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.
A network engineer is troubleshooting a connectivity issue between two switches, SW1 and SW2, connected via a trunk link. SW1 is a Cisco Catalyst 3850 running IOS-XE, and SW2 is a Cisco Catalyst 2960 running IOS. The trunk is configured as a dynamic desirable mode on SW1 and dynamic auto on SW2. The engineer notices that the trunk is not forming. 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
✓
The native VLAN is different on SW1 and SW2.
Option A is correct because when DTP modes are dynamic desirable (SW1) and dynamic auto (SW2), the trunk should form successfully if both switches support DTP and the encapsulation matches. However, if the native VLAN is mismatched, the trunk will not form because Cisco switches use DTP frames to negotiate trunking, and a native VLAN mismatch causes DTP to fail, preventing the trunk from coming up. This is a common issue that overrides the DTP negotiation.
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.
✓
The native VLAN is different on SW1 and SW2.
Why this is correct
Correct because a native VLAN mismatch can cause DTP frames to be dropped, preventing trunk negotiation.
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.
Incorrect because the 2960 only supports 802.1Q, and if SW1 were set to ISL, the trunk would not form, but the default is 802.1Q.
✗
VLAN 1 is not allowed on the trunk.
Why it's wrong here
Incorrect because VLAN 1 is allowed by default, and removing it would not prevent trunk formation, only traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that DTP modes alone guarantee trunk formation, but the trap here is that a native VLAN mismatch silently breaks DTP negotiation, overriding the dynamic desirable/auto combination.
Detailed technical explanation
How to think about this question
DTP frames are sent every 30 seconds on trunk-capable ports, and they include the native VLAN number in the TLV (Type-Length-Value) field. If the native VLANs differ, the receiving switch discards the DTP frame as invalid, causing the port to remain in access mode. In real-world scenarios, this often occurs after a switch replacement where the native VLAN was changed on one side but not the other, leading to a silent trunk failure that is hard to diagnose without checking 'show interfaces trunk'.
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: The native VLAN is different on SW1 and SW2. — Option A is correct because when DTP modes are dynamic desirable (SW1) and dynamic auto (SW2), the trunk should form successfully if both switches support DTP and the encapsulation matches. However, if the native VLAN is mismatched, the trunk will not form because Cisco switches use DTP frames to negotiate trunking, and a native VLAN mismatch causes DTP to fail, preventing the trunk from coming up. This is a common issue that overrides the DTP negotiation.
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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