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Understanding Burst Size Configuration on IQ2 and IQ2E Interfaces

You can explicitly configure the burst size for shapers in a traffic control profile for IQ2 and IQ2E interfaces. This feature is supported on M7i, M10i, M40e, M120, M320 routers and all T Series routers.

The shaping burst size determines the maximum number of bytes that can be sent through a shaper during a burst. The guaranteed burst size determines when the scheduler moves from green to yellow.

The burst size limits the number of credits that can be accumulated for scheduling. Configuring a burst size is only useful in the case when traffic is sent after a long lull period so that credits can be accumulated until the burst size limit is reached. When traffic is continuous, credits are not accumulated, and the burst size limit is not reached.

If no burst size value is specified when the shaping rate or guaranteed rate is configured, then a default burst size (expressed as a time value) is applied. The default shaping burst size is 10 ms of the shaping rate (that is, 10*shaping rate/1000 bytes). The minimum value is 2048 bytes to accommodate the minimum of 1 MTU.

The burst size value is adjusted and rounded off to meet the restrictions enforced by the hardware. Thus, the actual burst size in the hardware might vary slightly from the configured value.

To enable this feature, include the burst-size statement at the following hierarchy levels:


The guaranteed-rate burst size value cannot be greater than the shaping-rate burst size.