The answer is that VLAN 30 is pruned or not allowed on one side of the trunk. This is the most likely cause because an IEEE 802.1Q trunk only forwards frames for VLANs that are explicitly permitted in the allowed VLAN list on each switch interface; if VLAN 30 is missing from that list on one end, the trunk will drop those frames while still forwarding traffic for other VLANs like VLAN 10. On the CCNA 200-301 v2 exam, this scenario tests your understanding of trunk configuration and the difference between a VLAN being in the database versus being allowed on the trunk—a common trap is assuming that simply creating the VLAN on both switches is enough. Remember that the native VLAN only affects untagged frames and would not filter a tagged VLAN, and STP would block only the specific VLAN if a loop existed, not all VLANs. A useful memory tip: “Allowed list or it’s dismissed”—if a VLAN isn’t in the trunk’s allowed list, it gets dropped, no matter how many other VLANs work fine.
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: 802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.. 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.
Exhibit
SW1 show interfaces trunk
Port Vlans allowed on trunk: 10,30
SW2 show interfaces trunk
Port Vlans allowed on trunk: 10
Two switches are connected via an IEEE 802.1Q trunk. Hosts in VLAN 30 on opposite sides cannot communicate, yet hosts in VLAN 10 communicate normally. Both switches have VLAN 30 in their VLAN database, and the trunk link is operational. 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 30 is pruned or not allowed on one side of the trunk
If a VLAN is not allowed on one side of the trunk, frames for that VLAN are dropped even though other VLANs continue to work. Option A correctly identifies that VLAN 30 is either pruned or missing from the allowed list on one end. Option B is wrong because the native VLAN only affects untagged frames and would not cause the filtering of a tagged VLAN like VLAN 30. Option C is incorrect because STP can block individual VLANs per port, but it does not block all VLANs on a trunk due to a single VLAN’s issue—only the affected VLAN would be blocked. Option D is invalid because converting the trunk to access mode would break all VLAN trunking, including the working VLAN 10.
Key principle: 802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
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 not allowed on one side of the trunk
Why this is correct
That directly explains why VLAN 10 works and VLAN 30 does not.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
In a different scenario where a question specifies that all VLAN traffic must be tagged and requires the native VLAN to be set to a specific VLAN for proper communication, then setting the native VLAN to 30 would be necessary for hosts in that VLAN to communicate effectively.
✗
STP blocks all VLANs on the trunk when one VLAN fails
Why it's wrong here
STP is per-VLAN or per-instance and would not automatically block all VLANs because one VLAN has an allow-list issue.
When this WOULD be correct
In a different scenario, if a question stated that a trunk link was experiencing issues due to a misconfiguration where STP was incorrectly set to block all traffic, this option could be correct. For example, if the question described a network with multiple VLANs and a misconfigured STP that inadvertently blocked all VLANs due to a loop, then this option would apply.
✗
The trunk must be changed to access mode for VLAN 30
In a different scenario where a question states that VLAN 30 needs to be configured for a specific access port to allow devices to connect directly to it, then changing the trunk to access mode for VLAN 30 would be correct. This would imply that VLAN 30 is intended to operate as an access VLAN rather than a trunked VLAN.
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 not allowed on one side of the trunkCorrect answer▾
Why this is correct
That directly explains why VLAN 10 works and VLAN 30 does not.
✗The native VLAN must be set to 30Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because the native VLAN does not need to match the VLANs in use for communication; it simply serves as a default for untagged traffic. VLAN 30 can still operate correctly even if the native VLAN is set to a different value.
★ When this WOULD be the correct answer
In a different scenario where a question specifies that all VLAN traffic must be tagged and requires the native VLAN to be set to a specific VLAN for proper communication, then setting the native VLAN to 30 would be necessary for hosts in that VLAN to communicate effectively.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of the role of the native VLAN, believing that it must match the VLANs in use for communication to occur, leading to confusion about VLAN configurations.
✗STP blocks all VLANs on the trunk when one VLAN failsWrong answer — click to see why▾
Why this is wrong here
This option is wrong because Spanning Tree Protocol (STP) does not block all VLANs on a trunk due to a failure in one VLAN; it only blocks the specific VLAN that has issues, allowing other VLANs to continue functioning normally.
★ When this WOULD be the correct answer
In a different scenario, if a question stated that a trunk link was experiencing issues due to a misconfiguration where STP was incorrectly set to block all traffic, this option could be correct. For example, if the question described a network with multiple VLANs and a misconfigured STP that inadvertently blocked all VLANs due to a loop, then this option would apply.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of STP behavior, thinking that a failure in one VLAN would cause a cascading effect that impacts all VLANs on the trunk, reflecting a common misconception about STP functionality.
✗The trunk must be changed to access mode for VLAN 30Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because changing the trunk to access mode for VLAN 30 would not resolve communication issues between hosts in VLAN 30 on opposite sides of the trunk; access mode is not applicable to trunk links.
★ When this WOULD be the correct answer
In a different scenario where a question states that VLAN 30 needs to be configured for a specific access port to allow devices to connect directly to it, then changing the trunk to access mode for VLAN 30 would be correct. This would imply that VLAN 30 is intended to operate as an access VLAN rather than a trunked VLAN.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of VLAN configurations, believing that access mode is necessary for VLAN communication, especially if they confuse the roles of access and trunk ports.
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
Ensure you check the trunk configuration for allowed VLANs, not just VLAN existence or port assignments.
Detailed technical explanation
How to think about this question
802.1Q trunking is a Cisco and IEEE standard method that allows multiple VLANs to traverse a single physical link between switches by tagging Ethernet frames with VLAN identifiers. This tagging enables switches to segregate traffic logically, ensuring that frames belonging to different VLANs remain isolated even when sharing the same trunk link. The native VLAN is untagged by default, but all other VLANs must be explicitly allowed on the trunk to pass traffic.
When configuring trunks, each side must agree on which VLANs are allowed to traverse the link. Cisco switches use VLAN pruning or allowed VLAN lists to restrict which VLANs can send traffic over the trunk. If a VLAN is not included in the allowed VLAN list on one side, frames tagged with that VLAN ID are dropped, preventing communication for hosts in that VLAN. This explains why VLAN 10 communication works while VLAN 30 fails if VLAN 30 is pruned or disallowed on one side.
A common exam trap is confusing native VLAN settings or Spanning Tree Protocol (STP) behavior with VLAN pruning issues. Native VLAN mismatches do not block VLAN traffic but can cause security or tagging problems. STP operates per VLAN and does not block all VLANs due to one VLAN's failure. Understanding the allowed VLAN list on trunks and verifying VLAN membership is critical for troubleshooting VLAN connectivity issues in Cisco networks.
KKey Concepts to Remember
802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
Each side of a trunk link must allow the same VLANs for traffic to pass between switches for those VLANs.
If a VLAN is pruned or not allowed on one side of the trunk, traffic for that VLAN is dropped, causing communication failure.
The native VLAN is untagged on a trunk and does not need to match the VLAN experiencing connectivity issues.
Spanning Tree Protocol operates per VLAN and does not block all VLANs on a trunk due to one VLAN's failure.
Changing a trunk port to access mode disables multi-VLAN trunking and breaks connectivity for all VLANs except the access VLAN.
Cisco switches use VLAN pruning and allowed VLAN lists to control which VLANs can traverse a trunk link.
Verifying allowed VLANs on trunks is essential for troubleshooting VLAN connectivity issues in Cisco networks.
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
802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
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 802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches., 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 — 802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches..
What is the correct answer to this question?
The correct answer is: VLAN 30 is pruned or not allowed on one side of the trunk — If a VLAN is not allowed on one side of the trunk, frames for that VLAN are dropped even though other VLANs continue to work. Option A correctly identifies that VLAN 30 is either pruned or missing from the allowed list on one end. Option B is wrong because the native VLAN only affects untagged frames and would not cause the filtering of a tagged VLAN like VLAN 30. Option C is incorrect because STP can block individual VLANs per port, but it does not block all VLANs on a trunk due to a single VLAN’s issue—only the affected VLAN would be blocked. Option D is invalid because converting the trunk to access mode would break all VLAN trunking, including the working VLAN 10.
What should I do if I get this 200-301 question wrong?
Review 802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches., 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?
802.1Q trunking tags Ethernet frames to allow multiple VLANs to share a single physical link between switches.
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