Serial Link Interview Question

By certskills May 19, 2015 09:05

Today’s post poses a #CCNA question about a serial link, in the guise of an interview question. Imagine you walk into the interview, and the interviewer draws a small network diagram, and asks you to diagnose a problem? That’s the basic setup. This question focuses on troubleshooting point-to-point serial links. As usual, the question is below the fold, with the answer posted in a few days! Ask questions if you have them, but don’t spoil the answer yet for others.


You have just been ushered into a small conference room to begin an interview. The interviewer, Scott, got called away just as he was about to start asking questions. The marker board lists some network documentation, and some additional paper notes sit on the conference room table. Scott tells you that it’s ok to read over what’s there while he finishes what is hopefully a short call.

The board shows the following figure, along with a note that this small network has been built using real gear in the next conference room.

One page of documentation shows the output of the show interfaces s0/0/0 command on router R1. However, the documentation shows nothing about router R2. Additionally, above the figure on the board, the words “This figure is correct (Scott)!” are written. So, if you take the figure and command output as both being correct, which of the following answers lists items that might be a cause of the interface not being in an “up and up” state?

A. Either R1 or R2 is missing a required clock rate command on their respective serial interfaces

B. R2’s S0/0/1 interface is configured with encapsulation ppp

C. R2’s S0/0/1 interface has its cable unplugged

D. R2’s S0/0/1 interface is configured with shutdown


Answer Part 3: FR Global DLCIs
Answers: Serial Link Interview Question
By certskills May 19, 2015 09:05
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  1. Avinash Kumar May 19, 11:33

    Hello Sir,

    Option B.

    Many Thanks

    Reply to this comment
  2. Mike May 19, 18:55

    The example config was spell checked in Microsoft Word. 🙂

    Two options can be eliminated right away since the interface is “up”.
    But the line being “down” … well that’s another problem.

    Since the network diagram shows R1 connected to R2 without a cloud/telco network … well someone needs to provide timing! And the fun problems don’t necessarily stop there. 😉

    Reply to this comment
    • certskills Author May 19, 19:22

      Hi Mike – thanks for the note here, and at LinkedIn! Indeed, once you try this stuff on GNS3 or a simulator, the interface state stuff can skew a little away from what real gear does. Thanks for playing!

      Reply to this comment
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