Troubleshooting Layer 2 Switching: Answers

Wendell
By Wendell March 9, 2017 12:05

Can you think abstractly about how layer 2 switching works, and how VLANs and trunks impact their forwarding decisions? Or do you need to see the specific configurations to make sense of it?

This latest practice question pulls in a lot of concepts that impact layer 2 switching (forwarding), mostly related to VLANs, but with more of a conceptual approach. In some ways, it requires a little more mastery of the topic than if the question was more detailed and showed all the configuration.  As usual, check out the question first, then come here to consider my version of the answer. (And check out video 7.2 of the CCENT Exam Prep LiveLessons product for some related background.)

The Answers:

A

 

Analyzing the Question Stem

To do well on the exam, you need to be thoughtful and well-practiced at analyzing the question stem along with the answers. (To learning theorists, the question itself is called the stem.) When preparing, it helps to think about how to read the question stem, and practice your approach to figuring out what the stem is really asking. So for this post, I’ll spend a little space on some of that analysis for this question.

(Note that the CCENT Exam Prep LiveLessons product include a series of practice question videos which include analysis of the wording of each question stem.)

 

The Analysis

For this latest question, the first two sentences of the stem restrict the scope of the question a bit. The question really doesn’t start until the third sentence. Repeating and numbering those first two sentences:

  1. In this network, all hosts use the IP address/mask configurations as shown.
  2. All links physically work and all switches act as layer 2 switches.

Sentence 1 focuses on the layer 3 (IP/address) details, a tells us that it is implemented correctly. Sentence 2 moves on to the physical links, again working. Sentence two also helps interpret the icons in the figure: the switch icons are all icons for a layer 2 switch, but the question overtly states that the switches are acting as layer 2 switches.

What conclusions could you draw from these two set-up sentences? That whatever the problem is, it’s not based on IP details on the hosts, and it’s not an issue with any of the physical links.

If you add a quick glance at the answers to your analysis, you will see that the answers all must do with VLANs and VLAN trunking. VLANs and trunks of course impact how switches perform layer 2 switching. So from the stem’s first two sentences, plus the answers, it’s clear that the question focuses on issues related to VLANs.

 

Aside on Question Stem Style

Note that it would have been reasonable for an equivalent real question on the exam to just have left out those first two sentences in the stem, expecting you to infer the meaning of those first two sentences. That would have made the entire stem read like this:

Host 1 can successfully ping server A, but host 2’s ping of server A fails. Which answers list issue that could result these failure symptoms?

With the above shorter stem, you would need to rely on the answers a little more. For instance, if host 2’s link connected to SW3 was physically down, these same symptoms would exist. However, none of the answers give an option related to layer 1 or layer 3 issues, so the answers would have to give you more of the context.

Now back to the answers, and which ones are correct and incorrect!

 

Answer A – VLAN Answer – Correct

This first answer focuses on a simple concept that is sometimes overlooked: Layer 2 switches forward frames based on not only the destination MAC address of the frame, but also the VLAN of the frame.

In this case, the figure shows all the hosts (clients and servers) in VLAN 2. If SW3’s post connected to host 2 was configured to be in a different access VLAN other than VLAN 2, then when host 2 sent a frame, SW3 would forward the frame in that other VLAN. Assuming server A was assigned to VLAN 2, the layer 2 switches would never forward the frame to server A, causing host 2’s ping to fail.

Figure 1: The Network for the ARP Question, with Subnets Marked

 

Answers B and D – Trunking Answers – Incorrect

Both answers suggest an issue with the trunk between SW1 (the switch connected to server A) and SW3 (the switch connected to both hosts 1 and 2). In short, both problems would cause host 2’s ping to fail… but would also cause host 1’s ping to fail. Both answers describe a condition that would prevent VLAN 2 traffic from passing over the trunk, so both answers are incorrect, because they would not cause the symptoms described in the stem.

For answer B, by disallowed VLAN 2 on the trunk, the switches effectively choose to not forward VLAN 2 traffic on the trunk. Simple enough.

Answer D means that SW3 would not forward VLAN 2 traffic to SW1. Answer D’s implied configuration of switchport mode dynamic auto on both switches means that neither would begin the trunking negotiation process, so the link would not trunk.

 

Answers C and E – VLAN Answers – Incorrect

Likewise, both answers C and E cause an entire switch to no longer forward frames in VLAN 2. As a result, host 2’s ping would fail, but host 1’s ping would fail as well.

Specifically, Answer C (on switch SW1) states that SW1 has shutdown VLAN 2. That means that any frames received by SW1 for which SW1 considers the frame to be in VLAN 2 will not be forwarded. Simple enough.

For answer E, the same result occurs. With VLAN 2 deleted from switch SW3’s configuration, SW3 will not forward frames that it considers to be a part of VLAN 2 – even with two switch ports configured to be in access VLAN 2. Again, as a result, the pings from both host 1 and host 2 would fail.

Troubleshooting Layer 2 Switching
IPv6 Icky EUI-64 Drill 1
Wendell
By Wendell March 9, 2017 12:05
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6 Comments

  1. ninjasd March 10, 08:30

    Love the explanation about how the question is built…sometimes we fall on those slippery bananas due to lack of strong foundations, but we can also get distracted on other “non-relevant” information.

    Thank you!

    Reply to this comment
    • CCENTSkills March 14, 10:39

      Thanks! I really enjoyed doing that part of the CCENT Exam Prep videos (and I’m recording the CCNA/ICND2 product in a few weeks). I’ll try to put more of that in these blog posts. Thanks for the input!

      Reply to this comment
  2. tweety January 31, 11:06

    where can i find the question please

    Reply to this comment
    • CCENTSkills February 1, 09:40

      Look just below the post, just above the comments section where you see these comments. That area will generally list the previous and next post based on date. Because I try to post the answer post as the next post after posting the question, on this answer post, the question will be the previous post – and therefore linked at the bottom of the page. So look on the bottom right of the post area, just above the comments, and you’ll see “Troubleshooting Layer 2 Switching” and that’s the link.

      Reply to this comment
  3. Haadk February 28, 05:50

    Hello, I am going to take the CCENT , just wanted to know if i have to study the entire 36 chapters and then focus on the exam topics. Or can I just read the exam topics chapters and leave the rest. I need some clarity about if the CCNET study guide , which I bought , is enough study material or do I need to go somewhere else to review key information.

    Reply to this comment
    • CCENTSkills March 7, 10:05

      Hi Haadk,
      Somewhere in the Intro to the book, I state that the book covers all exam topics. Indeed it does, and I think at around 900 pages, it does that to some depth.
      For the exam, Cisco tells us at the top of the exam topics pages that they can ask about topics outside the exam topics. So it is literally impossible to know everything Cisco might ask on one of their exams, since they tell us they can ask anything. That said, I don’t think Cisco sets about to ask about topics outside the scope of the exam topics, but it can happen. Here’s a quote that is frequently listed in their web copy:

      “The following topics are general guidelines for the content likely to be included on the exam. However, other related topics may also appear on any specific delivery of the exam.”

      To summarize, the book is comprehensive to the exam topics, and I even include topics outside the exam topic list if I think it might be on the actual exam. However, logic dictates that no one study resource could cover every topic in the actual exam with an exam policy that allows Cisco to ask questions outside the exam topics.
      Hope this helps,
      Wendell

      Reply to this comment
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