Switch Forwarding and What Happens Next

certskills
By certskills April 7, 2015 08:05

Today’s post lists a question that comes with the ICND1 Official Cert Guide from Cisco Press. Here’s the story. I received a reader comment about this question, and I thought, wow, this one has enough little ins and outs to make a good blog discussion topic. Long story short, my friends at Cisco Press gave me permission to post a few of these questions in the blog from time to time in case any of you want to discuss them a little more deeply.

The question sits below the fold. I’ll get to the discussion on the next post. Enjoy!

This question is part of the exam database that you get when you buy the ICND1 Official Cert Guide (by me). This question has pieces that relate to many chapters – which is one of the reasons it makes for a good discussion question in the blog.

No need to go buy the book to use the question, though. Here it is!

The Question:

In the figure, all devices are statically configured with the IP addresses, masks, and default gateway information. The exhibit shows a pseudo MAC address table for SW1 and pseudo PC1 ARP table. (These pseudo tables have text that describes the MAC addresses instead of using the actual MAC addresses. An example is PC1-MAC, meaning PC1’s MAC address.)

Assume that SW1 is not blocking on any interfaces, and that all hosts have already completed DNS requests for all names, caching all name-to-IP-address maping. Which of the following statements are true about the next Ethernet frame sent in this network?

A) If the next frame is created as a result of PC1 trying to browse to the web server on PC5, the next frame that PC1 sends is forwarded by SW1 out its interfaces fa0/1, fa0/2, and fa0/3.

B) If the next frame is created as a result of PC1 trying to browse to the web server on PC5, the next frame sent by PC1 is forwarded by SW1 out its fa0/1 interface.

C) If the next frame is created as a result of PC1 trying to browse to the web server on PC5, the next frame sent by PC1 is forwarded by SW1 out its fa0/1 and fa0/2 interfaces, but not fa0/3.

D) If the next frame sent by PC1 was addressed to PC3’s MAC address, SW1 would only forward the frame out its fa0/2 interface.

E) If the next frame sent by PC1 was addressed to PC3’s MAC address, SW1 would forward the frame out its fa0/1 and fa0/2 interfaces.

Figure 1: Topology, Question SE-I1-822-07-001

Example 1: SW1 Pseudo Mac Table

Example 2: PC1 Pseudo ARP Table

Answer to the "Which Route..." question
Answers: Switch Forwarding and What Happens Next
certskills
By certskills April 7, 2015 08:05
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8 Comments

  1. Joel Mooke April 7, 09:08

    I will go with option B. Assuming switch 1 has learned all addresses it will no that to reach PC5 send all traffic to Fa0/1

    Reply to this comment
    • CCENTSkills April 7, 09:33

      Hi Joel! Thanks for the post. I’ll post the answer (and explanation) post here in a few days. Til then, I’ll not confirm nor deny your answer, just to not to spoil the answer for others. Thanks for playing! 🙂
      Wendell

      Reply to this comment
  2. Dadash April 7, 11:20

    hi everyone, answer is absolutely b, thank sir for the best book 🙂

    Reply to this comment
  3. Samir April 7, 20:42

    in my opinion answer must be “A”

    Reply to this comment
  4. Davy April 8, 05:10

    Since PC 5 is on another subnet, PC1 sends his frame to his default gateway which is known and forwarded out of fa0/1 of SW1.
    PC 1 is connected on port fa0/3 – so answer A

    If the next frame sent by PC1 was addressed to PC3’s MAC address, SW1 would forward the frame out its fa0/1 and fa0/2 interfaces because PC3’s Layer 2 address (MAC) is not yet known – so answer E

    Reply to this comment
  5. taofaj4u June 29, 03:21

    The answer is B, E

    Reply to this comment
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