Question: Forwarding and Encapsulation

certskills
By certskills 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.

Extended IPv4 ACL Drill 2 - Answers
Answer: Forwarding and Encapsulation
certskills
By certskills January 19, 2018 11:05
Write a comment

14 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).

    Reply to this comment
  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

    Reply to this comment
  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.

    Reply to this comment
  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 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

      Reply to this comment
    • CCENTSkills 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
  6. JAMES December 27, 19:32

    Hello Team,

    I chose A. Host A’s MAC Address!

    Reply to this comment
  7. Makoa July 19, 06:28

    Are all switches Layer 2 switches?

    Reply to this comment
    • certskills Author July 19, 06:47

      Nope. Many can be configured to act as layer 2 and layer 3, and it’s a per-port behavior. That said, a lot of CCNA uses switches that act only as layer 2 switches, and the icon shown in most figures are meant to imply “layer 2”. The layer 3 switch icon is closer to a cube shape.

      Reply to this comment
  8. Punya Athma April 20, 01:59

    Answer is D.

    First message sent by the server back to host A, the IP packet from the server encapsulated by the MAC address of R2’s Gi0/2 interface MAC address as the destination address. SW2 receives this frame on its Gi0/3 interface. R2’s Gi0/2’s MAC address is available on SW2’s MAC address table from the previous starting session, i.e, the passage of frames at the start from host A to the server. For SW2 not only this a known unicast frame but also no option for SW2 other than sending out through its Gi0/2 port because that is the only connected port to R2’s Gi0/2 port as shown on the diagram.

    Reply to this comment
    • certskills Author April 22, 09:56

      Hi Punya,
      Thanks for your recent posts! Keep’em coming – a great way to refine skills and to help others. Also, note that the QA posts typically have a question post like this one, and an answers post, linked below the post but above the comments – FYI in case you want to check your logic vs. mine.
      Wendell

      Reply to this comment
      • Punya Athma April 24, 05:21

        Dear Wendell, your words encourage me a lot. Plenty of thanks. I praise you also for pointing me out the location on this page that useful to match my answer, and, relevant reasoning with your mode or your style of deciding the correct answer/s.

        Reply to this comment
View comments

Write a comment

Comment; Identify w/ Social Media or Email

Subscribe

Subscribe to our mailing list and get interesting stuff and updates to your email inbox.

Thank you for subscribing.

Something went wrong.

Search

Categories