3Com 4200G 12-Port Configuration Manual page 368

Figure 1-8 Diagram for SP queuing
SP queuing is specially designed for mission-critical applications. The key feature of mission-critical
applications is that they require preferential service to reduce the response delay when congestion
occurs. Assume that there are eight output queues on the port and SP queuing classifies the eight
output queues on the port into eight classes, which are queue 7, queue 6, queue 5, queue 4, queue 3,
queue 2, queue 1, and queue 0 in the descending order of priority.
SP queuing schedules the eight queues strictly in the descending order of priority. It sends packets in
the queue with the highest priority first. When the queue with the highest priority is empty, it sends
packets in the queue with the second highest priority, and so on. By assigning mission-critical packets to
high priority queues and common service packets to low priority queues, you can ensure that the
mission-critical packets are always served prior to common service packets.
The disadvantage of SP queuing is that packets in the lower priority queues cannot get served if there
are packets in the higher priority queues for a long time when congestion occurs. This may cause low
priority traffic to starve to death.
2) WRR queuing
Figure 1-9 Diagram for WRR queuing
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