![]() In the next round, the queue with a negative weight value is not scheduled until its weight value becomes positive. If a packet is too long, WDRR allows a negative weight value so that long packets can be scheduled. Weighted Deficit Round Robin (WDRR) schedules packets based on the packet length, which is used as the weight. WRR scheduling, however, cannot schedule short-delay services in a timely manner. For example, if a queue is empty, WRR scheduling ignores this queue and starts to schedule the next queue. In addition, WRR scheduling can dynamically change the time of scheduling packets in queues. WRR scheduling offsets the disadvantage of PQ scheduling in which packets in queues with lower priories may be not processed for a long period of time if congestion occurs. That is, the switch schedules packets in queues 0 to 7 in sequence. If packets are scheduled in the same manner, large-sized packets obtain more bandwidth than small-sized packets.īy default, queues on the S5720I-SI, S5720-LI, S5720S-LI, S5720S-SI, S5720-SI, S5730S-EI, S5730-SI, S5735-L, S5735S-L, S5735S-L-M, S5735-S, S5735-S-I, and S5735S-S use the WRR scheduling mode, and the WRR weight for WRR scheduling is 1 for all these queues. WRR scheduling is performed on a per-packet basis, and there is no fixed bandwidth for each queue. A higher queue weight indicates a larger scheduling count. The statistics show that the scheduling count in each queue is proportional to the queue weight. Queue in the eleventh round of scheduling By doing this, queue 7 is not full and the scheduler can process packets in queues with lower priorities. To prevent starvation of packets in some queues, upstream devices need to accurately define service characteristics of data flows so that service flows mapped to queue 7 do not exceed a given percentage of the link capacity. ![]() For example, if data flows mapped to queue 7 arrive at a 100% link rate in a period, the scheduler does not process flows in queues 0 to 6. The PQ scheduling mechanism, however, may result in starvation of packets in queues with lower priorities. When packets of data flow X reach a node, the packets are processed first. Assume that data flow X is mapped to the queue of the highest priority on each node. PQ scheduling is valid for short-delay services. The packets in queue 5 are sent at the link rate when queue 6 and queue 7 are empty, and so on. ![]() The packets in queue 6 are sent at the link rate when packets in queue 6 need to be sent and queue 7 is empty. The scheduler processes packets in queue 6 only after queue 7 becomes empty. The packets in queue 7 are processed first. In Figure 1-1, the priorities of queues 7 to 0 are in descending order of priority. A switch enabled with PQ scheduling schedules packets based on priorities 7, 6, 5, 4, 3, 2, 1, and 0 in descending order. Packets in queues with a lower priority can be scheduled only after all packets in queues with a higher priority have been scheduled. Priority queuing (PQ) schedules packets in descending order of priority. In scenarios where the service traffic exceeds the bandwidth limit for a long period of time, you need to expand the network capacity or use dedicated devices to control services based on upper-layer applications. ![]() In scenarios where important services need to be preferentially processed upon burst traffic, queue scheduling (also called congestion management) and packet discarding (also called congestion avoidance) are recommended. On hardware devices with limited bandwidth resources, other methods are available for preventing, mitigating, or controlling network congestion. Increasing the link bandwidth is the best solution to prevent congestion. Congestion is common in a complex networking environment where IP packet switching and multi-service are deployed. For example, when a large amount of data flows from a high-bandwidth link to a low-bandwidth link, the outbound interface of the low-bandwidth link cannot process excess data flows. Congestion means the low data forwarding rate and extra delay resulting from insufficient network resources. ![]() Queue scheduling and packet discarding are used to cope with network congestion. ![]()
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