The correct answer is to remove the spanning-tree portfast trunk command from the interface. This is because enabling PortFast on a trunk port bypasses the normal spanning-tree listening and learning states, which causes the port to immediately transition to forwarding. When the core switch sends a BPDU across the trunk, the BPDU guard feature—often enabled by default with PortFast—detects this as a violation and places the port into errdisable state, leading to repeated state transitions. On the Cisco DevNet Associate 200-901 exam, this scenario tests your understanding of how spanning-tree features interact with trunk ports, and it is a common trap to assume PortFast is only for access ports. Remember that PortFast is designed for end-host ports, not for links between switches, so applying it to a trunk disrupts normal BPDU exchange. A useful memory tip: "PortFast on a trunk is a fast path to errdisable."
200-901 Network Fundamentals Practice Question
This 200-901 practice question tests your understanding of network fundamentals. 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
Refer to the exhibit.
"""
interface GigabitEthernet0/1
description Uplink to Core
switchport mode trunk
switchport trunk allowed vlan 10,20,30
spanning-tree portfast trunk
"""
Refer to the exhibit. A switch is configured with the shown trunk port. After connecting the uplink, the switch logs show repeated 'errdisable' state transitions on this port. The core switch is configured with the same allowed VLAN list. Which configuration change is most likely to resolve the issue?
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
✓
Remove the spanning-tree portfast trunk command from the interface.
The 'errdisable' state transitions on a trunk port are typically caused by a spanning-tree BPDU guard violation when PortFast is enabled. The 'spanning-tree portfast trunk' command enables PortFast on the trunk, which bypasses the normal listening/learning states and can cause the port to be placed into errdisable state if a BPDU is received from the core switch. Removing this command allows the trunk port to participate in standard spanning-tree convergence, preventing the repeated errdisable transitions.
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.
✗
Change the switchport mode to dynamic desirable.
Why it's wrong here
The core is likely configured as trunk, so dynamic desirable may cause negotiation issues, not errdisable.
✗
Add VLAN 1 to the allowed VLAN list.
Why it's wrong here
VLAN 1 is native by default and does not need to be explicitly allowed; adding it would not fix errdisable.
✓
Remove the spanning-tree portfast trunk command from the interface.
Why this is correct
Portfasttrunk is designed for host-facing trunks (e.g., to servers) and can cause STP issues when connecting to another switch.
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.
✗
Add the spanning-tree bpduguard enable command to the interface.
Why it's wrong here
BPDU guard would disable the port if it receives BPDUs, which is normal on a trunk to another switch, worsening the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that 'spanning-tree portfasttrunk' is safe for trunk ports, but the trap is that PortFast combined with BPDU guard (even if not explicitly configured, but enabled globally) causes errdisable when BPDUs are received, so the fix is to remove PortFast from the trunk.
Detailed technical explanation
How to think about this question
PortFast is designed for access ports to skip spanning-tree convergence, but when applied to a trunk port via 'spanning-tree portfast trunk', it still bypasses the listening/learning states. If BPDU guard is enabled (either globally or per-interface), receiving a BPDU on a PortFast-enabled port triggers errdisable. In this scenario, the core switch sends BPDUs on the trunk, causing the errdisable loop. Removing PortFast allows the trunk to properly process BPDUs and participate in STP, which stabilizes the link.
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.
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Remove the spanning-tree portfast trunk command from the interface. — The 'errdisable' state transitions on a trunk port are typically caused by a spanning-tree BPDU guard violation when PortFast is enabled. The 'spanning-tree portfast trunk' command enables PortFast on the trunk, which bypasses the normal listening/learning states and can cause the port to be placed into errdisable state if a BPDU is received from the core switch. Removing this command allows the trunk port to participate in standard spanning-tree convergence, preventing the repeated errdisable transitions.
What should I do if I get this 200-901 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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