- A
A) VLAN 10 has not been created on the new access switch
Correct. If the VLAN does not exist in the switch's database, the switch will not forward frames for that VLAN, even though the trunk is configured to allow it.
- B
B) Native VLAN mismatch on the trunk
Why wrong: Incorrect. A native VLAN mismatch can cause issues for untagged frames, but it would not specifically prevent VLAN 10 traffic if both sides allow it.
- C
C) Trunk encapsulation mismatch (ISL vs 802.1Q)
Why wrong: Incorrect. An encapsulation mismatch would likely cause the trunk to not come up or to flap, but the trunk is already operational (up/up).
- D
D) STP is blocking the VLAN 10 traffic on the trunk
Why wrong: Incorrect. STP blocks redundant paths to prevent loops, but on a single trunk link, STP should not block traffic unless there is a topology loop. The trunk is up, so STP is not blocking.
Quick Answer
The answer is that VLAN 10 has not been created on the new access switch. Even when a trunk port shows up/up and the allowed VLAN lists match on both sides, a switch will not forward traffic for a VLAN that does not exist in its local VLAN database; frames tagged with an unknown VLAN are simply dropped at the ingress port. This is a classic VLAN not created on switch trunk issue that tests your understanding of how a switch processes 802.1Q tags against its running configuration. On the CompTIA Network+ N10-009 exam, this scenario often appears as a trick where candidates assume a matching allowed list is sufficient, but the switch must first have the VLAN defined to participate in forwarding. A common memory tip is “create it before you trunk it”—the VLAN must exist in the switch’s database before any trunk can carry its traffic.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
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.
- ✓
A) VLAN 10 has not been created on the new access switch
Why this is correct
Correct. If the VLAN does not exist in the switch's database, the switch will not forward frames for that VLAN, even though the trunk is configured to allow it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
B) Native VLAN mismatch on the trunk
Why it's wrong here
Incorrect. A native VLAN mismatch can cause issues for untagged frames, but it would not specifically prevent VLAN 10 traffic if both sides allow it.
- ✗
C) Trunk encapsulation mismatch (ISL vs 802.1Q)
Why it's wrong here
Incorrect. An encapsulation mismatch would likely cause the trunk to not come up or to flap, but the trunk is already operational (up/up).
- ✗
D) STP is blocking the VLAN 10 traffic on the trunk
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco 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.
Detailed technical explanation
How to think about this question
When a VLAN is not created on a switch, the switch does not have a spanning-tree instance or a MAC address table for that VLAN, so any received tagged frames are discarded at the ingress. This is a common misconfiguration when adding a new switch: the trunk is configured correctly, but the VLAN database must be explicitly created (e.g., via 'vlan 10' in global config) before traffic can pass. In a real-world scenario, this often happens when using a switch that does not have VTP or when VTP is in transparent mode and the VLAN is not manually added.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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