Rate Policies

To rate-limit or classify packets, you must first define a rate policy. This rate policy defines the bandwidth thresholds and actions to take when traffic exceeds (or conforms to) your thresholds. For example, you can configure an interface to drop all FTP traffic exceeding 240 kbps.

The rate policy contains rate limits that define the bandwidth thresholds and the exceed or conform actions. Some of the exceed and conform actions you can define are

• Transmit—Transmit the packet.

• Continue—Go to the next rate limit in the list.

• Set precedence and transmit - Set the precedence of the packet to a value and then transmit it.

• Set precedence and continue—Set the precedence of the packet to a value and then go to the next rate limit in the list.

To define a rate limit, you must specify three numbers

• Average rate (in hps)—The bandwidth throughput allowed before traffic is subject to the exceed action. All traffic below the average rate is said to conform. Traffic above the average rate is allowed to burst or is said to exceed (this depends on how you configure the normal and excess burst sizes, described in the following two paragraphs).

• Normal burst size (in bytes)—The number of bytes allowed in a burst before some traffic is subject to the exceed action. Traffic is allowed to burst this many bytes above the average rate before the router applies the exceed action to some of the packets (drops the packet, reclassifies the packet, or anoUier action you define). Why "some" of the packets? Because traffic that bursts between the normal burst size and the excess burst size is dropped randomly in a RED-like manner (see "Random Early Detection," earlier in this chapter), helping to throttle back TCP-based flows before the excess burst size is reached.

• Excess burst size (in bytes)—The number of bytes allowed in a burst before all traffic is subject to the exceed action. Traffic is allowed to burst this many bytes above the average rate before the router applies the exceed action to all of the packets (drops the packet, reclassifies the packet, or another action you define). If the excess burst size is zero, the router applies the exceed action to all traffic that exceeds the average rate (no bursts allowed).

NOTE CAR might not be compatible with all router interfaces, such as PRI and tunnel interfaces. Check with Cisco for the most current restrictions.

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