The answer is the `show interfaces trunk` command, because it directly displays the allowed VLAN list on a trunk port, which is the exact information needed when verifying trunk allowed VLAN list configurations. When a trunk link is up/up but a specific VLAN fails to pass traffic, the most common cause is that the VLAN is missing from the allowed list on one or both ends, and this command reveals which VLANs are permitted, active, or pruned. On the CompTIA Network+ N10-009 exam, this scenario tests your ability to isolate Layer 2 trunking issues, often as a distractor from checking native VLAN mismatches or encapsulation mismatches. A common trap is to run `show vlan brief` or `show interfaces status`, but those do not show the per-trunk VLAN filter. Remember the memory tip: "Trunk talk shows the walk" — meaning `show interfaces trunk` walks you through every VLAN allowed on the trunk.
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. 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 administrator is troubleshooting communication between two switches connected via a trunk port. The trunk link is up/up, but devices in VLAN 20 cannot communicate across the trunk. The administrator has verified that both switches have VLAN 20 created and that the access ports are configured correctly. Which command should the administrator run on each switch to verify the trunk's allowed VLAN list?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Why wrong: This command shows VLAN information but does not display trunk-specific allowed VLAN lists for each trunk interface.
B
show interfaces trunk
Correct. This command displays detailed trunk status, including the allowed VLAN list, native VLAN, and trunking mode, helping to verify if VLAN 20 is allowed.
C
show running-config interface
Why wrong: While this shows the configuration, it may not show the default allowed VLAN list if not explicitly configured; 'show interfaces trunk' is more direct.
D
show mac address-table
Why wrong: This command shows MAC address table entries, not trunk configuration.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
show interfaces trunk
The 'show interfaces trunk' command displays the trunking status, including the allowed VLAN list on the trunk port. Since the trunk link is up/up but VLAN 20 traffic fails, the most likely cause is that VLAN 20 is not included in the allowed VLAN list on one or both switches. This command directly shows which VLANs are permitted, pruned, or active on the trunk.
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.
✗
show vlan brief
Why it's wrong here
This command shows VLAN information but does not display trunk-specific allowed VLAN lists for each trunk interface.
✓
show interfaces trunk
Why this is correct
Correct. This command displays detailed trunk status, including the allowed VLAN list, native VLAN, and trunking mode, helping to verify if VLAN 20 is allowed.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
show running-config interface
Why it's wrong here
While this shows the configuration, it may not show the default allowed VLAN list if not explicitly configured; 'show interfaces trunk' is more direct.
✗
show mac address-table
Why it's wrong here
This command shows MAC address table entries, not trunk configuration.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between 'show vlan brief' (which shows VLAN existence and access port assignments) and 'show interfaces trunk' (which shows trunk-specific VLAN permissions), leading candidates to mistakenly check VLAN existence instead of trunk VLAN filtering.
Trap categories for this question
Command / output trap
This command shows VLAN information but does not display trunk-specific allowed VLAN lists for each trunk interface.
Detailed technical explanation
How to think about this question
The 'show interfaces trunk' command reveals the trunking mode, encapsulation, and the allowed VLAN list (both configured and active). In a real-world scenario, a common issue is that VLAN 20 is created but not added to the trunk's allowed VLAN list, or it is being pruned by VTP. The command also shows the 'Vlans allowed on trunk' and 'Vlans active in management domain' fields, which help isolate whether the problem is a missing VLAN in the allowed list or a VTP pruning issue.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: show interfaces trunk — The 'show interfaces trunk' command displays the trunking status, including the allowed VLAN list on the trunk port. Since the trunk link is up/up but VLAN 20 traffic fails, the most likely cause is that VLAN 20 is not included in the allowed VLAN list on one or both switches. This command directly shows which VLANs are permitted, pruned, or active on the trunk.
What should I do if I get this N10-009 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: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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. After replacing a faulty switch, several users in the same VLAN report that they cannot communicate with the server that is on a different subnet. The switch is connected to the router via a trunk port. Which command should the administrator run on the router to verify that the VLAN is allowed on the trunk?
medium
A.show vlan
✓ B.show interfaces trunk
C.show mac address-table
D.show ip route
Why B: The `show interfaces trunk` command displays which VLANs are allowed on each trunk port. Since the switch is connected to the router via a trunk, this command verifies whether the VLAN of the affected users is permitted on that trunk. If the VLAN is missing from the allowed list, traffic to the server on a different subnet will be dropped at the router interface.
Last reviewed: Jun 11, 2026
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