Which mismatch can cause an OSPF neighbor to remain in EXSTART/EXCHANGE?

A mismatched MTU size is a common cause for OSPF neighbors to get stuck in the EXSTART/EXCHANGE state. During the EXSTART state, OSPF neighbors negotiate the MTU size to be used for Database Description (DBD) packets. If the MTU sizes don't match, the negotiation fails, preventing the neighbors from exchanging DBD packets and progressing to the FULL state. Mismatched OSPF link costs would not prevent neighbor adjacency from forming, but could lead to suboptimal routing. Mismatched OSPF network types (e.g., point-to-point vs. broadcast) would prevent adjacency from forming at all, often keeping neighbors in the INIT or 2-WAY state. Mismatched areas would also prevent adjacency, as OSPF requires neighbors to be in the same area to form a full adjacency.