IP telephony involves the use of an IP-based network to replace the legacy telephony services provided by PBX (private branch exchange) systems. This involves the use of an IP telephone, a call manager, and gateway services for the access to the main telephone network. The call manager will probably be located in a network server, and many of the gateway functions will be provided inside the networking equipment. Cisco’s 6500 series switches support many gateway functions.
IP packets traveling to and from the PC and to and from the phone share the same physical link to the same port of the switch. If the switch is already configured on a subnet-per-VLAN basis, this can cause problems if insufficient IP addresses are available.
One way of meeting the demands of mixed services, such as voice and data, is to allocate a VLAN specifically for the purpose of carrying voice traffic. These special VLANs are known as auxiliary VLANs. One advantage of auxiliary VLANs is that they can be used to ensure that data traveling across the shared link does not reduce the quality of service demanded by the IP phones. Another is that they allow the creation of a new VLAN with a new range of IP addresses just for the phones.
The IEEE 802.1p protocol is used to define quality of service at the MAC layer. Priorities are appended to the frame and these are regenerated at each layer 2 forwarding interface, based upon priorities established in the switches. This is covered in greater detail later in this book, but I just wanted to point out how QoS can be achieved with auxiliary VLANs.