CCENT Question 102: Switch MAC Learning

certskills
By certskills February 8, 2013 12:17

Here’s another #CCENT question that kicks up a small notch. PC1 successfully connects to a web server. What MAC address table entries are learned? To answer, you have to think about not just layer 2, but also layer 3, and even a tad bit about layer 4.

The Scenario: Question 102

I’ve numbered some of these questions just for my own long-term self-defense of keeping track of what’s what. This question, numbered 102, uses the same figure as the previous question. However, to focus on the ideas, instead of trivia, it shows each device’s MAC address as a pseudo-MAC address (eg, PC1-MAC for PC1’s MAC address).

Figure 102: Network Used for Question 102

Question

The user at PC1 opens a web browser and successfully connects to the web server hosted at PC4. None of the switches use port security, and none of the switches have statically configured any entries in their MAC address tables. All switch ports are configured to be in the default VLAN 1. Which of the following answers is true regarding the MAC table entries that should exist immediately after PC1 connects to the PC4 web server? (More than 1 correct answer may exist.)

a)     SW1: PC4-MAC, Port Fa0/1

b)     SW2: PC4-MAC, Port Fa0/5

c)     SW4: PC1-MAC, Port Gi0/2

d)     SW2: PC1-MAC, Port Fa0/4

 

Q101 Answer: Combining Protocols
CCENT Answer 102: Switch MAC Learning
certskills
By certskills February 8, 2013 12:17
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13 Comments

  1. John B February 8, 13:57

    If I type the command show mac address-table interface FastEthernet0/1 at SW1, I should see the MAC Address of PC1, in the real world lol

    Reply to this comment
  2. Scott Murphy February 8, 15:27

    I believe the correct answers are: B,C,D

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  3. Michel February 9, 11:21

    Yep B,C,D are the correct answers.

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  4. xcieja February 9, 16:53

    A and D –
    Why ?
    PC1 and PC4 are in two different networks.

    PC1 in order to send packet to PC4 sends frame to Def GW (R1).

    PC1 sends frame to R1 -it arrives at SW1 -SW1 saves source MAC address.
    Next it forwards frame to SW2 -> SW2 saves source MAC of PC1 and forwards frame to R1.

    Please note that it doesnt matter if SW1 and SW2 have already in MAC table MAC of R1 or they dont have….

    Reply to this comment
  5. CCENTSkills February 9, 19:24

    Hi folks – nice comments! I’ll post answers Monday.

    Reply to this comment
  6. wertz February 10, 17:42

    B,C,D

    B – when PC1 initially try to connects to PC4, the switch realizes it can go out port 0/5 to get to PC4

    C – To get back to PC1, the switches knows that it needs to go back out Port Gi0/2 to get back to PC1

    D – Lastly SW2 knows to go out port 0/4 to get back to pc 1

    A – The answer should be port 0/3

    Hope im correct lol

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  7. J Want February 11, 06:17

    My answer would be ‘D’

    The other three in my opinion are wrong as they reference layer 2 addresses outside of the LAN (ie beyond a router). So PC4’s mac would never make it into the left side LAN and PC1’s mac wouldn’t make it into the right side LAN

    I think…

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  8. CCENTSkills February 11, 09:41

    Hi all – thanks for the discussions – Answer post is now available here!
    http://blog.certskills.com/ccent/ccent-answer-102-switch-mac-learning/

    Wendell

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