Refer to the exhibit. Communication between R2 and R3 over FastEthenet1/1 falls. What is the root cause of the failure?

For two directly connected router interfaces to communicate, their IP addresses must be within the same IP network *and* configured with identical subnet masks. A mismatch in subnet masks will prevent proper IP communication, even if the physical link is up.

If the interface on R3 were not operational (e.g., administratively down or link protocol down), the communication would fail, but a subnet mask mismatch is a specific logical configuration error that can cause failure even when interfaces are physically up. C: An incorrect cable type would typically prevent the physical interface from coming up at all (down/down state), whereas a subnet mask issue allows the interface to be up/up but without IP connectivity. D: Disabling IP CEF would impact forwarding efficiency but would not entirely prevent basic communication between directly connected interfaces, only slow it down.