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

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!

Answers:

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.