- A
A: The switchport mode for VLAN 20 is set to access
Why wrong: The trunk port mode is configured for the uplink, not per-VLAN access mode. Setting the port to access would break the trunk entirely.
- B
B: VLAN 20 is not allowed on the trunk
If VLAN 20 is pruned or not included in the allowed list on the trunk, frames from VLAN 20 cannot reach the router.
- C
C: The router needs to be rebooted to apply the subinterface configuration
Why wrong: Router subinterfaces typically take effect immediately after configuration; a reboot is not required.
- D
D: The native VLAN on the trunk is misconfigured
Why wrong: The native VLAN handles untagged traffic, but both VLANs are tagged; native VLAN mismatch would cause a different issue, not complete lack of communication for a specific VLAN.
N10-009 Trunk links carry traffic for multiple VLANs. 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. A key principle to apply: trunk links carry traffic for multiple VLANs.. 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 configures VLAN 10 (Sales) and VLAN 20 (Engineering) on a switch. The switch is connected to a router via a trunk interface for inter-VLAN routing. Users in VLAN 10 can reach the router and external networks, but users in VLAN 20 cannot communicate outside their VLAN. The router's subinterface for VLAN 20 is configured correctly with an IP address and encapsulation dot1Q 20. What is the MOST likely cause of 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 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
B: VLAN 20 is not allowed on the trunk
The router's subinterface for VLAN 20 is correctly configured, so the issue lies on the switch side. If VLAN 20 is not explicitly allowed on the trunk interface connecting the switch to the router, frames from VLAN 20 will be dropped by the switch, preventing inter-VLAN routing. The default trunk allowed VLAN list often includes only VLAN 1, so VLAN 20 must be added with the 'switchport trunk allowed vlan add 20' command.
Key principle: Trunk links carry traffic for multiple VLANs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A: The switchport mode for VLAN 20 is set to access
Why it's wrong here
The trunk port mode is configured for the uplink, not per-VLAN access mode. Setting the port to access would break the trunk entirely.
- ✓
B: VLAN 20 is not allowed on the trunk
- ✗
C: The router needs to be rebooted to apply the subinterface configuration
Why it's wrong here
Router subinterfaces typically take effect immediately after configuration; a reboot is not required.
- ✗
D: The native VLAN on the trunk is misconfigured
Why it's wrong here
The native VLAN handles untagged traffic, but both VLANs are tagged; native VLAN mismatch would cause a different issue, not complete lack of communication for a specific VLAN.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between the router subinterface being correctly configured and the switch trunk not permitting the VLAN, leading candidates to incorrectly suspect the router or native VLAN settings.
Detailed technical explanation
How to think about this question
On a Cisco switch, the 'switchport trunk allowed vlan' command controls which VLANs are permitted to traverse a trunk link; by default, only VLAN 1 is allowed unless explicitly modified. If VLAN 20 is missing from this list, the switch will drop all frames tagged with VLAN 20, even if the router's subinterface is correctly configured with encapsulation dot1Q 20. In real-world scenarios, forgetting to add a new VLAN to the trunk allowed list is a common misconfiguration when scaling VLANs across multiple switches.
KKey Concepts to Remember
- Trunk links carry traffic for multiple VLANs.
- VLANs must be explicitly allowed on a trunk interface.
- VLAN pruning prevents specific VLAN traffic from traversing a trunk.
- Inter-VLAN routing requires VLAN traffic to reach the router's subinterface.
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
Trunk links carry traffic for multiple VLANs.
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 trunk links carry traffic for multiple VLANs., then practise related N10-009 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Implementation — This question tests Network Implementation — Trunk links carry traffic for multiple VLANs..
What is the correct answer to this question?
The correct answer is: B: VLAN 20 is not allowed on the trunk — The router's subinterface for VLAN 20 is correctly configured, so the issue lies on the switch side. If VLAN 20 is not explicitly allowed on the trunk interface connecting the switch to the router, frames from VLAN 20 will be dropped by the switch, preventing inter-VLAN routing. The default trunk allowed VLAN list often includes only VLAN 1, so VLAN 20 must be added with the 'switchport trunk allowed vlan add 20' command.
What should I do if I get this N10-009 question wrong?
Review trunk links carry traffic for multiple VLANs., then practise related N10-009 questions on the same topic to reinforce the concept.
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?
Trunk links carry traffic for multiple VLANs.
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Last reviewed: Jun 30, 2026
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
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