Answer to Serial Link Interview Question

certskills
By certskills June 3, 2011 09:52

Today’s post lists the answers to last week’s question about serial links. The question basically shows a diagram, plus some show command output, and asks what you know for sure based on the output in the command. Answer is below the fold.

Answer: D

The show interfaces command on R1 tells us nothing about the source of the clock on a serial link or the actual layer 1 clocking speed on a serial link. The show controllers command would tell us the type of serial cable (DTE or DCE), and if DCE, the clock rate used on the interface. However, the speed information (bandwidth 1544) in the output reflects the setting of R1’s bandwidth command, which has no influence on the actual speed at which bits are sent on the link at layer 1. So, none of the first three answers are correct. The facts listed in the commands may be true, but you cannot know that they are true from the output shown with the question.

R1’s show interfaces command lists a bandwidth of 1544, which means that R1’s bandwidth command setting is 1544. However, the bandwidth settings on the opposite ends of the link do not have to match, so the bandwidth 1544 setting on R1 tells us nothing about R2’s setting. Again, R2 may well be configured (or defaulted) to use bandwidth 1544, but you cannot know that fact from the output shown here.

Finally, for the one correct answer, you can know that R2’s S0/0/1 interface uses HDLC from the output on R1. R1’s S0/0/0, per the figure, connects to R2’s S0/0/1 interface. R1’s S0/0/0 is in an “up and up” state, and per the output, it uses HDLC. For R1’s serial interface to be in an up/up state, the other end must use the same data link protocol, so R2’s S0/0/1 interface must also use HDLC.

Subnetting Speed Practice #3
Subnetting Speed Practice #3: Answers
certskills
By certskills June 3, 2011 09:52
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1 Comment

  1. Rickosic July 7, 22:33

    Hi Wendell.

    with your explanatios if the status is up/up,is this correct?: “I can only deduce that one of the two has a correct clock rate command configured, but i can´t deduce which one exactly” ?

    by the way, if the link has not yet set the clock rate , can the line-status be in “up”?

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