Auxiliary VLANs

Auxiliary VLANs
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 (QoS) 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.