260 Cisco LAN Switching Configuration Handbook
The QoS information from other switches in a QoS domain can be trusted. This
assumes that every switch in the QoS domain has been configured to enforce
QoS policies consistently.
QoS is port-based on these connections because every VLAN carried over the
link will have its QoS values already examined and modified. A Layer 3 switch
can trust the inbound DSCP information, but a Layer 2 switch can trust only the
inbound CoS values.
c. (Optional) Don’t trust QoS from sources outside the QoS domain:
Frames that are untagged receive CoS value 0. This also causes the ingress DSCP val-
ues to be mapped to 0 by the CoS-to-DSCP mapping. (See the next step.)
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; Layer 3 only) Apply a QoS policy to the voice traffic.
a. Define matching traffic with an ACL:
(global) ip access-list extended acl-name
(access-list) permit tcp any any range 2000 2002 dscp 26
(access-list) exit
In this case, the 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.
If other voice protocols are used, you can change the ACL to match against the
appropriate port numbers.
b. (Layer 3 IOS only) Define the QoS policy:
(global) policy-map policy-name
(pmap) class class-name access-group acl-name
IOS L3
(interface) mls qos cos 0
(interface) no mls qos trust
IOS L2
(interface) mls qos cos 0
(interface) no mls qos trust
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