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Strict Priority Queuing
Strict Priority Queuing is specially designed to meet the demands of critical services or applications. Critical
services or applications such as voice are delay-sensitive and thus require to be dequeued and sent first before
packets in other queues are dequeued on a congested network. For example, assume that 4 egress queues 3, 2, 1 and
0 with descending priority are configured on a port.
Then under SP algorithm, the port strictly prioritizes packets from higher priority queue over those from lower
priority queue. Namely, only after packets in highest priority queue are emptied, can packets in lower priority
queue be forwarded. Thus High-priority packets are always processed before those of less priority.
Medium-priority packets are always processed before low-priority packets. The lowest priority queue would be
serviced only when highest priority queues had no packets buffered.
Disadvantages of SP: The SP queueing gives absolute priority to high-priority packets over low-priority traffic; it
should be used with care. The moment a higher priority packet arrived in its queue, however, servicing of the lower
priority packets would be interrupted in favor of the higher priority queue or packets will be dropped if the amount
of high-priority traffic is too great to emptied within a short time.
WRR
WRR queue scheduling algorithm ensures every queue a guaranteed service time by taking turns to schedule all
queues. Assume there are 4 egress queues on the port. The four weight values (namely, w3, w2, w1, and w0)
indicate the proportion of resources assigned to the four queues respectively. On a 100M port, if you set the weight
values of WRR queue-scheduling algorithm to 25, 15, 5 and 5(corresponding to w3, w2, w1, and w0 respectively).
Summary of Contents for G3224P
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