Dell PowerConnect 5548 User Guide - Page 676

QoS Policers Overview The rate of traffic that matches a pre-defined set of rules can be measured, and limits, such as limiting the rate of file-transfer traffic that is allowed on a port, can be enforced. This is done by using the ACLs in the class map(s) to match the desired pattern of traffic, and by using a policer to apply QoS on the matching traffic. A policer is configured with a QoS specification. There are two kinds of policers: • Single Policer - A single policer applies the QoS to a single class map, and to a single flow, based on the policer's QoS specification. When a class map, using a single policer, is bound to multiple ports, each port has its own instance of the single policer; each applying the QoS on the class map (flow) at ports that are otherwise independent of each other. A single policer is created in the Policy Table and Policy Class Maps pages. • Aggregate Policer - An aggregate policer applies QoS to one or more class maps, and to one or more flows. An aggregation policer can support class maps from various policies. An aggregate policer applies QoS to all its flow(s) in aggregation, regardless of policies and ports. An aggregate policer is created in the Aggregate Policer pages. An aggregate policer is defined if the policer is to be shared with more than one class. Each policer is defined with its own QoS specification, and is composed of a combination of the following parameters: • Committed Information Rate (CIR) - A maximum allowed rate of traffic, measured in Kbps. • Committed Burst Size (CBS) - An amount of traffic, measured in bytes, which is allowed to pass as a temporary burst, even if it is above the defined maximum rate. • Exceed Action - An action to be applied to frames that are over the limits (called out-of-profile traffic). These frames can be forwarded as is, dropped, or forwarded, after rewriting their DSCP value with a value that marks them as lower-priority frames for all subsequent handling within the device. A policer is assigned to a class map when a class map is added to a policy. 676 Dell PowerConnect 55xx Systems User Guide