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Question 471 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network administrator recently configured BPDU…
This 200-301 practice question tests your understanding of 200-301 exam topics. 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.
Exhibit
SW1# show interfaces status Port Name Status Vlan Duplex Speed Type Gi0/1 err-disabled 10 auto auto 10/100/1000BaseTX Gi0/2 err-disabled 10 auto auto 10/100/1000BaseTX Gi0/3 err-disabled 10 auto auto 10/100/1000BaseTX Gi0/4 err-disabled 10 auto auto 10/100/1000BaseTX Gi0/5 connected trunk auto auto 10/100/1000BaseTX Gi0/6 connected 1 auto auto 10/100/1000BaseTX Gi0/7 connected 1 auto auto 10/100/1000BaseTX SW1# show running-config interface gi0/1 Building configuration... Current configuration : 83 bytes ! interface GigabitEthernet0/1 switchport mode access switchport access vlan 10 spanning-tree bpduguard enable end SW1# show running-config | include bpduguard spanning-tree portfast bpduguard default
A network administrator recently configured BPDU Guard on all access ports of a switch to protect against rogue switches. After the change, users in VLAN 10 report intermittent connectivity issues and frequent link flaps. The administrator checks the switch and notices that several ports are in an err-disabled state. What is the most likely cause of the problem?
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
✓
BPDU Guard is enabled on ports that are not in PortFast mode, causing err-disable when BPDUs are received.
The global command 'spanning-tree portfast bpduguard default' enables BPDU Guard on all PortFast-enabled ports. However, the ports in question (Gi0/1-4) are not configured with PortFast, but BPDU Guard is explicitly enabled on the interface. When BPDU Guard is enabled on a port without PortFast, the switch still monitors for BPDUs but does not automatically place the port into PortFast mode. If a BPDU is received on that port (e.g., from a legitimate switch or a misconfigured device), the port is immediately err-disabled. In this scenario, the ports are access ports in VLAN 10, likely connected to end devices that may be generating BPDUs (e.g., a switch with BPDU Guard disabled). The correct fix is to ensure that BPDU Guard is only applied to ports that are in PortFast mode, or to remove BPDU Guard from ports that are not PortFast. Option B is correct because the ports are not in PortFast mode, so BPDU Guard should not be applied. Option A is wrong because the issue is not due to Root Guard. Option C is wrong because the problem is not related to Loop Guard. Option D is wrong because BPDU Guard is enabled, but the ports are not in PortFast mode, which is the root cause.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Root Guard is preventing the port from becoming a root port.
Why it's wrong here
Root Guard is used to prevent a port from becoming a root port; it would not cause err-disabled state.
- ✓
BPDU Guard is enabled on ports that are not in PortFast mode, causing err-disable when BPDUs are received.
Why this is correct
BPDU Guard is designed to work with PortFast; if enabled on non-PortFast ports, any BPDU received will err-disable the port.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
Loop Guard has detected a unidirectional link and placed the port into err-disabled state.
Why it's wrong here
Loop Guard prevents alternate or root ports from becoming designated in the absence of BPDUs, but it does not err-disable ports.
- ✗
BPDU Guard is globally enabled but not configured on the interface, so the port is err-disabled due to a BPDU received.
Why it's wrong here
BPDU Guard is explicitly enabled on the interface (spanning-tree bpduguard enable), not just globally. The global command only applies to PortFast ports.
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.
✓BPDU Guard is enabled on ports that are not in PortFast mode, causing err-disable when BPDUs are received.Correct answer▾
Why this is correct
BPDU Guard is designed to work with PortFast; if enabled on non-PortFast ports, any BPDU received will err-disable the port.
✗Root Guard is preventing the port from becoming a root port.Wrong answer — click to see why▾
Why this is wrong here
Root Guard does not err-disable ports; it only keeps the port in a blocked state if a superior BPDU is received.
✗Loop Guard has detected a unidirectional link and placed the port into err-disabled state.Wrong answer — click to see why▾
Why this is wrong here
Loop Guard places the port into a blocking state (not err-disabled) when BPDUs are missing.
✗BPDU Guard is globally enabled but not configured on the interface, so the port is err-disabled due to a BPDU received.Wrong answer — click to see why▾
Why this is wrong here
The interface configuration shows BPDU Guard enabled, so it is not a global-only issue.
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
BPDU Guard is explicitly enabled on the interface (spanning-tree bpduguard enable), not just globally. The global command only applies to PortFast ports.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Access ports place end devices into a single VLAN.
What is the correct answer to this question?
The correct answer is: BPDU Guard is enabled on ports that are not in PortFast mode, causing err-disable when BPDUs are received. — The global command 'spanning-tree portfast bpduguard default' enables BPDU Guard on all PortFast-enabled ports. However, the ports in question (Gi0/1-4) are not configured with PortFast, but BPDU Guard is explicitly enabled on the interface. When BPDU Guard is enabled on a port without PortFast, the switch still monitors for BPDUs but does not automatically place the port into PortFast mode. If a BPDU is received on that port (e.g., from a legitimate switch or a misconfigured device), the port is immediately err-disabled. In this scenario, the ports are access ports in VLAN 10, likely connected to end devices that may be generating BPDUs (e.g., a switch with BPDU Guard disabled). The correct fix is to ensure that BPDU Guard is only applied to ports that are in PortFast mode, or to remove BPDU Guard from ports that are not PortFast. Option B is correct because the ports are not in PortFast mode, so BPDU Guard should not be applied. Option A is wrong because the issue is not due to Root Guard. Option C is wrong because the problem is not related to Loop Guard. Option D is wrong because BPDU Guard is enabled, but the ports are not in PortFast mode, which is the root cause.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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