Enterasys Networks X-PeditionTM manual Describing Weighted Random Early Detection

Mechanisms Providing QoS

Figure 12-1 RED Drop Probability Calculation

Drop Probability	1
	MaxP
	0		Average Queue Size
	MinTh	MaxTh

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-limitand random-detectare mutually exclusive. If random-detectis set on a queue, queue-limitcannot 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

XSR User’s Guide 12-11