3Com Switch 7754, Configuration Manual

Page: 665 / 940

Overview 665
Figure 175 Diagram for WRR
3 WRR queue-scheduling algorithm schedules all the queues in turn and every
queue can be assured of a certain service time. Assume there are 8 priority queues
on the port. WRR configures a weight value for each queue, which are w7, w6,
w5, w4, w3, w2, w1, and w0. The weight value indicates the proportion of
obtaining resources. On a 100M port, configure the weight value of WRR
queue-scheduling algorithm to 50, 50, 30, 30, 10, 10, 10 and 10 (corresponding
to w7, w6, w5, w4, w3, w2, w1, and w0 in order). In this way, the queue with the
lowest priority can get 5 Mbps bandwidth at least, and the disadvantage of SP
queue-scheduling that the packets in queues with lower priority may not get
service for a long time is avoided. Another advantage of WRR queue is that:
though the queues are scheduled in order, the service time for each queue is not
fixed, that is to say, if a queue is empty, the next queue will be scheduled. In this
way, the bandwidth resources are made full use of.
Traffic-based Traffic
Statistics
The function of traffic-based traffic statistics is to use ACL rules in traffic
identifying and perform traffic statistics on the packets matching with the ACL
rules. You can get the statistics of the packets you are interested in through this
function.
RED When congestion is too serious, the switch can adopt the random early detection
(RED) algorithm to solve the problem of excessive congestion and avoid global TCP
synchronization caused by the tail-drop algorithm.
When packets of one or more TCP connections are dropped at random and the
traffic is gradually reduced, packets of other TCP connections can still be 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.
In the RED algorithm, an upper limit and a lower limit are set for each queue, and
it is stipulated that:
■
When the queue length is smaller than the lower limit, packets are not
dropped.
Queue 1 Weight 1
ĂĂ
Queue 2 Weight 2
Queue N-1 Weight N-1
Queue N Weight N
Packets to be sent through
this port
Sent packets
Interface
Queue
scheduling
Sending queue
Packet
classification