This 200-901 practice question tests your understanding of network fundamentals. 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.
During a network outage, a technician notices that hosts in VLAN 10 cannot reach the default gateway at 192.168.10.1, but hosts in VLAN 20 can. The switch interfaces are up, and the router is configured with subinterfaces. 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 router subinterface for VLAN 10 is down or misconfigured.
The router subinterface for VLAN 10 is down or misconfigured. Since hosts in VLAN 10 cannot reach the default gateway but hosts in VLAN 20 can, the issue is isolated to VLAN 10. The router uses subinterfaces to route between VLANs via a trunk link; if the subinterface for VLAN 10 is down (e.g., no 'no shutdown' command) or misconfigured (e.g., wrong VLAN ID or encapsulation), it will not process traffic for that VLAN, while other subinterfaces remain functional.
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 trunk link is administratively down.
Why it's wrong here
If the trunk were down, all VLANs would be affected.
✗
The switchport trunk native VLAN is mismatched.
Why it's wrong here
A native VLAN mismatch would likely cause issues for all VLANs or at least the native VLAN, not just VLAN 10.
✓
The router subinterface for VLAN 10 is down or misconfigured.
Why this is correct
A down or misconfigured subinterface prevents routing for that VLAN.
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 router does not have an IP address configured.
Why it's wrong here
The router must have IP on the subinterface; lack of IP would affect all VLANs.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a trunk link issue or native VLAN mismatch would affect all VLANs equally, when in fact a subinterface-specific problem (like being administratively down or misconfigured) can isolate a single VLAN.
Detailed technical explanation
How to think about this question
Router-on-a-stick uses subinterfaces with IEEE 802.1Q encapsulation to tag frames for specific VLANs. Each subinterface must be in 'no shutdown' state and have the correct encapsulation dot1Q <vlan-id> command; a common misconfiguration is forgetting to issue 'no shutdown' on the subinterface or using the wrong VLAN ID. In a real-world scenario, a technician might check the trunk and physical interfaces first, overlooking that a subinterface can be administratively down even when the parent interface is up.
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.
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router subinterface for VLAN 10 is down or misconfigured. — The router subinterface for VLAN 10 is down or misconfigured. Since hosts in VLAN 10 cannot reach the default gateway but hosts in VLAN 20 can, the issue is isolated to VLAN 10. The router uses subinterfaces to route between VLANs via a trunk link; if the subinterface for VLAN 10 is down (e.g., no 'no shutdown' command) or misconfigured (e.g., wrong VLAN ID or encapsulation), it will not process traffic for that VLAN, while other subinterfaces remain functional.
What should I do if I get this 200-901 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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