CCENT Answer 104 and 105: Answers Part 2

certskills
By certskills March 19, 2013 08:34

Wrapping up this set of five #CCENT questions today with the completion of the answers. Sorry it took a while for this last post – work went sideways unexpectedly last week. For those of you who don’t recall the flow: five related questions in the recent past, with the last two (104 and 105) focused on troubleshooting. Today’s post gives the why/wherefore on the rest of the answers I didn’t get to in answer part 1.

Literal Answer(s):

Question 104: C

Question 105: D

Figure Reference

The figure is just a repeat of the figure from the questions, for handy reference.

Figure 104: Network Used for Question 104 (and 105)

The rest of today’s post discusses the rest of the answers, other than the answers about port security. Those answers were frankly the most challenging to work through. This post has lots of short topics.

Question 104: Two Answers Break Both A’s and B’s Ping

Two of the incorrect answers in question 104 break PC2’s ping, which was one part of the question. However, both actions would also break PC1’s ping, which the question said that PC1’s ping worked.

One answer suggested that R1’s G0/0 interface mask be changed to /25, keeping the same 10.1.1.254 address. That would make R1 have a connected subnet of 10.1.1.128/25, range 10.1.1.129 – 10.1.1.254. R1’s connected route would no longer include PC2’s 10.1.1.2 IP address, do R1 could no longer forward packets to host PC2.

The problem? R1 no longer has a route to reach host PC1’s 10.1.1.1 address either. So this answer breaks both PC1’s and PC2’s ping, which isn’t allowed.

Question 104 – Correct Answer

The correct answer to 104 says “a speed mismatch on one link”. A couple of facts:

  • A speed mismatch prevents Ethernet frames from crossing the link. Note that a duplex mismatch does not.
  • The question suggests “one” link. You choose. Which link? The link from host PC2 to switch SW2.

Frankly, I think this question had stronger distractors (wrong answers) than correct answers. Break one Ethernet link – the link connected to host PC2 – and host PC1 still have a path to its default router.

Question 105 – Misconfig of R1’s G0/1 IP Address

This incorrect answer breaks PC2’s ping, but also breaks PC1’s ping.

The change is to make R1’s G0/1 mask /25, keeping the address at 10.1.2.254. Similar to one answer for question 104, that makes R1 have a connected route for 10.1.2.128/25, range 10.1.2.129 – 10.1.2.254. That address range does not include PC4’s IP address, so R1 would not have a route with which to forward packets to PC4’s 10.1.2.4 address. Both pings would fail.

Question 105 – Misconfig of R1’s Routing Protocol

This one’s like so many logic puzzles – painfully simple, once it hits you. Until then, maybe it’s not so obvious. I’m leaving it for questions. The answer is incorrect. Ask if you like…

Question 105 – Misconfig of PC2’s Mask

Finally, this is the one correct answer to question 105. Again, this one was a little more obvious than some of the distractor answers. Changing PC2’s mask to /25, keeping the address as 10.1.1.2, puts the default router in a different subnet than PC2. The default router should be in the same subnet.

 

CCENT Answer 104 and 105: Troubleshooting
Cisco Changes #CCENT and #CCNA for First Time in over 5 Years (Overview)
certskills
By certskills March 19, 2013 08:34
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6 Comments

  1. Rickosic July 7, 19:54

    Hi Wendell. Thanks for your hard questios

    I know that both nodes (PC2 and R1intG0/0) must be in the same broadcast domain and subnet, but I still make the simulation in Packe Tracer because I feel than PC2 aren´t smart enough (at least in Packe tracer) to realize that the gateway is in diferent subnet. In that simulation mode it seems to confirm that the ping between PC2 and PC4 is successfull (but should not)!!

    Reply to this comment
    • Em3xus October 2, 22:58

      Hi Rickosic,

      Curious of your experience I too ran this sim up in packet tracer. I didn’t expect the ping to work as with a mask of /25 on R1’s G0/0 interface, PC1 and PC2 would sit in a different subnet. After getting it all configured I was correct and the ping did not work.

      After this I put R1’s G0/0 interface to the correct setting, with a mask of /24 and issued the ping from PC1 again. This time it worked as expected.

      Lastly I put it back to the incorrect setting of a /25 mask I pinged again, this time not working, also as expected.

      I’m not sure what you may have done in your run up of the lab, but something else is amiss there.

      Reply to this comment
      • Bav October 28, 09:21

        I also ran this in PT and was able to ping. Em3xus your steps to re-create this don’t make sense.

        To start with you said “After getting it all configured I was correct and the ping did not work”. I think the issue here is that even when configuring it right the the ping DOES work.

        For your 2nd step you said “I put R1’s G0/0 interface to the correct setting”. R1’s config does not change at all in this scenario, only PC2 so R1 should be as per the diagram 10.1.1.254/24.

        I’ve just bought some lab gear so going to try this out.

        Reply to this comment
  2. Mansoor June 17, 15:53

    Hi Wendell,

    What if subnet-zero was allowed in this question.

    Would the pings have failed ?.

    Please can you give an explanation based on this scenario. Thanks.

    Reply to this comment
  3. Mansoor June 18, 13:56

    The scenario could be something like below.

    PC1 – IP 10.1.1.1 255.255.255.128
    PC2 – IP 10.1.1.2 255.255.255.128

    The reason is that the IP’s would be overlapping b/w zero-subnet and 1st subnetted address. This scenario is related to ‘Misconfig of PC2’s Mask’ in Q-105.

    My question is will the above setting done on PC1 & PC2 will make the ping work ?.

    Should we assume for the exam like in this question that zero subnet is not allowed ?.

    Reply to this comment
    • CCENTSkills June 22, 10:15

      Hi Mansoor,
      I can try, but I think I need to ask you for more details, and make a comment on subnet zero – maybe it’s just a misunderstanding on that point.
      I cannot see a reason why adding “ip subnet-zero” or “no ip subnet-zero” on the router(s) in the question would have any impact at all.
      In your second post, you suggest new masks (same addresses) for PC1 and PC2. As a result, those PCs are NOT in a zero subnet… are you thinking that they are?
      In this case, with addresses in network 10.0.0.0, the zero subnet’s subnet ID is 10.0.0.0, no matter what mask is used. The subnet ID for the case you describe is 10.1.1.0.

      Given all that, maybe you could post again and give me a more specific case of what you’re asking?
      Wendell

      Reply to this comment
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