DR and BDR election is accomplished via the Hello protocol. Hello packets are exchanged via IP multicast packets on each segment.
However, only segments that are broadcast and nonbroadcast multi-access networks (examples are Ethernet and Frame Relay) will perform DR and BDR elections. Point-to-point links, like a serial WAN for example, will not have a DR election process.
- On a broadcast or nonbroadcast multi-access network, the router with the highest OSPF priority on a segment will become the DR for that segment. This priority is shown with the show ip ospf interface command.
- The default priority for a router interface is one. If all routers have the
- default priority set, the router with the highest Router ID (RID) will win.
- The RID is determined by the highest IP address on any interface at the moment of OSPF startup. This can be overridden with a loopback (logical) interface. If you set a routers interface to a priority value of zero,
- that router wont participate in the DR or BDR election on that interface. The state of the interface with priority zero will then be DROTHER.