Cisco Catalyst 2000 Configuration Handbook page 272

Tip A Cisco IP Phone can use an 802.1Q trunk to transport packets from two VLANs:
the voice VLAN (voice packets) and the native VLAN (data packets, untagged). By default,
a Cisco IP Phone transports both its voice packets and the data packets from a connected
device over the native VLAN. All data is untagged.
After a switch has been configured to instruct an IP Phone to support a VVID number, the
switch and phone must use an 802.1Q trunk between them.
For Catalyst switches, a special-case 802.1Q trunk is negotiated with the IP Phone using
CDP and the DTP. When the phone is detected, the switch port becomes a vlan2-access
port, supporting only the two voice and data VLANs. The port won't be shown in trunking
mode from the show trunk command. In fact, it doesn't matter which trunking mode (auto,
desirable, on, or off) is configured on the port—the special trunk will be negotiated through
the DTP. Be sure that the trunk is not configured using the nonegotiate keyword because
DTP messages will not be sent or received, and the trunk will not be automatically estab-
lished.
The Spanning Tree Protocol (STP) is automatically supported over the IP Phone trunk as
well. The show spantree command displays the STP state for both of the VLANs on the
trunk.
Establish VLANs with the IP Phone.
2.
a. (Optional) Use a VLAN for data.
(Optional) Identify the switch-port access VLAN:
■
(interface) switchport access vlan vlan-id
You can configure switch ports to support both PCs and IP Phones. For the
■
case when a regular host (not an IP Phone) is connected to a switch port, the
access VLAN should be set to vlan-id (1 to 1000 or 1025 to 4094). When a
PC is connected, only the access VLAN is supported, and no special trunking
negotiations take place. See section "6-1: VLAN Configuration" in Chapter 6,
"VLANS and Trunking," for more information.
■ Identify the switch-port native VLAN:
(interface) switchport trunk native vlan vlan-id
■ Data from the access switch port on an IP Phone is carried over the native
VLAN (untagged) of the special 802.1Q trunk. Therefore, you should identify
the native VLAN number as vlan-id (1 to 1000 or 1025 to 4094).
b. (Optional) Instruct the phone to transport data and voice.
(Optional) Use an 802.1Q trunk with a voice VLAN:
■
(interface) switchport voice vlan vlan-id
The IP Phone is instructed to use an 802.1Q trunk. Voice frames are tagged
with VLAN vlan-id (1 to 4096 COS or 1 to 1001 IOS), whereas frames from
the phone's data port are sent untagged (the native VLAN). The CoS value of
the voice frames are carried in the 802.1p priority field.
Chapter 14: Voice 251