- A
The router's subinterfaces are not configured with IP addresses.
Why wrong: If subinterfaces lacked IP addresses, they would not be able to route, but the question states the router has configured subinterfaces, implying IPs are set. Additionally, the switch port misconfiguration is a more fundamental issue.
- B
The switch port connected to the router is not configured as a trunk.
This is the most likely cause. A trunk port is required to carry frames from multiple VLANs to the router for inter-VLAN routing. An access port only carries a single VLAN, preventing the router from receiving traffic from other VLANs.
- C
The VLANs are not created on the switch.
Why wrong: If the VLANs were not created, users would not be able to communicate within their own VLAN either. Since intra-VLAN communication works, the VLANs exist.
- D
The router's default route is missing.
Why wrong: A default route is used for traffic destined to external networks, not for inter-VLAN routing. The subnet routes for VLAN 10 and 20 are directly connected on the router's subinterfaces.
Quick Answer
The answer is that the switch port connected to the router must be configured as a trunk port. In a router-on-a-stick design, the single physical link between the switch and router carries traffic for multiple VLANs, so the switch port must be set to trunk mode using 802.1Q tagging. When the port is an access port in VLAN 1, it strips all VLAN tags and only forwards frames from VLAN 1, meaning the router’s subinterfaces for VLAN 10 and VLAN 20 never receive the tagged frames they need to route between the VLANs. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how inter-VLAN routing depends on trunking to preserve VLAN membership across the link. A common trap is assuming subinterfaces alone are enough, but the switch port configuration is the critical missing piece. Remember the memory tip: “No trunk, no truck—traffic can’t cross VLANs without a trunk port.”
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. 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 administrator configures a router-on-a-stick to route traffic between VLAN 10 and VLAN 20. Users in each VLAN can communicate within their own VLAN but cannot reach devices in the other VLAN. The router has subinterfaces configured, and the switch port connected to the router is configured as an access port in VLAN 1. What is the most likely cause of the inter-VLAN connectivity failure?
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 switch port connected to the router is not configured as a trunk.
The router-on-a-stick design requires the switch port connecting to the router to be configured as a trunk port. This allows the switch to forward frames from multiple VLANs (VLAN 10 and VLAN 20) to the router's subinterfaces, each tagged with the appropriate 802.1Q VLAN ID. When the port is set as an access port in VLAN 1, it only accepts untagged frames from VLAN 1, so the router's subinterfaces for VLAN 10 and VLAN 20 never receive traffic, breaking inter-VLAN routing.
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 router's subinterfaces are not configured with IP addresses.
Why it's wrong here
If subinterfaces lacked IP addresses, they would not be able to route, but the question states the router has configured subinterfaces, implying IPs are set. Additionally, the switch port misconfiguration is a more fundamental issue.
- ✓
The switch port connected to the router is not configured as a trunk.
Why this is correct
This is the most likely cause. A trunk port is required to carry frames from multiple VLANs to the router for inter-VLAN routing. An access port only carries a single VLAN, preventing the router from receiving traffic from 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.
- ✗
The VLANs are not created on the switch.
Why it's wrong here
If the VLANs were not created, users would not be able to communicate within their own VLAN either. Since intra-VLAN communication works, the VLANs exist.
- ✗
The router's default route is missing.
Why it's wrong here
A default route is used for traffic destined to external networks, not for inter-VLAN routing. The subnet routes for VLAN 10 and 20 are directly connected on the router's subinterfaces.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that a router-on-a-stick only needs subinterfaces with IP addresses, leading candidates to overlook the critical requirement that the switch port must be a trunk to carry multiple VLAN tags.
Detailed technical explanation
How to think about this question
In a router-on-a-stick topology, the router's subinterfaces use 802.1Q encapsulation to tag frames with the VLAN ID. The switch trunk port must be configured with 'switchport mode trunk' and allowed VLANs to pass tagged frames; if set as an access port, the switch strips any 802.1Q tags and only forwards untagged frames from the access VLAN, causing the router to never see frames from VLAN 10 or 20. Real-world misconfigurations often occur when an administrator forgets to change the default switchport mode from dynamic desirable to trunk, or when the native VLAN mismatch causes unexpected behavior.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Network Troubleshooting — study guide chapter
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The switch port connected to the router is not configured as a trunk. — The router-on-a-stick design requires the switch port connecting to the router to be configured as a trunk port. This allows the switch to forward frames from multiple VLANs (VLAN 10 and VLAN 20) to the router's subinterfaces, each tagged with the appropriate 802.1Q VLAN ID. When the port is set as an access port in VLAN 1, it only accepts untagged frames from VLAN 1, so the router's subinterfaces for VLAN 10 and VLAN 20 never receive traffic, breaking inter-VLAN routing.
What should I do if I get this N10-009 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 30, 2026
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.
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