When OSPF neighbours will not form

An OSPF adjacency that will not come up is one of the more systematic problems in networking, because the state it is stuck in names the failing check.

Stuck in INIT

Hellos are being sent but this router does not see itself in the neighbour’s hello. Traffic is one-way. Check for an ACL blocking OSPF, a layer 2 problem in one direction, or multicast not being forwarded.

Stuck in 2-WAY on a broadcast segment

This is often not a fault. On a broadcast network, only the designated router and backup form full adjacencies. Two non-DR routers sitting at 2-WAY is correct behaviour.

Stuck in EXSTART or EXCHANGE

Almost always MTU. The routers exchange database description packets, and if the MTU differs between them the larger packets are never received. Either match the MTU on both sides or, as a workaround, disable the MTU check with ip ospf mtu-ignore. Matching them is better.

No neighbour at all

Work down the list of parameters that must match:

• Area ID must be identical on both interfaces
• Hello and dead intervals must match
• Both must agree on the network type
• Authentication type and key must match
• Stub flags must agree
• Subnet masks must match — a /24 and a /25 will not neighbour
• Router IDs must be unique

show ip ospf interface Gi0/1     # area, timers, type, DR
show ip ospf neighbor
debug ip ospf adj                # on a lab or quiet device only

The two that catch people

A passive interface silently prevents adjacency while looking configured. And a mismatched subnet mask produces routers that can ping each other perfectly and will never neighbour, because OSPF requires them to agree that they are on the same network.