Medianet Reference Guide - Cisco

Chapter 4
Medianet QoS Design Considerations
Cisco QoS Toolset
Policing and Markdown Tools
Policers are used to monitor traffic flows and to identify and respond to traffic violations. Policers
achieve these objectives by performing ongoing, instantaneous checks for traffic violations and taking
immediate prescribed actions when such violations occur. For example, a policer can determine if the
offered load is in excess of the defined traffic rate and then drop the out-of-contract traffic, as illustrated
in Figure 4-13.
Figure 4-13
A Policing Action
Offered Traffic
Policing
Offered Traffic
Excess traffic is dropped
Policing Rate
Time
Time
226609
Alternatively, policers may be used to remark excess traffic instead of dropping it. In such a role, the
policer is called a marker. Figure 4-14 illustrates a policer functioning as a marker.
Figure 4-14
A Policer as a Marker
Excess traffic is remarked, but transmitted
Offered Traffic
Policing
Offered Traffic
Policing Rate
Time
Time
226610
The rate at which the policer is configured to either drop or remark traffic is called the Committed
Information Rate (CIR). However, policers may police to multiple rates, such as the dual rate policer
defined in RFC 2698. With such a policer, the CIR is the principle rate to which traffic is policed, but an
upper limit, called the Peak Information Rate (PIR), is also set. The action of a dual-rate policer is
analogous to a traffic light, with three conditional states—green light, yellow light, and red light. Traffic
equal to or below the CIR (a green light condition) is considered to conform to the rate. An allowance
for moderate amounts of traffic above this principal rate is permitted (a yellow light condition) and such
traffic is considered to exceed the rate. However, a clearly-defined upper-limit of tolerance (the PIR) is
also set (a red light condition), beyond which traffic is considered to violate the rate. As such, a dual-rate
RFC 2698 policer performs the traffic conditioning for RFC 2597 Assured Forwarding PHBs, as
previously discussed. The actions of such a dual-rate policer (functioning as a three-color marker) are
illustrated in Figure 4-15.
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Medianet Reference Guide
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