Cisco Catalyst 2000 Configuration Handbook page 279

258 Cisco LAN Switching Configuration Handbook
2. (Optional; Layer 3 only) Adjust the ingress QoS-to-DSCP mappings:
(global) mls qos map cos-dscp 0 8 16 26 32 46 48 56
(global) mls qos map ip-prec-dscp 0 8 16 26 32 46 48 56
You can make minor adjustments to the mappings so that CoS 3 maps to DSCP 26
(AF31) and CoS 5 maps to DSCP 46 (EF). The default values are slightly different
and are not the standard values expected for voice traffic.
3. (Optional) Extend QoS trust into the IP Phone.
a. Set the phone access-port trust:
b. Set the default phone access-port CoS value:
4. (Layer 3 only) Trust DSCP information on the uplink ports:
(interface) mls qos trust dscp
Because the distribution and core layer switches are also within the QoS domain and
are properly configured to follow the QoS requirements, you can safely assume that
any QoS information coming from them has been examined and adjusted to conform
to the QoS policies. As such, this information can be trusted over the uplink ports
on an access layer switch.
5. (Optional; Layer 3 only) Apply a QoS policy to the voice traffic.
a. Define matching traffic with an ACL:
(interface) switchport priority extend {trust | none}
A Cisco IP Phone has its own access layer switch port, where a PC can be con-
nected. This port is untrusted (IOS none) by default, causing the CoS and IP
Precedence values for inbound frames to be set to 0. To allow the PC to mark its
own packets with IP Precedence values, set the mode to trusted (IOS trust).
(interface) switchport priority
When the phone's access port is set to untrusted mode, the CoS value for
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all inbound data frames is set to cos-value (0 to 7, default 0) by the phone.
(global) ip access-list extended acl-name
(access-list) permit tcp any any range 2000 2002 dscp 26
(access-list) exit
In this case, SCCP voice control TCP ports 2000, 2001, and 2002 are matched.
These frames are given a DSCP value of 26 (AF31), even if this value was
already set. This matching ACL is also necessary so that the CoS trust can be
established on switch ports configured with the set port qos trust trust-cos
command.
If other voice protocols are used, you can change the ACL to match against the
appropriate port numbers.
extend cos cos-value