The answer is that the allowed VLAN list is empty, which is the most likely cause of the VLAN 10 traffic failure. When the `show interfaces trunk` command displays 'allowed VLANs: none', it means the trunk’s VLAN filter has been manually cleared or set to an empty list, blocking all VLAN frames—including VLAN 10—regardless of whether that VLAN exists on both switches. This scenario tests your understanding of 802.1Q trunk configuration and the critical role of the allowed VLAN list in controlling which VLANs traverse the link. On the CompTIA Network+ N10-009 exam, this is a common trap: students often overlook the allowed VLAN list and instead check for mismatched native VLANs or encapsulation errors, but the output explicitly confirms the filter is the culprit. A useful memory tip is to think of the allowed VLAN list as a bouncer at a club—if the list is empty, no traffic gets in, no matter who shows up.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. 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 configures an 802.1Q trunk between two switches. The trunk is up, but VLAN 10 traffic is not passing. The engineer checks and confirms that VLAN 10 exists on both switches. The show interfaces trunk command displays 'allowed VLANs: none'. What is the most likely cause?
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 allowed VLAN list is empty
The 'show interfaces trunk' output showing 'allowed VLANs: none' explicitly indicates that the allowed VLAN list on the trunk has been manually cleared or set to none, which blocks all VLAN traffic including VLAN 10. Even though VLAN 10 exists on both switches, the trunk port's VLAN filter prevents any frames from being forwarded. This is the most direct cause of the issue.
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.
✗
The trunk encapsulation is not set to dot1q
Why it's wrong here
If the trunk were up, the encapsulation is already set correctly. The issue is the allowed VLAN list.
✗
The native VLAN mismatch
Why it's wrong here
A native VLAN mismatch usually causes connectivity issues for untagged frames but does not result in 'allowed VLANs: none'.
✓
The allowed VLAN list is empty
Why this is correct
The output explicitly shows 'allowed VLANs: none', meaning no VLANs are permitted on the trunk. The engineer must add VLAN 10 to the allowed list.
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.
✗
VLAN 10 is not created on one of the switches
Why it's wrong here
The scenario states that VLAN 10 exists on both switches, so this is not the cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between 'VLAN not created' and 'VLAN not allowed on trunk' — the trap here is that candidates assume VLAN 10 not passing must mean it doesn't exist on one switch, ignoring that the trunk's allowed VLAN list can independently block traffic even when the VLAN is present on both sides.
Trap categories for this question
Scenario analysis trap
The scenario states that VLAN 10 exists on both switches, so this is not the cause.
Detailed technical explanation
How to think about this question
The 'allowed VLANs' list on an 802.1Q trunk is controlled by the 'switchport trunk allowed vlan' command; setting it to 'none' (via 'switchport trunk allowed vlan none' or 'remove' all) effectively prunes every VLAN from the trunk, causing the port to drop all tagged and untagged frames except those for the native VLAN (if not explicitly removed). In real-world scenarios, this often happens when an engineer accidentally applies 'switchport trunk allowed vlan none' while troubleshooting or misconfigures a VLAN ACL, leading to silent traffic loss that is easily missed without checking the trunk's allowed list.
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 Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The allowed VLAN list is empty — The 'show interfaces trunk' output showing 'allowed VLANs: none' explicitly indicates that the allowed VLAN list on the trunk has been manually cleared or set to none, which blocks all VLAN traffic including VLAN 10. Even though VLAN 10 exists on both switches, the trunk port's VLAN filter prevents any frames from being forwarded. This is the most direct cause of the issue.
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: "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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