350-401 Infrastructure Practice Question
A network administrator is configuring a Cisco IOS switch to support a new voice VLAN. The administrator wants to ensure that voice traffic is tagged with CoS 5 and data traffic is untagged. Which command should be applied to the interface connected to an IP phone?
⚠ Common exam trap
Many exam-takers confuse voice VLAN configuration with trunk configuration or QoS trust, which are related but do not by themselves enable voice VLAN separation.
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 voice vlan 10
The switchport voice vlan command enables the voice VLAN on an access port, allowing the switch to advertise the voice VLAN to the IP phone via CDP or LLDP. The phone then tags voice frames with the voice VLAN and CoS, while data from the attached PC remains untagged. Other commands like trunk encapsulation or access mode do not provide voice VLAN functionality. QoS trust may be needed but is not the primary configuration.
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 voice vlan 10
Why this is correct
This command enables the voice VLAN on the interface, allowing the switch to send Cisco Discovery Protocol (CDP) or Link Layer Discovery Protocol (LLDP) information to the IP phone. The phone can then tag voice traffic with the appropriate VLAN and CoS. It also allows data traffic from the PC to be untagged. This is the correct configuration for a voice VLAN.
- ✗
switchport trunk encapsulation dot1q
Why it's wrong here
This command sets the trunk encapsulation to 802.1Q, which is necessary for trunk ports but does not configure a voice VLAN. It is typically used on trunk links between switches, not on access ports connected to IP phones. Therefore, it does not meet the requirement.
- ✗
mls qos trust cos
Why it's wrong here
This command configures QoS to trust the CoS values of incoming frames, which is important for preserving priority markings. However, it does not configure the voice VLAN itself. It must be used in conjunction with the voice VLAN command. Alone, it does not enable voice traffic segregation.
- ✗
switchport mode access
Why it's wrong here
This command sets the interface to access mode, which is suitable for connecting a single device (like a PC) but does not enable voice VLAN functionality. Without the voice VLAN command, the switch will not recognize or tag voice traffic. Thus, it is insufficient for the scenario.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.