Brocade BigIron RX Series Configuration Guide - Manual (Page 572)

494 BigIron RX Series Configuration Guide
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Determining packet drop priority using WRED
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Determining packet drop priority using WRED
You can configure a device to monitor traffic congestion and drop packets according to a WRED
(Weighted Random Early Detection) algorithm. This algorithm enables the system to detect the
onset of congestion and take corrective action. In practice, WRED causes a Switch to start dropping
packets as traffic in the switch starts to back up. WRED provides various control points that can be
configured to change a system's reaction to congestion. The following variables are used when
calculating whether to drop or forward packets:
• Statistical Average-Q-Size – The statistical average size of the queue calculated over time on
the switch.
• Current-Q-Size – The current size of the queue as calculated on the switch.
• Wq – This variable specifies the weights that should be given to the current queue size and the
statistical average-q-size when calculating the size for WRED calculations.
• Max-Instantaneous-Q-Size – The maximum size up to which a queue is allowed to grow.
Packets that cause the queue to grow beyond this point are unconditionally dropped. This
variable is user configured.
• Min-Average-Q-Size – The average queue size below which all packets are accepted. This
variable is user configured.
• Max-Average-Q-Size – The average queue size above which all packets are dropped. This
variable is user configured.
• Pmax – The maximum drop probability when queue-size is at Max-Average-Q-Size. This variable
is user configured.
• Pkt-Size-Max – The packet size to which the current packet's size is compared as shown in the
algorithm below. This variable is user configured.
How WRED Operates
The graph in Figure 88 describes the interaction of the previously described variables in the
operation of WRED. When a packet arrives at a switch, the average queue size (q-size) is calculated
(note that this is not the statistical average queue size - (refer to
“Calculating avg-q-size” on
page 495). If q-size as calculated is below the configured Min. Average Queue Size, then the packet
MaxQSz Specifies the instantaneous queue size in KB.
DropPrec Specifies the drop precedence.
MinAvgQSz Specifies the minimum average queue size in KB.
MaxAvgQSz Specifies the maximum average queue size in KB.
MaxDropProb Specifies the maximum drop precedence in percentage.
MaxPktSz Specifies the maximum packet size in byte.
TABLE 102 Queueing configuration information (Continued)
Field Description