OSPF stuck in EXSTART / EXCHANGE
Two OSPF routers that reach EXSTART/EXCHANGE and never go FULL are past the hello stage: they agreed to form an adjacency but can't finish exchanging the database. The textbook cause is an interface MTU mismatch, with a few close runners-up.
What it means
EXSTART/EXCHANGE is where routers negotiate the master/slave relationship and swap database description (DBD) packets. If the MTUs differ, the larger DBD is dropped and the adjacency hangs there, flapping between the two states.
Most common causes
- MTU mismatch. Different interface MTUs on the two sides. Match them, or set `ip ospf mtu-ignore` if you can't.
- Duplicate router-id. Two routers sharing a router-id breaks the adjacency. Make each unique.
- ACL blocking OSPF (IP protocol 89). A filter dropping protocol 89 stops the DBD exchange.
- Network-type mismatch. Broadcast vs point-to-point mismatch can prevent the adjacency completing.
How to fix it
- Compare the interface MTU on both sides; match them or use `mtu-ignore`.
- Confirm each router has a unique OSPF router-id.
- Check for ACLs dropping IP protocol 89 on the link.
- Confirm both interfaces use the same OSPF network type.