N10-009 Network Implementation Practice Question
Two switches are connected via a trunk link configured with allowed VLANs 10, 20, and 30. Hosts on VLAN 20 can communicate across the trunk, but hosts on VLAN 30 cannot. Which of the following is the most likely cause?
⚠ Common exam trap
Watch out — candidates often confuse trunk allowed VLAN configuration with native VLAN settings or assume a spanning-tree issue is VLAN-specific, when in fact spanning-tree blocks the entire trunk, not individual VLANs.
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
✓
D
The most likely cause is that VLAN 30 is only created on one switch. For a trunk to forward frames for a VLAN, that VLAN must exist (be created) on both switches. If VLAN 30 is only configured on one switch, the other switch will drop frames from VLAN 30, preventing communication. This explains why VLAN 20 works (exists on both) and VLAN 30 does not.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A
Why it's wrong here
A native VLAN mismatch would cause issues for untagged traffic, but the problem is specific to VLAN 30, not all untagged traffic.
When this WOULD be correct
In a scenario where all VLANs fail to communicate across the trunk, and the trunk is not in an errdisable state, a physical issue like a faulty cable or incorrect cable type (e.g., crossover vs. straight-through) would be the correct answer.
- ✗
B
Why it's wrong here
Trunk encapsulation mismatches (e.g., ISL vs. 802.1Q) would affect all VLANs, not just one.
When this WOULD be correct
Option B would be correct if the question described that no hosts on any VLAN could communicate across the trunk, or if the trunk link was down entirely. For example: 'Two switches are connected via a trunk link, but hosts on all VLANs cannot communicate across the trunk. Which is the most likely cause?'
- ✗
C
Why it's wrong here
If VLAN 30 were not allowed on the trunk, traffic would not pass, but since it is in the allowed list, this is not the issue.
When this WOULD be correct
In a scenario where a trunk link is misconfigured with an incorrect native VLAN on one switch, causing VLAN mismatch, option C (if it were 'native VLAN mismatch') would be correct.
- ✓
D
Why this is correct
If VLAN 30 is only created on one switch, the other switch will not have the VLAN in its database and will drop frames from that VLAN.
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.
✓DCorrect answer▾
Why this is correct
If VLAN 30 is only created on one switch, the other switch will not have the VLAN in its database and will drop frames from that VLAN.
✗AWrong answer — click to see why▾
Why this is wrong here
Option A is not defined, so it cannot be evaluated. The question describes a trunk issue where only VLAN 30 fails, suggesting a misconfiguration specific to that VLAN, not a generic problem like a bad cable or port error.
★ When this WOULD be the correct answer
In a scenario where all VLANs fail to communicate across the trunk, and the trunk is not in an errdisable state, a physical issue like a faulty cable or incorrect cable type (e.g., crossover vs. straight-through) would be the correct answer.
Why candidates choose this
Candidates may choose this option because they assume any trunk problem is due to physical connectivity, overlooking VLAN-specific configuration errors like missing allowed VLANs or native VLAN mismatch.
✗BWrong answer — click to see why▾
Why this is wrong here
Option B is incorrect because the question states that hosts on VLAN 20 can communicate across the trunk, indicating the trunk is operational. The issue is specific to VLAN 30, so a general trunk misconfiguration (like encapsulation mismatch or trunk mode) would affect all VLANs, not just one.
★ When this WOULD be the correct answer
Option B would be correct if the question described that no hosts on any VLAN could communicate across the trunk, or if the trunk link was down entirely. For example: 'Two switches are connected via a trunk link, but hosts on all VLANs cannot communicate across the trunk. Which is the most likely cause?'
Why candidates choose this
Candidates may assume any trunk issue is due to a general trunk misconfiguration, overlooking that the problem is VLAN-specific. They might not carefully read that VLAN 20 works, which rules out a global trunk problem.
✗CWrong answer — click to see why▾
Why this is wrong here
Option C is not provided in the question, so it cannot be evaluated. The correct answer is D, which is undefined, indicating a missing or incorrect option.
★ When this WOULD be the correct answer
In a scenario where a trunk link is misconfigured with an incorrect native VLAN on one switch, causing VLAN mismatch, option C (if it were 'native VLAN mismatch') would be correct.
Why candidates choose this
Candidates might be tempted to choose an option that sounds plausible, like a VLAN mismatch or pruning issue, without verifying the actual options given.
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
Go deeper
Related to this question
Learn chapter
Ethernet Switching and VLANs
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
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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.