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 port forwards traffic for multiple VLANs by tagging frames with VLAN IDs 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.
Switch SW1 sends traffic for VLAN 30 across a trunk to SW2, but hosts in VLAN 30 on SW2 cannot communicate with hosts in VLAN 30 on SW1. Other VLANs work across the trunk. Which trunk issue is most likely?
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 30 is pruned or missing from the allowed VLAN list
If only one VLAN fails across an otherwise healthy trunk, a missing or filtered VLAN in the allowed list is a common cause. Native VLAN matching and encapsulation would affect broader trunk behavior, not usually just one VLAN in this way.
Key principle: A trunk port forwards traffic for multiple VLANs by tagging frames with VLANIDs 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 30 is pruned or missing from the allowed VLAN list
Why this is correct
Native VLAN settings can matter, but they do not best explain why other VLANs still work while VLAN 30 alone fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A trunk port forwards traffic for multiple VLANs by tagging frames with VLANIDs using protocols like IEEE 802.1Q.
✗
The native VLAN is set to 1 on both switches
Why it's wrong here
Correct. VLAN 30 likely is not being carried across the trunk.
When this WOULD be correct
In a different scenario, if the question specified that hosts in VLAN 30 were unable to communicate due to a mismatch in native VLAN settings, where one switch had a native VLAN of 1 and the other had a different native VLAN, then this option would be correct.
✗
The trunk uses 802.1Q encapsulation
Why it's wrong here
Using 802.1Q is normal and not the cause here.
When this WOULD be correct
In a different scenario where the question states that hosts in VLAN 30 cannot communicate due to a mismatch in encapsulation methods (e.g., if one switch is using ISL and the other is using 802.1Q), then this option would be correct as it would indicate a configuration issue preventing proper VLAN tagging.
✗
SW1 is the STP root bridge
Why it's wrong here
Being the STP root bridge does not by itself block one VLAN on a trunk.
When this WOULD be correct
In a different scenario, if the question specified that a VLAN was not forwarding due to STP blocking on a non-root bridge, then stating that SW1 is the STP root bridge could be correct, as it would imply that SW1 is managing the forwarding decisions for the VLANs.
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 30 is pruned or missing from the allowed VLAN listCorrect answer▾
Why this is correct
Native VLAN settings can matter, but they do not best explain why other VLANs still work while VLAN 30 alone fails.
✗The native VLAN is set to 1 on both switchesWrong answer — click to see why▾
Why this is wrong here
The native VLAN mismatch or setting does not cause a single VLAN to fail while others work. Native VLAN issues typically cause all traffic to be mis-tagged or dropped, not just one specific VLAN. Here, other VLANs work fine, so native VLAN is not the problem.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that hosts in VLAN 30 were unable to communicate due to a mismatch in native VLAN settings, where one switch had a native VLAN of 1 and the other had a different native VLAN, then this option would be correct.
Why candidates choose this
Students often confuse native VLAN with allowed VLAN lists. They may think that changing the native VLAN affects which VLANs are carried, but native VLAN is only for untagged traffic and does not restrict specific VLANs.
✗The trunk uses 802.1Q encapsulationWrong answer — click to see why▾
Why this is wrong here
802.1Q is the standard trunking encapsulation used in modern networks. Using 802.1Q is correct and does not cause a single VLAN to fail. Both switches must use the same encapsulation, but that is not the issue here.
★ When this WOULD be the correct answer
In a different scenario where the question states that hosts in VLAN 30 cannot communicate due to a mismatch in encapsulation methods (e.g., if one switch is using ISL and the other is using 802.1Q), then this option would be correct as it would indicate a configuration issue preventing proper VLAN tagging.
Why candidates choose this
Some students might think that using 802.1Q instead of ISL could cause problems, but 802.1Q is the default and works fine. They may confuse encapsulation type with VLAN pruning.
✗SW1 is the STP root bridgeWrong answer — click to see why▾
Why this is wrong here
STP root bridge status does not affect which VLANs are allowed on a trunk. STP prevents loops but does not block specific VLANs unless configured with VLAN-based STP (like PVST+). Even then, being the root does not block a VLAN; it only influences port roles.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that a VLAN was not forwarding due to STP blocking on a non-root bridge, then stating that SW1 is the STP root bridge could be correct, as it would imply that SW1 is managing the forwarding decisions for the VLANs.
Why candidates choose this
Students may think that the root bridge controls all VLAN traffic, but STP only manages loop prevention. They might confuse STP root with VLAN pruning or allowed VLAN lists.
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
Beware of assuming that common trunk issues like native VLAN mismatches affect only one VLAN; they typically affect all VLANs.
Detailed technical explanation
How to think about this question
VLAN trunks carry multiple VLANs between switches using tagging protocols like IEEE 802.1Q. Each VLAN's traffic is tagged with a VLAN ID so that the receiving switch can segregate and forward frames to the correct VLAN. The trunk link must allow all VLANs that need to communicate across it; otherwise, traffic for missing VLANs will be dropped. Switches maintain a list of allowed VLANs on trunks, and pruning or misconfiguration can exclude specific VLANs from the trunk.
When a VLAN is missing from the allowed VLAN list on a trunk, frames tagged with that VLAN ID are not forwarded across the link. This causes hosts in that VLAN on one switch to be unable to communicate with hosts in the same VLAN on the other switch, even though other VLANs function normally. The native VLAN setting affects untagged frames but does not selectively block a single VLAN's tagged traffic. Similarly, using 802.1Q encapsulation is standard and does not cause selective VLAN failures.
A common exam trap is to confuse native VLAN mismatches or STP root bridge roles with VLAN pruning issues. Native VLAN mismatches typically cause untagged frame drops or VLAN hopping risks but do not isolate a single tagged VLAN while leaving others functional. STP root bridge status affects path selection and blocking ports but does not block VLANs on trunks. The practical impact of missing VLANs on trunks is a silent failure where traffic for that VLAN never reaches the other switch, causing communication failures isolated to that VLAN.
KKey Concepts to Remember
A trunk port forwards traffic for multiple VLANs by tagging frames with VLAN IDs using protocols like IEEE 802.1Q.
Switches maintain an allowed VLAN list on trunks that determines which VLANs are permitted to cross the trunk link.
If a VLAN is pruned or missing from the allowed VLAN list, traffic for that VLAN is blocked from crossing the trunk.
Native VLAN settings affect untagged frames but do not selectively block tagged VLAN traffic on trunks.
Spanning Tree Protocol (STP) root bridge status influences port roles and states but does not block individual VLANs on trunks.
Using 802.1Q encapsulation is standard for VLAN tagging and does not cause selective VLAN communication failures.
A missing VLAN in the allowed VLAN list causes isolated VLAN communication failure despite other VLANs working normally.
Troubleshooting VLAN issues on trunks requires verifying allowed VLAN lists and pruning configurations to ensure all needed VLANs are permitted.
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 port forwards traffic for multiple VLANs by tagging frames with VLAN IDs 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 port forwards traffic for multiple VLANs by tagging frames with VLAN IDs 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 port forwards traffic for multiple VLANs by tagging frames with VLAN IDs using protocols like IEEE 802.1Q..
What is the correct answer to this question?
The correct answer is: VLAN 30 is pruned or missing from the allowed VLAN list — If only one VLAN fails across an otherwise healthy trunk, a missing or filtered VLAN in the allowed list is a common cause. Native VLAN matching and encapsulation would affect broader trunk behavior, not usually just one VLAN in this way.
What should I do if I get this 200-301 question wrong?
Review a trunk port forwards traffic for multiple VLANs by tagging frames with VLAN IDs 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 port forwards traffic for multiple VLANs by tagging frames with VLAN IDs using protocols like IEEE 802.1Q.
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