How does a VoIP (Voice over IP) phone handle VLAN (Virtual Local Area Network) tagging, and how can you verify a port's voice VLAN configuration?

The IP (Internet Protocol) phone acts as a 3-port switch — it tags its own voice traffic with the voice VLAN and CoS (Class of Service) priority, while PC data traffic goes on the access VLAN. Use show interfaces switchport to verify.

* An IP phone as a 3-port switch: voice is tagged on the voice VLAN, PC data stays untagged on the access VLAN. *

How the phone works:

1. The switch tells the phone (via CDP (Cisco Discovery Protocol)) which voice VLAN to use
2. The phone tags voice frames with the voice VLAN ID and a CoS priority value for QoS (Quality of Service)
3. The phone may or may not tag the PC's data frames — depending on configuration

Tagging behavior on the port:

Verification with show interfaces fa0/18 switchport:

Key fields to look for:

• Access Mode VLAN: 20 (student) — the data VLAN
• Voice VLAN: 150 (voice) — the voice VLAN
• Administrative Mode: static access — port is in access mode
• Trunking Native Mode VLAN: 1 (default) — native VLAN for trunk fallback

Go deeper:

• IEEE 802.1p — Class of Service (Wikipedia) — the CoS priority the phone stamps on its tagged voice frames.
• Power over Ethernet (Wikipedia) — the same cable that carries the tagged voice VLAN also powers the phone.