The answer is that untagged traffic may be placed into different VLANs on each switch. This occurs because on an 802.1Q trunk, the native VLAN is the only VLAN that does not carry an 802.1Q tag; when the native VLAN configuration differs on each end, frames sent untagged from one switch arrive on the other switch and are assigned to that switch’s configured native VLAN instead. On the CCNA 200-301 v2 exam, this concept tests your understanding of trunking mechanics and common misconfiguration pitfalls—a frequent trap is assuming a native VLAN mismatch will shut down the trunk entirely, but CDP will still report the mismatch while the link remains operational. A solid memory tip is to think of the native VLAN as the “default bucket” for untagged frames: if the bucket labels don’t match, traffic gets dumped into the wrong bucket on the far side.
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: an 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default.. 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.
Two switches are connected by an 802.1Q trunk. CDP reports a native VLAN mismatch. Which issue is most likely to appear because of this?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Untagged traffic may be placed into different VLANs on each switch.
A native VLAN mismatch can cause untagged traffic sent on one side to be placed into a different VLAN on the other side. That leads to confusing connectivity issues and can also create security concerns. It does not automatically disable the trunk.
Key principle: An 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Untagged traffic may be placed into different VLANs on each switch.
Why this is correct
That is the classic symptom of a native VLAN mismatch.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
An 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default.
✗
All tagged traffic on the trunk is dropped immediately.
Why it's wrong here
Tagged traffic can still pass normally if allowed VLANs match.
When this WOULD be correct
In a different question setup where the context is about a misconfigured trunk link that explicitly states all tagged traffic is incorrectly configured to be dropped due to a specific policy or ACL, this option could be correct. For example, if the question described a scenario where a switch is configured to drop all tagged frames due to security policies, then this option would apply.
In a different scenario where a question asks about the effects of misconfiguring STP on a trunk link, an option stating that STP is disabled would be correct if the configuration explicitly disables STP for that interface, leading to potential network loops.
✗
The trunk automatically converts to an access port.
Why it's wrong here
That is not normal switch behavior.
When this WOULD be correct
In a different scenario, if a question specifies that a trunk link is configured incorrectly and explicitly states that the switch is unable to maintain trunking due to configuration errors, then this option could be correct, indicating that the trunk has reverted to access port behavior.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Untagged traffic may be placed into different VLANs on each switch.Correct answer▾
Why this is correct
That is the classic symptom of a native VLAN mismatch.
✗All tagged traffic on the trunk is dropped immediately.Wrong answer — click to see why▾
Why this is wrong here
Tagged traffic is not dropped due to a native VLAN mismatch; only untagged frames are affected. Tagged frames carry VLAN information and are forwarded normally as long as the allowed VLAN list matches on both ends.
★ When this WOULD be the correct answer
In a different question setup where the context is about a misconfigured trunk link that explicitly states all tagged traffic is incorrectly configured to be dropped due to a specific policy or ACL, this option could be correct. For example, if the question described a scenario where a switch is configured to drop all tagged frames due to security policies, then this option would apply.
Why candidates choose this
Students might think that any mismatch on a trunk causes all traffic to fail, but the native VLAN mismatch specifically impacts only untagged frames, not tagged ones.
✗STP is disabled on the trunk link.Wrong answer — click to see why▾
Why this is wrong here
A native VLAN mismatch does not disable STP. STP operates independently of native VLAN configuration; however, a mismatch can cause STP to malfunction if BPDUs are sent untagged and end up in the wrong VLAN, potentially causing loops.
★ When this WOULD be the correct answer
In a different scenario where a question asks about the effects of misconfiguring STP on a trunk link, an option stating that STP is disabled would be correct if the configuration explicitly disables STP for that interface, leading to potential network loops.
Why candidates choose this
Because native VLAN mismatches can cause STP issues (e.g., BPDU misplacement), some might incorrectly assume STP is disabled entirely.
✗The trunk automatically converts to an access port.Wrong answer — click to see why▾
Why this is wrong here
A native VLAN mismatch does not cause a trunk to convert to an access port. The trunk remains operational for tagged traffic; only the handling of untagged frames is affected. Port mode changes only occur due to DTP misconfiguration or manual intervention.
★ When this WOULD be the correct answer
In a different scenario, if a question specifies that a trunk link is configured incorrectly and explicitly states that the switch is unable to maintain trunking due to configuration errors, then this option could be correct, indicating that the trunk has reverted to access port behavior.
Why candidates choose this
Students may confuse the symptom of untagged traffic being mishandled with a complete port mode change, but the trunk stays in trunk mode.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common exam trap is to believe that a native VLAN mismatch causes the trunk link to shut down or block all traffic. In reality, only untagged traffic is affected, and tagged VLAN traffic continues to pass normally if allowed VLANs match. Another mistake is thinking the trunk automatically converts to an access port, which Cisco switches do not do. Candidates may also overlook that STP remains enabled and functional despite the mismatch. Focusing only on trunk status without considering untagged traffic behavior leads to incorrect answers.
Detailed technical explanation
How to think about this question
802.1Q trunking allows switches to carry traffic for multiple VLANs over a single physical link by tagging frames with VLAN identifiers. The native VLAN is a special VLAN on a trunk port where frames are sent untagged. By default, VLAN 1 is the native VLAN on Cisco switches, but this can be changed. When two switches connect via an 802.1Q trunk, they must agree on the native VLAN to ensure untagged traffic is handled consistently.
If the native VLAN is mismatched between the two switches, untagged frames sent by one switch are assigned to the native VLAN configured on the receiving switch, which may be different. This causes untagged traffic to be placed into different VLANs on each side, leading to connectivity problems and VLAN leakage. CDP can detect this mismatch and alert administrators, but the trunk link remains operational for tagged VLAN traffic. STP continues to function normally, and the trunk does not revert to an access port automatically.
The exam trap is to assume that a native VLAN mismatch disables the trunk or blocks all traffic, but only untagged traffic is affected. Tagged VLAN traffic passes normally if allowed VLANs match. Another common mistake is to overlook the security implications of native VLAN mismatches, as untagged traffic could cross VLAN boundaries unintentionally. In practical networks, it is best practice to change the native VLAN to an unused VLAN and ensure both ends match to avoid these issues.
KKey Concepts to Remember
An 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default.
A native VLAN mismatch occurs when the native VLAN configured on one switch does not match the native VLAN on the connected switch’s trunk port.
When a native VLAN mismatch exists, untagged frames sent from one switch are assigned to a different VLAN on the receiving switch, causing connectivity issues.
Cisco Discovery Protocol (CDP) can detect and report native VLAN mismatches between connected switches to help identify configuration errors.
Tagged traffic on a trunk is not affected by a native VLAN mismatch as long as allowed VLANs and tagging are consistent on both ends.
A native VLAN mismatch does not disable Spanning Tree Protocol (STP) or cause the trunk to convert to an access port automatically.
Native VLAN mismatches can create security risks by allowing untagged traffic to be misclassified and potentially leak between VLANs.
Properly matching native VLANs on both ends of a trunk is essential to maintain VLAN separation and prevent untagged traffic misrouting.
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
An 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default.
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 200-301 question in full detail.
Review an 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — An 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default..
What is the correct answer to this question?
The correct answer is: Untagged traffic may be placed into different VLANs on each switch. — A native VLAN mismatch can cause untagged traffic sent on one side to be placed into a different VLAN on the other side. That leads to confusing connectivity issues and can also create security concerns. It does not automatically disable the trunk.
What should I do if I get this 200-301 question wrong?
Review an 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default., then practise related 200-301 questions on the same topic to reinforce the concept.
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?
An 802.1Q trunk link carries multiple VLANs by tagging frames except for the native VLAN, which is sent untagged by default.
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