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New Member

OSPF DR Question

Hello folks -

Consider the following network below:

RA-----------------------RB--------------------RC

RA-RB network: 10.10.10.0/24

RB-RC network: 172.16.10.0/24

Assume all router interfaces are in Area 0 and the network type is BROADCAST. If that's the case, will there be a DR for the 10.10.10.0 subnet and another DR for 172.16.10.0 subnet? In other words, is the DR is elected per subnet, and not per area, given the network type is broadcast? If yes, then in the case above, there should be two DR's elected, correct?

Thanks for your help.

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: OSPF DR Question

You are correct.

If for example you check the ''sh ip ospf interface'' command on RB you will see for both interfaces the RB could be the DR for one segment and the BDR for the other segment (just an example).

Federico.

New Member

Re: OSPF DR Question

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

3 REPLIES

Re: OSPF DR Question

You are correct.

If for example you check the ''sh ip ospf interface'' command on RB you will see for both interfaces the RB could be the DR for one segment and the BDR for the other segment (just an example).

Federico.

New Member

Re: OSPF DR Question

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

New Member

Re: OSPF DR Question

Great explanation, thank you!!

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