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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 29 Configuring QoS
Understanding QoS
Figure 29-4 Policing and Marking Flowchart
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 to derive the internal DSCP (a 6-bit
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.
46977
Yes
Yes
No
No
Pass
through
Drop
Mark
Read the DSCP
of the packet.
Is a policer configured
for this DSCP?
Check if the packet is in
profile by querying the policer.
Check out-of-profile action
configured for this policer.
Drop packet.
Modify DSCP according to the
policed-DSCP map.
Start
Done