CCNA IP Routing Practice Question
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?
⚠ Common exam trap
Cisco often tests the misconception that as long as the router sub-interfaces are up/up and have correct encapsulation, inter-VLAN routing should work, leading candidates to overlook the switch trunk configuration as the root cause.
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-on-a-stick configuration requires the switch port connecting to R1 to be configured as a trunk port that permits VLANs 10 and 20. Even though the router sub-interfaces are correctly configured with encapsulation dot1q and IP addresses, if the switch port is in access mode or does not allow the specific VLANs, frames from VLAN 10 or 20 will be dropped by the switch, preventing inter-VLAN routing. The next logical step is to verify the switch port configuration with commands like 'show interfaces trunk' or 'show running-config interface <port>'.
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.
- ✗
Verify the encapsulation dot1Q numbers on the sub-interfaces match the VLAN assignments.
Why it's wrong here
Because the stem already confirms the sub-interfaces are configured with `encapsulation dot1q 10` and `encapsulation dot1q 20`, this verification would only re-check a known-good setting. Even if the tags exactly match VLAN 10 and 20, the router cannot forward traffic if the switch port is not a trunk carrying those VLANs, so this check is redundant and cannot identify the actual fault. It also ignores that a mismatch on the switch side (e.g., VLANs not allowed or an access port) would produce identical symptoms.
- ✗
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?”
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Related to this question
Learn chapter
OSPFv3 Single-Area Configuration for IPv6
Key term
Encapsulation
Encapsulation is the process of wrapping data with protocol headers and trailers before sending it across a network.
Key term
Interface
An interface is a point of connection or interaction between two systems, devices, or software components that allows them to exchange information or signals.
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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.