Accommodating Voice Traffic on Campus Switches
What is a voice VLAN?

Some Cisco Catalyst switches offer a "voice VLAN" feature. The voice VLAN, also known as an auxiliary VLAN, provides automatic VLAN association for IP phones. By associating the phones, and therefore the phone traffic, with a specific VLAN, the phone traffic will be on different IP subnets even though voice and data co-exist on the same physical infrastructure.

When a phone is connected to the switch, the switch sends necessary voice VLAN information to the IP phone, placing it into the voice VLAN without end-user intervention. Placing phone traffic onto a distinct VLAN allows the phone traffic to be segmented from the data traffic; this facilitates better network management and troubleshooting. Additionally, QoS or security policies can be enforced specifically for the traffic traversing the phone VLANs without affecting the data traffic. If the phone is moved, the voice VLAN association occurs again. The voice VLAN information may change if the phone is moved.

In an implementation where a PC, or other IP device, is connected to the switch through the IP phone, and the phone is in an Auxiliary VLAN, Layer 2 frame type incompatibility may keep the phone and device from communicating. The IP phone and device cannot communicate if they are in the same VLAN and subnet but each is using a different frame type. Because the traffic between the two takes place on the same subnet, it will not be routed and therefore, the Layer 2 headers will not be altered. Also, switch commands cannot be used to configure the frame type being used by a device on the other side of the phone that is not directly attached to the switch.

In order for the device and the phone to communicate, one of the following must be true:

  • They both use the same Layer 2 frame type.
  • The phone uses 802.1p frames and the device uses untagged frames.
  • The phone uses untagged frames and the device uses 802.1p frames.
  • The phone uses 802.1Q frames, and the voice VLAN equals the native VLAN.