designated router
termThe router an OSPF broadcast network elects to speak for the segment, with a backup designated router ready to replace it.
On a LAN with several OSPF routers, each becomes adjacent only to the designated router and its backup, not to every other router, and the designated router originates the network-LSA for the segment. The election goes by priority, then router ID, and is not pre-emptive.
RFC 2328 gives the reason: a designated router reduces the number of adjacencies on a broadcast network, and with them routing traffic and database size. Five routers in a full mesh would need 10 adjacencies; with a designated router and a backup they need 7. Two routers that are neither stop at the 2-Way state with each other, which is normal. The designated router multicasts its updates to AllSPFRouters, 224.0.0.5, and both it and the backup also listen on AllDRouters, 224.0.0.6.
The highest Router Priority wins, ties go to the highest router ID, and priority 0 makes a router ineligible. A router that finds a designated router already in place accepts it regardless of priority, so a returning router with a higher priority comes back as backup, not as designated router. RFC 2328 accepts that unpredictability because each change costs a moment of lost connectivity while the databases converge again. The backup is already adjacent to everyone and takes over when the designated router fails.
A link that joins only two routers needs no election: configured as point-to-point, it forms its adjacency directly, and FRRouting lists such a neighbour as Full/- rather than Full/DR.
Also known as: dr, bdr, backup designated router, dr election, drother