Question 191 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. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 redundant Layer 2 network with multiple VLANs. The network uses Rapid PVST+ for STP. The engineer wants to ensure that different VLANs have different root bridges to optimize traffic flow. The distribution switches are Cisco Catalyst 9300s. The engineer has configured one distribution switch as the root for VLANs 10 and 20, and the other as the root for VLANs 30 and 40. However, after implementation, the engineer notices that all VLANs have the same root bridge. What is the most likely cause?

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 engineer used the 'spanning-tree root primary' command without specifying the VLAN, which sets the priority for all VLANs.
Correct because the command without the VLAN keyword applies to all VLANs, causing the same switch to be root for all VLANs.
B
The engineer enabled BPDU guard on all access ports, which prevents the switch from receiving superior BPDUs.
Why wrong: Incorrect because BPDU guard disables ports that receive BPDUs, but it does not affect the root bridge election process.
C
The engineer enabled PortFast on all trunk ports, which causes the switch to ignore BPDUs and become root.
Why wrong: Incorrect because PortFast is for access ports and does not affect root bridge election; it only speeds up transition to forwarding.
D
The engineer enabled UplinkFast, which forces the switch to become the root bridge for all VLANs.
Why wrong: Incorrect because UplinkFast is a Cisco proprietary feature that improves convergence after uplink failure, but it does not force a switch to become root.

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 engineer used the 'spanning-tree root primary' command without specifying the VLAN, which sets the priority for all VLANs.

The 'spanning-tree root primary' command without specifying a VLAN sets the bridge priority to 24576 for all VLANs on the switch, making it the root for every VLAN. Since the engineer intended different root bridges per VLAN, this command overrides any per-VLAN priority settings and causes all VLANs to elect the same root bridge.

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 engineer used the 'spanning-tree root primary' command without specifying the VLAN, which sets the priority for all VLANs.
Why this is correct
Correct because the command without the VLAN keyword applies to all VLANs, causing the same switch to be root for all VLANs.
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 engineer enabled BPDU guard on all access ports, which prevents the switch from receiving superior BPDUs.
Why it's wrong here
Incorrect because BPDU guard disables ports that receive BPDUs, but it does not affect the root bridge election process.
✗
The engineer enabled PortFast on all trunk ports, which causes the switch to ignore BPDUs and become root.
Why it's wrong here
Incorrect because PortFast is for access ports and does not affect root bridge election; it only speeds up transition to forwarding.
✗
The engineer enabled UplinkFast, which forces the switch to become the root bridge for all VLANs.
Why it's wrong here
Incorrect because UplinkFast is a Cisco proprietary feature that improves convergence after uplink failure, but it does not force a switch to become root.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the nuance that the 'spanning-tree root primary' command without a VLAN parameter applies to all VLANs, leading candidates to overlook the need for per-VLAN configuration when designing asymmetric root bridges.

Detailed technical explanation

How to think about this question

The 'spanning-tree root primary' macro sets the bridge priority to 24576 (or 4096 if the current root priority is higher) for the specified VLAN, but when no VLAN is specified, it applies the priority to all VLANs globally. This is a common misconfiguration when engineers assume the command is VLAN-specific by default, whereas the correct approach is to use 'spanning-tree vlan <vlan-id> root primary' to set per-VLAN root bridges. In Rapid PVST+, each VLAN runs its own STP instance, so priority must be set per VLAN to achieve different root bridges.

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 engineer used the 'spanning-tree root primary' command without specifying the VLAN, which sets the priority for all VLANs. — The 'spanning-tree root primary' command without specifying a VLAN sets the bridge priority to 24576 for all VLANs on the switch, making it the root for every VLAN. Since the engineer intended different root bridges per VLAN, this command overrides any per-VLAN priority settings and causes all VLANs to elect the same root bridge.

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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