A: LAN Switching Logic

certskills
By certskills August 31, 2015 09:05

It seems simple… and then you’re sitting at the #CCENT or #CCNA exam. Where does that Ethernet frame really go? I posted a question about LAN Switching a few days back. Here are the answers! Post with any related clarifications or comments.

Letter Answers

A, D

 

Explanation

 

Several key concepts must be applied to answer this question. First, consider the frames. The first frames goes from A to C, and the second is a return frame from C to A. As worded, the question also implies that these frames both are addressed using the unicast MAC addresses of the two hosts, so neither are broadcast frames.

The second key concept is that by the time the first frame arrives at host C, both SW1 and SW2 will have built correct MAC table entries for host A’s MAC address. The reason is that switches learn MAC addresses based on the source MAC address, and the source MAC of the first frame is host A’s MAC address.

The second frame has a destination address of host A’s MAC. As a result, this second frame, sent from host C to host A, will not be flooded because both switches already know the best port out which to forward frames sent to host A’s MAC. This path will be: to first enter SW2’s F0/9 interface; exit SW2’s G0/2 interface; enter SW1’s G0/1 interface; then exit SW1’s F0/1 interface.

The wrong answers are wrong simply because they are not part of the forwarding path from SW2, to SW1, and on to host A, once the two switches have learned of host A’s MAC address.

 

Answers: EIGRP and Bandwidth
Basic OSPFv3
certskills
By certskills August 31, 2015 09:05
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10 Comments

  1. bthethy April 26, 16:54

    Yay I got this right 1st time 🙂

    Reply to this comment
  2. Calvin November 16, 18:18

    Hi what is the question to this answer?

    Reply to this comment
  3. kevlim123 August 3, 13:34

    I’m a bit confused, I kind of get why the right answers are right but I’m not sure I understand why. Let me spell out my logic: On SW2 Port F0/9 is ignored, so the switch receives the frame, “skips” F0/9 & forwards the frame straight to port G0/2 (since the MAC address table is already built), SW1 receives the frame on port G0/1, “skips” past this port & forwards it straight out port F0/1 (again since the MAC address table is already built)
    It’s this concept of “skipping” I’m struggling with, I’m missing something because I thought it went port F0/9 >> port G0/2 >>> port G0/1 >>> port F0/1
    I’m probably over complicating matters & missing something I should already know, thanks!

    Reply to this comment
  4. kevlim123 August 3, 13:38

    Hi, I’m a little bit confused as to the switch logic when receiving know MAC address frames.
    I thought it would go F0/9 >>> G0/2 >>> G0/1 >>> F0/1
    My stab at the logic is this; because the destination MAC address is known, it’s like the frame “skips” the incoming port & goes straight out the correct port. Like the switch looks at the incoming frame, stops it before it comes in an incoming int (eg port G0/1 on SW1) & sends it straight out the correct port.
    I know I’m missing something I should already know but I want to get this stuff straight in my head!

    Reply to this comment
    • CCENTSkills August 3, 16:36

      Hi Kevlim,
      Your sequence of interfaces is correct for the interfaces that the frame enters AND exits.
      Enters SW2 F0/9
      Exits SW2 G0/2
      Enters SW1 G0/1
      Exits SW1 F0/1

      If you understand the true reasons for that, and why SW1 does NOT forward the frame out its ports F0/2 nor G0/2, then you’re probably fine. Assuming you understand all that, maybe you missed the question because it’s asking for the ports OUT which that 2nd frame is forwarded. So that’s only the two ports in my list with the word “exits” in them.

      Hope this helps,
      Wendell

      Reply to this comment
  5. Marc February 12, 13:23

    I´ve a question about the 1st frame send from A to C. Did SW1 broadcast the first frame cause the empty MAC table? How SW1 has learned the mac address from host C?

    Reply to this comment
    • CCENTSkills February 28, 15:13

      Marc,
      SW1 didn’t “broadcast” the frame, it “flooded” the frame – but I’m guessing that’s what you meant? Yes, SW1, with an empty MAC address table, upon receiving a frame, did not see the destination MAC address of the frame in its MAC address table, so it flooded the frame.
      SW1 will learn host C’s MAC when host C sends a frame because that frame would list host C’s MAC as the source MAC. (Switch learning happens based on the source MAC.)

      Reply to this comment
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