The answer is to verify the trunk allowed VLAN list on both switch interfaces. This is correct because a trunk port’s allowed VLAN list acts as an explicit filter, determining which VLANs can forward traffic across the link; even if both switches agree on the native VLAN, any VLAN not present in that list will have its frames silently dropped at the egress port. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of VLAN pruning and trunking misconfigurations—a common trap is assuming that simply setting the same native VLAN guarantees all VLAN traffic will pass, when in fact the allowed list must be manually configured or set to the default “all.” A strong memory tip is to think of the allowed VLAN list as a bouncer at a club: no matter how many VLANs are in the building, only those on the guest list (the allowed list) get through the trunk door.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. 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. 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 is configuring a trunk port between two switches. Both switches have been set with native VLAN 99. However, traffic from some VLANs is not passing over the trunk. What should the administrator verify?
Why wrong: VTP is not required for trunking, and mismatched VTP domains do not prevent trunk traffic.
B
The allowed VLAN list on the trunk port.
By default, trunk ports allow all VLANs, but administrators sometimes restrict the allowed VLAN list. If a VLAN is not in the allowed list, its traffic will not pass.
C
The speed and duplex settings are identical.
Why wrong: Speed/duplex mismatches can cause issues, but they would affect all traffic, not just specific VLANs.
D
The spanning tree protocol is disabled.
Why wrong: Spanning tree should be enabled to prevent loops; disabling it could cause issues but not selective VLAN blockage.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The allowed VLAN list on the trunk port.
The trunk port's allowed VLAN list explicitly controls which VLANs are permitted to traverse the link. Even when both switches agree on the native VLAN (99), if a particular VLAN is not included in the allowed list on either side, its traffic will be dropped. This is a common misconfiguration that prevents specific VLAN traffic from passing over 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.
✗
The VTP domain name matches on both switches.
Why it's wrong here
VTP is not required for trunking, and mismatched VTP domains do not prevent trunk traffic.
✓
The allowed VLAN list on the trunk port.
Why this is correct
By default, trunk ports allow all VLANs, but administrators sometimes restrict the allowed VLAN list. If a VLAN is not in the allowed list, its traffic will not pass.
Related concept
Read the scenario before looking for a memorised answer.
✗
The speed and duplex settings are identical.
Why it's wrong here
Speed/duplex mismatches can cause issues, but they would affect all traffic, not just specific VLANs.
✗
The spanning tree protocol is disabled.
Why it's wrong here
Spanning tree should be enabled to prevent loops; disabling it could cause issues but not selective VLAN blockage.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that native VLAN mismatch is the only cause of trunk issues, but here the native VLAN matches, so candidates might overlook the allowed VLAN list as the root cause.
Detailed technical explanation
How to think about this question
On a Cisco switch, the 'switchport trunk allowed vlan' command defines a list of VLANs that are permitted to traverse the trunk. If a VLAN is not in this list, its frames are discarded at the egress port. Additionally, the native VLAN (VLAN 99) is used for untagged traffic on the trunk, but it is also subject to the allowed VLAN list—if native VLAN 99 is not in the allowed list, even untagged frames will be dropped. This is a subtle behavior that often catches administrators.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
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: The allowed VLAN list on the trunk port. — The trunk port's allowed VLAN list explicitly controls which VLANs are permitted to traverse the link. Even when both switches agree on the native VLAN (99), if a particular VLAN is not included in the allowed list on either side, its traffic will be dropped. This is a common misconfiguration that prevents specific VLAN traffic from passing over 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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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