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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.

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