Question 592 of 2,015

Spanning Tree Protocol →hardMultiple ChoiceObjective-mapped

350-401 Spanning Tree Protocol Practice Question

This 350-401 practice question tests your understanding of spanning tree protocol. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

An engineer is designing a Layer 2 network with redundancy. The network uses MST (Multiple Spanning Tree) to reduce the number of STP instances. The engineer has configured two regions: Region 1 and Region 2. The engineer notices that switches in Region 1 are not forming a single MST region, and instead, they are treating each other as if they are in different regions. The engineer checks the configuration and finds that the region name and revision number are the same on all switches in Region 1, but the VLAN-to-instance mapping is different on one switch. What is the most likely cause of the issue?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Open the full VLAN trunking answer →

A
The VLAN-to-instance mapping is not consistent across all switches in Region 1.
Correct because MST requires identical VLAN-to-instance mapping, region name, and revision number for switches to be in the same region.
B
The root bridge for each MST instance is not configured correctly.
Why wrong: Incorrect because root bridge configuration does not affect region formation; it only determines the root within the region.
C
BPDU Guard is enabled on the inter-switch links, preventing BPDU exchange.
Why wrong: Incorrect because BPDU Guard is typically used on access ports, not on trunk links between switches, and would not cause region mismatch.
D
PortFast is enabled on the inter-switch links, causing the switches to ignore BPDUs.
Why wrong: Incorrect because PortFast is for access ports and should not be enabled on trunk links; it does not affect MST region formation.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The VLAN-to-instance mapping is not consistent across all switches in Region 1.

In MST, all switches within a region must agree on three parameters: the region name, the revision number, and the VLAN-to-instance mapping. Even if the region name and revision number match, a single mismatch in the VLAN-to-instance mapping causes the switches to treat each other as if they belong to different regions, preventing them from forming a single MST region.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
The VLAN-to-instance mapping is not consistent across all switches in Region 1.
Why this is correct
Correct because MST requires identical VLAN-to-instance mapping, region name, and revision number for switches to be in the same region.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The root bridge for each MST instance is not configured correctly.
Why it's wrong here
Incorrect because root bridge configuration does not affect region formation; it only determines the root within the region.
✗
BPDU Guard is enabled on the inter-switch links, preventing BPDU exchange.
Why it's wrong here
Incorrect because BPDU Guard is typically used on access ports, not on trunk links between switches, and would not cause region mismatch.
✗
PortFast is enabled on the inter-switch links, causing the switches to ignore BPDUs.
Why it's wrong here
Incorrect because PortFast is for access ports and should not be enabled on trunk links; it does not affect MST region formation.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the fact that all three components of the MST configuration (name, revision, and VLAN-to-instance mapping) must match exactly for switches to be in the same region, and candidates mistakenly think only the name and revision matter.

Detailed technical explanation

How to think about this question

MST uses an MST Configuration Identifier (a hash of the region name, revision number, and VLAN-to-instance mapping) to determine region membership. Switches exchange this identifier in BPDUs; if the identifiers do not match, they consider each other as part of different regions, even if only one VLAN mapping differs. This is critical in large networks where a single misconfigured switch can break the MST region, causing multiple spanning-tree instances to run unnecessarily.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 350-401 question test?

Spanning Tree Protocol — This question tests Spanning Tree Protocol — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The VLAN-to-instance mapping is not consistent across all switches in Region 1. — In MST, all switches within a region must agree on three parameters: the region name, the revision number, and the VLAN-to-instance mapping. Even if the region name and revision number match, a single mismatch in the VLAN-to-instance mapping causes the switches to treat each other as if they belong to different regions, preventing them from forming a single MST region.

What should I do if I get this 350-401 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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