3Com 2928 - Baseline Plus Switch PWR, Руководство пользователя

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2-8
Figure 2-7
Schematic diagram for WRR queuing
A typical switch provides eight output queues per port. WRR assigns each queue a weight value
(represented by w7, w6, w5, w4, w3, w2, w1, or w0) to decide the proportion of resources assigned to
the queue. On a 100 Mbps port, you can set the weight values of WRR queuing to 50, 30, 10, 10, 50, 30,
10, and 10 (corresponding to w7, w6, w5, w4, w3, w2, w1, and w0 respectively). In this way, the queue
with the lowest priority is assured of at least 5 Mbps of bandwidth, thus avoiding the disadvantage of SP
queuing that packets in low-priority queues may fail to be served for a long time.
Another advantage of WRR queuing is that while the queues are scheduled in turn, the service time for
each queue is not fixed, that is, if a queue is empty, the next queue will be scheduled immediately. This
improves bandwidth resource use efficiency.
You can assign the output queues to WRR priority queue group 1 and WRR priority queue group 2.
Round robin queue scheduling is performed for group 1 first. If group 1 is empty, round robin queue
scheduling is performed for group 2.
You can implement SP+WRR queue scheduling on a port by assigning some queues on the port to the
SP scheduling group when configuring WRR. Packets in the SP scheduling group are scheduled
preferentially by SP. When the SP scheduling group is empty, the other queues are scheduled by WRR.
Line Rate
Line rate is a traffic control method using token buckets. The line rate of a physical interface specifies
the maximum rate for forwarding packets (including critical packets). Line rate can limit all the packets
passing a physical interface.
Traffic evaluation and token bucket
A token bucket can be considered as a container holding a certain number of tokens. The system puts
tokens into the bucket at a set rate. When the token bucket is full, the extra tokens will overflow.