- A
The interface will forward traffic for VLANs 10, 20, and 30, and all untagged frames will be placed into VLAN 999.
Correct. The allowed VLAN list restricts traffic to those three VLANs, and the native VLAN is 999.
- B
The interface will forward traffic for all VLANs except 10, 20, and 30, and the native VLAN is 1.
Why wrong: Incorrect. The allowed VLAN list specifies which VLANs are permitted, not denied. The native VLAN is explicitly set to 999.
- C
The interface will operate as an access port in VLAN 999.
Why wrong: Incorrect. The command 'switchport mode trunk' makes it a trunk port, not an access port.
- D
The interface will forward traffic for VLANs 10, 20, and 30, and all frames will be tagged including the native VLAN.
Why wrong: Incorrect. Frames on the native VLAN are sent untagged by default.
350-401 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.
Given the following configuration on a Cisco IOS-XE switch:
interface GigabitEthernet0/1 switchport mode trunk switchport trunk native vlan 999 switchport trunk allowed vlan 10,20,30
What is the effect of this configuration?
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 interface will forward traffic for VLANs 10, 20, and 30, and all untagged frames will be placed into VLAN 999.
Option A is correct because the configuration sets the interface as a trunk port, explicitly allows only VLANs 10, 20, and 30 to traverse it, and designates VLAN 999 as the native VLAN. On a trunk, the native VLAN is used for untagged frames (e.g., DTP, CDP, or any traffic sent without an 802.1Q header), so all untagged frames received or sent on this interface will be associated with VLAN 999.
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 interface will forward traffic for VLANs 10, 20, and 30, and all untagged frames will be placed into VLAN 999.
Why this is correct
Correct. The allowed VLAN list restricts traffic to those three VLANs, and the native VLAN is 999.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The interface will forward traffic for all VLANs except 10, 20, and 30, and the native VLAN is 1.
Why it's wrong here
Incorrect. The allowed VLAN list specifies which VLANs are permitted, not denied. The native VLAN is explicitly set to 999.
- ✗
The interface will operate as an access port in VLAN 999.
Why it's wrong here
Incorrect. The command 'switchport mode trunk' makes it a trunk port, not an access port.
- ✗
The interface will forward traffic for VLANs 10, 20, and 30, and all frames will be tagged including the native VLAN.
Why it's wrong here
Incorrect. Frames on the native VLAN are sent untagged by default.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the native VLAN being untagged by default and the 'switchport trunk native vlan tag' command that forces tagging, leading candidates to incorrectly assume that all VLANs on a trunk are always tagged.
Trap categories for this question
Command / output trap
Incorrect. The command 'switchport mode trunk' makes it a trunk port, not an access port.
Detailed technical explanation
How to think about this question
In 802.1Q trunking, the native VLAN is the only VLAN that is not tagged in the Ethernet frame; this allows interoperability with devices that do not understand VLAN tagging. The 'switchport trunk allowed vlan' command uses a VLAN list that is applied as a filter on the trunk, and any VLAN not in the list is pruned from the trunk. A common real-world scenario is using a native VLAN other than VLAN 1 to mitigate VLAN hopping attacks, as the default native VLAN (1) is a well-known attack vector.
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.
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VLANs and Trunking — study guide chapter
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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: The interface will forward traffic for VLANs 10, 20, and 30, and all untagged frames will be placed into VLAN 999. — Option A is correct because the configuration sets the interface as a trunk port, explicitly allows only VLANs 10, 20, and 30 to traverse it, and designates VLAN 999 as the native VLAN. On a trunk, the native VLAN is used for untagged frames (e.g., DTP, CDP, or any traffic sent without an 802.1Q header), so all untagged frames received or sent on this interface will be associated with VLAN 999.
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
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Last reviewed: Jun 24, 2026
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.
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