WRED Configuration with MQC/HQF

WRED (Weighted Random Early Detect) is a feature in Cisco IOS MQC/HQF to detect aggressive TCP traffic queuing and drop some random packets before a tail drop occurs to prevent TCP global synchronization. Goal is to increase overall throughput which will be degraded by TCP global synchronization.

To learn about MQC/HQF of Cisco IOS please go here.
To learn about IP Precedence & DSCP codes please go here.

Following graph illustrates this scenario..

As you can see on the above graph, nothing will be dropped until the Average Queue Depth is 20.
After 20, it starts dropping and it will increase it as the Queue Depth increases until 45.
When the queue depth is 45, it will drop 25% of the traffic randomly.
After 45 everything will be dropped..

In WRED, the above values have the following terminology.

20 is the Minimum Threshold
45 is the Maximum Threshold
25% is the MPD (Mark Probability Denominator)

Because some traffic classes are more important than others, using WRED we can configure the thresholds for each and every traffic class to drop less important traffic more to keep much room to more important traffic..

WRED works with CBWFQ and Physical interface FIFO queue.

The default behavior of WRED is to take the decisions based on IP Precedence values.

Let's see this default behavior using a policy-map which calls the class-default.

R1(config)#policy-map WRED
R1(config-pmap)#class class-default
R1(config-pmap-c)#random-detect

Applying it to an interface;

R1(config-pmap)#int g0/1
R1(config-if)#service-policy output WRED

Following is the active policy-map I configured. This will show the statistics of the WRED.
(click on the image to view in full size)

As you can see, threshold values, MPD etc are already configured for each and every class of IP Precedence by default. We can change them too.

1/10 means in MPD is drop 1 packets out of every 10 packets (10%)

We only used random-detect command up to now. This command can be extended too.

We can change the IP Precedence based WRED to DSCP based as well as the Traffic Policy (threshold values).

Changing to DSCP based will not list all the DSCP values as it does by default for IP Precedence. Instead it will list the DSCP values it sees from traffic.