Question 1,324 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.

A network engineer is troubleshooting an STP issue in a network that uses Rapid PVST+. The network has a root bridge (SW1) and a secondary root bridge (SW2). The engineer notices that after a link failure between SW1 and SW2, the network takes longer than expected to converge. The engineer checks the configuration and finds that SW2 has the 'spanning-tree uplinkfast' command enabled. The engineer also notices that SW2 has a lower priority than SW1. What is the most likely cause of the slow convergence?

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 STP breakdown →

A
UplinkFast is enabled, which is incompatible with Rapid PVST+ and causes the switch to use legacy STP convergence.
Correct because UplinkFast is not needed with Rapid PVST+ and can actually degrade performance by forcing the switch to use slower convergence mechanisms.
B
SW2 has a lower priority than SW1, so it takes longer to become the root bridge after failure.
Why wrong: Incorrect because the root bridge election is based on priority, but the convergence time is not significantly affected by priority values; Rapid PVST+ converges quickly regardless.
C
BPDU Guard is enabled on the uplink ports, which prevents BPDU exchange.
Why wrong: Incorrect because BPDU Guard would disable the port if a BPDU is received, but it does not slow convergence.
D
Loop Guard is enabled on the uplink ports, which delays port transition.
Why wrong: Incorrect because Loop Guard prevents loops but does not significantly delay convergence; it may cause a port to stay blocked if BPDUs are not received.

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

✓

UplinkFast is enabled, which is incompatible with Rapid PVST+ and causes the switch to use legacy STP convergence.

UplinkFast is a legacy STP feature that is incompatible with Rapid PVST+. When enabled on a switch running Rapid PVST+, it forces the switch to revert to 802.1D STP convergence behavior on the affected ports, disabling the rapid transition mechanisms (such as proposal/agreement and sync). This causes the network to take longer to converge after a link failure, as the switch falls back to the slower listening and learning states.

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.

✓
UplinkFast is enabled, which is incompatible with Rapid PVST+ and causes the switch to use legacy STP convergence.
Why this is correct
Correct because UplinkFast is not needed with Rapid PVST+ and can actually degrade performance by forcing the switch to use slower convergence mechanisms.
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.
✗
SW2 has a lower priority than SW1, so it takes longer to become the root bridge after failure.
Why it's wrong here
Incorrect because the root bridge election is based on priority, but the convergence time is not significantly affected by priority values; Rapid PVST+ converges quickly regardless.
✗
BPDU Guard is enabled on the uplink ports, which prevents BPDU exchange.
Why it's wrong here
Incorrect because BPDU Guard would disable the port if a BPDU is received, but it does not slow convergence.
✗
Loop Guard is enabled on the uplink ports, which delays port transition.
Why it's wrong here
Incorrect because Loop Guard prevents loops but does not significantly delay convergence; it may cause a port to stay blocked if BPDUs are not received.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that UplinkFast is a harmless optimization that can be combined with Rapid PVST+, when in fact it forces a fallback to legacy STP behavior, causing slow convergence.

Detailed technical explanation

How to think about this question

UplinkFast works by immediately placing a blocked uplink into forwarding when the root port fails, but it uses a dummy multicast frame and a 30-second max-age timer to flush MAC tables, which is incompatible with Rapid PVST+'s fast convergence (which uses sync and agreement within seconds). In a real-world scenario, if an engineer mistakenly enables 'spanning-tree uplinkfast' on a switch running Rapid PVST+, the network may experience convergence times of 30-50 seconds instead of the expected sub-second failover, leading to application timeouts.

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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.

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: UplinkFast is enabled, which is incompatible with Rapid PVST+ and causes the switch to use legacy STP convergence. — UplinkFast is a legacy STP feature that is incompatible with Rapid PVST+. When enabled on a switch running Rapid PVST+, it forces the switch to revert to 802.1D STP convergence behavior on the affected ports, disabling the rapid transition mechanisms (such as proposal/agreement and sync). This causes the network to take longer to converge after a link failure, as the switch falls back to the slower listening and learning states.

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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