The answer is that the port is error-disabled by BPDU Guard. This outcome occurs because PortFast configures the interface as an edge port, allowing immediate forwarding, while BPDU Guard actively monitors for incoming spanning-tree BPDUs. When a small unmanaged switch is connected and sends BPDUs, BPDU Guard detects this violation of the edge port assumption and immediately places the port into an err-disabled state to protect the Layer 2 topology from potential loops. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how PortFast and BPDU Guard work together in enterprise access-layer design; a common trap is assuming the port simply blocks or ignores the BPDUs, but the correct behavior is a hard shutdown. Remember the mnemonic: "Guard shuts the door when BPDUs knock on PortFast."
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. 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. A key principle to apply: portFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity.. 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 port connected to an end host is configured with PortFast and BPDU Guard. What is the most likely result if a small unmanaged switch is connected and starts sending BPDUs?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The port is error-disabled by BPDU Guard.
The most likely result is that the port is placed into an err-disabled state by BPDU Guard. In practical terms, PortFast tells the switch to treat the interface like an edge port for a normal endpoint, which is why it starts forwarding quickly. BPDU Guard protects that assumption. If the port suddenly receives a spanning-tree BPDU, the switch treats that as a sign that the port is no longer connected to a simple end device.
This combination is common in enterprise access-layer design because it improves user startup time while still protecting the topology. The correct answer is the one that describes the port being shut down automatically when BPDUs appear unexpectedly.
Key principle: PortFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity.
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 port is error-disabled by BPDU Guard.
Why this is correct
This is correct because BPDU Guard disables an edge port when it receives a BPDU.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
PortFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity.
✗
The port automatically becomes the root port.
Why it's wrong here
This is wrong because BPDU Guard does not promote the port into STP forwarding roles.
When this WOULD be correct
In a different scenario where a switch is configured without BPDU Guard and is part of a larger network topology, a port could automatically become the root port if it is the lowest-cost path to the root bridge during the Spanning Tree Protocol (STP) convergence process.
✗
The port is converted into a trunk.
Why it's wrong here
This is wrong because BPDU receipt does not trigger automatic trunk conversion here.
When this WOULD be correct
In a different scenario where a switch port is configured to allow trunking and is explicitly set to negotiate trunking with connected devices, a small unmanaged switch could potentially cause the port to convert to a trunk if it supports dynamic trunking protocols. The question would need to specify that trunking is enabled and that the connected device is capable of sending DTP messages.
✗
The port ignores the BPDU because PortFast disables STP entirely.
In a different scenario where a switch port is configured with PortFast but not with BPDU Guard, a question could ask what happens when a small unmanaged switch sends BPDUs. In that case, the port would ignore the BPDUs and remain in the forwarding state, making this option correct.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The port is error-disabled by BPDU Guard.Correct answer▾
Why this is correct
This is correct because BPDU Guard disables an edge port when it receives a BPDU.
✗The port automatically becomes the root port.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because a port configured with PortFast and BPDU Guard will not automatically become the root port when it receives BPDUs; instead, it will be error-disabled due to BPDU Guard's protection mechanism.
★ When this WOULD be the correct answer
In a different scenario where a switch is configured without BPDU Guard and is part of a larger network topology, a port could automatically become the root port if it is the lowest-cost path to the root bridge during the Spanning Tree Protocol (STP) convergence process.
Why candidates choose this
Candidates may find this option tempting because they might confuse the behavior of STP with PortFast, thinking that the port's role in the STP topology can change dynamically based on BPDU reception.
✗The port is converted into a trunk.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because a port configured with PortFast does not automatically convert to a trunk port when it receives BPDUs; instead, it remains in access mode. BPDU Guard will cause the port to be error-disabled upon receiving BPDUs, preventing any trunking behavior.
★ When this WOULD be the correct answer
In a different scenario where a switch port is configured to allow trunking and is explicitly set to negotiate trunking with connected devices, a small unmanaged switch could potentially cause the port to convert to a trunk if it supports dynamic trunking protocols. The question would need to specify that trunking is enabled and that the connected device is capable of sending DTP messages.
Why candidates choose this
Candidates may find this option tempting because they might confuse the behavior of PortFast with trunking configurations, mistakenly believing that any BPDU received would trigger a trunk negotiation instead of causing the port to be error-disabled.
✗The port ignores the BPDU because PortFast disables STP entirely.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because PortFast does not disable Spanning Tree Protocol (STP) entirely; it merely allows the port to transition to the forwarding state immediately without participating in STP calculations. BPDUs are still processed, and BPDU Guard will take action if they are received.
★ When this WOULD be the correct answer
In a different scenario where a switch port is configured with PortFast but not with BPDU Guard, a question could ask what happens when a small unmanaged switch sends BPDUs. In that case, the port would ignore the BPDUs and remain in the forwarding state, making this option correct.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of PortFast's function, believing it completely disables STP and thus would not react to BPDUs, leading to the assumption that the port would ignore them.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Remember, BPDU Guard disables the port, it doesn't use spanning-tree states like blocking or learning.
Detailed technical explanation
How to think about this question
PortFast is a Cisco feature applied to switch ports connected directly to end devices, such as workstations or servers, to speed up the transition to the forwarding state by bypassing the usual Spanning Tree Protocol (STP) listening and learning states. This reduces the delay in port activation, allowing devices to connect and communicate faster. However, PortFast does not disable STP entirely; it only changes the port's behavior to assume it is an edge port, which should not receive Bridge Protocol Data Units (BPDUs).
BPDU Guard is a complementary STP security feature designed to protect the network topology by shutting down a PortFast-enabled port if it receives any BPDUs. This is because receiving a BPDU on a PortFast port indicates that the port is connected to another switch or device that could cause loops or topology changes. When BPDU Guard detects BPDUs on such a port, it places the port into an error-disabled state to prevent potential STP topology issues.
A common exam trap is misunderstanding the interaction between PortFast and BPDU Guard. Some may incorrectly believe that PortFast disables STP and thus ignores BPDUs, or that the port automatically becomes a root port or trunk upon receiving BPDUs. In reality, BPDU Guard enforces strict protection by disabling the port immediately, preventing loops and preserving network stability. This behavior is critical in enterprise access-layer designs to maintain fast connectivity while safeguarding the STP topology.
KKey Concepts to Remember
PortFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity.
BPDU Guard monitors PortFast-enabled ports and disables the port if any BPDUs are received, protecting the network from potential loops.
Receiving a BPDU on a PortFast port signals that the port is connected to another switch or device, which is unexpected and triggers BPDU Guard.
When BPDU Guard activates, it places the port into an error-disabled state, effectively shutting down the port to prevent topology changes.
PortFast does not disable STP entirely; it only changes port behavior to assume it is connected to an end device, not another switch.
A port receiving BPDUs does not automatically become a root port or convert to a trunk port; STP roles and port modes are not changed by BPDU Guard.
BPDU Guard is commonly used in enterprise access layers to combine fast host connectivity with protection against misconfigurations or unauthorized switches.
The error-disabled state caused by BPDU Guard requires manual intervention or errdisable recovery to restore port functionality.
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
PortFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review portFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — PortFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity..
What is the correct answer to this question?
The correct answer is: The port is error-disabled by BPDU Guard. — The most likely result is that the port is placed into an err-disabled state by BPDU Guard. In practical terms, PortFast tells the switch to treat the interface like an edge port for a normal endpoint, which is why it starts forwarding quickly. BPDU Guard protects that assumption. If the port suddenly receives a spanning-tree BPDU, the switch treats that as a sign that the port is no longer connected to a simple end device.
This combination is common in enterprise access-layer design because it improves user startup time while still protecting the topology. The correct answer is the one that describes the port being shut down automatically when BPDUs appear unexpectedly.
What should I do if I get this 200-301 question wrong?
Review portFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity., then practise related 200-301 questions on the same topic to reinforce the concept.
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?
PortFast enables a switch port to transition immediately to forwarding state, bypassing STP listening and learning states for faster host connectivity.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A switch port connected to an end host is configured with both PortFast and BPDU Guard. What is the most likely outcome if a small switch is connected there and starts sending BPDUs?
hard
✓ A.The port is error-disabled by BPDU Guard.
B.The port automatically becomes the root port.
C.The port converts into a trunk for the attached switch.
D.The port ignores the BPDU because PortFast disables STP entirely.
Why A: BPDU Guard places the port into an error-disabled state upon receiving a BPDU, because PortFast defines the port as an edge port that should never receive BPDUs. Option B is incorrect because receiving a BPDU does not automatically make a port a root port; root port selection depends on bridge ID and path cost, and BPDU Guard prevents further STP processing by disabling the port. Option C is incorrect because a port cannot convert to a trunk solely by receiving a BPDU; trunking requires manual configuration or Dynamic Trunking Protocol (DTP). Option D is incorrect because PortFast does not disable STP entirely; it only speeds up initial convergence, and BPDU Guard actively responds to BPDUs by error-disabling the port.
Last reviewed: May 17, 2026
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