- A
The native VLAN mismatch
Why wrong: A native VLAN mismatch can cause traffic issues, but typically the trunk would still forward other VLAN traffic. The trunk being up indicates the native VLAN mismatch is not the primary issue here.
- B
The trunk encapsulation is not set to 802.1Q
Why wrong: If the encapsulation is not 802.1Q, the trunk would likely fail to negotiate or be down. Since the trunk is up, encapsulation is likely correct.
- C
The allowed VLAN list on the trunk does not include the desired VLANs
The allowed VLAN list explicitly controls which VLANs are permitted on the trunk. If the desired VLANs are not in the allowed list, their traffic will be dropped.
- D
The switch port mode is set to access
Why wrong: If the port were in access mode, it would not form a trunk. The trunk being up indicates the port is configured as a trunk.
Quick Answer
The answer is that the allowed VLAN list on the trunk does not include the desired VLANs. This is the most likely cause because when a trunk port is configured, an administrator can explicitly restrict which VLANs are permitted to traverse the link using the switchport trunk allowed vlan command; if this list is empty or missing the required VLANs, the trunk will show as up but will drop all traffic for those VLANs, effectively isolating the new switch from the core network. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of VLAN trunk allowed list troubleshooting, often appearing as a trick where the trunk status is operational but connectivity fails—a common trap is assuming a default trunk allows all VLANs, forgetting that prior restrictions may have been applied. To remember this, think of the trunk as a bouncer with a guest list: if the VLAN isn’t on the allowed list, it doesn’t get in.
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 engineer needs to add a new switch to an existing network. The switch must be configured to support VLANs and trunking. The engineer connects the switch to the existing network via a trunk port. After configuration, the VLANs on the new switch are not receiving traffic from the core network. The core switch shows the trunk is up but no VLANs are allowed. 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
The allowed VLAN list on the trunk does not include the desired VLANs
The core switch shows the trunk is up but no VLANs are allowed, which directly indicates that the allowed VLAN list on the trunk port does not include the desired VLANs. By default, a trunk port permits all VLANs, but if an administrator explicitly restricts the allowed VLAN list (e.g., with the 'switchport trunk allowed vlan' command), only those VLANs are forwarded. Since the new switch is not receiving traffic, the core switch's trunk likely has an empty or incorrect allowed VLAN list, preventing the desired VLAN traffic from crossing 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 native VLAN mismatch
Why it's wrong here
A native VLAN mismatch can cause traffic issues, but typically the trunk would still forward other VLAN traffic. The trunk being up indicates the native VLAN mismatch is not the primary issue here.
- ✗
The trunk encapsulation is not set to 802.1Q
Why it's wrong here
If the encapsulation is not 802.1Q, the trunk would likely fail to negotiate or be down. Since the trunk is up, encapsulation is likely correct.
- ✓
The allowed VLAN list on the trunk does not include the desired VLANs
Why this is correct
The allowed VLAN list explicitly controls which VLANs are permitted on the trunk. If the desired VLANs are not in the allowed list, their traffic will be dropped.
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.
- ✗
The switch port mode is set to access
Why it's wrong here
If the port were in access mode, it would not form a trunk. The trunk being up indicates the port is configured as a trunk.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a trunk being 'up/up' means all VLANs are automatically allowed, when in fact the allowed VLAN list can be explicitly restricted or cleared, causing traffic loss without any physical or encapsulation issues.
Detailed technical explanation
How to think about this question
The 'switchport trunk allowed vlan' command on Cisco IOS creates a per-VLAN filter at the trunk port, using a VLAN bitmap that is checked before forwarding frames. If the list is empty (e.g., 'switchport trunk allowed vlan none'), the trunk will be operationally up but will drop all tagged frames except possibly the native VLAN. In real-world scenarios, this often happens when a network engineer copies a configuration from another switch without adjusting the allowed VLAN list, or when an automation script inadvertently removes all VLANs from the trunk.
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
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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: The allowed VLAN list on the trunk does not include the desired VLANs — The core switch shows the trunk is up but no VLANs are allowed, which directly indicates that the allowed VLAN list on the trunk port does not include the desired VLANs. By default, a trunk port permits all VLANs, but if an administrator explicitly restricts the allowed VLAN list (e.g., with the 'switchport trunk allowed vlan' command), only those VLANs are forwarded. Since the new switch is not receiving traffic, the core switch's trunk likely has an empty or incorrect allowed VLAN list, preventing the desired VLAN traffic from crossing 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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