4000 Router Series ACL and QoS Configuration Guide

When using FIFO queuing, 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.

Introduction to WRED configuration

WRED configuration approaches

WRED supports the following approaches:

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

router supports only this approach.

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

table to an interface.

Introduction to 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, no packet is 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

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