N10-009 Network Implementation Practice Question
A network engineer needs to add a new switch to an existing network. The switch must be configured to support VLANs and trunking. The engineer connects the switch to the existing network via a trunk port. After configuration, the VLANs on the new switch are not receiving traffic from the core network. The core switch shows the trunk is up but no VLANs are allowed. What is the most likely cause?
⚠ Common exam trap
The N10-009 exam often tests the misconception that a trunk being 'up/up' means all VLANs are automatically allowed, when in fact the allowed VLAN list can be explicitly restricted or cleared, causing traffic loss without any physical or encapsulation issues.
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 on the trunk does not include the desired VLANs
The core switch shows the trunk is up but no VLANs are allowed, which directly indicates that the allowed VLAN list on the trunk port does not include the desired VLANs. By default, a trunk port permits all VLANs, but if an administrator explicitly restricts the allowed VLAN list (e.g., with the 'switchport trunk allowed vlan' command), only those VLANs are forwarded. Since the new switch is not receiving traffic, the core switch's trunk likely has an empty or incorrect allowed VLAN list, preventing the desired VLAN traffic from crossing the trunk.
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 mismatch
Why it's wrong here
A native VLAN mismatch occurs when the untagged VLAN on one side of a trunk link differs from the untagged VLAN on the other side. While this mismatch can lead to security vulnerabilities or cause untagged traffic (e.g., control plane protocols) to be misdirected or dropped, it generally does not prevent the trunk from coming up or forwarding *tagged* traffic for other VLANs. Therefore, if the trunk is up but specific VLANs are not passing, a native VLAN mismatch is not the primary cause.
When this WOULD be correct
A network engineer configures a trunk between two switches, and after connecting, devices on the same VLAN cannot communicate across the trunk, but the trunk link is up. The core switch shows the trunk is up, but the native VLAN is different on each end. This would be the correct answer when the symptom is that untagged traffic (like management traffic) fails, but tagged traffic may work.
- ✗
The trunk encapsulation is not set to 802.1Q
Why it's wrong here
Trunk encapsulation protocols, such as IEEE 802.1Q, are fundamental for how VLAN tags are inserted into Ethernet frames for transmission across a trunk link. If there is a mismatch in the encapsulation type between two connected switches, the trunk link would fail to establish or remain in a down state, preventing any VLAN traffic from traversing it. The fact that the trunk is 'up' indicates that the encapsulation method is correctly configured and matched on both ends.
When this WOULD be correct
A network engineer connects two switches with a trunk, but the trunk fails to establish. The core switch uses 802.1Q, while the new switch defaults to ISL. In this scenario, setting the trunk encapsulation to 802.1Q would resolve the issue.
- ✓
The allowed VLAN list on the trunk does not include the desired VLANs
Why this is correct
The allowed VLAN list on a trunk port explicitly defines which VLAN IDs are permitted to traverse that link. Even if the trunk itself is operational and established, any VLAN traffic not included in this configured list will be filtered and dropped by the switch. This mechanism ensures strict control over VLAN propagation, making it a primary reason why specific desired VLANs might fail to communicate across an otherwise functional trunk.
- ✗
The switch port mode is set to access
Why it's wrong here
A switch port configured in access mode is designed to carry traffic for a single, untagged VLAN, typically connecting to an end device. If the port were in access mode, it would be incapable of establishing a trunk link, which requires the ability to carry multiple tagged VLANs. The problem statement implies the trunk is 'up,' which directly contradicts an access mode configuration, as a trunk would not form under such conditions.
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 on the trunk does not include the desired VLANsCorrect answer▾
Why this is correct
The allowed VLAN list on a trunk port explicitly defines which VLAN IDs are permitted to traverse that link. Even if the trunk itself is operational and established, any VLAN traffic not included in this configured list will be filtered and dropped by the switch. This mechanism ensures strict control over VLAN propagation, making it a primary reason why specific desired VLANs might fail to communicate across an otherwise functional trunk.
✗The native VLAN mismatchWrong answer — click to see why▾
Why this is wrong here
The question states the trunk is up but no VLANs are allowed, indicating the issue is with the allowed VLAN list, not a mismatch. A native VLAN mismatch would cause traffic issues for untagged frames, but the trunk is operational and the problem is that no VLANs are permitted.
★ When this WOULD be the correct answer
A network engineer configures a trunk between two switches, and after connecting, devices on the same VLAN cannot communicate across the trunk, but the trunk link is up. The core switch shows the trunk is up, but the native VLAN is different on each end. This would be the correct answer when the symptom is that untagged traffic (like management traffic) fails, but tagged traffic may work.
Why candidates choose this
Candidates often confuse native VLAN mismatch with allowed VLAN issues because both involve VLAN configuration on trunks. The term 'native VLAN mismatch' is a common troubleshooting concept, leading them to select it without carefully reading that the trunk is up and the problem is that no VLANs are allowed.
✗The trunk encapsulation is not set to 802.1QWrong answer — click to see why▾
Why this is wrong here
The question states the trunk is up, meaning the encapsulation is already set correctly (likely 802.1Q by default on modern switches). If encapsulation were mismatched, the trunk would not come up at all.
★ When this WOULD be the correct answer
A network engineer connects two switches with a trunk, but the trunk fails to establish. The core switch uses 802.1Q, while the new switch defaults to ISL. In this scenario, setting the trunk encapsulation to 802.1Q would resolve the issue.
Why candidates choose this
Candidates may recall that encapsulation mismatch can cause trunk issues, but they overlook that the trunk is already up, indicating encapsulation is not the problem here.
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 logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
Switchport
A switchport is a physical or virtual interface on a network switch that connects devices like computers, printers, or other switches to a local area network.
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JA
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.