A network administrator is troubleshooting an OSPF dynamic routing issue between two core routers connected via a point-to-point Gigabit Ethernet link. Router 1 has its interface configured with an MTU of bytes, while Router 2 has its interface configured with an MTU of bytes. Both interfaces share the same subnet, OSPF Area , Hello timer of seconds, and Dead timer of seconds. When reviewing the OSPF neighbor status on Router 1, the neighbor relationship remains stuck in the EXSTART/EXCHANGE state. Which of the following identifies the root cause of this neighbor state condition?
- The routers cannot negotiate master/slave roles or transmit Database Description (DBD) packets due to the MTU size mismatch.Answer
- BThe OSPF Hello packets are dropped at Layer 2 because of mismatched PDU frame encapsulation types across the physical link.
- CThe Hello and Dead timers are mismatched, preventing the routers from establishing basic bi-directional communication.
- DThe routers are attempting to create an automated PAT overload session across the point-to-point connection.
Answer
An MTU mismatch between adjacent router interfaces prevents the successful exchange of Database Description (DBD) packets, causing the OSPF neighbor state machine to stall in the EXSTART/EXCHANGE state.
The correct answer identifies that an MTU mismatch between adjacent OSPF router interfaces causes the neighbor state machine to become stuck in the EXSTART/EXCHANGE state. During EXSTART, routers elect master/slave roles and begin exchanging Database Description (DBD) packets containing interface MTU values. If the MTU values do not match, the routers cannot complete the DBD summary exchange and fail to transition to the LOADING or FULL states.
Step-by-Step Solution
Key Concept
OSPF Adjacency States and MTU Requirements