- A
switchport mode trunk; switchport trunk native vlan 99; switchport trunk allowed vlan 10,20,30
Correct because it sets the trunk, changes the native VLAN, and restricts allowed VLANs.
- B
switchport mode trunk; switchport trunk native vlan 99; switchport trunk allowed vlan except 10,20,30
Why wrong: Incorrect because 'except' would allow all VLANs except those listed, which is the opposite of the requirement.
- C
switchport mode dynamic desirable; switchport trunk native vlan 99; switchport trunk allowed vlan 10,20,30
Why wrong: Incorrect because dynamic desirable does not guarantee a trunk; it relies on DTP negotiation.
- D
switchport trunk encapsulation dot1q; switchport mode trunk; switchport trunk native vlan 99
Why wrong: Incorrect because it does not restrict allowed VLANs, and encapsulation command is unnecessary.
Quick Answer
The correct configuration is to apply the commands switchport mode trunk, switchport trunk native vlan 99, and switchport trunk allowed vlan 10,20,30 on the access switch’s uplink interface. This is correct because it explicitly sets the interface to trunk mode, changes the native VLAN from the default VLAN 1 to VLAN 99 for security, and uses the allowed VLAN list to restrict traffic to only the required Engineering, Sales, and Management VLANs. On the ENCOR 350-401 exam, this scenario tests your understanding of trunk security best practices, particularly the need to prune unnecessary VLANs to reduce broadcast overhead and to change the native VLAN to prevent VLAN hopping attacks. A common trap is forgetting to set the interface to trunk mode first, or using the switchport trunk allowed vlan add command instead of overwriting the list with the exact VLANs. Remember the mnemonic “MNA” for the three-step order: Mode, Native, Allowed.
CCNP VLANs and Trunking Practice Question
This 350-401 practice question tests your understanding of vlans and trunking. 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.
An engineer is configuring a new access switch for a branch office. The switch must support multiple VLANs for different departments: VLAN 10 (Engineering), VLAN 20 (Sales), and VLAN 30 (Management). The uplink to the distribution switch is a trunk. The engineer wants to ensure that only the required VLANs are allowed on the trunk and that the native VLAN is changed from the default to VLAN 99 for security reasons. Which configuration commands should the engineer apply on the access switch's uplink interface?
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
switchport mode trunk; switchport trunk native vlan 99; switchport trunk allowed vlan 10,20,30
Option A is correct because it explicitly sets the interface to trunk mode, changes the native VLAN from the default VLAN 1 to VLAN 99 for security, and uses the 'allowed vlan' command to permit only VLANs 10, 20, and 30 on the trunk. This ensures that only the required department VLANs are carried, reducing unnecessary broadcast traffic and preventing VLAN hopping attacks by changing the native VLAN.
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 mode trunk; switchport trunk native vlan 99; switchport trunk allowed vlan 10,20,30
Why this is correct
Correct because it sets the trunk, changes the native VLAN, and restricts allowed VLANs.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
switchport mode trunk; switchport trunk native vlan 99; switchport trunk allowed vlan except 10,20,30
Why it's wrong here
Incorrect because 'except' would allow all VLANs except those listed, which is the opposite of the requirement.
- ✗
switchport mode dynamic desirable; switchport trunk native vlan 99; switchport trunk allowed vlan 10,20,30
- ✗
switchport trunk encapsulation dot1q; switchport mode trunk; switchport trunk native vlan 99
Why it's wrong here
Incorrect because it does not restrict allowed VLANs, and encapsulation command is unnecessary.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between 'allowed vlan' and 'allowed vlan except' — candidates may confuse the syntax and select the option that excludes the required VLANs instead of permitting them.
Trap categories for this question
Command / output trap
Incorrect because it does not restrict allowed VLANs, and encapsulation command is unnecessary.
Detailed technical explanation
How to think about this question
The 'switchport trunk native vlan 99' command changes the native VLAN from the default VLAN 1 to VLAN 99, which helps mitigate VLAN hopping attacks where an attacker can double-tag frames to access VLAN 1. The 'switchport trunk allowed vlan' command uses a VLAN list to prune unwanted VLANs from the trunk, reducing broadcast overhead and improving security. In a real-world scenario, failing to restrict allowed VLANs can lead to unnecessary traffic crossing the trunk and potential security breaches if an unauthorized VLAN is extended.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
VLANs and Trunking — study guide chapter
Learn the concepts, then practise the questions
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VLANs and Trunking practice questions
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FAQ
Questions learners often ask
What does this 350-401 question test?
VLANs and Trunking — This question tests VLANs and Trunking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: switchport mode trunk; switchport trunk native vlan 99; switchport trunk allowed vlan 10,20,30 — Option A is correct because it explicitly sets the interface to trunk mode, changes the native VLAN from the default VLAN 1 to VLAN 99 for security, and uses the 'allowed vlan' command to permit only VLANs 10, 20, and 30 on the trunk. This ensures that only the required department VLANs are carried, reducing unnecessary broadcast traffic and preventing VLAN hopping attacks by changing the native VLAN.
What should I do if I get this 350-401 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer is configuring a new Cisco Catalyst 9300 switch to connect to an existing network. The uplink to the core switch is configured as a trunk. The engineer wants to ensure that all VLANs except VLAN 1 are allowed on the trunk, and that the native VLAN is set to VLAN 999. Which configuration should the engineer apply on the uplink interface?
easy- ✓ A.switchport mode trunk; switchport trunk native vlan 999; switchport trunk allowed vlan except 1
- B.switchport mode trunk; switchport trunk native vlan 999; switchport trunk allowed vlan remove 1
- C.switchport mode trunk; switchport trunk native vlan 999; switchport trunk allowed vlan 2-4094
- D.switchport mode trunk; switchport trunk native vlan 999; switchport trunk allowed vlan none
Why A: Option A is correct because the 'switchport trunk allowed vlan except 1' command explicitly permits all VLANs except VLAN 1 on the trunk, while the 'switchport trunk native vlan 999' command sets the native VLAN to 999, ensuring that untagged frames on the trunk belong to VLAN 999 instead of the default VLAN 1. This meets the requirement to exclude VLAN 1 from the allowed list and change the native VLAN.
Last reviewed: Jun 24, 2026
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