Cisco Systems CL-28826-01 Understanding the Token-Bucket Mechanism

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User Guide for Cisco Security Manager 4.4

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Chapter6 3 Configuring Quality of Service

Quality of Service on Cisco IOS Routers

meet downstream requirements, you can eliminate bottlenecks in topologies with data-rate

mismatches. Shaping can either be performed on selected QoS classes or at the interface level

(hierarchical shaping).

Both policing and shaping mechanisms use the traffic descriptor for a packet—indicated by the

classification of the packet (see Understanding Marking Parameters, page 63-3)—to ensure adherence

to the agreed upon level of service. Although policers and shapers usually identify traffic descriptor

violations in the same way, they differ in the way they respond to violations, as shown in Figure 63-2:

•A policer typically drops excess traffic. In other cases, it transmits the traffic with a different

(usually lower) priority.

•A shaper typically delays excess traffic using a buffer, or queuing mechanism, to hold packets and

shape the flow when the data rate of the source is higher than expected.

Figure 63-2 Traffic Policing vs. Traffic Shaping

For information about defining policing and shaping parameters in a QoS policy, see Defining QoS Class

Policing Parameters, page 63-17 and Defining QoS Class Shaping Parameters, page 63-18.