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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
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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