Network & Infrastructure / OSPF & Underlay Convergence
OSPF ExStart loops and database-description MTU mismatch
Two routers discover each other but repeatedly fail during database exchange. The interface can pass small pings while the adjacency never reaches a fully synchronized state.
Environment and inputs: OSPFv2 IPv4 underlays, with OSPFv3 explicitly identified where used; timers and diagnostics vary by platform. Diagnostic/design example; not tested on production equipment.
Collect the evidence
Record neighbor transitions, database-description messages, interface MTUs and encapsulation overhead at both ends. Compare physical, subinterface and tunnel settings rather than only the parent port.
Explain the result
An MTU disagreement can block database-description acceptance. Small packets demonstrate basic reachability but do not establish that the interface parameters required for adjacency are compatible.
Plan the fix
Correct the intended end-to-end MTU using the platform documentation. Treat an MTU-ignore option as an explicitly reviewed exception, because it does not repair a real forwarding-size mismatch.
Check the result and rollback
Require stable Full state where the topology expects it and test relevant packet sizes across the path. Restore previous settings if fragmentation, adjacency resets or application loss increases.