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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
Understanding QoS
Figure 20-2 Basic QoS Model
Classification
Classification is the process of distinguishing one kind of traffic from another by examining the fields
in the packet. Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is
globally disabled, so no classification occurs.
Note
Classification occurs only on a physical interface basis. No support exists for classifying packets at
the VLAN or the switch virtual interface level.
You specify which fields in the frame or packet that you want to use to classify incoming traffic.
For non-IP traffic, you have these classification options as shown in
Figure 20-3
:
•
Use the port default. If the frame does not contain a CoS value, assign the default port CoS value to
the incoming frame. Then use the configurable CoS-to-DSCP map to generate the internal DSCP
value.
•
Trust the CoS value in the incoming frame (configure the port to trust CoS). Then use the
configurable CoS-to-DSCP map to generate the internal DSCP value. Layer 2 ISL frame headers
carry the CoS value in the three least-significant bits of the 1-byte User field. Layer 2 802.1Q frame
headers carry the CoS value in the three most-significant bits of the Tag Control Information field.
CoS values range from 0 for low priority to 7 for high priority.
The trust DSCP and trust IP precedence configurations are meaningless for non-IP traffic. If you
configure a port with either of these options and non-IP traffic is received, the switch assigns the
default port CoS value and generates the internal DSCP from the CoS-to-DSCP map.
•
Perform the classification based on the configured Layer 2 MAC access control list (ACL), which
can examine the MAC source address, the MAC destination address, and the Ethertype field. If no
ACL is configured, the packet is assigned the default DSCP of 0, which means best-effort traffic;
otherwise, the policy map specifies the DSCP to assign to the incoming frame.
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Classification
Policing
Generate DSCP
Actions at ingress
Actions at egress
Mark
In profile or
out of profile
Inspect packet and
determine the DSCP
based on ACLs or
the configuration.
Map the Layer 2
CoS value to a
DSCP value.
Compare DSCP to
the configured
policer and
determine if the
packet is in profile or
out of profile.
Based on whether
the packet is in or
out of profile and the
configured
parameters,
determine whether
to pass through,
mark down, or drop
the packet. The
DSCP and CoS are
marked or changed
accordingly.
Queueing and
scheduling
Based on the CoS,
determine into which
of the egress
queues to place the
packet. Then service
the queues
according to the
configured weights.
