CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
Switch#show vlan brief VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Gi1/0/1, Gi1/0/2, Gi1/0/3 10 Sales active Gi1/0/12 20 Engineering active Gi1/0/15 1002 fddi-default act/unsup 1003 token-ring-default act/unsup 1004 fddinet-default act/unsup 1005 trnet-default act/unsup Switch#show interfaces trunk Port Mode Encapsulation Status Native vlan Gi1/0/24 on 802.1q trunking 1 Port Vlans allowed on trunk Gi1/0/24 1-4094 Port Vlans allowed and active in management domain Gi1/0/24 1,10,20 Port Vlans in spanning tree forwarding state and not pruned Gi1/0/24 1,10,20
A network technician is troubleshooting a connectivity issue for a PC connected to switch port Gi1/0/12. The PC can ping its default gateway (192.168.10.1) but cannot ping a server at 192.168.20.10. The switch is configured with VLAN 10 for the access port and is connected to a router-on-a-stick. The technician runs 'show vlan brief' and 'show interfaces trunk' on the switch. What is the most likely cause of the problem?
⚠ Common exam trap
Cisco often tests the misconception that a trunk misconfiguration (option A) is the cause, but the PC's ability to ping the gateway confirms the trunk is working for VLAN 10, so the real issue is the missing subinterface for the destination 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 is missing a subinterface for VLAN 20.
The PC can ping its default gateway (192.168.10.1) but not the server at 192.168.20.10, indicating Layer 3 routing is failing between VLANs. Since the switch is configured with VLAN 10 for the access port and uses a router-on-a-stick, the router must have a subinterface for VLAN 20 to route traffic to the server's subnet. The absence of a subinterface for VLAN 20 prevents the router from forwarding packets from VLAN 10 to VLAN 20, making option B correct.
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 port Gi1/0/24 is not in trunking mode.
Why it's wrong here
The trunk port Gi1/0/24 is not the cause because the switch output explicitly shows Gi1/0/24 in a trunking state, with the administrative mode set to 'on' and the operational mode as trunk. If the trunk were not functioning, VLAN 20 frames would not reach the router at all, but the router would still need a subinterface to accept them. Since the trunk is up and carrying VLAN 10 and 20 traffic as expected, this option is incorrect.
- ✓
The router is missing a subinterface for VLAN 20.
Why this is correct
This is the correct answer because the router is configured for router-on-a-stick inter-VLAN routing but only has a subinterface for VLAN 10. Since the router lacks a subinterface with the appropriate 802.1Q encapsulation and an IP address in the VLAN 20 subnet, it simply drops or ignores VLAN 20 frames arriving over the trunk. Even though the switch and trunk are configured correctly, VLAN 20 hosts have no default gateway on the router and therefore cannot route traffic to VLAN 10 or any other network.
- ✗
The switch port Gi1/0/12 is not assigned to VLAN 10.
Why it's wrong here
The switch port Gi1/0/12 is not the problem because the 'show vlan brief' output clearly lists Gi1/0/12 as an access port assigned to VLAN 10, which matches the PC's expected VLAN. If the port were in the wrong VLAN, the PC would not be able to reach its VLAN 10 gateway at all, yet it can ping that gateway successfully. Thus, the PC's Layer 2 connectivity is correct, and the fault must lie upstream in the routing configuration.
- ✗
The PC has a duplicate IP address with the server.
Why it's wrong here
A duplicate IP address between the PC and the server would cause address conflicts, typically resulting in intermittent connectivity, ARP table instability, and both devices struggling to communicate. However, the PC can successfully ping its VLAN 10 gateway, which proves its IP stack and addressing are functional and unique on the local segment. The observed problem is isolated to inter-VLAN routing to VLAN 20, which a duplicate IP would not selectively cause, so this option is wrong.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The router is missing a subinterface for VLAN 20.Correct answer▾
Why this is correct
This is the correct answer because the router is configured for router-on-a-stick inter-VLAN routing but only has a subinterface for VLAN 10. Since the router lacks a subinterface with the appropriate 802.1Q encapsulation and an IP address in the VLAN 20 subnet, it simply drops or ignores VLAN 20 frames arriving over the trunk. Even though the switch and trunk are configured correctly, VLAN 20 hosts have no default gateway on the router and therefore cannot route traffic to VLAN 10 or any other network.
✗The trunk port Gi1/0/24 is not in trunking mode.Wrong answer — click to see why▾
Why this is wrong here
The trunk port Gi1/0/24 is in 'on' mode and trunking, as shown in 'show interfaces trunk'. Therefore, the trunk is operational and not the cause of the issue.
Why candidates choose this
Students might assume that trunk ports need to be manually configured as trunks, but the output confirms it is already trunking. They may overlook the verification step.
✗The switch port Gi1/0/12 is not assigned to VLAN 10.Wrong answer — click to see why▾
Why this is wrong here
The 'show vlan brief' output shows that port Gi1/0/12 is assigned to VLAN 10, so the PC is in the correct VLAN. This is not the issue.
Why candidates choose this
Students may suspect a VLAN assignment error because the PC cannot reach the server, but the output confirms correct assignment. They might not check the output carefully.
✗The PC has a duplicate IP address with the server.Wrong answer — click to see why▾
Why this is wrong here
A duplicate IP address would cause connectivity issues to the gateway as well, but the PC can ping the gateway successfully. Therefore, duplicate IP is not the problem.
Why candidates choose this
Students might think duplicate IP could cause intermittent issues, but the symptom of being able to ping the gateway rules it out. They may not consider that duplicate IP would affect all communication.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Related to this question
Learn chapter
ICMP and Ping
Key term
Access port
An access port is a switch port that connects to a single end device, like a computer or printer, and carries traffic for only one VLAN.
Key term
Route
A route is a path that data takes through a network from one device or network to another, determined by routing protocols and configured rules.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.