OSPF Neighbor Stuck in ExStart/Exchange: How to Fix It
When two OSPF routers reach ExStart or Exchange and never become FULL, the cause is almost always an interface MTU mismatch. Here's how to confirm it in under a minute and fix it.
Step 1 — Confirm the stuck state
show ip ospf neighborIf the neighbor sits in EXSTART or EXCHANGE and never reaches FULL, the routers agree on the basics (hello/dead timers, area, subnet, authentication) but fail during database exchange — the classic signature of an MTU mismatch.
Step 2 — Check the MTU on both ends
OSPF includes the interface MTU in its Database Description (DBD) packets. If the two interfaces disagree, each side rejects the other's DBD and the adjacency hangs. Compare both interfaces:
show interface GigabitEthernet0/1 | include MTURun it on both routers. Even a small difference (often from an MPLS, tunnel, or jumbo-frame change on one side) breaks the adjacency.
Step 3 — Watch the adjacency form
debug ip ospf adjAn MTU mismatch shows messages like "Nbr ... has larger interface MTU" and repeated DBD retransmissions. (Disable with undebug all when done.)
Step 4 — The fix
The correct fix is to match the MTU on both interfaces. If you can't change one side (e.g. a provider link), tell OSPF to ignore the MTU check on that interface:
interface GigabitEthernet0/1
ip ospf mtu-ignoreAlso confirm router-IDs are unique — a duplicate router-ID can produce similar stuck states. Use show ip ospf to verify.
How Tech Matrix solves this in ~60 seconds
An ExStart hang is almost always an MTU mismatch — but proving it means checking both neighbors. Tech Matrix reads the OSPF state and interface MTU on both routers at once, names the mismatch, and gives you the exact fix for your platform — in seconds, with your approval on every command.
Frequently asked questions
Almost always an interface MTU mismatch between the two routers. OSPF carries the MTU in DBD packets and rejects the adjacency if they differ.
Match the interface MTU on both routers. If you can't, apply 'ip ospf mtu-ignore' on the interface as a workaround, and confirm router-IDs are unique.
'debug ip ospf adj' shows messages about a larger interface MTU and repeated DBD retransmissions during ExStart/Exchange.