N10-009 Network Implementation Practice Question
A network administrator has just connected a new access switch to the core switch via a trunk port. The administrator configured the trunk port on both switches with the same allowed VLAN list. However, hosts on VLAN 10 connected to the new access switch cannot communicate with hosts on VLAN 10 on the core switch. The trunk is operational and shows up/up. What is the most likely cause?
⚠ Common exam trap
The N10-009 exam often tests the misconception that a trunk with matching allowed VLAN lists is sufficient for traffic to pass, but candidates forget that the VLAN must exist in the local VLAN database on both switches for traffic to be forwarded.
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
✓
A) VLAN 10 has not been created on the new access switch
The most likely cause is that VLAN 10 has not been created on the new access switch. Even though the trunk port is up/up and the allowed VLAN list matches, a switch will not forward traffic for a VLAN that does not exist in its local VLAN database. Without the VLAN being created, the switch drops all frames tagged with VLAN 10, preventing communication between hosts on that VLAN across the trunk.
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) VLAN 10 has not been created on the new access switch
Why this is correct
If VLAN 10 has not been explicitly created and activated within the new access switch's VLAN database, the switch will not recognize or process frames tagged with VLAN 10. Even if the trunk port is configured to permit VLAN 10, the switch will drop incoming frames for an unknown VLAN ID, effectively preventing traffic flow. A switch must have a local understanding of a VLAN to forward frames associated with it, regardless of trunk allowance.
- ✗
B) Native VLAN mismatch on the trunk
Why it's wrong here
A native VLAN mismatch primarily affects untagged frames traversing a trunk link, causing them to be misdirected or dropped as they are assigned to different native VLANs on each side. However, traffic for VLAN 10, when sent over an 802.1Q trunk, is typically explicitly tagged with the VLAN 10 identifier. Therefore, a native VLAN mismatch would not specifically prevent the forwarding of properly tagged VLAN 10 frames, assuming VLAN 10 is allowed on the trunk and exists on both switches.
When this WOULD be correct
This would be correct if the question stated that hosts on the native VLAN (e.g., VLAN 1) cannot communicate, or if there were symptoms like CDP/LLDP not working or management access issues across the trunk, indicating a native VLAN mismatch.
- ✗
C) Trunk encapsulation mismatch (ISL vs 802.1Q)
Why it's wrong here
A trunk encapsulation mismatch, such as one switch configured for 802.1Q and the other for ISL, would prevent the two switches from correctly interpreting each other's trunking frames. This fundamental disagreement in frame formatting would typically cause the trunk link to fail to establish, continuously flap, or experience severe errors, making it impossible for any VLAN traffic, including VLAN 10, to pass reliably. The question implies the trunk is operational, making this an unlikely cause.
When this WOULD be correct
This option would be correct in a scenario where a trunk link between two switches is down or not passing any traffic, and the switches use different trunking protocols (e.g., one switch is Cisco using ISL and the other is a non-Cisco switch using 802.1Q). The encapsulation mismatch would prevent the trunk from forming.
- ✗
D) STP is blocking the VLAN 10 traffic on the trunk
Why it's wrong here
Spanning Tree Protocol (STP) is designed to prevent network loops by blocking redundant paths, ensuring a loop-free logical topology. On a single, non-redundant trunk link between two switches, STP would typically place the port in a forwarding state for all allowed VLANs, including VLAN 10, unless a loop is detected elsewhere in the topology. If the trunk is 'up/up,' it indicates the port is forwarding, meaning STP is not blocking traffic on this specific link.
When this WOULD be correct
In a scenario where two switches are connected via redundant trunk links, and one link is blocked by STP to prevent loops, causing VLAN 10 traffic to be blocked on that specific trunk. The question would indicate multiple connections between switches and that the trunk is up but not forwarding traffic.
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.
✓A) VLAN 10 has not been created on the new access switchCorrect answer▾
Why this is correct
If VLAN 10 has not been explicitly created and activated within the new access switch's VLAN database, the switch will not recognize or process frames tagged with VLAN 10. Even if the trunk port is configured to permit VLAN 10, the switch will drop incoming frames for an unknown VLAN ID, effectively preventing traffic flow. A switch must have a local understanding of a VLAN to forward frames associated with it, regardless of trunk allowance.
✗B) Native VLAN mismatch on the trunkWrong answer — click to see why▾
Why this is wrong here
A native VLAN mismatch would cause issues with untagged traffic, but the problem is that hosts on VLAN 10 cannot communicate. Since both switches have the same allowed VLAN list and the trunk is up/up, a native VLAN mismatch would not prevent tagged VLAN 10 traffic from passing.
★ When this WOULD be the correct answer
This would be correct if the question stated that hosts on the native VLAN (e.g., VLAN 1) cannot communicate, or if there were symptoms like CDP/LLDP not working or management access issues across the trunk, indicating a native VLAN mismatch.
Why candidates choose this
Candidates often confuse native VLAN mismatch with general VLAN communication issues, as it is a common trunk misconfiguration that can cause connectivity problems, but it specifically affects untagged traffic, not tagged VLANs like VLAN 10.
✗C) Trunk encapsulation mismatch (ISL vs 802.1Q)Wrong answer — click to see why▾
Why this is wrong here
The trunk is operational and shows up/up, indicating that the encapsulation is correctly negotiated or configured. If there were an encapsulation mismatch, the trunk would not come up, or at least show errors. The problem is specific to VLAN 10, not all VLANs.
★ When this WOULD be the correct answer
This option would be correct in a scenario where a trunk link between two switches is down or not passing any traffic, and the switches use different trunking protocols (e.g., one switch is Cisco using ISL and the other is a non-Cisco switch using 802.1Q). The encapsulation mismatch would prevent the trunk from forming.
Why candidates choose this
Candidates may confuse trunk encapsulation issues with VLAN-specific problems, especially if they have encountered encapsulation mismatches in the past that caused connectivity issues. They might assume that any trunk problem is due to encapsulation mismatch without considering that the trunk is up.
✗D) STP is blocking the VLAN 10 traffic on the trunkWrong answer — click to see why▾
Why this is wrong here
STP blocking VLAN 10 traffic on the trunk would prevent communication, but the question states the trunk is operational and up/up, and STP typically blocks redundant paths, not a single trunk unless it's a loop. The issue is more likely that VLAN 10 doesn't exist on the new switch.
★ When this WOULD be the correct answer
In a scenario where two switches are connected via redundant trunk links, and one link is blocked by STP to prevent loops, causing VLAN 10 traffic to be blocked on that specific trunk. The question would indicate multiple connections between switches and that the trunk is up but not forwarding traffic.
Why candidates choose this
Candidates may think STP is a common cause for VLAN traffic issues on trunks, especially if they've seen STP blocking ports in loop prevention scenarios, and they might overlook the simpler VLAN creation requirement.
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?”
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Related to this question
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Physical Network Infrastructure
Key term
Trunk port
A trunk port is a switch port configured to carry traffic for multiple VLANs, using a tagging protocol to identify which VLAN each frame belongs to.
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
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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.