Two routers on an Ethernet link repeatedly stall in ExStart/Exchange during adjacency formation after one side had its MTU changed. What is the most likely underlying cause?

An MTU mismatch on an OSPF link will cause routers to stall in the ExStart/Exchange state. During the ExStart state, routers negotiate the master/slave relationship and exchange Database Description (DBD) packets. These DBD packets contain a summary of the router's link-state database. If the MTU advertised in the DBD packet by one router is larger than the MTU configured on the receiving router's interface, the receiving router will drop the packet, preventing successful DBD exchange and thus stalling adjacency formation. Mismatched OSPF area IDs would prevent adjacency formation entirely, not just stall it at ExStart/Exchange. Duplicate router IDs would cause issues, but typically not a stall at this specific stage. Mismatched OSPF process IDs are locally significant and do not affect adjacency formation between routers.