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220-1201 LLDP-MED Practice Question
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?
⚠ Common exam trap
A+ candidates often confuse LLDP-MED with other protocols that manage VLANs between switches, such as GVRP, leading them to select GVRP instead of LLDP for endpoint VLAN assignment.
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
✓
LLDP
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
STP
Why it's wrong here
Spanning Tree Protocol (STP) is a fundamental Layer 2 protocol used to prevent network loops by creating a single active path between any two network devices. It achieves this by selectively blocking redundant links, thereby eliminating broadcast storms and MAC table instability. While essential for network resilience and preventing topology issues, STP's role is strictly limited to managing the physical topology and does not involve the advertisement of VLAN assignments or other configuration details to client devices like VoIP phones.
- ✓
LLDP
Why this is correct
Link Layer Discovery Protocol (LLDP), particularly its Media Endpoint Discovery (LLDP-MED) extension, enables network devices to advertise their identity, capabilities, and specific network policies to directly connected devices. For VoIP phones, LLDP-MED allows a switch to communicate critical information such as the voice VLAN ID, power requirements (PoE), and QoS settings. This dynamic advertisement simplifies phone deployment and ensures the phone automatically configures itself to the correct voice VLAN, optimizing call quality and network segmentation.
- ✗
RSTP
Why it's wrong here
Rapid Spanning Tree Protocol (RSTP) is designed to prevent network loops by intelligently blocking redundant paths, offering significantly faster convergence times compared to its predecessor, STP. While crucial for maintaining network stability and preventing broadcast storms, RSTP operates at Layer 2 to manage the topology of the switching fabric. It does not provide mechanisms for a switch to advertise specific VLAN assignments or network configurations directly to end-user devices like VoIP phones.
- ✗
GVRP
Why it's wrong here
GARP VLAN Registration Protocol (GVRP) is a Layer 2 protocol that facilitates the dynamic registration and propagation of VLAN information between switches. Its primary function is to automate the configuration of VLAN trunks, allowing switches to learn which VLANs are active on connected ports and dynamically add or remove them from trunk links. However, GVRP is not designed to advertise specific VLAN IDs or quality-of-service (QoS) parameters directly to individual endpoints, such as a VoIP phone, for their operational configuration.
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Related to this question
Learn chapter
Network Configuration: IP, DNS, DHCP
Key term
Link Layer Discovery Protocol
Link Layer Discovery Protocol (LLDP) is a vendor-neutral protocol used by network devices to advertise their identity, capabilities, and neighbors over a local area network.
Key term
Voice over IP
Voice over IP (VoIP) is a technology that lets you make phone calls using an internet connection instead of a traditional telephone line.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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