3Com S7902E, Configuration Manual

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6-2
z
When the queue size is between the lower threshold and the upper threshold, the received packets
are dropped at random. The longer a queue is, the higher the drop probability is. However, a
maximum drop probability exists.
Different from RED, WRED determines differentiated drop policies for packets with different IP
precedence values. Packets with a lower IP precedence are more likely to be dropped.
Introduction to WRED Configuration
WRED Configuration Approaches
On an S7900E series switch, WRED is implemented with WRED tables. WRED tables are created
globally in system view and then applied to interfaces.
Introduction to WRED Parameters
Determine the following parameters before configuring WRED:
z
The upper threshold and lower threshold: when the average queue size is smaller than the lower
threshold, no packet is dropped. When the average queue size is between the lower threshold and
the upper threshold, packets are dropped randomly. The longer a queue, the higher the drop
probability. When the average queue size exceeds the upper threshold, subsequent packets are
dropped.
z
Drop precedence: a parameter used for packet drop. The value 0 corresponds to green packets,
the value 1 corresponds to yellow packets, and the value 2 corresponds to red packets. Red
packets are dropped preferentially.
z
Denominator for drop probability calculation: the bigger the denominator is, the smaller the
calculated drop probability is.
The S7900E series switches do not support the upper drop threshold.
Configuring WRED on an Interface
The SA cards do not support WRED.
In a WRED table, drop parameters are configured on a per queue basis, because WRED regulates
packets on a per queue basis.
A WRED table can be applied to multiple interfaces. For a WRED table already applied to an interface,
you can modify the values of the WRED table, but you cannot remove the WRED table.