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.
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?
STP
Why wrong: STP prevents loops in a network but does not provide VLAN information to endpoints.
CDP
CDP advertises VLAN information to Cisco phones, allowing them to automatically use the correct voice VLAN.
RSTP
Why wrong: RSTP is a faster version of STP and also does not convey VLAN assignments.
VTP
Why wrong: VTP manages VLANs across switches but does not communicate VLAN info directly to endpoints like phones.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
CDP
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.
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.
STP
CDP
Why this is correct
CDP advertises VLAN information to Cisco phones, allowing them to automatically use the correct voice VLAN.
Related concept
Access ports place end devices into a single VLAN.
RSTP
VTP
Why it's wrong here
VTP manages VLANs across switches but does not communicate VLAN info directly to endpoints like phones.
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: CDP — 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.
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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Last reviewed: Jun 18, 2026
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