Cisco WS-C2950SX-48-SI Configuration Manual page 220

Configuring Auxiliary VLANs
Figure 10-2 Switch-to-Phone Connections
When the IP phone connects to a 10/100 port on the Catalyst 4500 series switch, the access port
(PC-to-phone jack) of the IP phone can be used to connect a PC.
Packets to and from the PC and to and from the phone share the same physical link to the switch and the
same port of the switch.
Introducing IP-based phones into existing switch-based networks raises the following issues:
• The current VLANs might be configured on an IP subnet basis, and additional IP addresses might
not be available to assign the phone to a port so that it belongs to the same subnet as other devices
(PC) that are connected to the same port.
• Data traffic present on the VLAN supporting phones might reduce the quality of VoIP traffic.
You can resolve these issues by isolating the voice traffic onto a separate VLAN on each of the ports
that are connected to a phone. The switch port that is configured for connecting a phone would have
separate VLANs that are configured for carrying the following:
• Voice traffic to and from the IP phone (auxiliary VLAN)
• Data traffic to and from the PC that is connected to the switch through the access port of the IP phone
(native VLAN)
Isolating the phones on a separate, auxiliary VLAN increases the quality of the voice traffic and allows
a large number of phones to be added to an existing network where there are not enough IP addresses.
A new VLAN means a new subnet and a new set of IP addresses.
You can configure switch ports to send Cisco Discovery Protocol (CDP) packets that instruct an attached
Cisco IP Phone to transmit voice traffic to the switch in these frame types:
802.1Q frames carrying the auxiliary VLAN ID and Layer 2 CoS set to 5 (the switch port drops all
•
802.1Q frames except those carrying the auxiliary VLAN ID).
Note
• 802.1p frames, which are 802.1Q frames carrying VLAN ID 0 and Layer 2 CoS set to 5 (enter the
set port auxiliaryvlan mod[/port] dot1p command)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
10-14
Reset the Cisco IP Phone if the auxiliary VLAN ID changes.
–
– Enter the set port auxiliaryvlan mod[/port] aux_vlan_id command.
We recommend that you use 802.1Q frames and a separate VLAN.
Chapter 10 Configuring VLANs
—
Release 8.1
78-15486-01