What is DROther?

A DROther is an OSPF router on a shared segment, such as an Ethernet LAN, that is neither the Designated Router (DR) nor the Backup DR (BDR).

On a broadcast or NBMA segment, OSPF elects a DR and a BDR to act as the hub for that segment. Every other router becomes a DROther. A DROther forms a full adjacency with the DR and the BDR, exchanging databases with them and sending its updates to the AllDRouters multicast address, 224.0.0.6, which only those two listen on. The DR then floods the change back out to everyone on 224.0.0.5. With other DROthers, a router only exchanges Hellos and stops at the 2-Way state: each can see the other, but they never synchronise databases directly. This keeps the number of adjacencies on a LAN of n routers close to 2n rather than n(n-1)/2, which matters on segments with many routers.

The most common confusion is treating 2-Way as a fault. On a broadcast segment, two DROthers sitting at 2-Way with each other is exactly what should happen, and neighbour listings typically show it as 2-Way/DROther. A neighbour stuck in 2-Way is only a problem if one side should be the DR or BDR, or if the link was meant to be point-to-point. You can make a router a permanent DROther by setting its OSPF priority to 0, which is useful for small or less reliable routers that should never become the hub. Because the election is not pre-emptive, which routers end up as DROthers can also depend on boot order. See DR and BDR for the other two roles; the OSPF Fundamentals lesson explains the election and the neighbour states.