Describing Weighted Random Early Detection - Enterasys Security Router X-PeditionTM User Manual

In the following example, class bus has a minimum threshold of 460. RED will start to randomly
(with a probability between 0 and 1/10) discard packets when its queue grows over 460 packets. It
will start to discard each packet when the queue holds more than 550 packets.
Note: Drop Tail and RED cannot be used on the same queue at the same time. - queue-limit and
random-detect are mutually exclusive. If random-detect is set on a queue, queue-limit cannot be set
on the same queue until RED is removed.

Describing Weighted Random Early Detection

Weighted Random Early Detection (WRED) is variant of RED that combines RED's capability of
selectively dropping packets with DSCP or IP precedence marking. WED's three parameters
(MinTh, MaxTh and MaxP), may have different values for different DSCP or IP precedence and, as
a consequence, dropping probability will differ between DSCP/IP precedence markings.
You can configure MinT, MaxT and MaxP parameters for each DSCP/IP precedence value. If no
parameters are defined, WRED uses default values. For a particular queue, WRED drops packets
based only on one type of marking: DSCP or IP Precedence, meaning WRED can be configured as
DSCP or IP precedence-based but not both at same time. WRED can be applied to priority or fair
bandwidth class.
In the following configuration, DSCP value af11 has a minimum threshold of 300. For all other
DSCP values, WRED parameters are set to 460, 550, and 10. When the queue for class bus grows
larger than 300 packets, WRED starts randomly (with a probability between 0 and 1/20)
discarding packets with a DSCP marking of af11.
For all other DSCP values, WRED starts randomly discarding packets with a probability between
0 and 1/10 when its queue grows larger than 460 packets. It discards all packets with DSCP af11
when the queue is larger than 350, and discards all other packets when the queue is larger than
550 packets. Clearly, packets with DSCP af11 will be discarded more often than packets with other
DSCP values.
XSR(config)#policy-map ppwe
XSR(config-pmap<ppwe>)#class voip
XSR(config-pmap-c<voip>)#priority high 64 1000
XSR(config-pmap<ppwe>)#class bus
XSR(config-pmap-c<bus>)#bandwidth percent 30
XSR(config-pmap-c<bus>)#random-detect dscp-based
XSR(config-pmap-c<bus>)#random-detect dscp af11 300 350 20
XSR(config-pmap-c<bus>)#random-detect dscp default 460 550 10
WRED is important because it complies with Diff Serve, whose specification has three drop levels
for AF classes. When congested, packets with a higher drop DSCP marking should be discarded
statistically more often than packets with a lower drop marking. This can be achieved by using
Figure 12-1 RED Drop Probability Calculation
1
MaxP
0
MinTh
Average Queue Size
MaxTh
Mechanisms Providing QoS
XSR User's Guide 12-11