Answers: Serial Link Interview Question

By certskills May 22, 2015 09:05

Did you pass the interview? Today’s post lists the answers to the previous interview-style question about serial links. The question posed a scenario in which a serial link wasn’t working, and you had to think about possible root causes. Answer is below the fold.

The original problem statement

Other Q&A in this blog


Answer: B


The command output shows the R1’s S0/0/0 interface has a line status of “up”, but a protocol status of “down”. As noted in the “Troubleshooting Serial Links” section of Chapter 12 of my ICND2 Exam Cert Guide book, some problems cause both status codes to be “down”, and others cause an “up/down” status, with line status as “up”, and protocol status as “down”. The reasons that lead to an up/down state include:

  1. The router on the other end has disabled keepalives (no keepalive), while the local end still has keepalives enabled
  2. When using PPP, a CHAP authentication failure
  3. A mismatched data link protocol – for instance, the configuration of encapsulation ppp on one end, with encapsulation hdlc on the other

With those details in mind, if either router did not have a clock rate command, then the link would fail to a down/down state, and R1’s interface is in an up/down state. Similarly, if R2’s serial interface were shutdown, R1’s matching serial interface would again be down/down. For these reasons, both answers A and D are incorrect.

Answers B and C clearly meet the criteria in the list for causes of an up/down state. Indeed, if R2 were configured with encapsulation ppp, it could cause a data link protocol mismatch; R1’s show command output shows HDLC as the data link protocol. Similarly, that same output shows that R1’s S0/0/0 has keepalives enabled, so that if R2 were configured to disable keepalives on the link, then R1’s interface would fail to an up/down state.

Serial Link Interview Question
Question: Traceroute to a subnet broadcast address - what happens?
By certskills May 22, 2015 09:05
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