CCENT Answer 103: ARP Tables

certskills
By certskills February 22, 2013 11:05

Today’ post gives the letter answer(s), and explains why, for this latest #CCENT question. The question focuses on ARP tables, but with enough noise to make you think about what really happens in the network. Don’t read here until you try the question!

Literal Answer(s):

D, E

Figure Reference

Figure 103 is just a repeat of the figure from the question, for handy reference.

Figure 103: Network Used for Question 103

General Discussion: ARP with Multiple Subnets

ARP is simple as an end to itself. Many books fall to the temptation to explain ARP at its simplest. However, the exams have the word “troubleshoot” scattered all over the exam topics, which means you have to be ready to figure out how ARP is used in scenarios like the one in this question.

ARP learns MAC addresses of other devices in the same LAN-based IPv4 subnet, but not the MAC addresses of devices on other subnets. First, the protocol itself uses messages that routers do not forward. Second, the goal of ARP is to learn MAC addresses, used in Ethernet headers – headers that routers discard before forwarding the encapsulated IPv4 packets. So, a host needs to know ARP table entries for other IPv4 addresses on the same subnet.

Also, the LAN switches have no role in the ARP process in this scenario, other than to forward the Ethernet frames that hold the ARP messages. The switches have IP addresses for management, but to allow PC1 and PC2 to connect to web server PC4, the switches need no ARP entries at all. In fact, the switches could even have zero IPv4 configuration, and the PCs would still be able to connect to the web server (PC4).

ARP Entries Needed on the Subnet on the Left

To support the two web connections, the following ARP table entries will be needed:

  • PC1: For its default router, 10.1.1.254, MAC address R1-G0/0-MAC
  • PC2: For its default router, 10.1.1.254, MAC address R1-G0/0-MAC
  • R1: For PC1, 10.1.1.1, MAC address PC1-MAC, listed off R1’s G0/0 interface
  • R1: For PC2, 10.1.1.2, MAC address PC2-MAC, listed off R1’s G0/0 interface

Of these four ARP table entries on the various devices, note that only one of the items (R1’s entry for PC1; answer E) happens to be listed in the answers.

Other items of note that affect the right and wrong answers as worded:

  • PC4  (the web server) will not learn PC1-MAC or PC2-MAC.
  • Other ARP table entries may exist, but they are not required for the two web connections described in the question, so those could not be a correct answer in this case.

ARP Entries Needed on the Subnet on the Right

The subnet on the right also requires some ARP information, but the MAC addresses will be for hosts with IPv4 addresses in the subnet on the right (10.1.2.0/24). To support the two web connections, the following ARP table entries will be needed:

  • PC4: For its default router, 10.1.2.254, MAC address R1-G0/1-MAC
  • R1: For PC4, 10.1.2.4, MAC address PC4-MAC, listed off R1’s G0/1 interface

Of these two ARP table entries on the various devices, only one of the items (PC4’s entry for R1’s G0/1 interface IP and MAC address; answer D) happens to be in the list of correct answers.

CCENT Question 103: IPv4 ARP Tables
CCENT Question 104: Troubleshooting Ping Failure
certskills
By certskills February 22, 2013 11:05
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10 Comments

  1. Chris November 15, 22:36

    This is an older post, but I don’t see any reference in the offered answers to PC2. I do see PC1:
    R1 ARP table lists 10.1.1.1, MAC PC1-MAC
    Just a small technicality.

    Reply to this comment
    • CCENTSkills November 16, 10:39

      Hi Chris,
      I agree. I just updated that part of the explanation to mention PC1 instead, and to explicitly mention the correct answer (E) there as well. Thanks for the heads up!

      Reply to this comment
  2. Arslan December 26, 22:05

    I didn’t think E would be answer because I thought router would have a single MAC, as opposed two shown on the image. Thought E was posted just to confuse us. But, Googling now and i see that routers can have different macs for each interface. Good to know. Thanks.

    Reply to this comment
  3. Daniel January 20, 04:49

    Hello,
    I think that the answer “C” is also correct:
    c) SW2 ARP table lists 10.1.1.1, MAC PC1-MAC
    because PC1 and SW2 devices are in the same subnet .
    I’ve check with Packet Tracer and SW2 learn mac and ip adresse of PC1 in his table.
    It’s the same as the answer “E”.

    Reply to this comment
    • CCENTSkills January 20, 16:21

      Hi Daniel,
      Thanks for the note. In this case, C is not correct. note that the question includes a phrase “that exist to support the web connection from PC1 and/or PC2?”. The switch (a) doesn’t need an ARP entry at all to support layer 2 forwarding, and (b) even if it did learn an ARP entry due to a management VLAN interface, that ARP entry would be unnecessary for the process of forwarding the traffic. In short, a layer 2 switch needs no layer 3 info for the purposes of forwarding the layer 2 frames (as required for this particular scenario.) hope this helps…
      Wendell

      Reply to this comment
  4. Taylor B May 31, 20:55

    Hey I am a little confused about the formatting of the website. I notice there is a next question button that relates to the same topic, yet I do not see the question anywhere else in the “new” ccent content other than hitting the next buttons.
    Do the navigation buttons at the bottom of the example, just list everything, old and new? Should I just focus on the content listed under the 100-105 Book Chapters drop down menu? I just want to be sure I am not missing anything. Thank you.

    Reply to this comment
    • CCENTSkills June 6, 09:44

      Hi Taylor,
      Great question. So…

      Links at the bottom: Those are chronological. Then, because I purposefully post an “answer” post soon after the matching “question” post, the links are effective for finding a matched pair. But they’re not the best way to find content that’s more of that type, or more related to a topic.

      Menu: By content type
      Use the Hands-On and Question menus to find content by the style of post. EG, find all config labs, or all the QA, etc.

      Menu: By book content area: Use the book chapter or book part menus. Those items let you choose blog posts based on the matching book chapters or book parts. (Each book part is a set of 3-5 chapters of related content, grouped for easier review.)

      Does that clear it up? Ask more if not.
      Wendell

      Reply to this comment
      • MorganScott April 28, 16:23

        First I want to be clear that I am very grateful for all the content that is available here and all the work you put in to make this recourse available and high quality. But to tag in on this question, it does seem there is at least some content available by scrolling left and or right that does not seem to appear in any of the menu pages under “Parts” or “Chapters” Ss an example, the CCENT 104 question is clearly meant to be part of the menu page linked here, but isn’t, but I can find it by scrolling left. So my question is, are there very many other examples of this, or is this a one off case? There is so much good content here I am trying not to miss any. https://blog.certskills.com/ccent/category/part100105/part6/

        Reply to this comment
  5. Djee-man July 7, 04:17

    If CCENT becomes a performance- based exam, this legalese line of formulating a question quickly becomes obsolete.

    Reply to this comment
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