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200-901 Practice Question: During a network outage, a technician notices…
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?
⚠ Common exam trap
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.
Answer choices
Why each option matters
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.
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
An administratively down trunk would break connectivity for every VLAN traversing it, so VLAN 20 hosts could not reach their gateway either. It is tempting because a down link is a common outage cause, but the stem states switch interfaces are up and VLAN 20 traffic succeeds, contradicting a disabled trunk.
- ✗
The switchport trunk native VLAN is mismatched.
Why it's wrong here
A native VLAN mismatch causes CDP warnings and can leak untagged traffic between VLANs, but it does not selectively block VLAN 10 from reaching a router subinterface while VLAN 20 works. It is tempting because native VLAN errors do disrupt trunking, yet they would affect both VLANs or cause spanning-tree inconsistencies, not one-way gateway failure.
- ✓
The router subinterface for VLAN 10 is down or misconfigured.
Why this is correct
VLAN 20 succeeding proves the router's trunk and subinterface configuration works generally, isolating the fault to VLAN 10's subinterface being down or misconfigured. This satisfies the stem's constraint by explaining why only VLAN 10 hosts cannot reach their gateway.
- ✗
The router does not have an IP address configured.
Why it's wrong here
A router with subinterfaces for both VLANs would still answer ARP on VLAN 20, so a wholly unconfigured router contradicts the working VLAN 20 hosts. This fits a router with no subinterfaces at all, where neither VLAN could route.
Visual reference
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-901 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-901 exam.