Voice VLAN

A Voice VLAN is a dedicated VLAN configured on a switch port to carry VoIP traffic separately from data traffic. It ensures phone calls get priority and aren't affected by congestion on the data network.

VoIP phones are sensitive to latency and jitter in ways that regular data traffic is not. A file transfer tolerates a 200ms delay; a phone call doesn't. Putting voice and data on the same VLAN with the same priority means a file download can degrade call quality.

The solution is to isolate voice traffic into its own VLAN with QoS prioritization. Most managed switches support a voice VLAN configuration where a single port carries both: the phone connects to the switch port, which sends tagged voice traffic into the voice VLAN and untagged data traffic into the data VLAN. The PC plugs into the phone's passthrough port and gets data VLAN access.

The switch uses LLDP-MED or CDP to automatically tell the phone which VLAN to use — the phone picks it up on boot without manual configuration. This is why LLDP (or CDP on Cisco gear) matters in VoIP deployments.

If you're running a VoIP system — whether on-premise or hosted — a properly configured voice VLAN is non-negotiable for reliable call quality.