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OSPF DR/BDR Election Myth

OSPF DR/BDR election process is something that is full of many details, i hope that in the following thread i can cover its main aspects.

Each multi-access segment (ex: Ethernet Segment), will have 1 DR and 1 BDR. Each router on the segment forms a Full adjacency with the DR/BDR. Keep in mind that a router might be a DR/BDR on one of its attached multi-access networks, and it might not be the DR/BDR on another of its attached multi-access networks. In other words, the DR/BDR is a property of a router’s interface, not the entire router.

Now, how are the DR/BDR elected ?

A router declares its priority value via the “Rtr Pri” field in the OSPF hello packet. This value is configured under the interface via the “ip ospf priority <0-255>” command, with 1 being the default, and 0 meaning that the router is ineligible to become a DR or a BDR, and the higher priority wins. The RID serves as a tie-breaker when the priority values are the same.One important thing to keep in mind, the DR/BDR election is not pre-empitive, thus the first router to come up will be the DR and the second will be the BDR, the rules of election are o­nly effective when o­ne of them fail. If the BDR fails, a new BDR is elected according to the rules of election. o­n the other hand if the DR fails, the BDR takes the DR role, and a new BDR is elected according to the rules of election.Using “ip ospf priority 0” is the strongest tool with the lowest impact if you wish to have an instantaneous action, it doesn’t require resetting the OSPF process. If configured o­n router that is a DR or a BDR, this router will resign its role immediately.

I hope that i’ve been informative.

BR,
Mohammed Mahmoud.

8 comments

  1. Hey,

    Great site,

    Ideal for learning new tricks or refreshing other knowledge

  2. Thanks for the article. I have been doing labs and the highest RID doesn’t always get elected DR. You have confirmed my suspicion that boot order plays a role.

  3. Nice article

  4. Great tip on forcing an election with minimal impact!

  5. Nice and useful article, thank you so much!!

  6. Nice article! Thank you.

    Now, is it possible to have an ethernet segment without DR/BDR and the priorities are not changed?

  7. Yes, if you configure the network type on the interfaces as point to point there will be no election for DR/BDR.

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