Dell Brocade DCX, Configuration Manual

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476 BigIron RX Series Configuration Guide
53-1001810-01
Determining packet drop priority using WRED
18
•
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 87 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 476). If q-size as calculated is below the configured Min. Average Queue Size, then the packet
is accepted. If the average queue size is above the Max. configured Average Queue Size threshold,
the packet is dropped. If the Average Queue size falls between the Min. Average Queue Size and
the Max. Average Queue Size, packets are dropped according to the calculated probability
described in
“Calculating packets that are dropped” on page 477.
FIGURE 87
WRED operation graph
Calculating avg-q-size
The algorithm first calculates the avg-q-size through the following equation.
avg-q-size = ( (1 - Wq) * Statistical Average-Q-Size) + (Wq * Current-Q-Size)
The Wq value is instrumental to the calculation and can be:
•
equal to the statistical average queue size (Wq == 0), or
•
equal to the current queue size (Wq == 1) or
•
be between 0 and 1 (0 < Wq < 1).
Pmax
Min. Average
Queue Size
Max. Average
Queue Size