What is DR?

A DR (Designated Router) is the router elected on an OSPF broadcast segment, such as an Ethernet LAN, to act as the hub for exchanging routing information.

Without one, every pair of routers on a shared segment would form a full adjacency and flood every change to each other, which grows quickly with the number of routers. Instead, OSPF elects a DR and a Backup Designated Router (BDR). Every other router, a DROther, forms full adjacencies only with those two. DROthers send updates to the AllDRouters multicast address 224.0.0.6, and the DR refloods them to everyone on 224.0.0.5. The DR also originates the segment's Network LSA (Type 2), which lists the routers attached to it. The election uses the interface priority (default 1; highest wins, and 0 means "never become DR"), with the highest router ID as the tie-breaker.

The election is not pre-emptive: a router with a higher priority that joins later does not take over, so the DR is often simply whichever router booted first. If the DR fails, the BDR is promoted and a new BDR is elected. Two points confuse people. First, two DROthers stay in the 2-Way state with each other, and that is normal, not a fault. Second, DRs only exist on broadcast and non-broadcast multi-access networks; on a link between just two routers, setting the OSPF network type to point-to-point skips the election and speeds up adjacency, which is common practice for router-to-router links. If you want a particular router to be DR, raise its priority, or set priority 0 on routers that should never be. The OSPF Fundamentals lesson covers the DR/BDR election and neighbour states with a working lab.