N10-009 Network Implementation Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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 encapsulation were incorrectly configured or mismatched (e.g., not set to 802.1Q), the trunk link would typically fail to establish or operate correctly, preventing any traffic flow. However, the output 'allowed VLANs: none' specifically points to a logical configuration within an *established* trunking interface, rather than a fundamental link-layer negotiation failure. The encapsulation type is a prerequisite for trunk functionality, not a control for which VLANs are permitted once the trunk is up.
When this WOULD be correct
In a scenario where a trunk fails to form or comes up but does not pass any traffic, and 'show interfaces trunk' does not show 'allowed VLANs: none' but instead shows the trunk is down or not operational, the most likely cause could be an encapsulation mismatch (e.g., one side set to dot1q and the other to ISL).
- ✗
The native VLAN mismatch
Why it's wrong here
A native VLAN mismatch between two trunking ports typically results in connectivity problems for untagged frames traversing the link, and potentially generates console error messages or STP inconsistencies. However, it does not inherently prevent all VLANs from being allowed on the trunk. The 'allowed VLANs: none' output specifically indicates a configuration issue with the permitted VLAN list, not an untagged frame handling discrepancy.
- ✓
The allowed VLAN list is empty
Why this is correct
The explicit output "allowed VLANs: none" directly indicates that the trunk port is currently configured to permit no VLANs to traverse it. Even if VLAN 10 exists on both switches, it cannot pass traffic across this trunk until it is explicitly added to the allowed VLAN list for that interface. This configuration prevents any tagged or untagged traffic from being forwarded over the trunk, effectively isolating the connected network segments.
- ✗
VLAN 10 is not created on one of the switches
Why it's wrong here
The problem statement explicitly indicates that VLAN 10 exists on both switches involved in the trunk configuration. If VLAN 10 were indeed missing on one of the switches, devices in that VLAN would experience connectivity issues to resources in VLAN 10 on the other switch, but the trunk itself would still be operational and potentially allow other VLANs. An absent VLAN definition does not cause the trunk's allowed VLAN list to become empty.
When this WOULD be correct
If the question stated that VLAN 10 traffic is not passing and the engineer did not verify VLAN existence, or if the show vlan command showed VLAN 10 missing on one switch, then this would be the correct answer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓The allowed VLAN list is emptyCorrect answer▾
Why this is correct
The explicit output "allowed VLANs: none" directly indicates that the trunk port is currently configured to permit no VLANs to traverse it. Even if VLAN 10 exists on both switches, it cannot pass traffic across this trunk until it is explicitly added to the allowed VLAN list for that interface. This configuration prevents any tagged or untagged traffic from being forwarded over the trunk, effectively isolating the connected network segments.
✗The trunk encapsulation is not set to dot1qWrong answer — click to see why▾
Why this is wrong here
The trunk is up and the 'show interfaces trunk' output shows 'allowed VLANs: none', which directly indicates an empty allowed VLAN list, not an encapsulation issue. If the encapsulation were wrong, the trunk would not form or would show encapsulation mismatch errors.
★ When this WOULD be the correct answer
In a scenario where a trunk fails to form or comes up but does not pass any traffic, and 'show interfaces trunk' does not show 'allowed VLANs: none' but instead shows the trunk is down or not operational, the most likely cause could be an encapsulation mismatch (e.g., one side set to dot1q and the other to ISL).
Why candidates choose this
Candidates often confuse trunk configuration issues; they know that 802.1Q encapsulation is required for VLAN tagging, so they assume an encapsulation mismatch is the problem without checking the allowed VLAN list first.
✗VLAN 10 is not created on one of the switchesWrong answer — click to see why▾
Why this is wrong here
The question states that VLAN 10 exists on both switches, so this option is factually incorrect based on the given information.
★ When this WOULD be the correct answer
If the question stated that VLAN 10 traffic is not passing and the engineer did not verify VLAN existence, or if the show vlan command showed VLAN 10 missing on one switch, then this would be the correct answer.
Why candidates choose this
Candidates may assume that VLAN traffic issues are often due to missing VLAN definitions, especially when troubleshooting inter-switch connectivity.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Related to this question
Learn chapter
Ethernet Switching and VLANs
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
802.1Q
802.1Q is the networking standard that allows multiple virtual LANs (VLANs) to share a single physical network link by tagging Ethernet frames with VLAN identification information.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.