Q: LAN Switching Logic

By certskills August 29, 2015 09:05

To get your #CCENT and #CCNA, you must have the skill to predict how switches forward Ethernet frames. This skill goes beyond a simple generic list of actions like forward/filter/flood. This next question gives you another chance to build those skills by applying those basic rules to another scenario. Enjoy!


The figure shows a small enterprise network. The switches all have default configuration, unless otherwise noted in the question. The PCs have been configured, as well as the routers, so that it is possible for all hosts to ping each other. That also means that all the cabling shown in the figure works, and all interfaces are up.

On the left side of the figure, all switch interfaces default to be in the same VLAN (VLAN 1). Similarly, on the right, all devices sit in the same VLAN (VLAN 2).



Switch SW1 receives an Ethernet frame from host A. This frame is destined for host C, whose MAC address is 0200.CCCC.CCCC.  SW1 has all default configuration, and it has not learned any dynamic entries for any tables yet. Host C reacts to the receipt of this first frame by sending back some reply, with that Ethernet frame using host A’s MAC address as the destination MAC address. That 2nd frame arrives back at host A. Which of the following are true statements about interfaces out which this 2nd Ethernet frame (the one sent by host C to host A) will be forwarded? (Note to all: the bold text fixes an original mistake in this post. Frame 1: A sends to C. Frame 2: C sends to A. The question now correctly asks about frame 2.)

A. SW1 F0/1

B. SW1 F0/2

C. SW1 G0/1

D. SW2 G0/2

E. SW2 F0/9

F. SW1 G0/2


Answers: EIGRP and Bandwidth
A: LAN Switching Logic
By certskills August 29, 2015 09:05
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  1. Boolve August 27, 11:22

    I’m not English and for some reason your questions are always most difficult to understand, they are confusingly mixup. But your book is quite OK to understand. I believe that on the exam they use more friendly English for foreign people.

    Reply to this comment
    • CCENTSkills August 27, 14:09

      Hi Boolve,
      Thanks for the feedback – I am always looking to improve.
      Your comment is interesting. We try very hard to make the English wording in the books accessible to the large number of people who read the book in English but who also happen to speak a different language as their primary language. In the blog, I write very much like I speak to another native speaker, so it’s interesting to me that you noticed the difference. I’ll try and be a little more precise on the core parts of the questions!

      Reply to this comment
      • Mau Ramirez June 22, 20:06

        2022 not english native speaker student here haha. Well Im reading the question even after the correction made and is still hard to understand. I did a guess for option A because it was related to the traffic flow but never actually understood what you were looking for. To me the confusing word used here is the 2nd “which”.

        Reply to this comment
    • Boolve September 2, 11:52

      Yes right, there was mistake, thats why.
      A and D.

      Reply to this comment
  2. russell August 27, 13:16

    c and f should be the switchpports flooded

    Reply to this comment
    • CCENTSkills August 27, 14:12

      Hey Russell,
      Thanks for playing – I’ll wait to confirm/deny with a follow up post in a few days, and avoid comment to avoid spoiling the answer for others.

      But one quick comment, you said flood. Is that literal? Just food for thought (and obscure so to avoid spoiling). 🙂
      Thanks again,

      Reply to this comment
  3. Victor August 27, 18:49

    Hi Wendell,

    When you talk about the “2nd Ethernet frame” you refer to the one sent by host C to host A, right ?


    Reply to this comment
    • CCENTSkills August 27, 19:24

      Hey Victor,
      You are right, frame 2 is the frame sent from C back to host A. And the wording at the end of the question flipped those. So… I just edited the end of the question to clarify, and left a note so those who have already read it will know. The question: what happens to the 2nd frame, the one from C sent to A?

      Sorry Victor and all for the mistake! And thanks Victor for the notice.

      Reply to this comment
  4. Dexter August 27, 23:10

    Answer : A.
    When Host A sends a frame to Host C, the Switche will put the source MAC and the interface on which it received the frame in the forward/filter table. When Host C replies with the 2nd frame, the Swicth will look up the destination in the forward/filter table (the destination will be there since the switch learned that destination from the 1st frame) and forward the frame only out of the port F0/1.

    Reply to this comment
  5. Deepak August 27, 23:25

    A and D, as frame will be received by SW2 first which is forwarded on G0/2 and then when it will be received by SW1 it will be sent directly to host A on F0/1

    Reply to this comment
  6. Shivam August 27, 23:48

    Answer is A

    Reply to this comment
  7. Seth Loomer August 29, 11:02

    A and D. The switches learned the MAC addresses in the first frame so it will go the direct route. Out sw2 Gi0/2 then sw1 fa0/1.

    Reply to this comment
  8. Ronny August 30, 20:51

    Question should be:

    Out of which interfaces will the 2nd frame be forwarded?

    The ‘statements’ part of the question doesn’t really make sense as the answers aren’t in the form of statements.

    Anyway – I think A and D.

    Reply to this comment
  9. NK September 25, 22:31

    Frame 2 will be received in Sw2, will be forwarded out its G0/2 to SW1 which will then forward it out FA0/1 to PC-A

    Reply to this comment
  10. Pouta April 11, 11:01

    My answers are C and D

    Reply to this comment
  11. Punya Atma March 22, 03:20

    The choice of answers is, (A). SW1 F0/1.

    SW1 learns the source mac address of the first frame sent by host A,
    on SW1’s Fa0/1 port. So this mac address is registered in the SW1’s mac address table. However this same frame flooded out all the other ports of SW1, except the port Fa0/1 it arrives because SW1 not yet learned any other mac addresses, and therefore the destination address of this frame is an unknown mac address for SW1.
    The frame reaches SW2’s G0/2 port, and switches out by the SW2’s F0/9 port. Now the frame reaches host C, and processed by host C, as the frame destination address is the host C’s mac address itself.
    Host C also learns the frame is from host A, and host A’s source address.
    Now host C, sends its reply to host A with the host A’s mac address as the destination address. The reply reaches back SW1, via SW2, but host A’s mac address already is on the mac address-table of SW1. So SW1 switches this frame out by its Fa0/1 port.

    Reply to this comment
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