A network administrator is troubleshooting an OSPF network. R1 and R2 are directly connected via a serial link. The `show ip ospf neighbor` command on R1 shows R2 in the `EXSTART/EXCHANGE` state. What is the most likely reason for this state, assuming all other OSPF parameters (timers, area ID, authentication) match?
- AR1 and R2 are configured with different OSPF network types.
- BThe OSPF process on R2 has not yet started.
- CAn OSPF router ID conflict exists between R1 and R2.
- DAn MTU mismatch exists on the serial link between R1 and R2.
Show answer & explanationAnswer & explanation
Correct answer: D. An MTU mismatch exists on the serial link between R1 and R2.
The `EXSTART` and `EXCHANGE` states in OSPF are part of the Database Description (DBD) exchange process. If an MTU (Maximum Transmission Unit) mismatch exists on the link, routers will fail to negotiate the master/slave roles and exchange DBD packets successfully, causing them to get stuck in these states. All other options would typically prevent adjacency from reaching even the `EXSTART` state or manifest differently.
Why the other options are wrong
- A. Different OSPF network types would prevent adjacency from forming, likely sticking in `INIT` or `2WAY` states, or not even appearing as a neighbor.
- B. If the OSPF process on R2 had not started, R1 would not even see R2 as a neighbor, or it would be stuck in `DOWN` state.
- C. A Router ID conflict would prevent adjacency formation much earlier, often leading to issues in the `INIT` or `2WAY` state.
OSPF MTU Mismatch
An MTU mismatch on a link between OSPF neighbors can prevent the successful exchange of Database Description (DBD) packets, causing neighbors to get stuck in the EXSTART or EXCHANGE states.
- MTU: Maximum Transmission Unit (largest packet size).
- OSPF uses MTU in DBD packets.
- Mismatch prevents master/slave negotiation and database synchronization.
- Can be resolved by setting matching MTU values or ignoring MTU checks (not recommended).
Memory trick: DBD exchange needs matching MTU, or EXSTART/EXCHANGE will be stuck like glue.