Answers to the “LAN Switching Logic” Question

CCENTSkills
By CCENTSkills June 6, 2014 09:05

I posted a question about LAN Switching a few days back. Here are the answers! Post with any related clarifications or comments.

The key to this question is that the scenario and the question tell us that the switches use all default configuration, and that the switches have not learned any information yet. That means that the switches have no dynamically-learned MAC table entries, and no statically-configured MAC table entries. Based on these facts, you can know that both SW1 and SW2 will flood the frame.

Flooding means that a switch forwards the frame out all ports in the same VLAN, except for the port in which the frame arrived. In this case, SW1 does no send the frame back out its F0/1 port, because the frame came in that port. SW1 does send it out it’s other three ports, F0/2, G0/,1 and G0/2.

SW2 receives the frame on its G0/2 port. SW2 uses the same flooding logic, because based on the scenario and question, SW2 has no MAC table entries. So, SW2 does not forward the frame out G0/2, but does forward it out F0/9.

Router R1 does not forward Ethernet frames, because routers remove the Ethernet header and trailer as part of the routing process. Just based on wording, routers in general do not forward Ethernet frames, but do forward the packets that might arrive at the router inside Ethernet frames. So, R1 will not forward the Ethernet frame, although R1 might forward the packet inside the frame. Note that this logic may seem a bit picky, but it is intended to help you notice the use of the words frame and packet, and recall that switches forward frames, and routers forward packets.

 

So, the answers in this case: B, C, D, and E.

 

CCENT Question: LAN Switching Logic
IPv4 Addresses 4
CCENTSkills
By CCENTSkills June 6, 2014 09:05
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11 Comments

  1. Gran t June 12, 12:18

    Your problem and answer made me think more about packets, frames and networking than I have by just reading your book. There is more to learning networking than reading a book.

    Networking jargon is confusing because it generalizes actions to make it easier to think about networking.

    When you say “routers forward packets” you do not really mean routers forward packets by themselves because an outgoing packet must be encapsulated in a different frame, right?

    Well, you could explain that the packet by itself is actually forwarded inside the router circuitry, but depending on the physical orientation of the router, the packet might actually be traveling backwards in relation to the physical destination of the packet, but we still say “forward” anyway.

    When you say “the packet” you do not really mean the actual packet that travels the wire in the form of an electrical signal or travels over fiber in the form of a light signal which is then converted back to an electrical signal that is further changed when it is stored in a buffer.

    So when we say a frame is flooded out all interfaces of the switch except the incoming interface, we really don’t mean “the” frame, we mean copies of the frame and we really don’t mean exact copies since any noise and attenuation arriving at the switches receiving interface are fixed.

    So you made me think about all these things, and now I just worry about what the folks writing the Cisco test really mean when they ask their questions so I can answer correctly.

    Enough of thinking about frames and packets until your next question.

    Reply to this comment
  2. CCENTSkills Author June 13, 08:20

    Hi Grant,
    Couple of comments back!
    Networking, like many fields of study, requires the ability to sift through language to get to the objective facts. EG, you can find a lot of kids who can work basic algebra problems, but the second you ask a “word problem” where they have to create the algebra problem based on words, they have a complete mental wall appear. So I agree – part of the learning has to be moving from an acceptance of a definition of some words, to being ready to parse, apply, and think through how they apply to what happens in a network.

    Your paragraph that begins “When you say “routers forward packets””… I think I made this point in the book, but routers do forward packets. That is, they do not forward Data Link frames because they discard the incoming data link header, and build a new data link header. That same process does indeed require layer 1 and 2 processing, so you can also describe it as “a router receives data link frames, removes the IP packet, forwards the IP packet in a new data link frame”, but for brevity, that’s the same as saying that routers route packets.

    Later,
    Wendell

    Reply to this comment
  3. Rickosic June 13, 22:26

    Wendell I was in the cisco ccent comunity (https://learningnetwork.cisco.com/community/certifications/ccent?view=all#/?tagSet=undefined) and I saw many coments about people who didn´t pass this new INCD1 because it had questions about RIP and WAN with Frame relay etc…Its dificult study all the new material and I didn´t know if also study old topics and some ICND2 topics. What do you think? Thanks a lot. Your help always help us.

    Reply to this comment
  4. CCENTSkills Author June 17, 08:17

    Hi Rickosic,
    Well, I can’t say much about topics that dive into what is and isn’t on the exam, because of NDA. That said:
    Like all Cisco Press Cert Guides, we believe the books cover all the topics that fall within Cisco’s stated exam objectives…
    …but if we think later that we missed the boat, we do have a mechanism to add topics of note, through an appendix. Actually, we’ve published a version of that appendix for ICND2, including more detail on RSTP. Just go to the book page at ciscopress.com for the ICND2 book, and you can find the appendix.

    As for your question, I looked at the posts at your link, and only found three related threads. One said nothing about the topics. One was from Jan 2013, which had to be for the old exams, which did have RIP. Only one made a short mention of RIP, without much detail. While RIP isn’t listed explicitly in the exam topics, I could imagine Cisco justifying mentioning RIP in a question that was actually a distance vector question.

    Fortunately, Cisco gives us the exam topics. Unfortunately, those exam topics do not go into the depth we’d all like them to go. But that’s the reality. As a result, anyone who makes an exam prep product must take their best shot at including the correct deeper interpretation of the exam topics. I can’t say more about how I go about that due to NDA; the products must simply speak for themselves.
    Thanks,
    Wendell

    Reply to this comment
  5. AreNET October 7, 05:34

    conversation was about SW1, and in last sentence of question ju have not mentioned anything. thats why my asnwer dould be B C D but not E.
    because english language is my third language thats me makes scarry to earn CCNA. sometimes questions are more dificult to understand but not network broblem in a question.

    Reply to this comment
  6. CCENTSkills Author October 7, 10:26

    Hi,
    Two comments on your note aout the language challenges:

    1) Cisco does support some other languages other than English for the exam.
    2) Cisco does also have a convention of giving extra time for those taking the exam in English when English is not their native language.

    I don’t know where the details are, but here’s a thread from CLN that implies that when you register, the extra time just happens.
    https://learningnetwork.cisco.com/message/395426#395426

    Hope this helps!
    Wendell

    Reply to this comment
  7. Noillirt March 16, 21:57

    His point was that your question did not mention anything about the state of SW2’s ARP table. You must have meant to, but you did not. It says that all devices have default configuration and SW1 has no ARP entries.

    Reply to this comment
  8. Noillirt March 17, 14:30

    I meant MAC table, not ARP table.

    Reply to this comment
  9. Anna August 25, 14:37

    Hi Wendell,
    I didnt go through the entire comments. but quickly Is there a typo in the statement captured;
    “SW2 receives the frame on its G0/1 port. SW2 uses the same flooding logic, because based on the scenario and question, SW2 has no MAC table entries. So, SW2 does not forward the frame out G0/1, but does forward it out F0/9.”
    I dont see a g0/1 port connected to SW2.
    Please clarify.

    Thanks,
    Anna

    Reply to this comment
    • CCENTSkills Author August 26, 08:06

      Hi Anna,
      Yep, you’re right. The explanation about SW2’s logic in this post was correct, but misreferenced the incoming port as G0/1 instead of G0/2.

      Note that I edited the post to fix it for future readers, rather than expect people to read into the comments to find the edit.
      Thanks,
      Wendell

      Reply to this comment
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