Quality of Service Guide QoS and QoS Policies
Edition: 01 3HE 11014 AAAC TQZZA 155
3.7.7.1 WRED MinThreshold and MaxThreshold Computation
CBS is configured in kilobytes through the CLI; MBS is configured in bytes or
kilobytes. These configured values are converted to the corresponding number of
buffers. The conversion factor is a non-user-configurable, fixed default value that is
equal to the system-defined maximum frame size, ensuring that even the largest
frames can be hosted in the allocated buffer pools. This type of WRED is called
buffer-based WRED.
User-defined minThreshold and maxThreshold values, each defined as a
percentage, are also converted to the number of buffers. The minT is converted to
the system-minThreshold, and the maxT is converted to the system-maxThreshold.
The system-minT must be the absolute closest value to the minT that satisfies the
formula below (2^x means 2 to the exponent x):
system-maxThreshold – system-minThreshold = 2^x
3.7.7.2 WRED on Bridging Domain (Ring) Queues
The bridging domain queues support the following DP (discard probability) values:
0% to 10%, 25%, 50%, 75%, and 100%. User-configured values are rounded down
to match these DP values.
For example, configuring a DP to be 74% means that the actual value used is 50%.
Note: The 6-port Ethernet 10Gbps Adapter card and the 7705 SAR-X Ethernet ports use
payload-based WRED (also called byte-based WRED); see Payload-based WRED. The
above “system-minT” calculation does not apply to payload-based WRED. The 7705 SAR-X
TDM ports use buffer-based WRED.
Note: Tail drop for out-of-profile traffic begins when the queue occupancy of the
out-of-profile traffic exceeds that of the committed buffer size.
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