- A
The switch port to the workstation is incorrectly configured as a trunk instead of an access port.
Why wrong: If the switch port were a trunk, the show command would display 'trunk' status, not 'up/up' as an access port. The output indicates it is correctly an access port.
- B
The native VLAN on the trunk between the switch and router is mismatched.
Why wrong: A native VLAN mismatch would cause problems for untagged frames (typically VLAN 1), but the workstation is in VLAN 10, which is tagged. The symptom of no inter-VLAN communication for tagged VLANs would not be caused by a native VLAN mismatch.
- C
VLAN 10 is not allowed on the trunk link.
Why wrong: If VLAN 10 were not allowed on the trunk, the sub-interface would still appear up/up, but frames would be dropped at the trunk. However, a missing allowed VLAN would affect all traffic for that VLAN, while the specific symptom paired with a missing encapsulation command is a more precise root cause; the allowed list is often left to permit all VLANs by default.
- D
The router sub-interface for VLAN 10 is missing the encapsulation dot1q command.
In router-on-a-stick, each sub-interface must be mapped to a VLAN with the 'encapsulation dot1q <vlan>' command. Without it, the sub-interface cannot identify or tag frames for VLAN 10, so it never processes them, despite showing up/up. This perfectly explains why the workstation can't reach other VLANs.
Quick Answer
The answer is that the router sub-interface for VLAN 10 is missing the encapsulation dot1q command. This is the most likely cause because a router-on-a-stick configuration relies on sub-interfaces to tag incoming frames with the correct VLAN ID; without the `encapsulation dot1q 10` command, the sub-interface will not process 802.1Q-tagged traffic from the switch trunk, causing inter-VLAN routing to fail even though the interface shows up/up. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how trunking and sub-interface configuration interact—a common trap is assuming a sub-interface is fully operational just because it is administratively up. Remember the memory tip: "No dot1q, no tag—your VLAN traffic will drag."
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.
A network engineer notices that a workstation in VLAN 10 cannot communicate with hosts in VLAN 20. The workstation is connected to an access layer switch port that shows 'up/up' in show ip interface brief. The switch's trunk port to the router is up, and the router's sub-interface for VLAN 10 is also in an up/up state. The router-on-a-stick configuration appears operational, but inter-VLAN traffic still fails. What is the most likely cause?
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.
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 sub-interface for VLAN 10 is missing the encapsulation dot1q command.
The router-on-a-stick configuration requires each sub-interface to use the `encapsulation dot1q <vlan-id>` command to tag traffic with the correct VLAN ID. Without this command, the sub-interface will not accept or forward frames tagged for VLAN 10, even if the interface is administratively up. Since the workstation in VLAN 10 can reach its local gateway but not VLAN 20, the missing encapsulation is the most likely cause.
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 switch port to the workstation is incorrectly configured as a trunk instead of an access port.
Why it's wrong here
If the switch port were a trunk, the show command would display 'trunk' status, not 'up/up' as an access port. The output indicates it is correctly an access port.
- ✗
The native VLAN on the trunk between the switch and router is mismatched.
Why it's wrong here
A native VLAN mismatch would cause problems for untagged frames (typically VLAN 1), but the workstation is in VLAN 10, which is tagged. The symptom of no inter-VLAN communication for tagged VLANs would not be caused by a native VLAN mismatch.
- ✗
VLAN 10 is not allowed on the trunk link.
Why it's wrong here
If VLAN 10 were not allowed on the trunk, the sub-interface would still appear up/up, but frames would be dropped at the trunk. However, a missing allowed VLAN would affect all traffic for that VLAN, while the specific symptom paired with a missing encapsulation command is a more precise root cause; the allowed list is often left to permit all VLANs by default.
- ✓
The router sub-interface for VLAN 10 is missing the encapsulation dot1q command.
Why this is correct
In router-on-a-stick, each sub-interface must be mapped to a VLAN with the 'encapsulation dot1q <vlan>' command. Without it, the sub-interface cannot identify or tag frames for VLAN 10, so it never processes them, despite showing up/up. This perfectly explains why the workstation can't reach other VLANs.
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.
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 sub-interface for VLAN 10 is missing the encapsulation dot1q command.Correct answer▾
Why this is correct
In router-on-a-stick, each sub-interface must be mapped to a VLAN with the 'encapsulation dot1q <vlan>' command. Without it, the sub-interface cannot identify or tag frames for VLAN 10, so it never processes them, despite showing up/up. This perfectly explains why the workstation can't reach other VLANs.
✗The switch port to the workstation is incorrectly configured as a trunk instead of an access port.Wrong answer — click to see why▾
Why this is wrong here
Candidates may confuse the access port configuration with a trunk, but the stem explicitly states the port shows 'up/up' in show ip interface brief, which does not indicate trunk status.
✗The native VLAN on the trunk between the switch and router is mismatched.Wrong answer — click to see why▾
Why this is wrong here
Native VLAN mismatch is a common troubleshooting issue, but it only affects untagged traffic on the native VLAN—not tagged VLANs like VLAN 10.
✗VLAN 10 is not allowed on the trunk link.Wrong answer — click to see why▾
Why this is wrong here
Candidates often assume the allowed VLAN list is the culprit, but default trunk behavior permits all VLANs unless explicitly pruned, and the stem gives no indication of pruning.
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 sub-interface being 'up/up' guarantees it is fully operational for inter-VLAN routing, when in fact the missing `encapsulation dot1q` command leaves the sub-interface unable to process tagged frames.
Trap categories for this question
Command / output trap
If the switch port were a trunk, the show command would display 'trunk' status, not 'up/up' as an access port. The output indicates it is correctly an access port.
Detailed technical explanation
How to think about this question
In a router-on-a-stick topology, the router's physical interface is divided into logical sub-interfaces, each associated with a specific VLAN via the `encapsulation dot1q` command. This command inserts a 4-byte 802.1Q tag into the Ethernet frame header, allowing the router to distinguish traffic from different VLANs. Without this encapsulation, the sub-interface operates as a standard routed interface and will only accept untagged frames, causing all tagged VLAN 10 traffic to be dropped at the router's ingress.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 200-301 question test?
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 sub-interface for VLAN 10 is missing the encapsulation dot1q command. — The router-on-a-stick configuration requires each sub-interface to use the `encapsulation dot1q <vlan-id>` command to tag traffic with the correct VLAN ID. Without this command, the sub-interface will not accept or forward frames tagged for VLAN 10, even if the interface is administratively up. Since the workstation in VLAN 10 can reach its local gateway but not VLAN 20, the missing encapsulation is the most likely cause.
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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Last reviewed: Jun 25, 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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