220-1101 Networking Practice Question
A technician is configuring a small office network with a managed switch and a router that supports inter-VLAN routing. A workstation on VLAN 10 (192.168.10.0/24) can ping other workstations on VLAN 10 but cannot ping a server on VLAN 20 (192.168.20.0/24). The switch has a trunk port connecting to the router, and the trunk is configured to allow VLANs 10 and 20. The workstation's default gateway is set to 192.168.10.1. The server's default gateway is set to 192.168.20.1. Which of the following is the MOST likely cause of the problem?
⚠ Common exam trap
CompTIA often tests the router-on-a-stick configuration where candidates assume a trunk is sufficient for inter-VLAN routing, forgetting that the router must have a subinterface with the correct encapsulation and IP address for each 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
✓
A. The router's subinterface for VLAN 20 is not configured or is incorrectly configured.
The workstation can communicate within VLAN 10 but cannot reach VLAN 20, indicating that inter-VLAN routing is failing. Since the trunk is correctly allowing VLANs 10 and 20, the most likely cause is that the router's subinterface for VLAN 20 (e.g., GigabitEthernet0/0.20) is missing, has an incorrect encapsulation dot1Q 20 command, or lacks an IP address in the 192.168.20.0/24 subnet. Without a properly configured subinterface, the router cannot route traffic to or from VLAN 20.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
A. The router's subinterface for VLAN 20 is not configured or is incorrectly configured.
Why this is correct
Without a subinterface dedicated to VLAN 20 on the router, there is no Layer 3 gateway for the 192.168.20.0/24 network, so the router cannot route packets from the workstation's VLAN 10 to the server's VLAN 20. Even if the switch trunk is correctly carrying both VLANs, the router must have an encapsulation dot1Q 20 subinterface with an IP address in the 192.168.20.0/24 subnet to act as the default gateway for that VLAN. A missing or misconfigured subinterface means the router has no route to the destination subnet and will silently drop or send an ICMP unreachable, which matches the symptom that local VLAN 10 communication works but cross-VLAN fails.
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B. The workstation's firewall is blocking ICMP echo requests.
Why it's wrong here
The workstation's internal firewall could theoretically filter ICMP, but since the workstation can successfully ping other hosts on VLAN 10, ICMP echo requests are not being blocked by the local firewall. The actual failure is isolated to traffic destined for the 192.168.20.0/24 subnet, which points to Layer 3 routing, specifically the router's missing or misconfigured subinterface for VLAN 20. Even if a firewall were blocking ICMP, it would only affect ping, not all communication, and would not explain why the workstation can reach other VLAN 10 hosts but not the server on VLAN 20.
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C. The switch port connecting the router is configured as an access port on VLAN 1.
Why it's wrong here
If the router port were an access port, only VLAN 1 traffic would pass, and the workstation on VLAN 10 would not even reach its default gateway. The problem description implies local VLAN 10 communication works, so the trunk must be operational.
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D. The server's default gateway is set to 192.168.20.254.
Why it's wrong here
The server's default gateway is correctly stated as 192.168.20.1 in the question. If it were 192.168.20.254, the server could not reply to traffic from outside its subnet, but the initial failure is from the workstation to the server, and the workstation's gateway is correctly set.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA 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 220-1101 exam.