OSPF DR and BDR

[utawakevou]

I got OSPF running with 3 areas. See attach diagram for my OSPF setup. I want the border routers to be BDR's for area 0 but they should be DR for area 1 and 2 respectively. I want my two Nexus to be DR for area 0. Just need guidance in doing that.  

Is it possible to use the vPC Peer-switch feature on the Cisco Nexus 9K switches to configure both at DR in OSPF ?

[cofee]

As far as DR/BDR election process is concerned in a broadcast or NBMA  you will just need to make sure that your DR is configured with the highest router-id and BDR with the second highest router-id. Even though during the election process OSPF speaking routers in broadcast/nbma networks look at the ospf priority first and router with the highest priority will become the DR, but this value should be set to 1 on all routers which is a default value unless it has been changed by the administrator.

Now you can configure a router with the highest router-id or priority that you want to be elected as a DR, but how all OSPF routers boot up on a shared segment may result in unpredictable election result. Now you can either boot up or connect your DR first and then BDR, all other routers can boot up or  be connected at the same time and that shouldn't make any difference and the second option would be to be set ospf priority to 0 on the routersthat you don't want to be DR/BDR so they can never participate in DR/BDR election process.

All recommended changes wouldn't take any effect on the network that already have DR/BDR routers because a new election process will be needed for any changed configuration to take effect.

[Georg Pauwen]

Hello,

in addition to the other post, each area can only have one DR and one BDR. If I understand you correctly, you are trying to somehow combine both NEXUS into one by configuring a VPC peer link ? This is not possible, as both NEXUS will remain independent OSPF units, so to speak, which form and adjacency. Why not configure one Nexus as DR and the other as BDR ?

[utawakevou]

Hi,

Yes, that's what I want i.e to have DR and BDR for each area. I have already got the VPC running and I have used the VPC feature to configure the root bridge. Ok what I have done so far is the following:

• Assign priority 100 to all the SVI and loopback (Area 0) of nexus01
• Assign priority 90 to all the SVI and loopback (Area 0) of nexus02
• Assign priority 80 to the LAN interaface and loopback (Area 0) of Cisco 2800
• Assign priority 0 to the LAN interaface and loopback (Area 0) of Ubiquiti Edge - wan-rtr1 as I don't want it to participate 
• Assign priority 0 to the LAN interaface and loopback (Area 0) of Ubiquiti Edge - wan-rtr3 as I don't want it to participate

What I want now is to have the Cisco 2800 as the DR for area 1 and Ubiquiti Edge - wan-rtr3 to be the BDR for area 1 and I want to have the Ubiquiti Edge - wan-rtr1 as the DR/BDR for area 2 

[Richard Burts]

We need to clarify something about OSPF DR and BDR. The DR and BDR are elected per multi access link and not per area. It is a fairly common misunderstanding about DR and BDR that they are per area. But this is not correct. They are elected per subnet and not per area.

HTH

Rick

[utawakevou]

Indeed this greatly helps. Clarifies things for me as I saw different results from what I was expecting when I was making the changes and clearing OSPF process

Thanks

[Georg Pauwen]

Richard,

my bad, you are absolutely right, one DR/BDR per multiaccess segment, not area.