3Com Switch 8800 Advanced Software V5 Configuration Guide

860 CHAPTER 65: CONGESTION AVOIDANCE
■ When the queue length exceeds the maximum threshold, all the incoming
packets are dropped.
■ When the queue length is between the maximum threshold and the minimum
threshold, the packets are dropped randomly. The longer the queue is, the
higher the drop probability is, but a maximum drop probability exists.
Unlike RED, the random numbers of WRED is generated based on priority. It uses
IP precedence to determine the drop policy, and thus the drop probability of
packets with high priority is relatively low.
RED and WRED employ the random packet drop policy to avoid global TCP
synchronization. When packets of a TCP link are dropped and sent at a low rate,
the other TCP links still send packets at high rates. There are always some TCP
links sending packets at high rates, thus improving link bandwidth utilization.
Average queue length
Dropping packets through comparing the queue length with the
maximum/minimum threshold treat the burst traffic unfairly and affect traffic
transmission. WRED compares the average queue length with the
maximum/minimum threshold to determine the drop probability.
The average queue length reflects the queue change tendency and is insensitive to
bursting change of the queue length, thus preventing the unfair treatment for the
bursting traffic. The average queue length is calculated using the following
formula: average queue length = (Previous average queue length × (1-2
-n
)) +
(Current queue length ×2
-n
), where n can be configured through the qos wred
weighting-constant command.
Relationship between WRED and queuing mechanisms
The relationship between WRED and queuing mechanisms is shown as in the
following figure:
Figure 254 Relationship between WRED and queuing mechanisms