Q101 Answer: Combining Protocols

By certskills February 1, 2013 09:05

A seemingly simple #CCENT question: PC1 pings an IP address. What protocols are used as a result? And does the topology matter? Today’s answers to the previous blog post’s CCENT question spells out the literal answer, and the steps.

Check here for the original question before looking at the answer.

Quick Answer


Analysis of the Words

First, take a look at the words in the question. It mentions the phrase “…protocol that must have sent messages…”. It’s always a good idea to key on words like always, never, must/might versus may, and so on. In this case, the question is asking which of these protocols must be used somewhere for this ping command to have worked.

Also, note that the question lists the command ping It does not show the command ping PC4. While the CCENT and CCNA exams tend to mostly ignore name resolution, in real life, keeping an eye out for whether the user action uses names, rather than addresses, directs you down an entirely other line of reasoning.

Finally, although the real Cisco exams will tell you the number of correct answers, this one did not. So, it’s up to you to decide how many of the answers are correct.

Now on to the specific answers!

The Right Answer(s)

Right answers: “ICMP”

The right answer is easy in this case; the harder part for this question is ruling out the wrong answers.

The ping command generates ICMP Echo Request messages, and the other host sends back ICMP Echo Reply messages. The ping command by definition uses ICMP. So it’s correct right away.

The (for Sure) Wrong Answers

CDP: Cisco Discovery Protocol (CDP) may be used in this network, but it has nothing to do with the ICMP messages, or any overhead work to allow the ICMP messages to flow.

NDP: Neighbor Discovery Protocol (NDP) is a protocol that does many tasks for IPv6 (including replacing ICMP for IPv4 functions). However, the ping command clearly uses an IPv4 address, no NDP does not matter here.

The Trickier Wrong Answers

The rest of the wrong answers require a little more thought.

DHCP: Both PC1 and PC4 could have relied on DHCP to learn their IPv4 address and other IPv4 settings. However, remember that wording about “must” in the question? DHCP could have been used, before the ping. Alternately, both hosts could have used static IPv4 configuration. (Also, the question stem asked what happen from the point the command was issued, until it finished, and DHCP should have finished before the command was issued, and not in reaction to it.)

ARP: PC1 and PC4 will both rely on ARP – at some point. However, the question again asks what *must* have happened in reaction to the ping. If ARP were needed, it would indeed happen in reaction to the command. However, if the router, and PC1 and PC4, have all their ARP table entries already, then no ARP messages would have flowed in reaction to the ping command.

Any routing protocol: R1 definitely needs working routes to both the subnets in the design, or the ping will not work. (The question states that the ping works.) However, R1 can learn those routes as connected routes off each LAN interface, without using a routing protocol. So, while R1 may have a routing protocol configured, those messages did not matter to the routes required for the working ping command.

That’s it!

CCENT Practice Question 101: Combining Protocols
CCENT Question 102: Switch MAC Learning
By certskills February 1, 2013 09:05
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