CCNA Switching and Network Access Practice Question
A switch trunk must carry VLANs 10, 20, and 30, but traffic for VLAN 20 is failing. The trunk allowed list on one side is `10,30`. What is the most likely cause?
⚠ Common exam trap
Focus on the allowed list configuration, not on VLAN existence or trunk mode. Misconfigurations in allowed lists are a common trap.
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
✓
VLAN 20 is missing from the allowed VLAN list on one side of the trunk.
The most likely cause is that VLAN 20 is not in the allowed VLAN list on one side of the trunk. Option B is incorrect because converting the trunk to an access port would block all other VLANs, not just fix VLAN 20. Option C is incorrect because there is no requirement that VLAN 20 must be the native VLAN; native VLAN is unrelated to allowed list filtration. Option D is incorrect because ISL vs 802.1Q does not affect per-VLAN filtering; the allowed list is a separate configuration independent of the encapsulation type.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
VLAN 20 is missing from the allowed VLAN list on one side of the trunk.
Why this is correct
The allowed VLAN list is a per-trunk filter that determines which VLANs are permitted to traverse the link. If VLAN 20 is omitted from the 'switchport trunk allowed vlan' command on either side, frames tagged for VLAN 20 will be discarded at that interface, even though the trunk itself is up. The fix is to explicitly add VLAN 20 to the allowed list on the affected side. Since this is a static configuration error, no other trunk parameter or encapsulation change is needed.
- ✗
The trunk must be converted to an access port for VLAN 20 to work.
Why it's wrong here
An access port is designed to carry a single untagged VLAN, so converting the trunk to an access port would force it to carry only the VLAN assigned as the access VLAN, such as VLAN 20. This would drop the tags for VLANs 10 and 30, making them unreachable across the link, which defeats the purpose of a trunk that must carry all three VLANs. Furthermore, trunk ports are the standard mechanism for inter-switch multi-VLAN traffic, and a misconfigured allowed list is a filtering problem, not a port-mode problem.
When this WOULD be correct
In a different scenario where the question states that VLAN 20 is not functioning due to a configuration error, and the configuration requires VLANs to be on access ports for specific devices, then converting the trunk to an access port for VLAN 20 would be the correct solution.
- ✗
VLAN 20 must always be the native VLAN.
Why it's wrong here
The native VLAN on an 802.1Q trunk is the VLAN that is transmitted without an 802.1Q tag; it is an untagged path used for control traffic and for devices that do not support VLAN tagging. VLAN 20 can be carried either as a tagged VLAN or as the native VLAN, but setting it as native would simply change the tagging behavior, not the allowed list. If VLAN 20 is missing from the allowed list, even untagged frames for the native VLAN will not be forwarded. Thus, native VLAN assignment is independent from the VLAN membership filtering issue.
When this WOULD be correct
In a different scenario where the question specifies that VLAN 20 must be the native VLAN for compatibility with legacy devices, this option would be correct if the trunk configuration did not set VLAN 20 as the native VLAN.
- ✗
The switches must both use ISL instead of 802.1Q.
Why it's wrong here
ISL and 802.1Q are different trunking encapsulations; both use their own tagging format and both are capable of carrying VLAN 20 across a trunk. The problem described is that VLAN 20 is not in the allowed list on one side, which is a VLAN filter applied at the interface level regardless of the encapsulation protocol. Even if both switches were configured for ISL, the missing VLAN would still be omitted from the allowed list, so the traffic would still be blocked. Additionally, most Cisco switches today default to 802.1Q, and mixing encapsulations would break the trunk entirely.
When this WOULD be correct
In a different scenario, if a question stated that VLAN 20 was configured on both switches but traffic was still failing due to compatibility issues between ISL and 802.1Q, then stating that both switches must use ISL could be correct. This would imply that the trunking method is incompatible with the VLAN configuration.
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.
✓VLAN 20 is missing from the allowed VLAN list on one side of the trunk.Correct answer▾
Why this is correct
The allowed VLAN list is a per-trunk filter that determines which VLANs are permitted to traverse the link. If VLAN 20 is omitted from the 'switchport trunk allowed vlan' command on either side, frames tagged for VLAN 20 will be discarded at that interface, even though the trunk itself is up. The fix is to explicitly add VLAN 20 to the allowed list on the affected side. Since this is a static configuration error, no other trunk parameter or encapsulation change is needed.
✗The trunk must be converted to an access port for VLAN 20 to work.Wrong answer — click to see why▾
Why this is wrong here
An access port can only carry a single VLAN, so converting the trunk to an access port would prevent VLANs 10 and 30 from passing as well. The trunk is the correct interface type for carrying multiple VLANs, and the issue is specifically with the allowed list, not the port mode.
★ When this WOULD be the correct answer
In a different scenario where the question states that VLAN 20 is not functioning due to a configuration error, and the configuration requires VLANs to be on access ports for specific devices, then converting the trunk to an access port for VLAN 20 would be the correct solution.
Why candidates choose this
Students may confuse the concept of a trunk carrying multiple VLANs with an access port being simpler, and might think that changing to an access port would fix the issue for VLAN 20, not realizing it would break the other VLANs.
✗VLAN 20 must always be the native VLAN.Wrong answer — click to see why▾
Why this is wrong here
The native VLAN is used for untagged traffic on an 802.1Q trunk, but any VLAN can be designated as native. There is no requirement that VLAN 20 must be the native VLAN for it to traverse a trunk; the native VLAN is simply a default VLAN for untagged frames.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that VLAN 20 must be the native VLAN for compatibility with legacy devices, this option would be correct if the trunk configuration did not set VLAN 20 as the native VLAN.
Why candidates choose this
Some students might incorrectly associate the native VLAN with special behavior or assume that a VLAN must be native to pass over a trunk, especially if they have encountered scenarios where native VLAN mismatch causes issues.
✗The switches must both use ISL instead of 802.1Q.Wrong answer — click to see why▾
Why this is wrong here
The encapsulation type (ISL vs. 802.1Q) is not the root cause here; the problem is that VLAN 20 is explicitly omitted from the allowed list. Changing encapsulation would not add VLAN 20 to the allowed list and could introduce additional compatibility issues if both switches do not support the same encapsulation.
★ When this WOULD be the correct answer
In a different scenario, if a question stated that VLAN 20 was configured on both switches but traffic was still failing due to compatibility issues between ISL and 802.1Q, then stating that both switches must use ISL could be correct. This would imply that the trunking method is incompatible with the VLAN configuration.
Why candidates choose this
Students may recall that ISL and 802.1Q are different trunking protocols and might think that switching to ISL could resolve VLAN issues, but the allowed list is independent of the encapsulation type.
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?”
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Key term
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A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco 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 200-301 exam.