This 350-401 practice question tests your understanding of vlans and trunking. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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.
An engineer is troubleshooting a problem where a trunk link between two Cisco switches is not passing traffic for VLAN 10, but other VLANs are working. The trunk is configured with switchport mode trunk on both sides. The engineer checks the allowed VLAN list and sees VLAN 10 is included. The native VLAN is set to 1 on both sides. 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 10 is not created in the VLAN database on one of the switches.
The most likely cause is that VLAN 10 is not created in the VLAN database on one of the switches. Even if VLAN 10 is included in the allowed VLAN list on the trunk, a switch will not forward traffic for a VLAN that does not exist in its local VLAN database. The trunk interface will be operationally down for that specific VLAN, preventing traffic from passing.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 10 is not created in the VLAN database on one of the switches.
Why this is correct
Correct because the VLAN must exist on both switches for traffic to pass.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
VTP pruning has removed VLAN 10 from the trunk.
Why it's wrong here
Incorrect because if VTP pruning were active, VLAN 10 would not be in the allowed list.
✗
The native VLAN is mismatched.
Why it's wrong here
Incorrect because native VLAN mismatch would affect the native VLAN, not VLAN 10.
✗
Spanning Tree Protocol is blocking VLAN 10 on the trunk.
Why it's wrong here
Incorrect because STP blocks entire interfaces, not individual VLANs, unless using PVST+ with a loop, which is not indicated.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between a VLAN being allowed on a trunk and a VLAN being created in the VLAN database, leading candidates to focus on trunk configuration rather than verifying the VLAN's existence on both switches.
Detailed technical explanation
How to think about this question
On Cisco switches, a VLAN must exist in the VLAN database (created via 'vlan 10' in global config) before the switch can forward traffic for that VLAN. The trunk interface's allowed VLAN list only controls which VLANs are permitted to traverse the link, but if the VLAN is not locally defined, the switch will drop frames for that VLAN. This is a common misconfiguration when VLANs are added to trunks before being created on the switch, especially in environments without VTP or with VTP in transparent mode.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
VLANs and Trunking — This question tests VLANs and Trunking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VLAN 10 is not created in the VLAN database on one of the switches. — The most likely cause is that VLAN 10 is not created in the VLAN database on one of the switches. Even if VLAN 10 is included in the allowed VLAN list on the trunk, a switch will not forward traffic for a VLAN that does not exist in its local VLAN database. The trunk interface will be operationally down for that specific VLAN, preventing traffic from passing.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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