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 connectivity issue between two switches, SW1 and SW2, connected via a trunk. The trunk is configured with switchport mode trunk on both sides. The engineer notices that some VLANs are not passing traffic, even though they are in the allowed list. The output of 'show interfaces trunk' on SW1 shows that VLANs 10, 20, and 30 are in the allowed list and are active. However, hosts in VLAN 30 cannot reach the distribution switch. 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 30 is not created in the VLAN database on SW2.
VLAN 30 must exist in the VLAN database on both switches for traffic to be forwarded across the trunk. Even if VLAN 30 is in the allowed list and active on SW1, if it has not been created on SW2, SW2 will discard frames tagged with VLAN 30 because it has no VLAN 30 interface or forwarding table entry. This is a common misconfiguration where the VLAN is allowed on the trunk but not present on the remote switch.
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 30 is not created in the VLAN database on SW2.
Why this is correct
Correct because a VLAN must exist in the VLAN database on both ends of a trunk 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.
✗
The native VLAN is mismatched between SW1 and SW2.
Why it's wrong here
Incorrect because a native VLAN mismatch would affect the native VLAN traffic, not a specific non-native VLAN.
✗
VTP pruning is removing VLAN 30 from the trunk.
Why it's wrong here
Incorrect because if VTP pruning were active, VLAN 30 would not appear in the allowed list on the trunk.
✗
The trunk is not forming due to DTP negotiation.
Why it's wrong here
Incorrect because both sides are set to trunk mode, so DTP is not needed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that being in the allowed list on the trunk is sufficient for traffic to pass, when in fact the VLAN must be created in the VLAN database on both ends of the trunk.
Detailed technical explanation
How to think about this question
When a switch receives a tagged frame on a trunk port, it checks whether the VLAN ID exists in its local VLAN database. If the VLAN is not created, the frame is dropped at the ingress, even if the trunk allows that VLAN. This behavior is defined in IEEE 802.1Q and is independent of VTP; VTP only propagates VLAN information but does not automatically create VLANs on a switch in transparent mode. In real-world troubleshooting, 'show vlan brief' on SW2 would confirm the absence of VLAN 30.
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 30 is not created in the VLAN database on SW2. — VLAN 30 must exist in the VLAN database on both switches for traffic to be forwarded across the trunk. Even if VLAN 30 is in the allowed list and active on SW1, if it has not been created on SW2, SW2 will discard frames tagged with VLAN 30 because it has no VLAN 30 interface or forwarding table entry. This is a common misconfiguration where the VLAN is allowed on the trunk but not present on the remote switch.
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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