OSPF - DR-BDR election per broadcast domain

An OSPF DR/BDR election will take place for every broadcast domain or network segment within a topology. An election does not take place on a per-area basis, a fact that is often misunderstood.

Take a look at the following network diagram:

ospf-3-routers-multi-access.png Here we have a single network segment or broadcast domain with three OSPF routers in it. A DR/BDR election will take place and one router will be elected as the DR, another as the BDR, and the third will be designated a DROTHER.

In contrast, take a look at the following network topology:

ospf-r1-r2-r3-two-broadcast-domains.png In this case, we have R2 connected to two different broadcast domains. In this case, there will be two DR/BDR elections, one between R1 and R2, and another between R2 and R3. That means that R2 may be a DR for one network segment, and a BDR for the other.

Note that we only have one OSPF area, but two DR/BDR elections.