What layer of the OSI model would you assume the problem is in if you type show interface serial 1 and receive the following message? "Serial1 is down, line protocol is down."
Answer: A
If you see that a serial interface and the protocol are both down, then you have a Physical layer problem. If you see serial1 is up, line protocol is down, then you are not receiving (Data Link) keepalives from the remote end.