DR and BDR election occurs in multi-access Broadcast and Non-Broadcast network types.

The purpose of the DR is to be the Router in charge to maintain the topology table and to distribute updates to other routers within the same segment.

All other routers (called DROTHERs) will form adjacencies only with the elected DR and BDR.

OSPF Routers advertise updates to the “AllDR” multicast address of 224.0.0.6; then the DR advertises the Link-State advertisements (LSAs) via the “AllSPF” multicast address 224.0.0.5.

With the use of DR is possible to reduce the network traffic between neighbors by providing a single source of updates.

DR and BDR Election:

The default DR election criteria are as follows:

• The Router configured with the highest priority wins the election.  The default priority is 1 and the possible values range between 0 – 255.
• If the priority is set to 0, the router will not participate in the DR/BDR election.
• If the routers participating in the election tie with their configured priority; then the router will use highest Router ID (RID) as tie-breaker.
• The Router with the second highest priority value becomes the BDR.
• If a router with the higher priority comes online after the election has taken place; it will not become DR or BDR until DR and BDR fail.
• If the DR fails, the BDR will take over.  Then another election will take place to elect a new BDR.
• Preemption is not supported.  Thus, the first router to come up will be the DR and the second will be the BDR.

In topologies, such hub-and-spoke with a single hub is recommended to suppress the election of BDR manually setting the hub with high priority and the spokes with priority 0.

OSPF Manual Priority Configuration

To configure the OSPF priorities use the ip ospf priority <0-255> interface command.

!
config t
interface e0/0
ip ospf priority 100
exit
!

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