Method used to configure congestion avoidance and congestion management on S series switches


For S series switches (except the S1700), congestion avoidance prevents a network from being overloaded using a packet discarding policy. Congestion management ensures that high-priority services are preferentially processed based on the specified packet scheduling sequence.
On a traditional network, quality of services (QoS) is affected by network congestion. Congestion means the low data forwarding rate and delay resulting from insufficient network resources. Congestion results in delay of packet transmission, low throughput rate, and high resource consumption. Congestion often occurs in a complex networking environment where packet transmission and provision of various services are both required.
Congestion avoidance and congestion management are two flow control mechanisms for resolving congestion on a network.
For the configuration of congestion management on S series fixed switches, see "Example for Configuring Congestion Management (Schedule Template Mode)" in the Typical QoS Configuration.
For the comprehensive configuration example of S series fixed and modular switches, see "Example for Configuring Congestion Avoidance and Congestion Management (Using WRR Scheduling and an SRED Policy)" or "Example for Configuring Congestion Avoidance and Congestion Management (Using PQ+DRR Scheduling and a WERD Profile)" in the Typical QoS Configuration.

Other related questions:
How to configure congestion avoidance on an AR
When congestion occurs on a network, an AR router enabled with congestion management determines the packet forwarding sequence based on the configured scheduling policy to ensure that high-priority services are sent preferentially.

Configuration examples of congestion management and congestion avoidance on an AR
For details about the comprehensive examples for configuring congestion avoidance and congestion management on an AR, see "4.7.1 Example for Configuring Congestion Management and Congestion Avoidance" on "CLI-based Configuration Guide - QoS", link for ""

What impact does the queue length have
A longer queue buffers more packets but introduces a longer delay. If congestion intermittently occurs on a network, buffering more packets prevents unnecessary packet loss. If congestion constantly occurs on a network, increasing the queue length cannot solve this problem. You need to increase the bandwidth.

After congestion management is configured on an S series switch, why is the delay increased when the switch is fully loaded
After you configure congestion management on an S series switch (except the S1700), delay increase will occur if the traffic exceeds the bandwidth of an interface. This is a normal phenomenon and is related to congestion management.

Congestion management mechanism on an S series switch
On S series switches (except S1700), each physical port has eight sending queues numbered from 7 to 0 in descending order of priority. Queue 7 has the highest priority and queue 0 has the lowest priority. Sending ports support the following queue scheduling modes: PQ, WRR, DRR, PQ+WRR, and PQ+DRR. When PQ+WRR or PQ+DRR is used, packets in PQ queues are scheduled first. The PQ queue with the highest priority is scheduled preferentially and then PQ queues with lower priorities are scheduled in succession. After all PQ queues are scheduled, WRR or DRR queues are then scheduled. For WRR and DRR queues, the queues are scheduled to ensure bandwidth first and then scheduled based on weights. The S5720HI fixed switch and X series cards of S series modular switches do not support WRR and PQ+WRR scheduling.

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