26-9
Catalyst 3750 Metro Switch Software Configuration Guide
78-15870-01
Chapter 26 Configuring QoS
Understanding QoS
You create and name a policy map by using the policy-map global configuration command. When you
enter this command, the switch enters the policy-map configuration mode. In this mode, you use the
class policy-map configuration command to name the traffic class associated with the traffic policy. If
you specify class-default as the class name in the class policy-map configuration command, packets that
fail to meet any of the matching criteria are classified as members of the default traffic class. You can
manipulate this class (for example, police it and mark it) just like any traffic class, but you cannot delete
it. After you name the traffic class with the class command, the switch enters policy-map class
configuration mode, and you can specify the actions to take on this traffic class.
An ingress policy-map can include police, police aggregate, trust, or set policy-map class configuration
commands. You attach the ingress policy-map to a port by using the service-policy input interface
configuration command.
For more information, see the “Ingress Policing and Marking” section on page 26-9. For configuration
information, see the “Configuring an Ingress QoS Policy” section on page 26-48.
Ingress Policing and Marking
After a packet is classified and has a DSCP-based or CoS-based QoS label assigned to it, the traffic
policing and marking process can begin. Figure 26-4 on page 26-11 shows an ingress, single-rate traffic
policer.
Ingress traffic policing controls the maximum rate of traffic received on a port. The policer defines the
bandwidth limitations of the traffic, and the action to take if the limits are exceeded. It is often configured
on ports at the edge of a network to limit traffic into or out of the network. In most policing
configurations, traffic that falls within the rate parameters is sent. Traffic that exceeds the parameters is
considered to be out of profile or nonconforming and is dropped or sent with a different priority.
Note All traffic, regardless of whether it is bridged or routed, is subjected to a configured policer. As a result,
bridged packets might be dropped or might have their DSCP or CoS fields modified when they are
policed and marked.
You can create these types of ingress policers:
• Individual (single-rate)
QoS applies the bandwidth limits specified in the policer separately to each matched traffic class.
You can configure a single-rate policer within a policy map by using the police policy-map class
configuration command.
• Aggregate
QoS applies the bandwidth limits specified in an aggregate policer cumulatively to all matched
traffic flows. You configure this type of policer by specifying the aggregate policer name within a
policy map by using the police aggregate policy-map class configuration command. You specify the
bandwidth limits of the policer by using the mls qos aggregate-policer global configuration
command. In this way, the aggregate policer is shared by multiple classes of traffic within a policy
map.
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