The answer is three. In a VLAN trunk setup, each VLAN represents a separate Layer 2 broadcast domain, but a Layer 3 broadcast domain only exists where a router interface—or subinterface—provides a default gateway for that VLAN. Since the router has subinterfaces configured for VLANs 10, 20, and 30, each of those three VLANs becomes its own Layer 3 broadcast domain. VLAN 99, the native VLAN, has no router subinterface, so it remains strictly a Layer 2 broadcast domain with no Layer 3 boundary. On the CompTIA Network+ N10-009 exam, this question tests your understanding of the relationship between VLANs, trunking with 802.1Q, and router-on-a-stick subinterfaces. A common trap is to count every VLAN as a Layer 3 domain, forgetting that a router must be present to route traffic for that VLAN. Remember the memory tip: “No router, no Layer 3”—if a VLAN lacks a subinterface, it cannot have a Layer 3 broadcast domain.
N10-009 Networking Concepts Practice Question
This N10-009 practice question tests your understanding of networking concepts. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 has configured a switch with four VLANs: VLAN 10, 20, 30, and 99 (native). The switch is connected to a router via an 802.1Q trunk link. The router has subinterfaces for VLANs 10, 20, and 30, each with an IP address. VLAN 99 is used for management and does not have a router subinterface. How many Layer 3 broadcast domains exist in this network?
Why wrong: Incorrect. A single broadcast domain would imply all devices can communicate via broadcast, which is not the case with multiple VLANs.
B
2
Why wrong: Incorrect. There are more than two broadcast domains because each VLAN, including the native VLAN 99, is isolated.
C
3
Why wrong: Incorrect. This count assumes only VLANs with router subinterfaces are broadcast domains, but VLAN 99 without a subinterface still constitutes a broadcast domain.
D
4
Correct. Each of the four VLANs (10, 20, 30, 99) is a separate broadcast domain, regardless of router subinterface configuration.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
4
Each VLAN is a separate Layer 2 broadcast domain, and a router subinterface provides the default gateway for that VLAN, creating a corresponding Layer 3 broadcast domain. VLAN 99 (native) has no router subinterface, so it does not have a Layer 3 broadcast domain. Therefore, only VLANs 10, 20, and 30 have Layer 3 broadcast domains, totaling 3.
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.
✗
1
Why it's wrong here
Incorrect. A single broadcast domain would imply all devices can communicate via broadcast, which is not the case with multiple VLANs.
✗
2
Why it's wrong here
Incorrect. There are more than two broadcast domains because each VLAN, including the native VLAN 99, is isolated.
✗
3
Why it's wrong here
Incorrect. This count assumes only VLANs with router subinterfaces are broadcast domains, but VLAN 99 without a subinterface still constitutes a broadcast domain.
✓
4
Why this is correct
Correct. Each of the four VLANs (10, 20, 30, 99) is a separate broadcast domain, regardless of router subinterface configuration.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between Layer 2 broadcast domains (one per VLAN) and Layer 3 broadcast domains (one per routed subnet), leading candidates to incorrectly count all VLANs as Layer 3 domains even when a native VLAN lacks a subinterface.
Detailed technical explanation
How to think about this question
In a router-on-a-stick configuration, each subinterface is configured with encapsulation dot1Q <vlan-id> and an IP address, creating a distinct Layer 3 interface. The native VLAN (99) is untagged on the trunk and is typically used for management traffic; without a subinterface, the router cannot route for that VLAN. Broadcast domains at Layer 3 are defined by the subnet assigned to each subinterface, and ARP requests are confined to that subnet.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Networking Concepts — This question tests Networking Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 4 — Each VLAN is a separate Layer 2 broadcast domain, and a router subinterface provides the default gateway for that VLAN, creating a corresponding Layer 3 broadcast domain. VLAN 99 (native) has no router subinterface, so it does not have a Layer 3 broadcast domain. Therefore, only VLANs 10, 20, and 30 have Layer 3 broadcast domains, totaling 3.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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