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Managing Transient Traffic Bursts by Configuring Weighted RED Buffer Occupancy

By default, RED is performed based on instantaneous buffer occupancy information. However, IQ-PICs can be configured to use weighted average buffer occupancy information. This option is configured on a per-PIC basis and applies to the following IQ-PICs:

  • Channelized T1/T3

  • Channelized E1/E3

  • Channelized OC3/STM1

  • Channelized OC12

If you configure this feature on an unsupported PIC, you see an error message.

If you configure this feature on a channelized OC12 intelligent queuing (IQ) interface, the PIC reboots.

When weighted average buffer occupancy is configured, you configure a weight value for averaged buffer occupancy calculations. This weight value is expressed as a negative exponential value of 2 in a fractional expression. For example, a configured weight value of 2 would be expressed as 1/( 2²) = 1/4. If a configured weight value was configured as 1 (the default), the value would be expressed as 1/( 2¹) = 1/2.

This calculated weight value is applied to the instantaneous buffer occupancy value to determine the new value of the weighted average buffer occupancy. The formula to derive the new weighted average buffer occupancy is:

new average buffer occupancy = weight value * instantaneous buffer occupancy + (1 – weight value) * current average buffer occupancy

For example, if the weight exponent value is configured as 3 (giving a weight value of 1/2³ = 1/8), the formula used to determine the new average buffer occupancy based on the instant buffer usage is:

new average buffer occupancy = 1/8 * instantaneous buffer occupancy + (7/8) * current average buffer occupancy

The valid operational range for the weight value on IQ-PICs is 0 through 31. A value of 0 results in the average buffer occupancy being the same as the instantaneous buffer occupancy calculations. Values higher than 31 can be configured, but in these cases the current maximum operational value of 31 is used for buffer occupancy calculations.


The show interfaces command with the extensive option displays the configured value for the RED buffer occupancy weight exponent. However, in all such cases, the current operational maximum value of 31 is used internally.

To configure weighted average buffer occupancy:

  1. Specify the FPC slot number and Q-PIC number on which you want to configure RED weighted average buffer occupancy calculations:
  2. Specify the weight exponent value.
  3. Verify your configuration.

    For example:

  4. Save your configuration.