CCNA Switching and Network Access Practice Question
Exhibit
SW1: switchport trunk allowed vlan 10,20,30 SW2: switchport trunk allowed vlan 10,20
Two switches are connected by an 802.1Q trunk. Hosts in VLAN 30 cannot communicate across the link, but VLAN 10 works. What is the most likely cause?
⚠ Common exam trap
Beware of confusing native VLAN issues with allowed VLAN list configurations. Native VLAN problems affect untagged traffic, not specific VLANs.
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 missing from the allowed VLAN list on SW2
When one VLAN works across a trunk but another does not, the most likely cause is that the failing VLAN is missing from the allowed VLAN list on one or both switches. This is often confirmed by a 'show interfaces trunk' command. Option B is incorrect because removing VLAN 10 would break an already working VLAN and does not address VLAN 30. Option C is incorrect because both ISL and 802.1Q carry multiple VLANs; the issue is not the encapsulation protocol. Option D is incorrect because changing a trunk port to access mode would disable the trunk entirely, preventing all VLAN traffic.
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 missing from the allowed VLAN list on SW2
Why this is correct
SW2's trunk port has an allowed VLAN list that includes only VLANs 10 and 20, so tagged frames for VLAN 30 are not forwarded from SW1 to SW2. The switchport trunk allowed vlan command acts as a filter, and VLAN 30 is excluded, causing the hosts in VLAN 30 to lose communication across the trunk. To fix the issue, the allowed list on SW2 must be updated to include VLAN 30.
- ✗
VLAN 10 must be removed for VLAN 30 to pass
Why it's wrong here
A single trunk can carry multiple VLANs concurrently, so VLAN 10 does not need to be removed for VLAN 30 to pass. The allowed VLAN list on a trunk is a filter that permits specific VLAN IDs; leaving out VLAN 30 means frames with that tag are discarded at SW2's ingress. Removing VLAN 10 would be unnecessary and would disrupt any devices that rely on VLAN 10 connectivity.
When this WOULD be correct
In a different scenario where the question states that only one VLAN can be active on the trunk due to a specific configuration limitation, removing VLAN 10 could be necessary to allow VLAN 30 to pass traffic. This could occur in a legacy system or a misconfigured switch environment.
- ✗
The trunk should use ISL instead of 802.1Q
Why it's wrong here
The trunk's encapsulation is already 802.1Q, which is the IEEE standard and is supported by both switches in the exhibit. Replacing it with ISL, a Cisco-proprietary protocol that is largely obsolete and not a CCNA focus, would not add VLAN 30 to SW2's allowed list. The root cause is the VLAN filter applied on SW2's trunk interface, not the tagging protocol in use.
When this WOULD be correct
In a scenario where a question specifies that a network is using only ISL for trunking and VLAN 30 is configured correctly on both switches, but communication still fails, the correct answer could be that ISL is required instead of 802.1Q due to compatibility issues with legacy equipment.
- ✗
The port on SW1 should be changed to access mode
Why it's wrong here
Changing SW1's trunk port to access mode would strip the 802.1Q tag and send frames as untagged on a single VLAN. This would eliminate VLAN separation across the link, and SW2's trunk interface would not process the frames correctly, because trunking relies on tagged frames. The actual fault is not the mode on SW1, but the missing VLAN 30 permission on SW2's trunk port.
When this WOULD be correct
In a different scenario where the question states that both VLANs are configured on the switches but VLAN 30 is not receiving any traffic, the correct answer could be that the port on SW1 needs to be set to access mode to allow VLAN 30 traffic to be processed correctly.
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 missing from the allowed VLAN list on SW2Correct answer▾
Why this is correct
SW2's trunk port has an allowed VLAN list that includes only VLANs 10 and 20, so tagged frames for VLAN 30 are not forwarded from SW1 to SW2. The switchport trunk allowed vlan command acts as a filter, and VLAN 30 is excluded, causing the hosts in VLAN 30 to lose communication across the trunk. To fix the issue, the allowed list on SW2 must be updated to include VLAN 30.
✗VLAN 10 must be removed for VLAN 30 to passWrong answer — click to see why▾
Why this is wrong here
Multiple VLANs can coexist on a trunk; removing VLAN 10 is not required for VLAN 30 to pass. The allowed VLAN list can include any combination of VLANs, and VLAN 10's presence does not block VLAN 30.
★ When this WOULD be the correct answer
In a different scenario where the question states that only one VLAN can be active on the trunk due to a specific configuration limitation, removing VLAN 10 could be necessary to allow VLAN 30 to pass traffic. This could occur in a legacy system or a misconfigured switch environment.
Why candidates choose this
A student might think that only one VLAN can be active at a time on a trunk, confusing the concept of native VLAN or misunderstanding that VLANs are independent.
✗The trunk should use ISL instead of 802.1QWrong answer — click to see why▾
Why this is wrong here
The problem is not related to the trunking protocol; 802.1Q is the industry standard and works correctly when configured properly. Changing to ISL would not resolve the issue if VLAN 30 is simply not allowed on the trunk.
★ When this WOULD be the correct answer
In a scenario where a question specifies that a network is using only ISL for trunking and VLAN 30 is configured correctly on both switches, but communication still fails, the correct answer could be that ISL is required instead of 802.1Q due to compatibility issues with legacy equipment.
Why candidates choose this
Some students may recall that ISL is an older Cisco proprietary trunking protocol and might think switching protocols could fix VLAN issues, but the root cause here is a missing VLAN in the allowed list.
✗The port on SW1 should be changed to access modeWrong answer — click to see why▾
Why this is wrong here
Changing the port to access mode would break the trunk entirely, preventing all VLAN traffic (including VLAN 10) from passing. The trunk is functioning for VLAN 10, so the issue is specific to VLAN 30, not the trunk mode.
★ When this WOULD be the correct answer
In a different scenario where the question states that both VLANs are configured on the switches but VLAN 30 is not receiving any traffic, the correct answer could be that the port on SW1 needs to be set to access mode to allow VLAN 30 traffic to be processed correctly.
Why candidates choose this
A student might think that if a VLAN is not working, the port should be in access mode for that VLAN, but that would isolate the port to a single VLAN and not solve the inter-switch communication problem.
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
Trunk port
A trunk port is a switch port configured to carry traffic for multiple VLANs, using a tagging protocol to identify which VLAN each frame belongs to.
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.
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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.