The answer is a missing subinterface for VLAN 20 on the router. This is correct because in a router-on-a-stick configuration, the router must have a separate logical subinterface for each VLAN that needs inter-VLAN routing. Since the PC can ping its gateway (VLAN 10) but not the server in VLAN 20, the Layer 3 path is broken between the two VLANs—the switch is correctly trunking both VLANs, but without a subinterface for VLAN 20, the router has no way to receive or forward traffic destined for that subnet. On the CCNA 200-301 v2 exam, this scenario tests your understanding of router-on-a-stick missing subinterface troubleshooting, a classic Layer 3 connectivity issue. A common trap is assuming the problem is on the switch side, but the switch’s ‘show vlan brief’ and ‘show interfaces trunk’ would confirm VLAN 20 exists and is trunked. The real culprit is the router’s missing subinterface. Memory tip: “No sub, no routing between VLANs—every VLAN needs its own dot1q door.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. 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.
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?
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.
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
The trunk port Gi1/0/24 is not in trunking mode.
Why wrong: The output shows the trunk port is in 'on' mode and trunking, so this is not the issue.
B
The router is missing a subinterface for VLAN 20.
The PC in VLAN 10 can ping its gateway, but VLAN 20 traffic cannot be routed because the router lacks a subinterface for VLAN 20.
C
The switch port Gi1/0/12 is not assigned to VLAN 10.
Why wrong: The 'show vlan brief' output shows Gi1/0/12 is in VLAN 10, so the PC is in the correct VLAN.
D
The PC has a duplicate IP address with the server.
Why wrong: Duplicate IP would cause connectivity issues to the gateway as well, but the PC can ping the gateway successfully.
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.
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 port Gi1/0/24 is not in trunking mode.
Why it's wrong here
The output shows the trunk port is in 'on' mode and trunking, so this is not the issue.
✓
The router is missing a subinterface for VLAN 20.
Why this is correct
The PC in VLAN 10 can ping its gateway, but VLAN 20 traffic cannot be routed because the router lacks a subinterface for VLAN 20.
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 switch port Gi1/0/12 is not assigned to VLAN 10.
Why it's wrong here
The 'show vlan brief' output shows Gi1/0/12 is in VLAN 10, so the PC is in the correct VLAN.
✗
The PC has a duplicate IP address with the server.
Why it's wrong here
Duplicate IP would cause connectivity issues to the gateway as well, but the PC can ping the gateway successfully.
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
The PC in VLAN 10 can ping its gateway, but VLAN 20 traffic cannot be routed because the router lacks a subinterface for VLAN 20.
✗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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Trap categories for this question
Command / output trap
The output shows the trunk port is in 'on' mode and trunking, so this is not the issue.
Detailed technical explanation
How to think about this question
In a router-on-a-stick configuration, the router uses subinterfaces (e.g., Gi0/0.10 for VLAN 10, Gi0/0.20 for VLAN 20) with 802.1Q encapsulation to route between VLANs. The switch trunk must allow both VLANs, and the router must have a subinterface for each VLAN that needs inter-VLAN routing; missing a subinterface for VLAN 20 means the router has no Layer 3 interface for that subnet, so it drops packets destined for 192.168.20.10. A real-world scenario is when a network engineer adds a new VLAN but forgets to configure the corresponding subinterface on the router, causing one-way or no connectivity to that VLAN.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 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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