Answers to a Root Question with No Direct Link to the Root

By certskills August 2, 2015 09:05

This post answers the Spanning Tree question linked here. If you want to noodle the question a little longer, check out the answer to a similar question that showed three switches in a triangle. To solve this question, just use the same concepts shown with that switch triangle. Then start ruling in the correct answers, and ruling out the incorrect answers. Details and answers below the fold!


A, B, E


Figure 2: Four Switches, S4 as the Root


Ruling in the Right Answers

First, you can (and probably should) repeat the triangle analysis from the previous question. Why? In this question, a triangle exists between root (S4), S1, and S2. The choices made by both S1 and S2 then determine the Hello details they send to S3, and the current question is about S3’s choices. So, the same logic seen in the earlier question, with a single switch triangle, can be used to analyze S1’s and S2’s choices of root port in this question, which in turn affects S3’s choice of root port.

So, repeating that same analysis described in the previous question’s answer, first think about the S1, S2, S4 triangle. From that, you can determine what settings matter to S1’s and S2’s choice of root port, which in turn impacts S3’s choice.

S1’s logic includes S1’s port cost on ports F0/4 and F0/2, as part of two possible root paths

S2’s logic includes S2’s port cost on ports F0/4 and F0/1, as part of two possible root paths

To choose the root, all three switch’s bridge ID’s (BIDs) are considered

From this analysis, we can rule in answers A, B and E. If you want more background on whys and wherefores, check out the answer to the previous STP question, while spells out more of those details.

Rule Out the Rest!

If you understood the analysis from the previous question, you could actually rule out the rest of the incorrect answers as incorrect, without thinking about switch S3 at all.  Why? First, two of the incorrect answers mention the port priority, and as pointed out in the answers to the previous question, the port priority is examined as a tiebreaker – but only in cases when a switch has multiple parallel links between two switches. In this question, no such parallel links exist. As a result, the tiebreaker of lowest-neighbor-Bridge-ID will break any ties first, before needing to consider port priority. So you can rule out both answers that mention port priority.

The other incorrect answer mentions the root switch (S4) and the port cost on one the root switch’s interfaces. As discussed in the answer to the previous question, the cost of the root’s ports does not impact the other switches’ calculations of root cost. For those same reasons, this answer can be ruled out as incorrect.

Basic VLANs
An STP Root Question with No Direct Link to the Root
By certskills August 2, 2015 09:05
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  1. Victor October 30, 18:21

    Hi. I have a question about the involvement of F0/1 on SW2 in choosing the root port for S3. I thought that the first step in choosing a root port for a non-root switch was to find the route with the smallest cost to the root switch adding all the outgoing ports costs. For example if we are talking about S3 there would be three posibilities:
    SW3 F0/2 -> SW2 F0/4 -> S4
    SW3 F0/1 -> SW1 F0/4 -> S4
    SW3 F0/1 -> SW1 F0/2 -> SW2 F0/4 -> S4
    And if there was a tie, the tiebreaker would be the lowest-neighbor-Bridge-ID. So I’m not seeing SW2 F0/1’s involvement anywhere in the decision of choosing S3’s root port. I’m sure that I am probably wrong but I would be glad if you could tell me why. Great questions, by the way :).

    Reply to this comment
    • certskills Author November 1, 11:13

      Hi Victor,
      Glad you like these questions! They’re so good, I have to think hard about them when I get questions like yours. 🙂
      Anyway, you missed on combination: S3 – S2 – S1 – S4. That path uses outgoing port F0/1 on S2 (which leads to S1).

      Reply to this comment
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