Question 65 of 1,389
CCNA Network Services and Security Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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
When two switches have different native VLANs configured on the same 802.1Q trunk, each switch treats untagged frames as belonging to its own locally configured native VLAN. The sending switch places untagged frames into its native VLAN, while the receiving switch places them into its own native VLAN, which may be a different VLAN number. This mismatch causes untagged broadcast, multicast, and unknown unicast traffic to be placed into different VLAN domains on each switch, creating a classic native VLAN mismatch symptom.
- ✗
All tagged traffic on the trunk is dropped immediately.
Why it's wrong here
On an 802.1Q trunk, tagged frames carry a VLAN ID and are forwarded based on that tag, independent of the native VLAN configuration. A native VLAN mismatch only affects untagged frames, because each switch assigns untagged traffic to its own local native VLAN. Therefore, tagged traffic continues to traverse the trunk normally for all allowed VLANs, and it is not dropped immediately simply because the native VLANs differ.
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.
- ✗
STP is disabled on the trunk link.
Why it's wrong here
Spanning Tree Protocol (STP) operates independently of the native VLAN configuration on an 802.1Q trunk. Bridge Protocol Data Units (BPDUs) are sent as untagged frames and are processed on the native VLAN, but a mismatch in native VLAN numbers does not stop STP from exchanging BPDUs or making forwarding decisions. STP continues to run on the trunk link, detecting loops and placing ports into forwarding or blocking states based on the spanning-tree topology, regardless of the native VLAN mismatch.
When this WOULD be correct
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
A switchport configured as an 802.1Q trunk stays in trunking mode even if the native VLAN settings differ between the two ends. The Dynamic Trunking Protocol (DTP) is responsible for negotiating whether a link becomes an access or trunk port, but DTP is not triggered by a native VLAN mismatch. Switches do not automatically change an administratively configured trunk to an access port based on native VLAN disagreement, so the trunk link remains operational while carrying tagged frames correctly.
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
When two switches have different native VLANs configured on the same 802.1Q trunk, each switch treats untagged frames as belonging to its own locally configured native VLAN. The sending switch places untagged frames into its native VLAN, while the receiving switch places them into its own native VLAN, which may be a different VLAN number. This mismatch causes untagged broadcast, multicast, and unknown unicast traffic to be placed into different VLAN domains on each switch, creating a classic native VLAN mismatch symptom.
✗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?”
Visual reference
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Last reviewed: May 17, 2026
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