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User Guide for Cisco Security Manager 4.4
OL-28826-01
Chapter 63 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.
Related Topics
• Understanding the Token-Bucket Mechanism, page 63-7
• Understanding Marking Parameters, page 63-3
• Understanding Queuing Parameters, page 63-4
• Defining QoS Policies, page 63-10
• Quality of Service on Cisco IOS Routers, page 63-1
Understanding the Token-Bucket Mechanism
Both policing and shaping use a token-bucket mechanism to regulate data flow. A token bucket is a
formal definition of a rate of transfer. It has three components: a burst size, a mean rate, and a time
interval (Tc). Any two values may be derived from the third using this formula:
Policing
Shaping
Traffic Rate
Traffic Rate
Traffic Rate
TimeTime
Traffic Rate
TimeTime
TrafficTraffic
TrafficTraffic
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