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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17 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
  6. Raam Prabha April 17, 00:28

    the answer is B

    Reply to this comment
    • Raam Prabha April 18, 02:24

      PC1 is in a different LAN from PC5. Therefore PC1 sends the intended frame to its default gateway R1. PC1 encapsulates
      the IP packet with destination MAC address of R1’s Fa0/0 interface. The frame enters SW1’s Fa0/3 interface, SW1 compares this frame destination address with its MAC address table and SW1 sees this address is there in the table. For SW1 this address is a known unicast address. Therefore SW1 sends the frame only out of its Fa0/1 interface.

      Reply to this comment
  7. Chris Dedman-Rollet July 24, 18:49

    A is not correct because (If I understood) fa0/3 and fa0/2 are the enters and fa0/1 is the output.
    According to the table, PC1 has the MAC Address of R1 ,Sw1 has also the MAC Addres of R1.
    That’s why I choose answer B.

    Reply to this comment
  8. Israel Peña October 29, 15:35

    Answers B and E.

    Why B?
    Because PC5 its not in the same subnet as PC1, so PC1 must send the encapsulated IP packet to its default gateway (10.1.1.254), that is, to R1’s f0/0 MAC address.
    So when SW1 receives the frame it can find an entry for that interface through port F0/1. And because the entry exists, its a “known unicast frame” so the switch doesn’t have to make flooding. It will send the frame out that only interface.

    Why E?
    Because PC3 its in the same subnet as PC1, no router is involved and because PC1 have an entry in its ARP table for PC3, the frame can go directly to PC3 through SW1. However, SW1 does not have an entry for PC3 in its MAC address table, so it have to flood the frame to all its interfaces (f0/1 & f0/2) except to the interface who receive the frame (f0/3).

    Reply to this comment
  9. Punya Atma March 23, 03:41

    The choices of answer to this question are, B, and E.

    PC1, learns from its routing table, that, PC5 is in a different subnet.
    So, PC1 sends the frame to its default gateway, R1. To do this, PC1 encapsulates the IP pkt by the destination mac-address of router R1, as this mac-address is available on PC1 pseudo ARP table, (R1-fa0/0-MAC). This Ethernet frame has PC1’s mac-address as its source address.
    This frame reaches SW1’s Fa0/3 port. However this frame also has a known mac-address to SW1, as, SW1 Pseudo Mac table has R1’s Fa0/0 mac-address, and the frame leaves only out of SW1’s Fa0/1 port.
    So here one of the choices for answers, is, ‘B’.

    Now, the next choice for the next part, of the question:-

    The frame sent by PC1, addressed to PC3.

    PC3’s mac-address is listed in PC1’s pseudo ARP table. So PC1 encapsulate the IP pkt with PC3’s mac-address as the destination address. PC1 sends this frame on SW1’s Fa0/3 port.
    However there is no PC3-MAC, in SW1 Pseudo Mac Table.
    Thereby this frame’s destination address is an unknown mac-address for SW1. So SW1 floods out this frame out of the other two ports, fa0/1, and, fa0/2.
    Therefore the choice answer here is, ‘E’.

    Reply to this comment
  10. imikcisco August 13, 04:38

    B, E

    Reply to this comment
  11. Peter Lundgren August 25, 08:36

    B and E

    Reply to this comment
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