- A
DHCP
Why wrong: DHCP assigns IP addresses, but does not tag traffic for VLAN separation.
- B
802.1Q
802.1Q is the VLAN tagging protocol used to mark frames for different VLANs, enabling voice and data to share a port.
- C
DNS
Why wrong: DNS resolves hostnames to IP addresses, not relevant to VLAN tagging.
- D
ARP
Why wrong: ARP maps IP addresses to MAC addresses, not used for VLAN separation.
Question 572 of 1,020
802.1Q VLAN Tagging for VoIP Phone Traffic Separation
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?
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.
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
✓
802.1Q
802.1Q is the IEEE standard for VLAN tagging, which allows a single switch port to carry traffic for multiple VLANs. In this scenario, the switch port must be configured as an 802.1Q trunk or use a voice VLAN feature that tags VoIP traffic with a specific VLAN ID, separating it from the untagged data traffic. This ensures the VoIP phone can communicate with the call server on its designated voice VLAN while the computer continues to work on the data 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.
- ✗
DHCP
- ✓
802.1Q
Why this is correct
802.1Q is the VLAN tagging protocol used to mark frames for different VLANs, enabling voice and data to share a port.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
DNS
- ✗
ARP
Common exam traps
Common exam trap: answer the scenario, not the keyword
The CompTIA A+ exam often tests the distinction between protocols that manage traffic (like DHCP) and protocols that separate traffic (like 802.1Q), leading candidates to mistakenly choose DHCP because it is commonly associated with VoIP phone configuration.
Detailed technical explanation
How to think about this question
When a Cisco switch port is configured with the 'switchport voice vlan' command, it uses 802.1Q tagging to mark voice frames with the specified VLAN ID, while data frames remain untagged on the native VLAN. The VoIP phone typically uses CDP (Cisco Discovery Protocol) or LLDP-MED to learn the voice VLAN ID from the switch, then tags its packets accordingly. Without this tagging, the phone's traffic would be placed in the same VLAN as the computer, preventing registration if the call server resides on a different VLAN.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this 220-1201 question test?
Network Protocols — This question tests Network Protocols — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 802.1Q — 802.1Q is the IEEE standard for VLAN tagging, which allows a single switch port to carry traffic for multiple VLANs. In this scenario, the switch port must be configured as an 802.1Q trunk or use a voice VLAN feature that tags VoIP traffic with a specific VLAN ID, separating it from the untagged data traffic. This ensures the VoIP phone can communicate with the call server on its designated voice VLAN while the computer continues to work on the data VLAN.
What should I do if I get this 220-1201 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.
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Same concept, more angles
1 more ways this is tested on 220-1201
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. 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?
medium- A.STP
- ✓ B.LLDP
- C.RSTP
- D.GVRP
Why B: Link Layer Discovery Protocol (LLDP) is essential because it allows the switch to inform the VoIP phone of the configured voice VLAN ID. When a switch port is in trunk mode, the phone uses LLDP (specifically LLDP-MED) to learn which VLAN to tag its voice traffic with, enabling it to communicate with the call manager. Without LLDP, the phone would not know which VLAN to use and would fail to register.
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Last reviewed: Jul 4, 2026
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