- A
Another switch was connected to the access port, causing BPDU Guard to disable the port.
Correct because BPDU Guard disables a PortFast-enabled port if a BPDU is received, which happens when another switch is connected.
- B
A broadcast storm occurred due to a loop in the network.
Why wrong: Incorrect because a broadcast storm would cause high CPU but not necessarily put the port in errdisable state due to BPDU Guard.
- C
The host connected to the port caused a duplex mismatch.
Why wrong: Incorrect because a duplex mismatch can cause errors but does not trigger BPDU Guard to disable the port.
- D
The cable connecting the host is faulty, causing link flaps.
Why wrong: Incorrect because link flaps can cause errdisable due to link flapping, but the logs specifically mention BPDU Guard, indicating a BPDU was received.
Quick Answer
The answer is that another switch was connected to the access port, causing BPDU Guard to disable the port. This occurs because PortFast places the port into the forwarding state immediately, bypassing the normal STP listening and learning phases, but BPDU Guard actively monitors for any incoming Bridge Protocol Data Units. When a rogue switch is plugged into that access port, it sends BPDUs, which BPDU Guard interprets as a potential bridging loop, triggering the errdisable state to protect the network. On the ENCOR 350-401 exam, this scenario tests your understanding of how PortFast and BPDU Guard interact—a common trap is assuming BPDU Guard prevents loops by blocking traffic, when in fact it error-disables the port. Remember the memory tip: "PortFast skips the wait, BPDU Guard seals the fate."
CCNP 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 a connectivity issue in a switched network. The network uses Rapid PVST+ with multiple VLANs. The engineer notices that a host connected to an access port on SW1 cannot communicate with the default gateway, which is on a distribution switch. The access port is configured with PortFast and BPDU Guard. The engineer checks the switch logs and sees that the port went into errdisable state. What is the most likely cause of the errdisable state?
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.
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
Another switch was connected to the access port, causing BPDU Guard to disable the port.
The access port is configured with PortFast and BPDU Guard. PortFast immediately transitions the port to forwarding, but BPDU Guard monitors for incoming BPDUs. When another switch is connected to this access port, it sends BPDUs, triggering BPDU Guard to error-disable the port to prevent a potential bridging loop. This matches the log entry showing the port went into errdisable state.
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.
- ✓
Another switch was connected to the access port, causing BPDU Guard to disable the port.
Why this is correct
Correct because BPDU Guard disables a PortFast-enabled port if a BPDU is received, which happens when another switch is connected.
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.
- ✗
A broadcast storm occurred due to a loop in the network.
Why it's wrong here
Incorrect because a broadcast storm would cause high CPU but not necessarily put the port in errdisable state due to BPDU Guard.
- ✗
The host connected to the port caused a duplex mismatch.
Why it's wrong here
Incorrect because a duplex mismatch can cause errors but does not trigger BPDU Guard to disable the port.
- ✗
The cable connecting the host is faulty, causing link flaps.
Why it's wrong here
Incorrect because link flaps can cause errdisable due to link flapping, but the logs specifically mention BPDU Guard, indicating a BPDU was received.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between BPDU Guard (which reacts to BPDUs) and other errdisable causes like loop guard, UDLD, or link-flap; the trap here is assuming that any errdisable on an access port must be due to a physical issue (duplex, cable) rather than a deliberate STP protection mechanism.
Detailed technical explanation
How to think about this question
BPDU Guard is a Spanning Tree Protocol (STP) enhancement that works with PortFast. When enabled on an access port, any received BPDU (even a single one) immediately places the port into errdisable state. This prevents accidental connection of a switch to an end-user port, which could cause a bridging loop. The errdisable state can be recovered manually by issuing 'shutdown' followed by 'no shutdown' on the interface, or automatically via errdisable recovery cause bpduguard.
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.
- →
Spanning Tree Protocol — study guide chapter
Learn the concepts, then practise the questions
- →
Spanning Tree Protocol practice questions
Targeted practice on this topic area only
- →
All 350-401 questions
2,015 questions across all exam domains
- →
ENCOR 350-401 study guide
Full concept coverage aligned to exam objectives
- →
350-401 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 350-401 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Architecture practice questions
Practise 350-401 questions linked to Architecture.
Enterprise Network Design practice questions
Practise 350-401 questions linked to Enterprise Network Design.
SD-Access Architecture practice questions
Practise 350-401 questions linked to SD-Access Architecture.
SD-WAN Architecture practice questions
Practise 350-401 questions linked to SD-WAN Architecture.
QoS Architecture practice questions
Practise 350-401 questions linked to QoS Architecture.
Virtualization practice questions
Practise 350-401 questions linked to Virtualization.
Network Function Virtualization practice questions
Practise 350-401 questions linked to Network Function Virtualization.
Virtual Machines and Hypervisors practice questions
Practise 350-401 questions linked to Virtual Machines and Hypervisors.
VRF and Path Isolation practice questions
Practise 350-401 questions linked to VRF and Path Isolation.
Infrastructure practice questions
Practise 350-401 questions linked to Infrastructure.
OSPF practice questions
Practise 350-401 questions linked to OSPF.
BGP practice questions
Practise 350-401 questions linked to BGP.
Practice this exam
Start a free 350-401 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
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: Another switch was connected to the access port, causing BPDU Guard to disable the port. — The access port is configured with PortFast and BPDU Guard. PortFast immediately transitions the port to forwarding, but BPDU Guard monitors for incoming BPDUs. When another switch is connected to this access port, it sends BPDUs, triggering BPDU Guard to error-disable the port to prevent a potential bridging loop. This matches the log entry showing the port went into errdisable state.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Last reviewed: Jun 24, 2026
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.