R2511-HP MSR Router Series ACL and QoS Configuration Guide(V5)

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

lower threshold, and drop probability for packets with different precedence values to provide

differentiated drop policies.

With FIFO queuing, PQ, or CQ 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 22 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 using one of the following approaches:

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

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

table to an interface.

WRED parameters

Determine the following parameters before configuring WRED:

• The 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 is, the higher the

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

are dropped.

• The exponent used for average queue size calculation—The bigger the exponent is, the less

sensitive the average queue size is to real-time queue size changes.