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.

An STP Root Question with No Direct Link to the Root
Q: Parsing Layer and Header Terminology
By certskills August 2, 2015 09:05
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October 30, 2017 6:21 pm

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  certskills
November 1, 2017 6:27 pm

Yes, I see it now. Thank you so much for answering.

September 7, 2018 11:22 am

Hi Wendell,

I got the answers but tackled them slightly differently. Instead of looking at them like triangles I instead mapped out all the various paths to the root from S3, and then added up the default interface costs. Similar to how you tackled the STP puzzle.

From there I scanned the interfaces in the questions to see if they were used in any of the various paths and what the impact could be if I changed the default setting.


S3 – S2 – S4 = 19 + 19 = 38
S3 – S1 – S4 = 19 + 19 = 38
S3 – S1 – S2 – S4 = 19 + 19 + 19 = 57
S3 – S2 – S1 – S4 = 19 + 19 + 19 19 = 57

Was this a correct way to answer this?

Barnabas Busa
Barnabas Busa
May 20, 2020 11:26 am

Hello Wendell,

In the book, page 232 Table 9-10 you have made a typo regarding RSTP state for port administratively disabled state you put discarding, while it should say disabled.

best regards