IOS and Security Device Manager

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."

A Data Link layer

B Physical layer

C None. It is a router problem.

D Network layer

ANS:A - Physical layer

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.

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