- A
1
Why wrong: Incorrect. One broadcast domain would be the case if all devices were on the same subnet.
- B
2
Why wrong: Incorrect. Two broadcast domains is not accurate given the three routed subinterfaces.
- C
3
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.
- D
4
Why wrong: Incorrect. This mistakenly counts VLAN 99, which lacks a router subinterface, as a Layer 3 broadcast domain.
Question 211 of 520
N10-009 Layer 3 broadcast domain 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. A key principle to apply: layer 3 broadcast domain. 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?
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.
Key principle: Layer 3 broadcast domain
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.
- ✓
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.
Related concept
Layer 3 broadcast 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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
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
- Layer 3 broadcast domain
- 802.1Q trunk
- Router-on-a-stick
- Native VLAN
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
Layer 3 broadcast domain
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.
Visual reference
What to study next
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Review layer 3 broadcast domain, then practise related N10-009 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Networking Concepts — This question tests Networking Concepts — Layer 3 broadcast domain.
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this N10-009 question wrong?
Review layer 3 broadcast domain, then practise related N10-009 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Layer 3 broadcast domain
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Last reviewed: Jun 30, 2026
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