To maximize efficiency, OSPF does not form an adjacency with every neighbor that is detected, because the maintenance of an adjacency requires compute cycles and because on multiaccess networks such as LANs, a full mesh of adjacencies is largely redundant. On multiaccess networks, an election algorithm is performed to first elect a designated router (DR), and then a backup designated router (BDR). The DR functions to represent the LAN itself and forms an adjacency with the BDR and all other compatible neighbors (DRother) on the LAN segment. The DRother routers form two adjacencies across the LAN—one to the DR and one to the BDR. The neighbor state for DRother neighbors on a DRother router itself is expected to remain in the "two-way" state. This simply means that the various DRothers have detected each other as neighbors, but an adjacency has not been formed.
The DR is responsible for flooding LSAs that reflect the connectivity of the LAN. This means that loss of one neighbor on a 12-node LAN results in a single LSA that is flooded by the DR, as opposed to each remaining router flooding its own LSA. The reduced flooding results in reduced network bandwidth consumption and reduced OSPF processing overhead. If the DR fails, the BDR will take over and a new BDR is elected.
OSPF elects a DR and BDR based on a priority setting, with a lower value indicating a lesser chance at winning the election; a setting of 0 prevents the router from ever becoming the DR. In the event of a tie, the router with the highest router ID (RID) takes the prize. The OSPF DR Election algorithm is nondeterministic and nonrevertive, which means that adding a new router with a higher, more preferred priority does not result in the overthrow of the existing DR. In other words, router priority matters only during active DR/BDR election. This behavior minimizes the potential for network disruption/LSA flooding when new routers are added to the network. Thus, the only way to guarantee that a given router is the DR is to either disable DR capability in all other routers (set their priority to 0), or ensure that the desired router is powered on first and never reboots. Where possible, the most stable and powerful router should be made the DR/BDR, and a router should ideally be the DR for only one network segment.
