The correct answer is that the native VLAN ID must match on both ends of an IEEE 802.1Q trunk. This is because the native VLAN is the only VLAN that carries untagged traffic across the trunk; if the IDs differ, frames from one switch’s native VLAN will be misinterpreted by the other switch, leading to traffic being placed into the wrong VLAN, which can cause misrouting and even dangerous layer 2 loops. On the CompTIA Network+ N10-009 exam, this concept often appears in troubleshooting scenarios where a trunk link is up but traffic fails, and the common trap is assuming all VLANs must match—when in reality only the native VLAN ID is mandatory for basic functionality. A solid memory tip: think of the native VLAN as the “default language” of the trunk—if both sides don’t speak the same language, the conversation breaks down.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 administrator is configuring an IEEE 802.1Q trunk between two switches. Which of the following must match on both ends for the trunk to function correctly?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The native VLAN ID
For an IEEE 802.1Q trunk to function correctly, the native VLAN ID must match on both ends. The native VLAN is the VLAN that carries untagged traffic across the trunk; if the IDs differ, frames from one switch's native VLAN will be placed into a different VLAN on the other switch, causing traffic misrouting and potential layer 2 loops. This is a fundamental requirement of the 802.1Q standard, and mismatched native VLANs are a common source of trunk failures.
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 native VLAN ID
Why this is correct
A mismatched native VLAN can cause traffic on the untagged VLAN to be placed in the wrong VLAN on the other switch.
Related concept
Read the scenario before looking for a memorised answer.
✗
The trunk port speed and duplex
Why it's wrong here
Speed and duplex must match for any link, but this is not a trunk-specific requirement.
✗
The encapsulation type
Why it's wrong here
Both ends must use 802.1Q, but this is a standard setting and less likely to be misconfigured than the native VLAN.
✗
The allowed VLAN list
Why it's wrong here
The allowed VLAN list can be pruned differently on each end; the trunk will still pass traffic for VLANs allowed on both sides.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the requirement for matching native VLANs with the need for matching encapsulation type, mistakenly thinking that both ends must be set to 'dot1q' when in fact 802.1Q is the default and only option for modern trunks, making the native VLAN the critical matching parameter.
Detailed technical explanation
How to think about this question
Under the hood, the native VLAN ID is inserted into the 802.1Q tag field as a VLAN ID of 0 for untagged frames, and both switches must agree on which VLAN those untagged frames belong to. In a real-world scenario, if one switch has native VLAN 1 and the other has native VLAN 99, a broadcast storm can occur because the switches will interpret the untagged traffic as belonging to different VLANs, potentially creating a bridging loop. The 'show interfaces trunk' command on Cisco IOS reveals the native VLAN, and mismatches are flagged as 'native VLAN mismatch' in logs.
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 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 native VLAN ID — For an IEEE 802.1Q trunk to function correctly, the native VLAN ID must match on both ends. The native VLAN is the VLAN that carries untagged traffic across the trunk; if the IDs differ, frames from one switch's native VLAN will be placed into a different VLAN on the other switch, causing traffic misrouting and potential layer 2 loops. This is a fundamental requirement of the 802.1Q standard, and mismatched native VLANs are a common source of trunk failures.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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