OSPF neighbors won't form — what has to match for adjacency?
show ip ospf neighbor is empty on both routers even though they can ping each other. What is the checklist of things OSPF requires to agree before neighboring?
1 Answer
Ping proving L3 works rules out the easy stuff — OSPF is pickier. On the shared link these must match: area number, hello timer, dead timer, subnet/mask, authentication (type and key), stub/area flags, and MTU (mismatched MTU famously sticks neighbors in EXSTART/EXCHANGE). These must differ: router IDs — duplicate RIDs refuse adjacency (or corrupt the LSDB between non-neighbors). Also verify the interface is actually in OSPF (network statement wildcard or ip ospf area), isn't passive-interface, and nothing filters multicast 224.0.0.5/.6 protocol 89. Debug order: show ip ospf interface on both ends and compare line by line — it prints area, timers, and network type in one screen, and the mismatch is usually staring at you.