The answer is that VLAN 50 is not allowed on the trunk from SwitchA, which is the most likely cause of the isolated VLAN. On a Cisco trunk, the allowed VLAN list explicitly controls which VLANs can traverse the link; if a VLAN is missing from that list, its traffic is blocked while all other VLANs continue to function normally. This scenario tests your understanding of trunk configuration and the allowed VLAN list, a core topic in the CCNA 200-301 v2 exam where you must distinguish between issues like native VLAN mismatches, encapsulation problems, or pruning. A common trap is assuming a native VLAN mismatch would isolate a specific VLAN, but that error affects the native VLAN or causes spanning-tree loops, not a single non-native VLAN. Remember the memory tip: "Allowed list is a gate—if a VLAN's not on the list, it's locked out of the trunk."
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 VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.. 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
SwitchA# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/2 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/2 10,20,30
SwitchB# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/2 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/2 10,20,30,50
Two switches are connected by a trunk. VLAN 50 exists on both switches, but hosts in VLAN 50 cannot communicate across the link. All other VLANs work. Based on the exhibit, 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.
SwitchA# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/2 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/2 10,20,30
SwitchB# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi0/2 on 802.1q trunking 1
Port Vlans allowed on trunk
Gi0/2 10,20,30,50
A
VLAN 50 is not allowed on the trunk from SwitchA.
This is correct because VLAN 50 is missing from SwitchA’s allowed list.
B
The native VLAN is mismatched.
Why wrong: This is wrong because both sides show native VLAN 1.
C
802.1Q cannot carry VLAN 50.
Why wrong: This is wrong because 802.1Q can carry normal VLAN tags, including VLAN 50.
D
VLAN 50 must be configured as the native VLAN first.
Why wrong: This is wrong because a VLAN does not need to be native to cross a trunk.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 50 is not allowed on the trunk from SwitchA.
Option A is correct because VLAN 50 is missing from the allowed VLAN list on the trunk, which blocks only that VLAN while allowing others. Option B is incorrect: a native VLAN mismatch would affect the native VLAN (usually VLAN 1) or cause STP issues, not specifically VLAN 50. Option C is incorrect: 802.1Q encapsulation supports all VLANs from 1 to 4094, including VLAN 50. Option D is incorrect: VLANs do not need to be the native VLAN to be transported over a trunk; any VLAN can traverse a trunk if it is allowed.
Key principle: A VLANtrunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
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 50 is not allowed on the trunk from SwitchA.
Why this is correct
This is correct because VLAN 50 is missing from SwitchA’s allowed list.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A VLANtrunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
✗
The native VLAN is mismatched.
Why it's wrong here
This is wrong because both sides show native VLAN 1.
When this WOULD be correct
In a scenario where two switches are configured with different native VLANs and a question specifies that only one VLAN is affected, a mismatch in the native VLAN would cause communication issues for that specific VLAN while allowing others to function normally.
✗
802.1Q cannot carry VLAN 50.
Why it's wrong here
This is wrong because 802.1Q can carry normal VLAN tags, including VLAN 50.
When this WOULD be correct
In a different scenario, if the question stated that the switch was using a proprietary trunking protocol that does not support VLAN 50 or has a limitation on the VLAN range, then option C would be correct, indicating a protocol limitation.
✗
VLAN 50 must be configured as the native VLAN first.
Why it's wrong here
This is wrong because a VLAN does not need to be native to cross a trunk.
When this WOULD be correct
In a different scenario where the question specifies that VLAN 50 must be the native VLAN for proper communication, this option would be correct. For example, if the question stated that all VLANs except the native VLAN are unable to communicate, then configuring VLAN 50 as the native VLAN would resolve the issue.
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 50 is not allowed on the trunk from SwitchA.Correct answer▾
Why this is correct
This is correct because VLAN 50 is missing from SwitchA’s allowed list.
✗The native VLAN is mismatched.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because a native VLAN mismatch would typically affect all VLANs, not just VLAN 50. Since other VLANs are functioning correctly, it indicates that the native VLAN configuration is not the issue.
★ When this WOULD be the correct answer
In a scenario where two switches are configured with different native VLANs and a question specifies that only one VLAN is affected, a mismatch in the native VLAN would cause communication issues for that specific VLAN while allowing others to function normally.
Why candidates choose this
Candidates may choose this option due to a common misconception that native VLAN mismatches universally disrupt all VLAN traffic, leading them to overlook the specific context of the question.
✗802.1Q cannot carry VLAN 50.Wrong answer — click to see why▾
Why this is wrong here
802.1Q is a standard that can carry VLANs up to 4096, including VLAN 50. Therefore, the issue of VLAN 50 not communicating is not due to the protocol's inability to carry it.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that the switch was using a proprietary trunking protocol that does not support VLAN 50 or has a limitation on the VLAN range, then option C would be correct, indicating a protocol limitation.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of VLAN tagging protocols and their capabilities, leading them to incorrectly assume that certain VLANs cannot be carried over 802.1Q.
✗VLAN 50 must be configured as the native VLAN first.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because VLAN 50 can exist on a trunk without being the native VLAN; it does not need to be configured as such to communicate across the trunk link.
★ When this WOULD be the correct answer
In a different scenario where the question specifies that VLAN 50 must be the native VLAN for proper communication, this option would be correct. For example, if the question stated that all VLANs except the native VLAN are unable to communicate, then configuring VLAN 50 as the native VLAN would resolve the issue.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of VLAN configurations and the role of native VLANs, thinking that all VLANs must be treated the same without recognizing the specific requirements for trunk links.
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 differentiate between general trunk issues and VLAN-specific configurations. Don't confuse access port settings with trunk link issues.
Trap categories for this question
Command / output trap
This is wrong because both sides show native VLAN 1.
Detailed technical explanation
How to think about this question
A VLANtrunk is a point-to-point link between two switches that carries traffic for multiple VLANs simultaneously using tagging protocols such as IEEE 802.1Q. The trunk encapsulates frames with VLAN tags so that receiving switches can identify the VLAN membership of each frame and forward it appropriately. For hosts in the same VLAN to communicate across switches, the VLAN must be allowed and active on the trunk link. If a VLAN is not permitted on the trunk, frames tagged with that VLAN ID will be dropped or ignored, preventing inter-switch communication for that VLAN.
Cisco switches use a default configuration that allows all VLANs on a trunk unless explicitly restricted by the "allowed VLAN" list. Network administrators can limit VLANs on trunks to reduce unnecessary traffic or for security reasons. When a VLAN is missing from the allowed VLAN list on one side of the trunk, frames tagged with that VLAN are filtered out, causing connectivity issues only for that VLAN. Troubleshooting such issues requires verifying the allowed VLAN lists on both trunk interfaces and ensuring VLAN consistency.
The exam trap in this scenario is to confuse a trunk link being operational with all VLANs passing. The trunk can be up and carry traffic for many VLANs, but selective filtering of VLANs can cause partial failures. Candidates often mistake native VLAN mismatches or tagging protocol limitations as causes, but the key is the allowed VLAN list. Practically, this selective VLAN filtering is common in enterprise networks to control broadcast domains and enforce segmentation, making it a critical concept for CCNA candidates to master.
KKey Concepts to Remember
A VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
Cisco switches allow all VLANs on a trunk by default unless the allowed VLAN list is explicitly configured to restrict them.
If a VLAN is missing from the allowed VLAN list on a trunk port, frames tagged with that VLAN are dropped and cannot cross the link.
Hosts in the same VLAN on different switches require that VLAN to be allowed and active on the trunk connecting those switches.
A trunk link can be operational and carry traffic for some VLANs while filtering out others due to allowed VLAN restrictions.
Native VLAN mismatches do not prevent VLAN traffic from crossing a trunk unless the VLAN is explicitly filtered out.
802.1Q tagging supports all VLAN IDs from 1 to 4094, so it does not inherently block any VLAN traffic.
Selective VLAN filtering on trunks is a common network design practice to control broadcast domains and improve security.
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 VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
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 VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously., 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 VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously..
What is the correct answer to this question?
The correct answer is: VLAN 50 is not allowed on the trunk from SwitchA. — Option A is correct because VLAN 50 is missing from the allowed VLAN list on the trunk, which blocks only that VLAN while allowing others. Option B is incorrect: a native VLAN mismatch would affect the native VLAN (usually VLAN 1) or cause STP issues, not specifically VLAN 50. Option C is incorrect: 802.1Q encapsulation supports all VLANs from 1 to 4094, including VLAN 50. Option D is incorrect: VLANs do not need to be the native VLAN to be transported over a trunk; any VLAN can traverse a trunk if it is allowed.
What should I do if I get this 200-301 question wrong?
Review a VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously., 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 VLAN trunk link uses 802.1Q tagging to carry multiple VLANs between switches simultaneously.
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