The answer is a native VLAN mismatch. This is the correct choice because on a trunk link, the native VLAN carries frames without an 802.1Q tag, and if each switch expects a different native VLAN, the receiving switch will either drop those untagged frames or misinterpret them as belonging to a different VLAN, breaking communication even when the allowed VLAN lists match. On the CompTIA Network+ N10-009 exam, this scenario is a classic trap: the trunk is up and the allowed VLANs are identical, so many candidates overlook the native VLAN configuration. The exam tests your understanding that a trunk’s operational state does not guarantee correct data flow for the native VLAN. A simple memory tip is “untagged means native”—if untagged frames arrive at a switch expecting a different native VLAN, they get misclassified or dropped, so always verify that the native VLAN ID is identical on both ends of the trunk.
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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 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?
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.
If the native VLAN configured on the trunk interfaces differs between the switches, traffic for that VLAN may not pass correctly.
B
Speed or duplex mismatch
Why wrong: Speed/duplex mismatch would cause CRC errors, late collisions, or intermittent connectivity, not a complete failure of a specific VLAN.
C
Spanning Tree Protocol blocking the port
Why wrong: STP would block the entire port to prevent loops, affecting all VLANs, not just one.
D
VLAN 10 is not created on one of the switches
Why wrong: 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.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
If the native VLAN configured on the trunk interfaces differs between the switches, traffic for that VLAN may not pass correctly.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Speed or duplex mismatch
Why it's wrong here
Speed/duplex mismatch would cause CRC errors, late collisions, or intermittent connectivity, not a complete failure of a specific VLAN.
✗
Spanning Tree Protocol blocking the port
Why it's wrong here
STP would block the entire port to prevent loops, affecting all VLANs, not just one.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
On a Cisco trunk, the native VLAN (default VLAN 1) is used for untagged traffic, and both ends must agree on the native VLAN to avoid misclassification. When a native VLAN mismatch occurs, frames from the native VLAN are received with a different VLAN ID or as untagged on the other side, causing them to be dropped or placed into the wrong VLAN. This is a common misconfiguration because the 'show interfaces trunk' command displays the native VLAN, but administrators often forget to verify it matches on both ends.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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