Cisco Catalyst 2000 Configuration Handbook page 278

Tip A Cisco IP Phone marks its SCCP voice control packets with CoS 3, ToS 3, and DSCP
26 (AF31). The RTP voice bearer packets are marked with CoS 5, ToS 5, and DSCP 46 (EF).
These are carried over the frames in the voice VLAN (VVID) of the 802.1Q trunk.
The IP Phone also marks traffic from its access switch port if instructed to do so. By
default, these frames are carried untagged over the native VLAN of the 802.1Q trunk and
have their ToS and DSCP values set to 0.
b. (Optional) Don't trust QoS from a PC running Cisco SoftPhone:
IOS L3
(interface) mls qos cos 0
(interface) no mls qos trust
IOS L2
(interface) mls qos cos 0
(interface) no mls qos trust
Although a SoftPhone PC produces voice control and bearer data packets, other
applications running can attempt to mark the CoS in nonvoice packets. Because of
this, you should not trust the QoS information coming from the PC. Set these
switch ports to an untrusted state and configure Layer 3 switches in your QoS
domain to classify and mark the voice control and bearer packets appropriately.
Tip The Cisco SoftPhone application marks its SCCP voice control packets with CoS 0,
ToS 0, and DSCP 0 (default). The RTP voice bearer packets are marked with CoS 5, ToS 5,
and DSCP 46 (EF). These are carried over the access VLAN untagged because no inherent
trunk is used.
c. (Optional) Don't trust QoS from a regular data-only host:
IOS L3
(interface) mls qos cos 0
(interface) no mls qos trust
IOS L2
(interface) mls qos cos 0
(interface) no mls qos trust
Frames that are untagged or that do not match any QoS-classifying access control lists
(ACL) will be marked with CoS value 0. This also causes the ingress DSCP values to be
mapped to 0 by the CoS-to-DSCP mapping. (See the next step.)
Chapter 14: Voice 257