OSPF Neighbor Calculator
Count OSPF adjacencies on any network type and see how much DR/BDR election cuts against a full mesh.
Inputs
Full Adjacencies Formed
21
Adjacencies Without DR/BDR
66
Reduction from DR/BDR Election
68.18%
Hello Packets per Second
1.200pps
Dead-to-Hello Ratio
4.0
Election Behaviour
DR and BDR elected by highest priority then highest router ID; DROthers peer only with them
Step by step
Values used
Routers on the segment = 12 routers; OSPF network type = Broadcast — DR/BDR elected; Hello interval = 10 s; Dead interval = 40 s
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.
Full Adjacencies Formed
= 21
Adjacencies Without DR/BDR
= 66
Reduction from DR/BDR Election
= 68.18
Hello Packets per Second
= 1.200 pps
Dead-to-Hello Ratio
= 4.0
Election Behaviour
= DR and BDR elected by highest priority then highest router ID; DROthers peer only with them
How it works
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.
Formulas
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.
- n
- Routers on the segment
- 2n − 3
- Each DROther peers with DR and BDR, plus the DR–BDR adjacency
- dead interval
- Time without a hello before the neighbour is declared down — 4 × hello by default
Neighbour failure detection
Failure detection takes up to the dead interval, which defaults to 4 × hello.
Frequently Asked Questions
How is OSPF Neighbor calculated?
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.
Why does OSPF Neighbor matter?
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.
What values do I need to enter?
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.
Should I tune hellos down or use BFD?
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.