CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: a trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.. 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 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?
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
✓
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.
Key principle: A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
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
This is correct because the trunk is explicitly not permitting VLAN 20 on that side.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
✗
The trunk must be converted to an access port for VLAN 20 to work.
Why it's wrong here
This is wrong because a trunk is the correct inter-switch mechanism for carrying multiple VLANs.
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
This is wrong because a VLAN does not need to be native in order to traverse a trunk.
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
This is wrong because the key issue here is the allowed list, not a required change in encapsulation type.
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
This is correct because the trunk is explicitly not permitting VLAN 20 on that side.
✗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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Focus on the allowed list configuration, not on VLAN existence or trunk mode. Misconfigurations in allowed lists are a common trap.
Detailed technical explanation
How to think about this question
A trunk link in Cisco networking is a point-to-point link between switches that carries traffic for multiple VLANs simultaneously. Trunks use tagging protocols like IEEE 802.1Q to identify VLAN membership of frames as they traverse the link. This allows switches to maintain VLAN separation across a shared physical connection, enabling devices in the same VLAN but on different switches to communicate. The trunk configuration includes specifying which VLANs are allowed to pass through the trunk, controlling VLAN traffic flow and security.
When configuring trunks, the allowed VLAN list determines which VLANs can send traffic across the trunk link. If a VLAN is not included in the allowed list on either side of the trunk, traffic for that VLAN will be blocked and fail to traverse the link, even if the VLAN is active locally on both switches. This selective filtering is crucial for managing VLAN traffic and preventing unauthorized VLANs from crossing the trunk. The correct troubleshooting approach is to verify that the allowed VLAN lists match and include all required VLANs on both ends.
A common exam trap is to confuse trunk operation with native VLAN or encapsulation type issues. While native VLAN mismatches or encapsulation differences can cause problems, the selective VLAN filtering caused by allowed VLAN lists is a frequent and specific cause of VLAN traffic failure. Practically, a trunk can appear operational for some VLANs but fail for others if the allowed list excludes certain VLANs. Understanding this selective filtering behavior helps avoid misdiagnosis and ensures VLAN traffic flows correctly across trunks.
KKey Concepts to Remember
A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
The allowed VLAN list on a trunk controls which VLANs can send traffic across the trunk link between switches.
If a VLAN is missing from the allowed VLAN list on either side of the trunk, traffic for that VLAN will be blocked on the trunk.
Trunks must have matching allowed VLAN lists on both ends to ensure VLAN traffic can traverse the link end to end.
The native VLAN setting does not affect whether a VLAN is allowed on a trunk; it only determines untagged frame handling.
Encapsulation type mismatches (e.g., ISL vs. 802.1Q) can cause trunk failures but are unrelated to VLAN allowed list filtering.
A trunk can operate correctly for some VLANs while blocking traffic for others due to selective VLAN filtering in the allowed list.
Troubleshooting VLAN traffic failures on trunks should start by verifying the allowed VLAN lists include all required VLANs.
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
A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
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 a trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Review a trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q., 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?
A trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers using protocols like IEEE 802.1Q.
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