Refer to the exhibit. Routers A and B are directly connected and running OSPF, but they are
unable to form a neighbor relationship. What is the most likely cause?

A.
The routers are not on the same network.

B.
The network statements do not match.

C.
The process number does not match.

D.
The MTU does not match.

E.
The OSPF cost does not match.

F.
There is a physical issue with the cable.

Explanation:

OSPF sends the interface MTU in a database description packet. If there is a MTU mismatch,
OSPF will not form an adjacency and they are stuck in exstart/exchange state. The interface MTU
option was added in RFC
2178. Previously, there was no mechanism to detect the interface MTU mismatch. This option was
added in Cisco IOS Software Release 12.0.3 and later.
If the router with the higher MTU sends a packet larger that the MTU set on the neighboring router,
the neighboring router ignores the packet and the neighbor state remains in exstart.
Note: By default, the MTU for Ethernet is 1500 bytes. We can check the OSPF adjacency process
with the command “show ip ospf neighbor”.

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