Quick Recap of the DR/BDR Election Fundamentals:
- The OSPF DR/BDR election is based on the interface level OSPF priority value (range:0 - 255; default is 1) along with the Router-ID. The interface with the highest priority value is elected the DR, and the one w/the second highest priority the BDR. If there is a tie, the router with the Highest RID is most likely to be elected the DR or BDR.
- There is no preemption in OSPF much like ISIS does with its DIS election. This means once a DR always a DR even when a new router comes up w/a higher interface priority or higher RID unless the entire OSPF process is cleared (manually or via the box rebooting).
- DR is an attribute of the interface (segment) and not the routing process. So, yes in this example, the box in the middle could be a DR for one segment and the BDR or DROther on the other or neither (based on which box comes up first or if you disable the participation via setting the priority to 0).
- Lastly, what's shown is a common example in a Service Provider environment. i.e. the circuits are Gig or 10 Gig link and thus the network type is Broadcast. In the event, another device will never be added to this segment, it is best practice to change the network type to point-to-point under the respective interfaces, thus avoiding a DR/BDR election and reducing overhead.
Cheers,
ugot2nome
