How do the DR and BDR get elected? The OSPF DR and BDR election decision is based on the following criteria, in sequential order:

1. The routers in the network elect the router with the highest interface priority as the DR. The router with the second highest interface priority is elected as the BDR. The priority can be configured to be any number between 0 – 255. The higher the priority, the likelier the router will be selected as the DR. If the priority is set to 0, the router is not capable of becoming the DR. The default priority of multiaccess broadcast interfaces is 1. Therefore, unless otherwise configured, all routers have an equal priority value and must rely on another tie breaking method during the DR/BDR election.

2. If the interface priorities are equal, then the router with the highest router ID is elected the DR. The router with the second highest router ID is the BDR.

Recall that the router ID is determined in one of three ways:

Note: In an IPv6 network, if there are no IPv4 addresses configured on the router, then the router ID must be manually configured with the router-id rid command; otherwise, OSPFv3 does not start.

In the figure, all Ethernet router interfaces have a default priority of 1. As a result, based on the selection criteria listed above, the OSPF router ID is used to elect the DR and BDR. R3 with the highest router ID becomes the DR; and R2, with the second highest router ID, becomes the BDR.

Note: Serial interfaces have default priorities set to 0; therefore, they do not elect DR and BDRs.

The DR and BDR election process takes place as soon as the first router with an OSPF-enabled interface is active on the multiaccess network. This can happen when the routers are powered on, or when the OSPF network command for that interface is configured. The election process only takes a few seconds. If all of the routers on the multiaccess network have not finished booting, it is possible that a router with a lower router ID becomes the DR. (This can be a lower-end router that takes less time to boot.)