Quick Answer
The answer is IEEE 802.1Q, the standard VLAN tagging protocol that must be configured on the switch port to separate voice traffic from data traffic. This protocol works by inserting a 4-byte tag into the Ethernet frame header, which identifies which VLAN the traffic belongs to, allowing a single physical port to carry both a VoIP phone’s voice traffic and a computer’s data traffic on separate logical networks. On the CompTIA A+ Core 1 220-1201 exam, this concept often appears in troubleshooting scenarios where a phone fails to register but a PC works, testing your understanding that without 802.1Q tagging, the switch cannot distinguish voice from data frames. A common trap is confusing 802.1Q with 802.1X (port-based authentication) or assuming a simple VLAN assignment on the port is enough—remember, tagging is required when two VLANs share one link. Memory tip: think “Q for Queue” because 802.1Q tags prioritize voice traffic in its own VLAN.
This 220-1201 practice question tests your understanding of network protocols. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 customer reports that their new VoIP phone cannot register with the company's call server, but their computer on the same switch port works fine. The network uses VLANs for voice and data traffic. Which protocol must be configured on the switch port to separate the voice traffic from the data traffic?
DHCP
Why wrong: DHCP assigns IP addresses, but does not tag traffic for VLAN separation.
802.1Q
802.1Q is the VLAN tagging protocol used to mark frames for different VLANs, enabling voice and data to share a port.
DNS
Why wrong: DNS resolves hostnames to IP addresses, not relevant to VLAN tagging.
ARP
Why wrong: ARP maps IP addresses to MAC addresses, not used for VLAN separation.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
802.1Q
This question tests knowledge of VLAN tagging protocols used to separate voice and data traffic on a single switch port. IEEE 802.1Q is the standard VLAN tagging protocol that allows multiple VLANs to share the same physical link, which is essential for VoIP deployments where a phone and PC share a port.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
DHCP
802.1Q
DNS
ARP
Common exam traps
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
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
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 220-1201 questions on switching, trunking, and access-port configuration.
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FAQ
Network Protocols — This question tests Network Protocols — Access ports place end devices into a single VLAN..
The correct answer is: 802.1Q — This question tests knowledge of VLAN tagging protocols used to separate voice and data traffic on a single switch port. IEEE 802.1Q is the standard VLAN tagging protocol that allows multiple VLANs to share the same physical link, which is essential for VoIP deployments where a phone and PC share a port.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 220-1201 questions on switching, trunking, and access-port configuration.
Access ports place end devices into a single VLAN.
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Same concept, more angles
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Variation 1. During a network upgrade, a technician configures a new VoIP phone to use a dedicated VLAN for voice traffic. After connecting the phone to a switch port configured for voice VLAN, the phone powers on but cannot register with the call manager. The switch port is set to trunk mode. Which protocol is essential for the phone to obtain its VLAN assignment?
mediumWhy B: VoIP phones often use CDP (Cisco Discovery Protocol) or LLDP (Link Layer Discovery Protocol) to receive voice VLAN information from a switch. In this scenario, the switch port is in trunk mode, but without CDP/LLDP, the phone won't know which VLAN to use for voice traffic. CDP is Cisco-proprietary and commonly used in such environments to dynamically assign the voice VLAN.
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Last reviewed: Jun 18, 2026
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