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Routing & Switching · Intermediate

OSPF MTU Mismatch

A condition where OSPF neighbors have different interface MTUs, leaving the adjacency stuck in ExStart or Exchange.

Definition

DBD packets carry the interface MTU; a router that receives a DBD with a larger MTU than its own rejects it. The fix is to match MTUs or, less ideally, configure ip ospf mtu-ignore. Hellos still succeed so neighbors appear in show ip ospf neighbor.

Example

ip mtu 1500 on both sides, or ip ospf mtu-ignore

Exam tip

Stuck in EXSTART/EXCHANGE is the textbook symptom of MTU mismatch.

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