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At which layer of the OSI model would you assume the problem is if you type show interface serial 1 and receive the following message? Serial 1 is down, line protocol is down

Physical layer
Data Link layer
Network layer
None

Correct Answer: Physical layer

Explanation:

If you see that a serial interface and the protocol are both down, then you have a physical layer problem. If you see serial 1 is up, line protocol is down, then you are not receiving (Data Link) Keepalives from the remote end.

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