The answer is three broadcast domains. This is correct because each VLAN creates its own isolated Layer 2 broadcast domain, and with VLANs 10, 20, and 30 configured on a single switch, broadcasts from devices in one VLAN never reach devices in another VLAN. The router’s three subinterfaces provide inter-VLAN routing at Layer 3, but they do not merge the broadcast domains; instead, each subinterface belongs to a distinct VLAN, preserving the separation. On the CompTIA Network+ N10-009 exam, this question tests your understanding that broadcast domains are defined by VLANs, not by the number of switches or routers—a common trap is thinking a router reduces broadcast domains, but it actually keeps them separate while enabling communication between them. A helpful memory tip is “one VLAN, one broadcast domain,” so for three VLANs, you always count three.
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 has a single switch with VLANs 10, 20, and 30 configured. The switch is connected to a router that has three subinterfaces, each in a different VLAN. How many broadcast domains are present?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
3
Each VLAN is a separate Layer 2 broadcast domain. With VLANs 10, 20, and 30 configured on the switch and a router using subinterfaces to route between them, there are exactly three broadcast domains — one per VLAN. Broadcasts are confined to their VLAN and do not cross VLAN boundaries without a Layer 3 device.
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
A broadcast domain is isolated per VLAN; a single broadcast domain would mean all devices are in the same VLAN.
✓
3
Why this is correct
Each VLAN (10, 20, 30) creates its own broadcast domain. Router subinterfaces do not combine them.
Related concept
Read the scenario before looking for a memorised answer.
✗
4
Why it's wrong here
There are only three VLANs, so there cannot be four broadcast domains unless there is an additional unmentioned VLAN.
✗
5
Why it's wrong here
There is no fifth broadcast domain; the router does not create additional broadcast domains.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often count the router subinterfaces as separate broadcast domains, not realizing that broadcast domains are strictly Layer 2 constructs and that the router only provides inter-VLAN routing without adding new broadcast domains.
Detailed technical explanation
How to think about this question
A broadcast domain is defined by the set of devices that receive a broadcast frame sent by any member. In a VLAN-based switched network, each VLAN corresponds to an isolated broadcast domain at Layer 2. The router’s subinterfaces (e.g., GigabitEthernet0/0.10, .20, .30) operate as the default gateways for each VLAN, but they do not create additional broadcast domains; they simply route traffic between the existing Layer 2 domains. In real-world deployments, misconfiguring subinterface encapsulation (e.g., 802.1Q) can cause VLAN leaks, inadvertently merging broadcast domains.
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: 3 — Each VLAN is a separate Layer 2 broadcast domain. With VLANs 10, 20, and 30 configured on the switch and a router using subinterfaces to route between them, there are exactly three broadcast domains — one per VLAN. Broadcasts are confined to their VLAN and do not cross VLAN boundaries without a Layer 3 device.
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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