- A
Verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20.
This directly addresses the most probable cause: a missing or misconfigured trunk on the switch side. Even with router sub-interfaces up/up, the link must be a trunk carrying the correct VLANs for inter-VLAN routing to function.
- B
Verify the encapsulation dot1Q numbers on the sub-interfaces match the VLAN assignments.
Why wrong: The stem explicitly states the sub-interfaces are configured with 'encapsulation dot1q 10' and 'encapsulation dot1q 20' – this verification has already been done and is not the issue.
- C
Verify the default gateway settings on hosts in VLANs 10 and 20.
Why wrong: While a wrong default gateway could cause reachability issues, the more fundamental problem is likely the trunk link. The router's sub-interface IPs are the gateways, but if the trunk is not carrying the VLANs, the hosts cannot even send traffic to the gateway. Checking host settings before confirming Layer 2 connectivity skips essential steps.
- D
Check the physical interface G0/0 for interface errors or duplex mismatches.
Why wrong: Errors or duplex mismatches could cause performance issues, but the interface is already up/up and no mention of errors or drops exists. This is a lower-probability cause and should be investigated only after ruling out trunk misconfiguration.
Quick Answer
The answer is to verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20. This is correct because the router’s sub-interfaces showing up/up only confirm the physical link and Layer 2 protocol are active using the native VLAN, but they do not prove that tagged frames for VLANs 10 and 20 can actually traverse the link. Without a properly configured trunk, the switch will drop or ignore those 802.1Q-tagged frames, breaking inter-VLAN communication at Layer 2 before any routing can occur. On the CCNA 200-301 v2 exam, this scenario tests your understanding that router-on-a-stick trunk troubleshooting must start at the switch port, not the router—a common trap is to immediately check IP routing or ACLs when the real issue is a missing trunk or allowed VLAN list. Remember the mnemonic “Trunk First, Route Later” to avoid wasting time on Layer 3 when Layer 2 is the bottleneck.
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. 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.
A network technician is troubleshooting a router-on-a-stick configuration. R1 has sub-interface G0/0.10 with encapsulation dot1q 10 and IP 192.168.10.1/24, and sub-interface G0/0.20 with encapsulation dot1q 20 and IP 192.168.20.1/24. Hosts in VLAN 10 cannot reach hosts in VLAN 20. The physical interface G0/0 is up and no shutdown. Both sub-interfaces show up/up. What should the technician do next?
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
Verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20.
The router's sub-interfaces being 'up/up' only confirms the physical link and Layer 2 protocol (using the native VLAN untagged, often VLAN 1) are active. It does not prove that the switch port is a trunk, nor that VLANs 10 and 20 are allowed across the link. Without a properly configured trunk, tagged frames for those VLANs will not traverse. Verifying the switch port trunk configuration and allowed VLANs is the most logical next step to resolve inter-VLAN communication at Layer 2 before investigating Layer 3 settings.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20.
Why this is correct
This directly addresses the most probable cause: a missing or misconfigured trunk on the switch side. Even with router sub-interfaces up/up, the link must be a trunk carrying the correct VLANs for inter-VLAN routing to function.
Related concept
Access ports place end devices into a single VLAN.
- ✗
Verify the encapsulation dot1Q numbers on the sub-interfaces match the VLAN assignments.
Why it's wrong here
The stem explicitly states the sub-interfaces are configured with 'encapsulation dot1q 10' and 'encapsulation dot1q 20' – this verification has already been done and is not the issue.
- ✗
Verify the default gateway settings on hosts in VLANs 10 and 20.
Why it's wrong here
While a wrong default gateway could cause reachability issues, the more fundamental problem is likely the trunk link. The router's sub-interface IPs are the gateways, but if the trunk is not carrying the VLANs, the hosts cannot even send traffic to the gateway. Checking host settings before confirming Layer 2 connectivity skips essential steps.
- ✗
Check the physical interface G0/0 for interface errors or duplex mismatches.
Why it's wrong here
Errors or duplex mismatches could cause performance issues, but the interface is already up/up and no mention of errors or drops exists. This is a lower-probability cause and should be investigated only after ruling out trunk misconfiguration.
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.
✓Verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20.Correct answer▾
Why this is correct
This directly addresses the most probable cause: a missing or misconfigured trunk on the switch side. Even with router sub-interfaces up/up, the link must be a trunk carrying the correct VLANs for inter-VLAN routing to function.
✗Verify the encapsulation dot1Q numbers on the sub-interfaces match the VLAN assignments.Wrong answer — click to see why▾
Why this is wrong here
Candidates may think the encapsulation numbers might be swapped, but the stem confirms they are correctly assigned to the respective VLAN IDs.
✗Verify the default gateway settings on hosts in VLANs 10 and 20.Wrong answer — click to see why▾
Why this is wrong here
Many candidates jump to end-host configuration, assuming the router is fully reachable because interfaces are up/up, but the trunk is the prerequisite for any communication between VLANs.
✗Check the physical interface G0/0 for interface errors or duplex mismatches.Wrong answer — click to see why▾
Why this is wrong here
Candidates might think any communication loss warrants a physical layer check, but here the symptoms point strongly toward a Layer 2 trunking issue.
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Verify the switch port connected to R1 is configured as a trunk and allows VLANs 10 and 20. — The router's sub-interfaces being 'up/up' only confirms the physical link and Layer 2 protocol (using the native VLAN untagged, often VLAN 1) are active. It does not prove that the switch port is a trunk, nor that VLANs 10 and 20 are allowed across the link. Without a properly configured trunk, tagged frames for those VLANs will not traverse. Verifying the switch port trunk configuration and allowed VLANs is the most logical next step to resolve inter-VLAN communication at Layer 2 before investigating Layer 3 settings.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 14, 2026
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.
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