A network administrator cannot connect to a remote router by using SSH. Part of the show
interfaces command is shown.
router#show interfaces
Serial0/1/0 is up, line protocol is down
At which OSI layer should the administrator begin troubleshooting?

A.
physical

B.
data link

C.
network

D.
transport

Explanation:
https://learningnetwork.cisco.com/thread/12389
I think the indication here is “Serial 0 is up, line protocol is down”. What causes this indication?
Correct me if I am wrong. When you have this indication, a cable unplugged is not a correct
answer.If you check the output of your “show interface serial 0” command again, you should
notice it as “Serial 0 is down, line protocol is down. Under the “show ip int brief” you should
see status = down and protocol = down as opposed to up, down. Because you disconnected the
cable, layer 1 will go down, which is indicated by the serial 0 down status. The line protocol
status is for layer 2. So, a cable unplugged is not a correct answer to “Serial 0 is up, line
protocol is down”. Hope this helps.