Question 304 of 1,389
CCNA Switching and Network Access Practice Question
Exhibit
SwitchA# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,40 SwitchB# show interfaces trunk Port Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 10,20,30,40
PCs in VLAN 30 on SwitchA cannot reach PCs in VLAN 30 on SwitchB. VLAN 30 exists on both switches and all other VLANs work across the same link. Based on the exhibit, what is the most likely cause?
⚠ Common exam trap
Be careful not to confuse general trunk issues with specific VLAN forwarding problems. Always check the allowed VLAN list when specific VLANs fail to pass.
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 not allowed on the trunk from SwitchA.
The trunk is up, but VLAN 30 is missing from the allowed list on SwitchA. In plain language, the hallway between the switches is open, but one side is refusing to carry that specific VLAN through the hallway. Since the other VLANs are working, the failure is selective rather than total. That strongly points to an allowed-VLAN problem rather than a broader trunk outage. This is a classic CCNA switching scenario because it tests whether you can separate trunk health from per-VLAN forwarding. A trunk can be operational and still block one VLAN if that VLAN is not permitted on one side. The native VLAN and encapsulation are not the issue shown here — the mismatch in the allowed list is.
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 not allowed on the trunk from SwitchA.
Why this is correct
VLAN 30 is not included in SwitchA's trunk allowed list, so frames from VLAN 30 are not forwarded over the trunk to SwitchB. Unlike the native VLAN, which is untagged, VLAN 30 must be explicitly permitted with the switchport trunk allowed vlan command on both ends. Without this entry, all VLAN 30 traffic remains isolated to the local switch, preventing PCs on SwitchB from reaching those on SwitchA.
- ✗
The native VLAN is mismatched.
Why it's wrong here
A native VLAN mismatch could cause spanning-tree and untagged traffic problems, but both switches are configured with native VLAN 1, so this is not the issue. Additionally, a mismatch would affect any untagged traffic, not selectively block only VLAN 30, which would normally be tagged. Therefore, this option does not explain the reported symptom.
When this WOULD be correct
In a scenario where a question specifies that PCs in VLAN 30 cannot communicate due to a native VLAN mismatch, and both switches are configured to use different native VLANs, this option would correctly identify the cause of the issue.
- ✗
The trunk must use ISL instead of 802.1Q.
Why it's wrong here
The trunk is already using 802.1Q, which is the IEEE standard and is successfully carrying the other VLANs between the switches. ISL is a Cisco-proprietary encapsulation that is not needed for this scenario, and converting the trunk to ISL would not add VLAN 30 to the allowed list. Therefore, the problem lies in the VLAN filtering configuration, not the trunk encapsulation type.
When this WOULD be correct
In a different scenario, if the question specified that the switches were configured to use ISL and that VLAN 30 was not configured to use ISL, then this option would be correct. For example, if the question stated that VLAN 30 was only configured for 802.1Q on both switches, this could lead to a failure in communication.
- ✗
VLAN 30 must be configured as the native VLAN.
Why it's wrong here
Configuring VLAN 30 as the native VLAN is not a requirement for crossing a trunk; the native VLAN is used only for untagged frames, while all other VLANs are encapsulated with 802.1Q tags. VLAN 30 can be carried normally as a tagged VLAN as long as it is in the allowed list. Forcing VLAN 30 to be native would not fix the missing allowed-list entry and could complicate trunk encapsulation.
When this WOULD be correct
In a different scenario, if the question stated that PCs in VLAN 30 were unable to communicate due to untagged traffic being misrouted, then configuring VLAN 30 as the native VLAN could resolve the issue, making this option correct.
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 not allowed on the trunk from SwitchA.Correct answer▾
Why this is correct
VLAN 30 is not included in SwitchA's trunk allowed list, so frames from VLAN 30 are not forwarded over the trunk to SwitchB. Unlike the native VLAN, which is untagged, VLAN 30 must be explicitly permitted with the switchport trunk allowed vlan command on both ends. Without this entry, all VLAN 30 traffic remains isolated to the local switch, preventing PCs on SwitchB from reaching those on SwitchA.
✗The native VLAN is mismatched.Wrong answer — click to see why▾
Why this is wrong here
The exhibit shows both switches have native VLAN 1 configured, so there is no mismatch. A native VLAN mismatch would cause all VLAN traffic to fail or be misdirected, not just a single VLAN.
★ When this WOULD be the correct answer
In a scenario where a question specifies that PCs in VLAN 30 cannot communicate due to a native VLAN mismatch, and both switches are configured to use different native VLANs, this option would correctly identify the cause of the issue.
Why candidates choose this
Students often confuse native VLAN issues with VLAN pruning or allowed lists, and may assume that a single VLAN problem is due to native VLAN mismatch, especially if they have seen such issues in labs.
✗The trunk must use ISL instead of 802.1Q.Wrong answer — click to see why▾
Why this is wrong here
Since other VLANs are working across the same trunk, the trunk encapsulation (802.1Q) is functioning correctly. Changing to ISL would not fix the issue and would break connectivity for all VLANs.
★ When this WOULD be the correct answer
In a different scenario, if the question specified that the switches were configured to use ISL and that VLAN 30 was not configured to use ISL, then this option would be correct. For example, if the question stated that VLAN 30 was only configured for 802.1Q on both switches, this could lead to a failure in communication.
Why candidates choose this
Some test-takers might think that ISL is required for certain VLANs or that 802.1Q has limitations, but in modern networks 802.1Q is the standard and ISL is obsolete.
✗VLAN 30 must be configured as the native VLAN.Wrong answer — click to see why▾
Why this is wrong here
A VLAN does not need to be the native VLAN to traverse a trunk; native VLAN is only for untagged traffic. Making VLAN 30 the native VLAN would not solve the problem and could introduce other issues.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that PCs in VLAN 30 were unable to communicate due to untagged traffic being misrouted, then configuring VLAN 30 as the native VLAN could resolve the issue, making this option correct.
Why candidates choose this
Students may confuse the concept of native VLAN with the idea that a VLAN must be 'allowed' or 'special' to pass over a trunk, leading them to think that changing the native VLAN would help.
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?”
Visual reference
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Last reviewed: May 17, 2026
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