You configure a port with both an access VLAN and a voice VLAN. The access VLAN handles PC traffic. The voice VLAN handles phone traffic. The switch tells the phone which VLAN to use via CDP or LLDP. The phone tags its frames with that VLAN ID and often sets PCP to for priority. Without this setup, voice and data compete equally, causing call quality issues during congestion.
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$ curl repovive.com/roadmaps/network-design/switching-and-vlans/voice-vlan-configuration
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