N10-009 Network Troubleshooting Practice Question
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?
⚠ Common exam trap
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.
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.
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.
When this WOULD be correct
In a scenario where a router-on-a-stick is configured but subinterfaces lack IP addresses, hosts in different VLANs cannot communicate because the router has no gateway for each VLAN. This would be the cause if the administrator forgot to assign IPs.
- ✓
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.
- ✗
The VLANs are not created on the switch.
Why it's wrong here
If VLANs were not properly created and assigned on the switch, devices within the same intended VLAN would be unable to communicate with each other, as their frames would not be correctly tagged or forwarded within the isolated broadcast domain. The ability for users to communicate *within* their respective VLANs confirms that the VLANs themselves have been successfully established and configured on the switch ports. This issue specifically pertains to inter-VLAN communication, implying the VLANs exist but cannot reach the router correctly.
When this WOULD be correct
In a scenario where users cannot communicate within their own VLAN, and the switch has no VLAN configuration, the most likely cause would be that the VLANs are not created on the switch. For example, if a new switch is deployed without VLAN definitions, intra-VLAN traffic would fail.
- ✗
The router's default route is missing.
Why it's wrong here
A default route (0.0.0.0/0) is primarily configured to direct traffic for destinations not explicitly listed in the router's routing table, typically towards an upstream router or the internet. For inter-VLAN routing, the router only needs direct knowledge of the VLAN subnets, which are established when IP addresses are assigned to its subinterfaces. Therefore, a missing default route would not prevent communication between directly connected VLANs on the same router-on-a-stick setup, as these routes are directly connected.
When this WOULD be correct
In a scenario where a router is used to connect a local network to the internet, and hosts in VLAN 10 cannot access external networks (e.g., the internet) while inter-VLAN communication works, a missing default route on the router would be the likely cause.
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 N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓The switch port connected to the router is not configured as a trunk.Correct answer▾
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.
✗The router's subinterfaces are not configured with IP addresses.Wrong answer — click to see why▾
Why this is wrong here
The router's subinterfaces are configured, so they likely have IP addresses. The issue is the switch port is an access port in VLAN 1, preventing tagged traffic from VLANs 10 and 20 from reaching the router.
★ When this WOULD be the correct answer
In a scenario where a router-on-a-stick is configured but subinterfaces lack IP addresses, hosts in different VLANs cannot communicate because the router has no gateway for each VLAN. This would be the cause if the administrator forgot to assign IPs.
Why candidates choose this
Candidates may assume that missing IP addresses on subinterfaces is a common misconfiguration, but the question states subinterfaces are configured, implying IPs are set. They overlook the trunking issue.
✗The VLANs are not created on the switch.Wrong answer — click to see why▾
Why this is wrong here
The question states that users within each VLAN can communicate, which means the VLANs are already created on the switch. The issue is inter-VLAN routing, which fails because the switch port to the router is an access port in VLAN 1, not a trunk carrying both VLANs.
★ When this WOULD be the correct answer
In a scenario where users cannot communicate within their own VLAN, and the switch has no VLAN configuration, the most likely cause would be that the VLANs are not created on the switch. For example, if a new switch is deployed without VLAN definitions, intra-VLAN traffic would fail.
Why candidates choose this
Candidates may think that VLANs must be explicitly created for inter-VLAN routing to work, overlooking that the problem here is the trunk port misconfiguration, not the existence of VLANs.
✗The router's default route is missing.Wrong answer — click to see why▾
Why this is wrong here
The router's default route is used for reaching networks not directly connected, not for inter-VLAN routing. Since the router has subinterfaces for VLAN 10 and 20, it can route between them directly without a default route. The issue is that the switch port is an access port, preventing VLAN-tagged traffic from reaching the router.
★ When this WOULD be the correct answer
In a scenario where a router is used to connect a local network to the internet, and hosts in VLAN 10 cannot access external networks (e.g., the internet) while inter-VLAN communication works, a missing default route on the router would be the likely cause.
Why candidates choose this
Candidates may confuse inter-VLAN routing with routing to external networks, assuming that a default route is necessary for any routing between VLANs. They might overlook that the router's directly connected subinterfaces provide routes for the VLANs.
Analysis generated from the official N10-009blueprint 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
Troubleshoot: Network Connectivity
Key term
VLAN
A VLAN (Virtual Local Area Network) is a logical grouping of network devices that behave as if they are on the same physical network segment, regardless of their actual physical location.
Key term
Inter-VLAN routing
Inter-VLAN routing is the process of forwarding network traffic between different VLANs to enable communication across logically separated broadcast domains.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.