Reasons for OSPF Neighborship in EXSTART/EXCHANGE State

D.

OSPF (Open Shortest Path First) is a routing protocol that uses a link-state routing (LSR) algorithm to determine the best path for data packets through a network. OSPF routers form neighbor relationships with other OSPF routers to exchange routing information and build a topology of the network.

The EXSTART/EXCHANGE state is a transitional state that occurs during the process of establishing an OSPF neighborship. During this state, the OSPF routers exchange Database Descriptor (DBD) packets to compare their link-state databases and synchronize them. The EXSTART/EXCHANGE state is the second state in the four-step process of establishing an OSPF neighborship, which includes the following steps:

1. Down state: OSPF routers do not have any information about their neighbors.
2. Init state: OSPF routers have received Hello packets from their neighbors and exchange Database Descriptor (DBD) packets to synchronize their databases.
3. Exstart/Exchange state: OSPF routers exchange DBD packets to compare and synchronize their databases.
4. Full state: OSPF routers have fully synchronized their databases and can exchange routing information.

Out of the given options, the reason that could cause an OSPF neighborship to be in the EXSTART/EXCHANGE state is option D - mismatched MTU size. The Maximum Transmission Unit (MTU) is the largest packet size that can be transmitted over a network. If the MTU sizes of two OSPF routers are different, the router with the larger MTU size will fragment the packets to fit the smaller MTU size, causing inefficient use of network resources. This mismatch can cause the OSPF neighborship to remain in the EXSTART/EXCHANGE state.

Option A (mismatched OSPF link costs), Option B (mismatched OSPF network type), and Option C (mismatched areas) can also cause OSPF neighborship issues, but they typically do not cause the EXSTART/EXCHANGE state. Instead, these issues can cause the neighborship to remain in the Init state or cause the neighborship to fail altogether.