Count OSPF adjacencies on any network type and see how much DR/BDR election cuts against a full mesh.
OSPF avoids an O(n²) flooding mesh on multi-access media by electing a DR that relays LSAs, with a BDR standing by. Every DROther forms a full adjacency only with those two, so state grows linearly rather than quadratically. The 2n − 3 figure is what makes a large broadcast segment survivable, and the hello/dead pair is the single biggest term in OSPF convergence — 40 seconds of detection dwarfs the milliseconds SPF takes.
OSPF Neighbor
On a broadcast or NBMA segment with a DR and BDR the adjacency count is 2n − 3; without election it is the full mesh n(n − 1) ÷ 2.
Neighbour failure detection
Failure detection takes up to the dead interval, which defaults to 4 × hello.
On a broadcast or NBMA segment with a DR and BDR the adjacency count is 2n − 3; without election it is the full mesh n(n − 1) ÷ 2. OSPF avoids an O(n²) flooding mesh on multi-access media by electing a DR that relays LSAs, with a BDR standing by. Every DROther forms a full adjacency only with those two, so state grows linearly rather than quadratically.
The 2n − 3 figure is what makes a large broadcast segment survivable, and the hello/dead pair is the single biggest term in OSPF convergence — 40 seconds of detection dwarfs the milliseconds SPF takes.
This calculator takes 4 inputs: Routers on the segment, OSPF network type, Hello interval, Dead interval. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Use BFD. Sub-second hellos raise control-plane load on every router and are vulnerable to CPU spikes; BFD runs a lightweight dedicated session, typically 300 ms × 3, and just tells OSPF the neighbour is gone.