR, it considers that the DR is down and runs the election process. The election process first elects the BDR. If a BDR already exists, then a new BDR is not selected; the existing BDR remains. Then, the election process tries to elect the DR. If there are no routers that have elected themselves DR at that point (which is the case you have), it will promote any existing BDR to DR. Then, it will re-run the process to elect the DR. The former BDR is now the DR so no BDR exists now. Therefore, the router with the highest priority is made BDR.

Hope that helps - pls rate the post if it does.

Paresh