N10-009 Network Implementation Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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 occurs when the untagged VLAN configured on one end of a trunk link differs from the other. This condition primarily affects untagged frames, which are typically management traffic or frames from devices not aware of VLANs, causing them to be dropped or misdirected. However, 802.1Q tags explicitly identify VLAN 20 traffic, and a native VLAN mismatch would not selectively block only this tagged VLAN while allowing others to pass.
When this WOULD be correct
If the question described that all VLAN traffic fails or only untagged traffic fails across the trunk, and both switches have the same VLANs configured, then a native VLAN mismatch would be the most likely cause.
- ✓
VLAN 20 is not included in the allowed VLAN list on the trunk.
Why this is correct
Trunk links, by default, permit all VLANs to traverse them. However, network administrators often configure an 'allowed VLAN list' to restrict which VLANs are permitted on a specific trunk port for security or traffic management purposes. If VLAN 20 is explicitly excluded or simply not included in this configured list on either end of the trunk, its traffic will be dropped, preventing communication for devices in that VLAN across the link.
- ✗
The trunk port is set to dynamic desirable mode.
Why it's wrong here
Dynamic Desirable is a Cisco-proprietary Dynamic Trunking Protocol (DTP) mode used to actively attempt to form a trunk link with a neighboring switch. While DTP helps in negotiating the trunking state, it does not possess any functionality to filter or block specific VLANs from traversing an established trunk. Therefore, this mode would not be the cause of selective blocking of VLAN 20 traffic, as its role is solely in trunk link establishment.
When this WOULD be correct
A question where two switches fail to form a trunk link entirely, and one switch is set to dynamic desirable while the other is set to dynamic auto or access mode, preventing trunk establishment.
- ✗
The encapsulation type is ISL instead of 802.1Q.
Why it's wrong here
Encapsulation type refers to the tagging protocol used on a trunk link, with 802.1Q being the industry standard and ISL being Cisco proprietary. If one switch is configured for 802.1Q and the other for ISL, or if there's any mismatch in the configured encapsulation, the trunk link would fail to establish or pass *any* VLAN-tagged traffic effectively. This fundamental incompatibility would prevent all VLAN communication, not just selectively block VLAN 20 while others function normally.
When this WOULD be correct
If the question described a scenario where two switches are configured with different trunk encapsulation types (e.g., one using ISL and the other 802.1Q), then the mismatch would prevent trunking and cause traffic issues across all VLANs.
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.
✓VLAN 20 is not included in the allowed VLAN list on the trunk.Correct answer▾
Why this is correct
Trunk links, by default, permit all VLANs to traverse them. However, network administrators often configure an 'allowed VLAN list' to restrict which VLANs are permitted on a specific trunk port for security or traffic management purposes. If VLAN 20 is explicitly excluded or simply not included in this configured list on either end of the trunk, its traffic will be dropped, preventing communication for devices in that VLAN across the link.
✗The native VLAN is mismatched between the switches.Wrong answer — click to see why▾
Why this is wrong here
A native VLAN mismatch would cause traffic on the native VLAN (typically VLAN 1) to fail, not specifically VLAN 20. Since VLAN 10 works, the native VLAN is likely correct.
★ When this WOULD be the correct answer
If the question described that all VLAN traffic fails or only untagged traffic fails across the trunk, and both switches have the same VLANs configured, then a native VLAN mismatch would be the most likely cause.
Why candidates choose this
Candidates often confuse native VLAN mismatch with general VLAN traffic issues, assuming any trunk problem is due to native VLAN mismatch without considering allowed VLAN lists.
✗The trunk port is set to dynamic desirable mode.Wrong answer — click to see why▾
Why this is wrong here
In this scenario, VLAN 10 traffic works fine, indicating the trunk is operational. Dynamic desirable mode would not cause selective VLAN failure; it affects trunk negotiation, not per-VLAN traffic filtering.
★ When this WOULD be the correct answer
A question where two switches fail to form a trunk link entirely, and one switch is set to dynamic desirable while the other is set to dynamic auto or access mode, preventing trunk establishment.
Why candidates choose this
Candidates may confuse dynamic trunking protocol (DTP) modes with VLAN traffic filtering, assuming that dynamic desirable mode could restrict specific VLANs, when it only controls trunk negotiation.
✗The encapsulation type is ISL instead of 802.1Q.Wrong answer — click to see why▾
Why this is wrong here
The question states that the trunk is configured with 802.1Q, so ISL encapsulation is not in use. If ISL were the issue, the trunk would likely not form or would have problems with all VLANs, not just VLAN 20.
★ When this WOULD be the correct answer
If the question described a scenario where two switches are configured with different trunk encapsulation types (e.g., one using ISL and the other 802.1Q), then the mismatch would prevent trunking and cause traffic issues across all VLANs.
Why candidates choose this
Candidates may confuse encapsulation mismatches with VLAN-specific issues, or they might think that ISL and 802.1Q incompatibility could selectively affect certain VLANs.
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?”
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Related to this question
Learn chapter
Ethernet Switching and VLANs
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
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.