Configuring Wred - H3C S3610-28P Operation Manual

Operation Manual – QoS
H3C S3610&S5510 Series Ethernet Switches
When the queue length is in the range of the upper limit and the lower limit, the
inbound packets are dropped at random. In this case, a number is assigned to
each inbound packet and then compared with the drop probability of the current
queue. If the number is bigger than the drop probability, the inbound packet is
dropped. The longer a queue is, the higher the drop probability is. However, there
is a top drop probability.
Compared with the RED algorithm, the WRED algorithm generates precedence-based
random numbers. It adopts IP precedence in determining drop policy and takes the
benefits of high-precedence packets into consideration, so the drop probability is
comparatively low.
In the RED and WRED algorithm, packets are dropped at random so that global TCP
synchronization is avoided. When packets in a TCP connection are dropped and sent
at a low rate, packets in other TCP connections are still sent at a high rate. In this way,
packets in a part of connections are sent at a high rate in any case. Thus, the utilization
rate of bandwidth is improved.
III. Queue length
In the following cases, you can increase the queue length to buffer more packets and
improve packet forwarding performance:
Network with dense broadcast or multicast traffic and large burst traffic
Packets of high-speed links are forwarded through low-speed links, or packets
received through multiple equal-rate interfaces at the same time are forwarded to
one interface that is of the same rate.
By increasing the queue length, you can reduce the packet loss ratio and packet
processing capability of the device in the cases mentioned above. Although increasing
queue length improves the ability to process burst traffic, it reduces the ability of
congestion avoidance.

6.2 Configuring WRED

6.2.1 Configuration Prerequisites
Prepare a network environment where congestion may occur.
Chapter 6 Congestion Avoidance
6-2