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 host in VLAN 20 cannot communicate with a host in VLAN 30, even though both are connected to the same access switch. The access switch is configured with VLANs 20 and 30, and the uplink to the distribution switch is a trunk that allows both VLANs. The distribution switch has SVIs for both VLANs and IP routing is enabled. The engineer verifies that the trunk is up and both VLANs are allowed. What is the most likely cause of the communication failure?
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 hosts are not configured with the correct default gateway pointing to the SVI on the distribution switch.
Hosts in different VLANs must communicate through a Layer 3 device. The correct default gateway for each host should be the IP address of the SVI on the distribution switch for its respective VLAN. If the hosts are configured with an incorrect or no default gateway, traffic cannot be routed between VLAN 20 and VLAN 30, even though the trunk and SVIs are properly configured.
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 hosts are not configured with the correct default gateway pointing to the SVI on the distribution switch.
Why this is correct
Correct because hosts need a default gateway to route traffic to other VLANs; if misconfigured, inter-VLAN communication fails.
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 trunk is not allowing VLAN 20 or VLAN 30.
Why it's wrong here
Incorrect because the engineer verified the trunk allows both VLANs.
✗
Spanning Tree Protocol is blocking the SVI interfaces.
Why it's wrong here
Incorrect because SVIs are logical interfaces and are not affected by STP.
✗
The native VLAN mismatch on the trunk is causing the issue.
Why it's wrong here
Incorrect because the trunk is up, so native VLAN mismatch is not present.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a properly configured trunk and SVIs alone guarantee inter-VLAN communication, when in fact the hosts must have the correct default gateway configured to reach the SVI.
Detailed technical explanation
How to think about this question
For inter-VLAN routing, hosts must send traffic to their default gateway (the SVI IP) which then performs routing between VLANs. The 'ip routing' command enables Layer 3 forwarding on the distribution switch. A common real-world scenario is when hosts are configured with a static IP but the default gateway is omitted or points to the wrong SVI, causing all off-subnet traffic to fail.
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 hosts are not configured with the correct default gateway pointing to the SVI on the distribution switch. — Hosts in different VLANs must communicate through a Layer 3 device. The correct default gateway for each host should be the IP address of the SVI on the distribution switch for its respective VLAN. If the hosts are configured with an incorrect or no default gateway, traffic cannot be routed between VLAN 20 and VLAN 30, even though the trunk and SVIs are properly configured.
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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