N10-009 Layer 3 broadcast domain Practice Question
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?
⚠ Common exam trap
Candidates often assume every VLAN forms a Layer 3 broadcast domain, but without a router subinterface (or equivalent L3 termination) a VLAN only creates a Layer 2 broadcast domain.
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
✓
3
A Layer 3 broadcast domain corresponds to a routed IP subnet. Without a router subinterface, VLAN 99 has no Layer 3 termination, so only VLANs 10, 20, and 30 (with subinterfaces) form separate broadcast domains. Thus, there are 3 Layer 3 broadcast domains.
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. One broadcast domain would be the case if all devices were on the same subnet.
When this WOULD be correct
If the switch had no VLANs configured (single flat network) and the router had a single interface with one IP address, there would be one Layer 3 broadcast domain.
- ✗
2
Why it's wrong here
Incorrect. Two broadcast domains is not accurate given the three routed subinterfaces.
When this WOULD be correct
If the question stated that VLAN 99 is not used and the trunk only carries VLANs 10, 20, and 30, and the router has subinterfaces for only two of them (e.g., VLAN 10 and 20), then there would be 2 Layer 3 broadcast domains (one per subinterface).
- ✓
3
Why this is correct
Correct. Only VLANs with a router subinterface (10, 20, 30) create Layer 3 broadcast domains. VLAN 99 (native) has no subinterface, so it does not form a Layer 3 domain.
- ✗
4
Why it's wrong here
Incorrect. This mistakenly counts VLAN 99, which lacks a router subinterface, as a Layer 3 broadcast domain.
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.
✓3Correct answer▾
Why this is correct
Correct. Only VLANs with a router subinterface (10, 20, 30) create Layer 3 broadcast domains. VLAN 99 (native) has no subinterface, so it does not form a Layer 3 domain.
✗1Wrong answer — click to see why▾
Why this is wrong here
Each VLAN is a separate Layer 3 broadcast domain. With VLANs 10, 20, 30, and 99, there are four broadcast domains, not one.
★ When this WOULD be the correct answer
If the switch had no VLANs configured (single flat network) and the router had a single interface with one IP address, there would be one Layer 3 broadcast domain.
Why candidates choose this
Candidates may think all traffic passes through a single router interface, overlooking that each VLAN creates its own broadcast domain even without a subinterface.
✗2Wrong answer — click to see why▾
Why this is wrong here
Each VLAN is a separate Layer 3 broadcast domain. VLANs 10, 20, and 30 each have a router subinterface, and VLAN 99 is a native VLAN without a subinterface, but it still constitutes its own broadcast domain because it is a distinct VLAN. Thus, there are 4 broadcast domains, not 2.
★ When this WOULD be the correct answer
If the question stated that VLAN 99 is not used and the trunk only carries VLANs 10, 20, and 30, and the router has subinterfaces for only two of them (e.g., VLAN 10 and 20), then there would be 2 Layer 3 broadcast domains (one per subinterface).
Why candidates choose this
Candidates may mistakenly think that only VLANs with router subinterfaces create broadcast domains, ignoring the native VLAN's separate broadcast domain, or they may incorrectly assume the native VLAN is part of another VLAN's domain.
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?”
Visual reference
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Related to this question
Learn chapter
Network Virtualization and VXLAN
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
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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.