Adjacency troubleshooting
EXSTART entered the chat
The topology was cute. The database exchange had questions.
Okay, but what does it mean?
An MTU mismatch can keep an adjacency in ExStart or Exchange because database-description packets are not accepted as expected. Compare the actual interface MTUs on both sides. This is a clue, not a diagnosis: packet loss and other negotiation problems can produce similar symptoms.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.