The answer is that VLAN 20 is not included in the allowed VLAN list on the trunk. This is the most likely cause because an 802.1Q trunk link, while operational for VLAN 10, will only forward traffic for VLANs explicitly permitted in its allowed VLAN list; if that list was manually configured or pruned after the trunk was established, VLAN 20 would be silently dropped even though it exists on both switches. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding that a trunk’s allowed VLAN list acts as a traffic filter—a common trap is assuming a working trunk passes all VLANs by default, when in fact administrators often restrict the list for security or efficiency. Remember the memory tip: “Allowed means permitted, not present”—just because a VLAN is created on both switches doesn’t mean the trunk will carry it.
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 technician configures an 802.1Q trunk link between two switches. After the configuration, VLAN 20 traffic is not passing across the trunk, although VLAN 10 traffic works fine. The technician verifies that both switches have VLAN 20 created and that the trunk is up. 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.
The native VLAN is mismatched between the switches.
Why wrong: A native VLAN mismatch causes issues for untagged frames, but it would not block a specific VLAN if the allowed list is properly configured.
B
VLAN 20 is not included in the allowed VLAN list on the trunk.
By default, trunk ports allow all VLANs, but if an allowed VLAN list is configured, only listed VLANs are forwarded. VLAN 20 is likely missing.
C
The trunk port is set to dynamic desirable mode.
Why wrong: Dynamic Desirable is a DTP mode that negotiates trunking; it does not filter VLANs.
D
The encapsulation type is ISL instead of 802.1Q.
Why wrong: If both switches are configured for 802.1Q, the encapsulation matches, and this would not cause selective VLAN blocking.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 20 is not included in the allowed VLAN list on the trunk.
The most likely cause is that VLAN 20 is not included in the allowed VLAN list on the trunk. By default, an 802.1Q trunk allows all VLANs, but if the allowed VLAN list has been manually configured or pruned, VLAN 20 may have been excluded. Since VLAN 10 works, the trunk is operational, and both switches have VLAN 20 created, the issue is specifically that the trunk is not permitting VLAN 20 traffic.
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 native VLAN is mismatched between the switches.
Why it's wrong here
A native VLAN mismatch causes issues for untagged frames, but it would not block a specific VLAN if the allowed list is properly configured.
✓
VLAN 20 is not included in the allowed VLAN list on the trunk.
Why this is correct
By default, trunk ports allow all VLANs, but if an allowed VLAN list is configured, only listed VLANs are forwarded. VLAN 20 is likely missing.
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.
✗
The trunk port is set to dynamic desirable mode.
Why it's wrong here
Dynamic Desirable is a DTP mode that negotiates trunking; it does not filter VLANs.
✗
The encapsulation type is ISL instead of 802.1Q.
Why it's wrong here
If both switches are configured for 802.1Q, the encapsulation matches, and this would not cause selective VLAN blocking.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that a native VLAN mismatch is the default cause of VLAN-specific traffic failures, but the real issue is usually the allowed VLAN list being misconfigured or pruned.
Detailed technical explanation
How to think about this question
On Cisco switches, the 'switchport trunk allowed vlan' command defines which VLANs are permitted on a trunk. If VLAN 20 is not in this list, frames tagged with VLAN 20 are dropped at the egress. This is often caused by an administrator using 'switchport trunk allowed vlan 10' (which implicitly removes all other VLANs) or by VTP pruning if enabled. The 'show interfaces trunk' command would reveal the allowed VLAN list and whether VLAN 20 is pruned or excluded.
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: VLAN 20 is not included in the allowed VLAN list on the trunk. — The most likely cause is that VLAN 20 is not included in the allowed VLAN list on the trunk. By default, an 802.1Q trunk allows all VLANs, but if the allowed VLAN list has been manually configured or pruned, VLAN 20 may have been excluded. Since VLAN 10 works, the trunk is operational, and both switches have VLAN 20 created, the issue is specifically that the trunk is not permitting VLAN 20 traffic.
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.
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. Two switches are connected via a trunk link configured with allowed VLANs 10, 20, and 30. Hosts on VLAN 20 can communicate across the trunk, but hosts on VLAN 30 cannot. Which of the following is the most likely cause?
medium
A.A
B.B
C.C
✓ D.D
Why D: Option D is correct because the most likely cause is that VLAN 30 is not allowed on the trunk link. Even though the trunk is configured with allowed VLANs 10, 20, and 30, if VLAN 30 is not present in the allowed VLAN list on one of the switch ports (e.g., due to a missing 'switchport trunk allowed vlan add 30' command or a pruning issue), frames from VLAN 30 will be dropped at the trunk. This explains why hosts on VLAN 20 can communicate while those on VLAN 30 cannot.
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Question Discussion
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