Question: Forwarding and Encapsulation

CCENTSkills
By CCENTSkills January 19, 2018 11:05

You can learn how a LAN switch forwards frames in just a few minutes, and then explore that idea in different contexts throughout your entire span of studying for #CCENT and #CCNA Routing and switching. This latest practice question asks about just that but requires you to think about encapsulations and addresses before you can think about a switch’s forwarding logic. Have fun!

Question:

Consider a web session from host A to web server B, and focus on the first message sent by the server back to host A. Which facts does switch SW2 use when making its forwarding decision for that message?

  1. Host A’s MAC address
  2. Host B’s MAC address
  3. One of R1’s interface MAC addresses
  4. One of R2’s interface MAC addresses
  5. Host A’s IP address
  6. Server B’s IP address

I’ll post the answer post in just a few days; once that happens, the bottom of this page will show a link to the answer post.

IPv6 Shrinker 1 - Answers
Answer: Forwarding and Encapsulation
CCENTSkills
By CCENTSkills January 19, 2018 11:05
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8 Comments

  1. Bav January 20, 08:00

    I’ve gone for “D. One of R2’s interface MAC addresses”. Specifically R2’s G0/2 Ethernet address. My reasoning is as follows:

    When web server B replies back to host A the layer 3 addressing will be source 10.1.4.4 and destination 10.1.1.1. 10.1.1.1 however is on a remote subnet so web server B needs to send this to it’s default gateway. The next hop (default gateway) is on the same subnet and the R2’s G0/2 MAC address is also in the ARP table so the source mac is web server B and destination mac is R2’s G0/2 mac.

    When Switch 2 receives this frame it compares the destination mac address with it’s mac address table and sees it has an entry for R2’s G0/2 interface mac address and sends it out its G0/2 interface. It will have learned this entry previously when R2 forwarded the initial DNS query from host A to the web server.

    (hope I’m right).

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  2. Hume January 26, 09:17

    D. One of R2’S interface MAC address. My reasoning behind my answer is because to transmit the reply back to the host the message needs to be routed out of the LAN and only a Router can do that and since the web server is on the right side clearly the answer would be R2’S mac address

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  3. Christopher January 29, 20:55

    The answer is D. I chose this one because when SW2 receives the frame from the web server B, SW2 looks at the destination MAC (that in this case corresponds to R2 Gi0/2 MAC) to compare with its MAC table and decide where to send this frame. In this case the destination MAC is the R2 Gi0/2 MAC because PC A is in a different subnet than the server so its logic tells it to send the frame to the default Gateway.

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  4. Cristhopher January 29, 21:02

    The answer is D. I chose this one because when SW2 receives the frame from the web server B, SW2 looks at the destination MAC (that in this case corresponds to R2 Gi0/2 MAC) to compare with its MAC table and decide where to send this frame. In this case the destination MAC is the R2 Gi0/2 MAC because PC A is in a different subnet than the server so its logic tells it to send the frame to the default Gateway.

    Reply to this comment
  5. Aldo September 27, 14:49

    In some questions of the software (I have 1.0.29.3), I started to see this issues specifically on Chapter 7 Exam and Part review questions; the questions are hard to understand, and when you show the answers, the answer explanation say things like: “as you can see in the output, or according to the provided outputs” and that’s why I failed the questions because the exhibit don’t provide any output code, it only shows diagram. Please add the output code for questions that need it to be analyzed.

    Reply to this comment
    • CCENTSkills Author October 2, 08:43

      Hi Aldo,
      If you’re seeing some questions that (a) claim there is output and (b) there’s no output, then that’s a mistake on our part. My apologies. If you’ll click the feedback button, the app creates an email that takes an image of the window and collects the question ID. That forwards to a human that works with the product. If it’s an administrative issue, they handle it, and if it needs author input, it flows to me. But if you’ll start with that process, we’ll have all the doc needed to fix the issue.
      Note that just this week we uncovered a bug that appeared with a small software release that causes some issues in exhibits (aka the feature that shows command output), so it could well be that you saw a question or two that had issues due to that bug. If you can take the extra minute to send feedback from that question, it would help. Thanks much.
      Wendell

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    • CCENTSkills Author October 2, 08:45

      Aldo,
      And on your other re-post… note that your first several comments do not appear until I see them and approve them as a method of spam filtering. It just took me a few days to check the blog – sorry about that.
      Wendell

      Reply to this comment
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