Answers: Switch Forwarding and What Happens Next

certskills
By certskills April 9, 2015 09:05

This post discusses the answers to the questions listed in the previous post. But what makes this question so challenging for someone headed for the ICND1 exam? Three things, in my opinion:

1) Application: it requires the application of concepts to a scenario that was not specifically listed in the book.

2) Forwarding w/ Multiple Switches: Many people get the ideas of what a single switch does to forward traffic, but get confused the first few times they have to think through stringing together multiple switches together.

3) Multiple layers and processes: The question requires you to think about ARP and switch forwarding, plus host forwarding logic, and how known information in the ARP and MAC tables impact those choices.

This post walks through the answers. Ask away if something strikes your interest!

The first three answers, A, B, and C, all use the same traffic: PC1 next wants to send an IP packet to PC5. Reviewing host routing logic, a host sends IP traffic to its default gateway when the destination is in another subnet. In addition, the only possible default gateway is R1, 10.1.1.254, and the figure shows PC1’s Default GW setting as 10.1.1.254.

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

To send a packet to PC5, PC1 will need to build an ip packet with destination IP 10.1.2.5. The destination MAC, however, will need to be R1’s F0/0 MAC address, so that the LAN will deliver the frame from PC1 to R1. For PC1 to know R1’s MAC address, PC1 either needs to ARP, or to reply on an existing ARP entry.

That last point is a huge key, because the question asks about the next frame PC1 sends. That frame may hold an ARP request, or it may hold the IP packet destined to PC5 (10.1.2.5).

Per the examples, PC1 has an entry for 10.1.1.254 (R1’s IP address) in PC1’s ARP table, so PC1 does not need to ARP again for that info. The next frame sent by PC1 will hold the IP packet, in a frame with destination MAC address of R1’s Fa0/0 MAC.

If you work through the above, now you know the destination MAC of the next frame sent by PC1, and can then think about what SW1 does. Because the frame has a unicast MAC address as the destination, SW1 either:

1) Floods the frame (if SW1 does not list the MAC in its MAC table) or

2) Forwards out the one correct interface (if SW1 does list the MAC in its table)

Per the examples, SW1 does list R1’s Fa0/0 MAC, off SW1’s port Fa0/1. So, Answer B is correct, and answers A and C are incorrect.

You actually could have ruled out A without knowing any other information… I’ll leave that as another question.

At this point, you have to consider answers D and E. I’ll leave the literal answer to whether D and/or E is correct to the next answer post. In the mean time… answers D and E start with the assumption that the next frame lists PC3’s MAC as the destination. What would have happened if PC1 issued a ping 10.1.1.3 command, assuming PC3’s IP address is indeed 10.1.1.3? Would indeed the next frame have been addressed to PC3’s MAC address, or not? I’ll wrap this discussion up with one more post in a few days.

Switch Forwarding and What Happens Next
More Answers: Switch Forwarding and What Happens Next
certskills
By certskills April 9, 2015 09:05
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3 Comments

  1. Samir April 9, 10:48

    Hi mr Odom, in answer 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 through fa0/3) in this question which means “through fa0/3” ? through sw1 fa0/3 or sw3 fa0/3 ?

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    • CCENTSkills April 9, 11:16

      Hi Samir,
      Ha, I see the confusion in my wording. by “F0/1 through Fa0/3” I meant “Fa0/1, Fa0/2, and Fa0/3”, with “through” meaning a range of interfaces numbers on switch 1. But I can see how you might think it meant something else. I’ll edit that answer for future readers to help avoid confusion. Thanks for the note, and sorry about the wording.
      Wendell

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