Which misconfiguration would cause an OSPF neighbor ship to be stuck in an ExStart state?

• A. Authentication mismatch
• B. Timer mismatch
• C. Subnet mismatch
• D. MTU mismatch

Answer: (D)

In an OSPF (Open Shortest Path First) environment, the establishment of a neighbor relationship progresses through several states, one of which is the ExStart state. When OSPF routers are stuck in this state, it indicates that they are having issues agreeing on certain parameters necessary for forming a neighbor relationship.

The primary reason for a neighbor relationship remaining in the ExStart state is an MTU (Maximum Transmission Unit) mismatch. MTU defines the largest size a packet can be for transmission over the network. If two OSPF neighbors have different MTU settings, they cannot successfully synchronize their database descriptors (DBDs) during the neighbor formation process, which is crucial during the ExStart state. OSPF requires both routers to have the same MTU to avoid fragmentation issues, ensuring that the communication is efficient and no packets are dropped or fragmented improperly.

This lack of agreement on MTU prevents the OSPF routers from moving past the ExStart state, as they are unable to proceed to send each other the necessary DBDs that confirm the information they have about the network. Therefore, for OSPF neighbor relationships to successfully form, matching MTU configurations are essential to avoid being stuck in the ExStart state.