Queue scheduling mechanism on S series switches

On S series switches (except the S1700), each physical interface has eight transmission queues. Queue 7 has the highest priority whereas Queue 0 has the lowest priority. Transmitting interfaces support PQ, WRR, and DRR scheduling as well as their combinations such as PQ+WRR and PQ+DRR.
If PQ+WRR or PQ+DRR scheduling is configured, a switch first schedules packets in PQ queues. During PQ scheduling, the switch first schedules packets in the queue of the highest priority, and then schedules packets in queues of a lower priority. After PQ scheduling is complete, the switch performs WRR or DRR scheduling.
When scheduling packets in WRR/DRR queues, the switch first ensures the bandwidth, and then schedules packets in WRR/DRR queues according to their weights.
Notes:
- X series cards of S series modular switches do not support WRR and PQ+WRR scheduling.
- The S5720HI does not support WRR and PQ+WRR scheduling.

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