An STP Root Question with No Direct Link to the Root

 In 200-301 V1 Ch09: Spanning Tree, 200-301 V1 Part 3: VLANs, STP, Q&A

Here’s another Spanning Tree question, one that creates a topology in which one switch does not have a direct link to the root. What happens in this case? Note that as with this similar question, this question is meant for study prep, not for assessment, so don’t worry about how much time it takes, or fret if you get it wrong – just use it to learn.

In the switched network shown in the figure, the switches use PVST+, which creates an STP instance per VLAN. This question focuses on the STP state in VLAN 2. Currently, switch S4 is the root.

Figure 2: Four Switches, S4 as the Root

Continuing with the conditions and assumptions… there are no other devices or links in the network. All the links physically work and are enabled (no shutdown). All the links use 802.1Q trunking. The switches do not necessarily use all default STP settings.

The point here is that, with switch S4 as the root, which STP settings in this network could impacts S3’s choice of root port. To choose its root port, switch S3 will compare all received Hello BPDUs, and choose the best (least cost) path to reach the root. Think about all the switches, interfaces, and port priority settings. Which of those settings (as listed in the answers) could impact S3’s choice of root port here in VLAN 2?

A) S1’s F0/4 interface cost for VLAN 2

B) S2’s F0/1 interface cost for VLAN 2

C) S3’s F0/1 port priority for VLAN 2

D) S2’s F0/3 port priority for VLAN 2

E) S1’s F0/2 interface cost for VLAN 2

F) S4’s F0/1 interface cost for VLAN 2

Answers: ACL Drill Set 2
Answers to a Root Question with No Direct Link to the Root
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Rico

A) S1’s F0/4 int cost for VLAN 2

D) S2’s F0/3 port priority for VLAN 2

E) S1’s F0/2 int cost for VLAN 2

based on these are output ports for a frame traveling from S3 to the root switch. (?)

Rico

Oops … not D) … I meant to say C)

Matteo

Hi,
For me the correct answer for the question is D,
I think in a defoult configuration the best path is from S1’s F0/3 int and S4’s F0/1 int, so to change the topology The only answer is to change the prority of S2’s F0/3 int.

[…] post answers the Spanning Tree question linked here. But first, remember that triangle analysis of STP from last question? Well, all you need to know […]

srinivasan

The root port for S3 is a choice between f0/2 and f0/3.
The lowest cost route would determine the root port.
Changing S1 F0/4 interface cost and changing S2 F0/3 port priority do not give the same result. Clearly changing S1 F0/4 interface cost makes it the lowest cost route.

Gabriel Moran

Why is S2 F0/1 considered in this? If S1 already picked its own root port as F0/4 and then lost the tie breaker (assuming default values) on F0/2 therefore blocking on that port.

Please clarify

Gabriel Moran

Got it, I think I am thinking too much inside the box vs outside the box. Thanks for the reply.

Gabriel Moran

Wendell, sorry to post this here but there doesn’t seem to be any other way to get a hold of you regarding webpage problems. I went to view the labs for 200-105 Chapter 5 and it returns an error, when I click to contact/report an issue it does not do anything at all. Thanks

Gabriel Moran

That is the same thing I am seeing. It wasn’t specific in saying there was no labs for Chapter 5, I assumed the page link was broken. Thanks.

Punya Atma

Answers: A, and C.

Punya Atma

For the answer A). Primarily, S1’s interface cost to S4 is the lowest.

For the answer C). S3’s F0/1 port priority is in S3’s BID.

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