Explain the missing neighbor.
The useful OSPF question is not just what an adjacency is. It is why an expected adjacency is absent.
Separate link health from adjacency
Routers can communicate directly while disagreeing about the OSPF area. Confirm addressing first, then compare the area's configuration on both ends.
What does passive actually do?
A passive transit interface prevents the neighbor exchange you need. Inspect passive-interface settings when a hardening change precedes the outage.
Healthy neighbors, missing routes
An adjacency alone does not prove that every service network participates in the process. Compare connected prefixes with network statements and the remote routing table.
Protocol reference: OSPFv2 neighbors and areas — RFC 2328.
A neighbor gone quiet
New York data center. User, partner and service networks meet at one monitored hub. The directly connected service-edge link responds, but the OSPF adjacency across it is absent. Other adjacencies and monitoring traffic remain healthy. Investigate the two service networks from R1: 10.201.64.0/24 to 10.201.68.0/24 (TCP/443), and 10.201.65.0/24 to 10.201.69.0/24 (TCP/8443).
Link addressing (3)
- R1 R2
10.201.80.0/30 - R2 R3
10.201.81.0/30 - R2 R4
10.201.82.0/30
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