Objectives

Upon completing this lesson, you will be able to explain how to use traffic policing and traffic shaping to condition traffic. This ability includes being able to meet these objectives:

■ Describe the purpose of traffic conditioning using traffic policing and traffic shaping

■ List key benefits of traffic conditioning using traffic policing and traffic shaping

■ Differentiate between the features of traffic policing and traffic shaping

■ Explain how a token bucket can be used by network devices to measure traffic rates

■ Explain how traffic can be policed using a single token bucket scheme

■ Explain how traffic can be policed using a dual token bucket scheme

■ Explain how traffic can be policed using a dual-rate metering scheme

■ Explain how traffic can be shaped using a single token bucket scheme

■ Describe the key traffic policing and shaping mechanisms available in Cisco IOS software and how each compares to the others

■ Identify the points in a network where rate-limiting can most effectively be employed

This topic describes the purpose of traffic conditioning using traffic policing and traffic shaping.

Traffic Policing and Shaping Overview

These mechanisms must classify packets before policing or shaping the traffic rate.

Traffic shaping queues excess packets to stay within the desired traffic rate.

These mechanisms must classify packets before policing or shaping the traffic rate.

Traffic shaping queues excess packets to stay within the desired traffic rate.

Traffic policing typically drops or marks excess traffic to stay within a traffic rate limit.

Both traffic shaping and policing mechanisms are traffic-conditioning mechanisms that are used in a network to control the traffic rate. Both mechanisms use classification so that they can differentiate traffic. They both measure the rate of traffic and compare that rate to the configured traffic-shaping or traffic-policing policy.

The difference between traffic shaping and policing can be described in terms of their implementation:

■ Traffic shaping buffers excessive traffic so that the traffic stays within the desired rate. With traffic shaping, traffic bursts are smoothed out by queuing the excess traffic to produce a steadier flow of data. Reducing traffic bursts helps reduce congestion in the network.

■ Traffic policing drops excess traffic in order to control traffic flow within specified rate limits. Traffic policing does not introduce any delay to traffic that conforms to traffic policies. Traffic policing can cause more TCP retransmissions, because traffic in excess of specified limits is dropped.

Traffic-policing mechanisms such as class-based policing or committed access rate (CAR) also have marking capabilities in addition to rate-limiting capabilities. Instead of dropping the excess traffic, traffic policing can alternatively mark and then send the excess traffic. This allows the excess traffic to be re-marked with a lower priority before the excess traffic is sent out. Traffic shapers, on the other hand, do not remark traffic; these only delay excess traffic bursts to conform to a specified rate.

This topic describes the key benefits of traffic conditioning using traffic policing and traffic shaping.

Traffic policing is typically used to satisfy one of these requirements:

■ Limiting the access rate on an interface when high-speed physical infrastructure is used in transport. Rate limiting is typically used by service providers to offer customers sub-rate access. For example, a customer may have an OC-3 connection to the service provider but pay only for a T1 access rate. The service provider can rate-limit the customer traffic to T1 speed.

■ Engineering bandwidth so that traffic rates of certain applications or classes of traffic follow a specified traffic rate policy. For example, rate-limiting traffic from file-sharing applications to 64 kbps maximum.

■ Re-marking excess traffic with a lower priority at Layer 2 and Layer 3, or both, before sending the excess traffic out. Cisco class-based traffic policing can be configured to mark packets at both Layer 2 and Layer 3. For example, excess traffic can be re-marked to a lower differentiated services code point (DSCP) value and also have the Frame Relay discard eligible (DE) bit set before the packet is sent out.

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