CCNA Switching and Network Access Practice Question
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?
⚠ Common exam trap
Ensure you check the trunk configuration for allowed VLANs, not just VLAN existence or port assignments.
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 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.
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
VLAN 30 is either pruned via VTP pruning or excluded by the switchport trunk allowed vlan list on at least one of the interconnected switches. Because the 802.1Q trunk carries VLAN 10 successfully, the physical link and trunk parameters are operational, but VLAN 30's frames are deliberately filtered before crossing the trunk, so hosts in VLAN 30 experience complete connectivity loss while VLAN 10 remains unaffected.
- ✗
The native VLAN must be set to 30
Why it's wrong here
The native VLAN is used exclusively for untagged frames on an 802.1Q trunk and defaults to VLAN 1; user data VLAN traffic is normally tagged, so setting the native VLAN to 30 would not affect traffic delivery for VLAN 30. Since the problem is that VLAN 30 frames are not permitted or are pruned on the trunk, adjusting the native VLAN would neither allow those frames onto the trunk nor resolve the allowed-list mismatch.
When this WOULD be correct
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
Spanning Tree Protocol operates on a per-VLAN basis (PVST/PVST+) or per-instance in MST, so STP independently evaluates blocking or forwarding state for each VLAN on the trunk. A failure such as VLAN 30 not being allowed on one side is an administrative filtering issue, not a Layer 2 loop condition, and STP would not respond by blocking all VLANs on the trunk merely because one VLAN's frames are not being transmitted.
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
Why it's wrong here
Configuring the trunk as an access port would force the link to carry traffic for only a single, untagged VLAN, eliminating the multi-VLAN 802.1Q trunk entirely. This would break connectivity not only for VLAN 30 but also for VLAN 10 and any other VLANs currently traversing the link, so it is not a valid fix for the reported issue and would degrade the network further.
When this WOULD be correct
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
VLAN 30 is either pruned via VTP pruning or excluded by the switchport trunk allowed vlan list on at least one of the interconnected switches. Because the 802.1Q trunk carries VLAN 10 successfully, the physical link and trunk parameters are operational, but VLAN 30's frames are deliberately filtered before crossing the trunk, so hosts in VLAN 30 experience complete connectivity loss while VLAN 10 remains unaffected.
✗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?”
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Related to this question
Learn chapter
Configuring Switch Ports for Desktops, VoIP Phones, APs, IoT, and Virtualized Hosts
Key term
802.1Q
802.1Q is the networking standard that allows multiple virtual LANs (VLANs) to share a single physical network link by tagging Ethernet frames with VLAN identification information.
Key term
Native VLAN
A native VLAN is the default VLAN assigned to a trunk port that carries untagged traffic for backwards compatibility with devices that do not understand VLAN tagging.
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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.