- A
The trunk is using 802.1Q encapsulation and the remote switch expects ISL
Why wrong: Encapsulation mismatch would prevent the trunk from forming altogether, not just cause errors on native VLAN frames. The trunk is up and errors occur specifically on native VLAN tagged frames.
- B
The remote switch is sending frames on VLAN 99 as untagged
Why wrong: If the remote switch sends untagged frames, that is correct for the native VLAN; no error would occur. The error occurs because the remote switch is sending tagged frames for VLAN 99.
- C
The remote switch is tagging frames on the native VLAN
When a trunk is configured with native VLAN 99, frames for that VLAN should be sent untagged. If the remote switch sends them tagged, the receiving switch may log errors or drop the frames because it expects untagged frames for the native VLAN.
- D
The allowed VLAN list does not include VLAN 99
Why wrong: If VLAN 99 is not in the allowed list, frames would be dropped entirely, not generate an error. The error indicates a tagging/untagging mismatch, not a missing VLAN.
Quick Answer
The answer is a native VLAN tagging mismatch, specifically that the remote switch is tagging frames on the native VLAN. This triggers an error because the 802.1Q trunk standard mandates that frames belonging to the native VLAN must be sent untagged; the receiving switch expects untagged traffic on that VLAN and logs a "native VLAN tagged frame error" when it sees a tag. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of trunk port behavior and the common pitfall of mismatched native VLANs between switches—a frequent cause of connectivity issues and error logs. A classic trap is assuming all VLAN traffic on a trunk must be tagged, but the native VLAN is the deliberate exception. For the exam, remember the golden rule: native VLAN frames travel naked. If you see a tag where none belongs, the native VLANs don't match.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. 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 switch port is configured as a trunk with native VLAN 99. When a frame tagged with VLAN 99 is received on the trunk, the switch displays an error. What is the most likely 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.
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 remote switch is tagging frames on the native VLAN
The native VLAN is the VLAN that carries untagged traffic on an 802.1Q trunk. By design, frames belonging to the native VLAN should be sent untagged. When a switch receives a frame tagged with the native VLAN ID (VLAN 99), it indicates a mismatch in native VLAN configuration between the two ends of the trunk. The switch logs an error because it expects native VLAN frames to be untagged, but the remote switch is incorrectly tagging them.
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.
- ✗
The trunk is using 802.1Q encapsulation and the remote switch expects ISL
Why it's wrong here
Encapsulation mismatch would prevent the trunk from forming altogether, not just cause errors on native VLAN frames. The trunk is up and errors occur specifically on native VLAN tagged frames.
- ✗
The remote switch is sending frames on VLAN 99 as untagged
Why it's wrong here
If the remote switch sends untagged frames, that is correct for the native VLAN; no error would occur. The error occurs because the remote switch is sending tagged frames for VLAN 99.
- ✓
The remote switch is tagging frames on the native VLAN
Why this is correct
When a trunk is configured with native VLAN 99, frames for that VLAN should be sent untagged. If the remote switch sends them tagged, the receiving switch may log errors or drop the frames because it expects untagged frames for the native VLAN.
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.
- ✗
The allowed VLAN list does not include VLAN 99
Why it's wrong here
If VLAN 99 is not in the allowed list, frames would be dropped entirely, not generate an error. The error indicates a tagging/untagging mismatch, not a missing VLAN.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that native VLAN frames are always tagged, when in fact they must remain untagged on an 802.1Q trunk, and the error occurs because the remote switch is tagging them.
Detailed technical explanation
How to think about this question
In 802.1Q trunking, the native VLAN is defined in IEEE 802.1Q-2014 as the VLAN that does not have a tag inserted in the frame. When a switch receives a tagged frame with the native VLAN ID, it may log a 'native VLAN mismatch' error (e.g., 'CDP-4-NATIVE_VLAN_MISMATCH') and drop the frame or treat it inconsistently. This is a common misconfiguration when one switch is set to tag native VLAN traffic (e.g., via the 'vlan dot1q tag native' global command) and the other is not, leading to connectivity issues for devices in that VLAN.
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.
What to study next
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The remote switch is tagging frames on the native VLAN — The native VLAN is the VLAN that carries untagged traffic on an 802.1Q trunk. By design, frames belonging to the native VLAN should be sent untagged. When a switch receives a frame tagged with the native VLAN ID (VLAN 99), it indicates a mismatch in native VLAN configuration between the two ends of the trunk. The switch logs an error because it expects native VLAN frames to be untagged, but the remote switch is incorrectly tagging them.
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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Last reviewed: Jun 11, 2026
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.
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