Predicting ARP Messages 1

By certskills January 31, 2018 09:05

#CCNA candidates learn about ARP early in the ICND1 half of their studies, and it takes just a few minutes to learn the basics. At the same time, people who write CCNA exam questions can ask us about ARP in many ways that require us to think about ARP in the context of many other networking concepts. You need to be ready to take that basic knowledge and answer these kinds of applied ARP questions – and here’s a sample.


In quick succession, host A successfully pings host B, the engineer clears the MAC table on switch SW2, and host A again successfully pings host B. Which answers describe the ARP Requests sent as a result of the second ping?

  1. 1 ARP Request is sent by hosts A and B combined
  2. 2 or more ARP Requests are sent by hosts A and B combined
  3. 1 ARP Request is sent by switches SW1 and SW2 combined
  4. 2 or more ARP Requests are sent by switches SW1 and SW2 combined
  5. 1 ARP Request is sent by routers R1 and R2 combined
  6. 2 or more ARP Requests are sent by routers R1 and R2 combined
  7. 0 ARP Requests are sent

As usual, I’ll post the answer in just a few days. At that time, a link to the answer will appear towards the bottom of this page, just above where the comments begin.

Interpreting show stp (1) - Answers
Answer: Predicting ARP Messages 1
By certskills January 31, 2018 09:05
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  1. ciscoccna January 31, 10:13

    Hello mister Odom …I guess the Answer is B.

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  2. CYRIL February 1, 18:47


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  3. Jack Mac February 10, 14:36

    ARP is used by devices at Layer 3, so the switches would not be issuing ARP requests.
    The host devices rely upon their individual ARP caches, and the routers also rely upon individual ARP caches before issuing any ARP requests. Since no ARP caches have been cleared, these devices will not be issuing ARP requests.
    What will happen as a result of of the MAC table being cleared? Switch 2 will need to flood the incoming ICMP Echo Request out ALL ports except the port, G0/1, that is tied to R2 since it won’t know where the MAC address for host B “lives”.
    The answer to the question, as noted by CYRIL, is G – 0 ARP requests will be sent.

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  4. Cross April 20, 22:39

    Hello Wendell,

    I noticed the Q&A hasn’t been updated in a while which answers my intended question as to why some of the questions don’t conform to the new 200-301 study guides.

    I was particularly excited by this question because, even though I managed to get the answer correctly, it was more due to an elimination process than knowing the actual reason. The explanation you gave perfectly cleared that up. The part 1 of the new study guide doesn’t mention switches and MAC address table building process.

    It would be great to see more questions in future, especially those that can be found in the appropriate sections in the 200-301 study guide.

    Thanks a lot!

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    • certskills Author April 22, 10:02

      Hi Cross,
      Thanks for the note. If I reduce the suggestion part of it, I think you’re saying this question might be better associated with a later chapter/part, so that you would have already read about LAN switching as well. Right? I see your point. You’ll see more about ARP in CCNA 200-301 Volume 1 chapter 16. I’m going to leave it here, in part as a sort of learning exercise, sort of how you experienced it. But I agree, as an assessment, it would be unfair as something asked after chapters 2 and 3. Thanks much!

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