When the command show ospf neighbor shows one neighbor in the exStart state, what is the most likely cause?

When the command "show ospf neighbor" displays a neighbor in the exStart state, it indicates that the OSPF (Open Shortest Path First) protocol is attempting to establish an adjacency with that neighbor but has not yet succeeded in doing so. The exStart state is the initial stage of the OSPF neighbor relationship establishment process where the routers are negotiating which router will be responsible for creating the database description (DBD) packets.

The most likely cause for a neighbor being stuck in the exStart state is that the MTU (Maximum Transmission Unit) does not match between the two OSPF neighbors. In OSPF, all routers forming an adjacency must have the same MTU size configured on the interfaces through which they communicate. If the MTUs are mismatched, they will not be able to complete the negotiations necessary to proceed to the loading state, which prevents them from becoming fully adjacent. As a result, they remain in the exStart state, unable to exchange routing updates.

Therefore, the correct answer points to the MTU mismatch as the most relevant issue at this state, highlighting the importance of ensuring that router interfaces have compatible MTU configurations when establishing OSPF adjacencies.