STP Puzzle #2

By certskills December 9, 2011 13:12

Here’s the 2nd STP Puzzle. As with most of these drill-style practice problems, take a look at the problem statement, and work it out. No need to rush on this one – just get it right. These STP puzzles give you part of the info about a LAN topology, and your job is to figure out what you can figure out about the rest of the STP details. It’s practice man, practice!

If you’ve not yet learned about STP, this exercise won’t help much. But you can go read more in my ICND2 book. Then try out the exercise!

I promise I’ll get the broader review of STP, and how to attack these problems, posted in the next week or so.

STP Puzzle Rules – Skip if You Know the Drill

For each puzzle, you will be given a problem statement that includes the following kinds of facts. For any facts that the problem statement does not tell you, figure out everything else on the list.

  • Switch Bridge ID
  • Switch priority (a part of the bridge ID)
  • Root ports
  • Designated ports
  • Blocking ports
  • Per-VLAN port cost
  • Per-VLAN port priority
  • Notes about a switch using default settings

Some rules:

  1. The puzzles use CCNA-level concepts only.
  2. The problem lists partial information, so you may not be able to determine all STP facts. Part of your job is to figure out what you cannot tell from the information given.
  3. Unless otherwise stated, assume that the problem relates to the STP topology for VLAN 1
  4. Unless otherwise stated, assume all switch-to-switch links are up and working physically, are performing VLAN trunking in the chosen VLAN. That is, the trunking state is not preventing STP from using the link.
  5. Do *not* assume that the switches use default configuration.
  6. If you have questions or comments, make sure and list you reasoning that leads up to the question or point.

STP Puzzle 2 – Problem Statement

Figure 1 shows most of the key facts in summary form. Additionally, make sure you remember or review the rules listed above, in particular, note that the configurations are not necessarily all default configurations. Also, the following facts are true for this probem:

  • All facts have to do with VLAN 2; answer about the STP topology for VLAN 2 only.
  • Switch S4’s base MAC address, used to form its Bridge ID (BID), is higher (worse) than S3’s.
  • S1 uses all default port costs in VLAN 2.

Figure 1 – Given Information, STP Puzzle 2

That’s it. Ask questions on the process here if you have them, and engage on the puzzles as I get them posted. Enjoy!

STP Puzzle #1, Answer Part 3
STP Puzzle #2 - Answer
By certskills December 9, 2011 13:12
Write a comment


  1. Mark M. December 12, 13:09


    I have a few questions. First, you show 2 different links blocking on the topology, Based on what I see, there is not enough info to determine which port is blocking for which vlan. (IE..F0/2 of S4 is blocking for vlan 1 and F0/2 of S3 is blocking for vlan 2) Could you please clarify?

    Second, could you please provide BID for S1 and S2?

    Reply to this comment
    • Wendell Odom of Certskills Author December 12, 20:09

      Hey Mark,
      On your #1: it’s all for VLAN 2 only in this one. I’m sticking with single VLANs until I get a few reps out there, so to speak.
      Your #2: I could give you the BID for S1 and S2, but you can figure out which is root without it. Could I make an alternate suggestion? Chase the RP, DP, and “neither” roles for the interfaces. Look at today’s new post, in the section about choosing the DPs, for some hints. EG, the port on the other end of a link that blocks will always be a DP. That should let you decide on the root, even without knowing the BIDs. I’ll do an answer for the root choice in the next day, and I’ll list a BID for each then, to answer you, but in the mean time, try the above.

      Reply to this comment
      • Mark M. December 12, 22:07


        After further review, here is my answer.

        S2 is root bridge. F0/1, F0/3 and F0/4 DP.

        S1 – F0/2 RP, F0/3 DP , F0/4 DP

        S4 – F0/1 RP F0/2 BP

        S3 – F0/1 RP F0/2 BL

        Look forward to your answer.


        Reply to this comment
  2. Wendell Odom of Certskills Author December 12, 20:12

    All – Check out this post, which is the “how to attack” post for this kind of question in the blog. FYI!

    Reply to this comment
  3. Wendell Odom of Certskills Author December 13, 18:04

    Hey Mark – you’re 100% right on your answer. Just posted the answer post, by the way. Thanks for playing!

    Reply to this comment
  4. Isis October 17, 08:46

    Before doing this puzzle, I hadn’t even thought of a non-root switch having all its ports in a FWD state.

    Thank you for making me notice this possibility, Wendell.

    I love your methods.

    Reply to this comment
  5. Isis October 17, 08:53

    Actually, this situation is pretty common.
    I think I got confused somewhere…X.x

    Reply to this comment
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