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Catalyst 3750 SwitchSoftware Configuration Guide
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Chapter35 Configuring QoS
Understanding QoS
Mapping Tables
During QoS processing, the switch represents the priority of all traffic (including non-IP traffic) with an
QoS label based on the DSCP or CoS value from the classification stage:
•During classification, QoS uses configurable mapping tables to derive a corresponding DSCP or
CoS value from a received CoS, DSCP, or IP precedence value. These maps include the
CoS-to-DSCP map and the IP-precedence-to-DSCP map. You configure these maps by using the mls
qos map cos-dscp and the mls qos map ip-prec-dscp global configuration commands.
On an ingress port configured in the DSCP-trusted state, if the DSCP values are different between
the QoS domains, you can apply the configurable DSCP-to-DSCP-mutation map to the port that is
on the boundary between the two QoS domains. You configure this map by using the mls qos map
dscp-mutation global configuration command.
•During policing, QoS can assign another DSCP value to an IP or a non-IP packet (if the packet is
out of profile and the policer specifies a marked-down value). This configurable map is called the
policed-DSCP map. You configure this map by using the mls qos map policed-dscp global
configuration command.
•Before the traffic reaches the scheduling stage, QoS stores the packet in an ingress and an egress
queue according to the QoS label. The QoS label is based on the DSCP or the CoS value in the packet
and selects the queue through the DSCP input and output queue threshold maps or through the CoS
input and output queue threshold maps. In addition to an ingress or an egress queue, the QOS label
also identifies the WTD threshold value. You configure these maps by using the mls qos srr-queue
{input | output} dscp-map and the mls qos srr-queue {input | output} cos-map global
configuration commands.
The CoS-to-DSCP, DSCP-to-CoS, and the IP-precedence-to-DSCP maps have default values that might
or might not be appropriate for your network.
The default DSCP-to-DSCP-mutation map and the default policed-DSCP map are null maps; they map
an incoming DSCP value to the same DSCP value. The DSCP-to-DSCP-mutation map is the only map
you apply to a specific port. All other maps apply to the entire switch.
For configuration information, see the “Configuring DSCP Maps” section on page35-67.
For information about the DSCP and CoS input queue threshold maps, see the “Queueing and
Scheduling on Ingress Queues” section on page35-16. For information about the DSCP and CoS output
queue threshold maps, see the “Queueing and Scheduling on Egress Queues” section on page35-17.