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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter20 Configuring QoS Understanding QoS
Mapping Tables
During QoS processing, the switch represents the priority of all traffic (including non-IP traffic) with an
internal DSCP value:
•During classification, QoS uses configurable mapping tables t o deri ve t he in ter nal DSCP ( a 6-bi t
value) from received CoS or IP precedence (3-bit) values. These maps include the CoS-to-DSCP
map and the IP-precedence-to-DSCP map.
On an ingress interface 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
interface that is on the boundary between the two QoS domains.
•During policing, QoS can assign another DSCP value to an IP or non -IP packet (if the packet is out
of profile and the policer specifies a marked down DSCP value). This configurable map is called the
policed-DSCP map.
•Before the traffic reaches the scheduling stage, QoS uses the configurable DSCP-to-CoS map to
derive a CoS value from the internal DSCP value. Through the CoS-to-egress-queue map, the CoS
values select one of the four egress queues for output processing .
The CoS-to-DSCP, DSCP-to-CoS, and the IP-precedence-to-DSCP map 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 ma ps; t hey ma p
an incoming DSCP value to the same DSCP value. The DSCP-to-DSCP-mutation map is the only map
you apply to a specific Gigabit-capable Ethernet port or to a group of 10/100 Ethernet ports . All other
maps apply to the entire switch.
For configuration information, see the “Configuring DSCP Maps” section on page 20-39.