Designated & Backup Designated Router
Based on the network type, OSPF router can elect one router to be a Designated Ruter (DR) and one router to be a Backup Designated Router (BDR). For example, on multiaccess broadcast networks (such as LANs) routers defaults to elect a DR and BDR. DR and BDR serve as the central point for exchanging OSPF routing information. Each non-DR or non-BDR router will exchange routing information only with the DR and BDR, instead of exchanging updates with every router on the network segment. DR will then distribute topology information to every other router inside the same area, which greatly reduces OSPF traffic.
To send routing information to a DR or BDR the multicast address of 220.127.116.11 is used. DR sends routing updates to the multicast address of 18.104.22.168. If DR fails, BDR takes over its role of redistributing routing information.
Every router on a network segment will establish a full neighbor relationship with the DR and BDR. Non-DR and non-BDR routers will establish a two way neighbor relationship between themselves.