Why does voice traffic require its own VLAN, and what are the QoS (Quality of Service) requirements for VoIP (Voice over IP)?

Voice traffic needs a separate VLAN (Virtual Local Area Network) because it has strict quality requirements — assured bandwidth, high QoS (Quality of Service) priority, ability to avoid congestion, and latency under 150 ms end-to-end.

VoIP (Voice over IP) requirements:

• Assured bandwidth — voice calls need a guaranteed minimum bandwidth
• High QoS priority — voice packets must be prioritized over data
• Congestion avoidance — voice must bypass congested queues
• Delay < 150 ms — human conversation breaks down above ~150 ms one-way latency

How it works with Cisco:

1. The switch port uses CDP (Cisco Discovery Protocol) to tell the IP (Internet Protocol) phone which voice VLAN to use
2. The phone tags its own voice traffic with the voice VLAN ID and a CoS (Class of Service) priority value
3. The phone acts as a 3-port switch — one port to the switch, one to the phone's internal circuitry, one to a connected PC
4. PC data traffic goes out on the access (data) VLAN, voice traffic on the voice VLAN

The show interfaces fa0/18 switchport command confirms both the access VLAN and voice VLAN assignments on a port.

Go deeper:

• Quality of service (Wikipedia) — the latency/jitter/bandwidth guarantees voice needs (breaks down above ~150 ms).
• IEEE 802.1p — Class of Service (Wikipedia) — the 3-bit PCP/CoS field a phone sets (voice = priority 5).