R0106-HP MSR Router Series ACL and QoS Configuration Guide(V7)

With FIFO queuing used, you can set the exponent for average queue size calculation, upper threshold,

lower threshold, and drop probability for each queue to provide differentiated drop policies for different

classes of packets.

Relationship between WRED and queuing mechanisms

Figure 18 Relationship between WRED and queuing mechanisms

Through combining WRED with WFQ, the flow-based WRED can be realized. Because each flow has its

own queue after classification, a flow with a smaller queue size has a lower packet drop probability,

when a flow with a larger queue size has a higher packet drop probability. In this way, the benefits of the

flow with a smaller queue size are protected.

WRED configuration approaches

You can configure WRED by using one of the following approaches:

• Interface configuration—Configure WRED parameters on an interface and enable WRED.

• WRED table configuration—Configure a WRED table in system view and then apply the WRED

table to an interface.

The router supports only the interface configuration approach.

WRED parameters

Determine the following parameters before configuring WRED:

• Upper threshold and lower threshold—When the average queue size is smaller than the lower

threshold, packets are not dropped. When the average queue size is between the lower threshold

and the upper threshold, the packets are dropped at random. The longer the queue, the higher the

drop probability. When the average queue size exceeds the upper threshold, subsequent packets

are dropped.

• 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.

Packets to be sent

through this interface

Packets sent

Sending queue

Interface

Classify

Schedule

……

Queue 1 weight 1

Queue 2 weight 2

Queue N-1 weight N-1

Queue N weight N

……

WRED drop

Packets

dropped