N10-009 Network Troubleshooting Practice Question
A network technician is troubleshooting communication between two switches. The trunk link between them is up, and both switches have the same list of allowed VLANs. However, devices in VLAN 10 on one switch cannot communicate with devices in VLAN 10 on the other switch. What is the MOST likely cause of this issue?
⚠ Common exam trap
CompTIA often tests the native VLAN mismatch scenario by presenting a trunk that is up and has matching allowed VLANs, leading candidates to overlook the native VLAN configuration and incorrectly choose options like STP blocking or VLAN not created.
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
✓
Native VLAN mismatch
A native VLAN mismatch is the most likely cause because when two switches have different native VLANs configured on a trunk, they will incorrectly tag or fail to tag frames for that VLAN. In this scenario, devices in VLAN 10 cannot communicate because the native VLAN frames (which are sent untagged) are being dropped or misinterpreted by the receiving switch, even though both switches list VLAN 10 as allowed. The trunk is up and the allowed VLAN list matches, so the issue points directly to a mismatch in the native VLAN configuration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Native VLAN mismatch
Why this is correct
A native VLAN mismatch occurs when the untagged VLAN configured on one end of an 802.1Q trunk link differs from the untagged VLAN on the other end. This causes untagged frames, including those from the native VLAN and potentially control traffic, to be misidentified and assigned to the wrong VLAN upon reception. Consequently, traffic for the specific native VLAN fails to communicate correctly across the trunk, while other VLANs that are explicitly tagged continue to function normally, precisely matching a problem affecting only one VLAN.
- ✗
Speed or duplex mismatch
Why it's wrong here
A speed or duplex mismatch between two connected network devices would cause severe performance degradation across the entire physical link. Symptoms typically include a high rate of CRC errors, late collisions, frame discards, and extremely slow or intermittent connectivity for *all* traffic traversing that link, irrespective of VLAN. This issue would not selectively prevent communication for only a single VLAN while other VLANs remain operational.
When this WOULD be correct
A speed or duplex mismatch would be the correct answer if the question described symptoms like intermittent connectivity, high error rates, or a link that is up but flapping, and the trunk link is not stable. For example: 'Two switches are connected via a trunk, but the link frequently goes up and down. What is the most likely cause?'
- ✗
Spanning Tree Protocol blocking the port
Why it's wrong here
Spanning Tree Protocol (STP) is designed to prevent Layer 2 switching loops by logically blocking redundant paths. If STP were to block a trunk port, it would prevent *all* traffic, for *all* configured VLANs, from passing through that specific port. STP operates at a lower layer and does not differentiate between individual VLANs when making blocking decisions; its action is comprehensive across the entire link, thus it would not cause a communication issue limited to only one VLAN.
When this WOULD be correct
In a scenario where a switch port is in a blocking state due to STP, causing no communication at all on that port, and the question specifies that the port is administratively up but no data passes, STP blocking would be the correct answer.
- ✗
VLAN 10 is not created on one of the switches
Why it's wrong here
If VLAN 10 did not exist on a switch, that switch would not be able to forward frames for that VLAN, but the question states both switches have the same allowed VLAN list, implying VLAN 10 exists.
When this WOULD be correct
A question where a trunk is configured with allowed VLANs, but one switch lacks the VLAN definition (e.g., 'no vlan 10'), causing traffic to be dropped even though the VLAN is in the allowed list.
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.
✓Native VLAN mismatchCorrect answer▾
Why this is correct
A native VLAN mismatch occurs when the untagged VLAN configured on one end of an 802.1Q trunk link differs from the untagged VLAN on the other end. This causes untagged frames, including those from the native VLAN and potentially control traffic, to be misidentified and assigned to the wrong VLAN upon reception. Consequently, traffic for the specific native VLAN fails to communicate correctly across the trunk, while other VLANs that are explicitly tagged continue to function normally, precisely matching a problem affecting only one VLAN.
✗Speed or duplex mismatchWrong answer — click to see why▾
Why this is wrong here
A speed or duplex mismatch would cause connectivity issues at Layer 1, such as packet loss or link instability, but the trunk link is up and both switches have the same allowed VLANs. The symptom of VLAN-specific communication failure points to a Layer 2 problem like native VLAN mismatch, not a physical mismatch.
★ When this WOULD be the correct answer
A speed or duplex mismatch would be the correct answer if the question described symptoms like intermittent connectivity, high error rates, or a link that is up but flapping, and the trunk link is not stable. For example: 'Two switches are connected via a trunk, but the link frequently goes up and down. What is the most likely cause?'
Why candidates choose this
Candidates may confuse Layer 1 issues (speed/duplex) with Layer 2 trunking problems, especially when the trunk is up but traffic fails. They might think physical mismatch could cause VLAN-specific issues, but it typically affects all traffic equally.
✗Spanning Tree Protocol blocking the portWrong answer — click to see why▾
Why this is wrong here
Spanning Tree Protocol blocking the port would prevent all traffic on the trunk, not just VLAN 10 traffic. Since the trunk is up and other VLANs may work, STP is not the issue.
★ When this WOULD be the correct answer
In a scenario where a switch port is in a blocking state due to STP, causing no communication at all on that port, and the question specifies that the port is administratively up but no data passes, STP blocking would be the correct answer.
Why candidates choose this
Candidates may confuse STP blocking with a VLAN-specific issue, or they might think STP could selectively block a VLAN, which is not the case.
✗VLAN 10 is not created on one of the switchesWrong answer — click to see why▾
Why this is wrong here
The question states both switches have the same list of allowed VLANs, so VLAN 10 is allowed on the trunk. If VLAN 10 were not created on one switch, it would not appear in the allowed list, contradicting the given condition.
★ When this WOULD be the correct answer
A question where a trunk is configured with allowed VLANs, but one switch lacks the VLAN definition (e.g., 'no vlan 10'), causing traffic to be dropped even though the VLAN is in the allowed list.
Why candidates choose this
Candidates may confuse 'allowed VLANs on trunk' with 'VLAN existence on the switch', assuming a missing VLAN definition could cause communication failure despite the trunk configuration.
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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Troubleshooting Methodology
Key term
Allowed VLANs
Allowed VLANs are the specific VLANs whose traffic is permitted to pass over a specific trunk link between switches, acting as an access control filter for VLAN traffic on a port.
Key term
Native VLAN mismatch
A Native VLAN mismatch occurs when two connected switches have different Native VLANs configured on the trunk port, causing control traffic like CDP, DTP, and BPDUs to be sent on the wrong VLAN and potentially creating security vulnerabilities or connectivity issues.
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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.