Cisco 100-105 Exam

At which OSI layer should the administrator begin troubleshooting?

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
Answer: Option B.
Explanation: 

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”.

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