%OSPF-5-ADJCHG: Process 1, Nbr 10.0.0.2 on GigabitEthernet0/1 from FULL to DOWN, Neighbor Down: Dead timer expired
In plain English: Two routers that were exchanging routes with OSPF stopped hearing from each other.
Platform: Cisco IOS / IOS XE routers (OSPFv2 per RFC 2328; the 40-second dead interval named here is the default on broadcast networks).
What it means
Two routers that were exchanging routes with OSPF stopped hearing from each other. 'Dead timer expired' means this router received nothing from the neighbor for the full dead interval (40 seconds by default) — the neighbor died, its interface died, or the path between them stopped passing traffic.
Likely causes, in order
The neighbor rebooted or its interface went down — check its side first if you can.
The link between them is failing quietly: a bad optic or cable can pass some traffic and drop OSPF's small hello packets.
Someone changed OSPF settings on one side only: hello/dead timers, area number, or authentication.
Heavy congestion ate the hello packets — rare, but real on saturated links.
Check, in order
show ip ospf neighborWhich neighbors are FULL (healthy) and which are missing or stuck in a partial state like EXSTART or INIT.
show ip ospf interface Gi0/1The timers and settings this side is using, to compare against the other side.
show interfaces Gi0/1Errors, drops, and whether the interface itself has been flapping.
The fix
Restore basic reachability first: interface up on both ends, then matching area, timers, and authentication on both sides.
If the neighbor is stuck in EXSTART rather than DOWN, suspect an MTU mismatch — that has its own full case, linked below.
If drops repeat at the same time of day, look for congestion or a scheduled job on the path before blaming OSPF.
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Full case: this error is one stop in a bigger fault. Work the complete playbook: OSPF Adjacency Stuck in EXSTART — checks, ranked causes, the fix, and verification.