The answer is the `switchport trunk allowed vlan 10,20,30` command set. This is correct because the `switchport trunk allowed vlan` command explicitly restricts which VLANs are permitted to traverse an IEEE 802.1Q trunk, pruning all others from the link to conserve bandwidth and improve security. On the CompTIA Network+ N10-009 exam, this tests your understanding of VLAN tagging and trunk configuration, often appearing in a scenario where you must distinguish between the default behavior (allowing all VLANs) and a restricted list. A common trap is confusing this with the `switchport mode trunk` command, which only enables trunking but does not limit VLANs. For a memory tip, think of the command as a "whitelist" for your trunk: you explicitly list what is allowed, and everything else is automatically blocked.
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 configure a trunk link between two Cisco switches so that only VLANs 10, 20, and 30 are allowed. Which command set will accomplish this?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
switchport trunk allowed vlan 10,20,30
Option A is correct because the 'switchport trunk allowed vlan' command explicitly defines which VLANs are permitted to traverse the trunk link. By specifying '10,20,30', only traffic from those VLANs is forwarded, while all other VLANs are pruned from the trunk. This is the standard method for restricting VLANs on an IEEE 802.1Q trunk between Cisco switches.
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.
✓
switchport trunk allowed vlan 10,20,30
Why this is correct
This command explicitly allows only the listed VLANs on the trunk. All other VLANs are denied.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
switchport mode trunk
Why it's wrong here
This command sets the interface to trunk mode but does not restrict which VLANs are allowed; by default all VLANs are allowed.
✗
switchport trunk native vlan 1
Why it's wrong here
This sets the native VLAN for untagged traffic, usually VLAN 1. It does not control which VLANs are allowed on the trunk.
✗
switchport trunk encapsulation dot1q
Why it's wrong here
This sets the encapsulation protocol to IEEE 802.1Q but does not filter VLANs.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between configuring trunk mode ('switchport mode trunk') and restricting VLANs ('switchport trunk allowed vlan'), leading candidates to mistakenly think that setting the interface to trunk alone is sufficient to limit VLAN traffic.
Trap categories for this question
Command / output trap
This command sets the interface to trunk mode but does not restrict which VLANs are allowed; by default all VLANs are allowed.
Detailed technical explanation
How to think about this question
Under the hood, the 'switchport trunk allowed vlan' command modifies the VLAN allowed list in the switch's VLAN Trunking Protocol (VTP) or local database, which controls the forwarding of BPDUs and data frames. A subtle behavior is that if you use 'switchport trunk allowed vlan add' or 'remove', you must be careful not to accidentally prune the native VLAN, as some switches will drop untagged traffic if the native VLAN is removed. In real-world scenarios, restricting VLANs on a trunk is critical for security and broadcast domain segmentation, especially in multi-tenant environments.
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: switchport trunk allowed vlan 10,20,30 — Option A is correct because the 'switchport trunk allowed vlan' command explicitly defines which VLANs are permitted to traverse the trunk link. By specifying '10,20,30', only traffic from those VLANs is forwarded, while all other VLANs are pruned from the trunk. This is the standard method for restricting VLANs on an IEEE 802.1Q trunk between Cisco switches.
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: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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