- A
The router subinterface for VLAN 20 is not enabled.
Why wrong: If the subinterface were not enabled, pings to the VLAN 10 subinterface would also likely fail. The issue is at the switch port, not the router.
- B
The switch port connecting to the router should be configured as a trunk.
A trunk port carries traffic for multiple VLANs. Since the router uses subinterfaces with 802.1Q encapsulation, the switch must allow tagged frames for both VLANs.
- C
The hosts in VLAN 20 do not have a default gateway configured.
Why wrong: The problem is that traffic from VLAN 20 cannot reach the router's subinterface at all; the default gateway is not reachable.
- D
The router's VLAN 20 subinterface has an incorrect IP address.
Why wrong: The subinterface is configured correctly; the hosts cannot ping it because the switch port is blocking VLAN 20 traffic.
Quick Answer
The answer is that the switch port connecting to the router must be configured as a trunk. In a router-on-a-stick design, the router uses a single physical interface divided into subinterfaces, each tagged with a specific VLAN ID via 802.1Q encapsulation. The switch port must be a trunk to carry those VLAN tags; an access port strips all tags and only permits one VLAN, which is why hosts in VLAN 20 cannot reach the router—their frames are dropped at the switch port. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of VLAN tagging and trunk port requirements for inter-VLAN routing. A common trap is assuming an access port works because one VLAN succeeds, but the router needs tagged frames from all VLANs. Remember the memory tip: "One router, many VLANs—trunk the port, not the access plan."
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. 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 is configuring a router-on-a-stick to route between two VLANs (VLAN 10 and VLAN 20). The router has two subinterfaces: GigabitEthernet0/1.10 with encapsulation dot1Q 10 and IP 10.10.10.1/24, and GigabitEthernet0/1.20 with encapsulation dot1Q 20 and IP 10.10.20.1/24. The switch port connected to the router is configured as an access port in VLAN 10. Hosts in VLAN 10 can ping the router's VLAN 10 interface, but hosts in VLAN 20 cannot ping the router's VLAN 20 interface. 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 switch port connecting to the router should be 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, not an access port. An access port only carries traffic for a single VLAN (VLAN 10 in this case), so frames from VLAN 20 are dropped at the switch port before reaching the router. Configuring the port as a trunk with allowed VLANs 10 and 20 would enable the router's subinterfaces to receive and forward traffic for both VLANs.
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 subinterface for VLAN 20 is not enabled.
Why it's wrong here
If the subinterface were not enabled, pings to the VLAN 10 subinterface would also likely fail. The issue is at the switch port, not the router.
- ✓
The switch port connecting to the router should be configured as a trunk.
Why this is correct
A trunk port carries traffic for multiple VLANs. Since the router uses subinterfaces with 802.1Q encapsulation, the switch must allow tagged frames for both 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 hosts in VLAN 20 do not have a default gateway configured.
Why it's wrong here
The problem is that traffic from VLAN 20 cannot reach the router's subinterface at all; the default gateway is not reachable.
- ✗
The router's VLAN 20 subinterface has an incorrect IP address.
Why it's wrong here
The subinterface is configured correctly; the hosts cannot ping it because the switch port is blocking VLAN 20 traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between access and trunk ports in router-on-a-stick scenarios, trapping candidates who assume that configuring subinterfaces alone is sufficient without ensuring the switch port is set to trunk mode.
Detailed technical explanation
How to think about this question
In a router-on-a-stick topology, the router uses 802.1Q trunking to receive and send frames tagged with different VLAN IDs. The switch port must be set to 'switchport mode trunk' and, if needed, 'switchport trunk allowed vlan 10,20' to pass the 802.1Q-tagged frames. Without trunking, an access port strips VLAN tags and only forwards untagged frames belonging to its configured VLAN, effectively isolating the router from any other VLAN traffic.
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 N10-009 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The switch port connecting to the router should be 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, not an access port. An access port only carries traffic for a single VLAN (VLAN 10 in this case), so frames from VLAN 20 are dropped at the switch port before reaching the router. Configuring the port as a trunk with allowed VLANs 10 and 20 would enable the router's subinterfaces to receive and forward traffic for both VLANs.
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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