What are OSPF DR/BDR for, and how is the election decided?

On a multi-access segment with n routers, full-mesh adjacencies would mean n(n−1)/2 relationships all flooding to each other. The DR (designated router) centralizes it: everyone syncs with the DR (and BDR as hot standby) instead of each other — flooding scales linearly. Election: highest interface priority wins (0–255, default 1; 0 = never DR); tie broken by highest router ID. Two gotchas. First, the election is non-preemptive: a "better" router joining later does NOT take over until the current DR dies — so boot order matters, and to force your choice you set priorities before adjacency (ip ospf priority 100) or clear the process. Second, point-to-point links skip DR/BDR entirely — seeing a DR on a p2p link means the network type is misconfigured.