Prerequisites For Queuing And Scheduling; Guidelines And Limitations - Cisco Nexus 9000 Series Configuration Manual

Prerequisites for Queuing and Scheduling

Traffic shaping regulates and smooths out the packet flow by imposing a maximum traffic rate for each port's
egress queue. Packets that exceed the threshold are placed in the queue and are transmitted later. Traffic
shaping is similar to traffic policing, but the packets are not dropped. Because packets are buffered, traffic
shaping minimizes packet loss (based on the queue length), which provides better traffic behavior for TCP
traffic.
Using traffic shaping, you can control access to available bandwidth, ensure that traffic conforms to the policies
established for it, and regulate the flow of traffic to avoid congestion that can occur when the egress traffic
exceeds the access speed of its remote, target interface. For example, you can control access to the bandwidth
when policy dictates that the rate of a given interface should not, on average, exceed a certain rate even though
the access rate exceeds the speed.
Queue length thresholds are configured using the WRED configuration.
Note
Traffic shaping is not supported on ALE enabled device 40G front panel ports. When traffic shaping is
configured for the system level, the setting is ignored and no error message is displayed. When traffic shaping
commands are configured for the port level, the setting is rejected and an error message is displayed.
Prerequisites for Queuing and Scheduling
Queuing and scheduling have the following prerequisites:
• You must be familiar with using modular QoS CLI.
• You are logged on to the device.

Guidelines and Limitations

Queuing and scheduling have the following configuration guidelines and limitations:
• show commands with the internal keyword are not supported.
• The device supports a system-level queuing policy, so all ports in the system are impacted when you
• A type queuing policy can be attached to the system or to individual interfaces for input or output traffic.
• Changes are disruptive. The traffic passing through ports of the specified port type experience a brief
• Performance can be impacted. If one or more ports of the specified type do not have a queuing policy
• Traffic shaping might increase the latency of packets due to queuing because it falls back to
• Traffic shaping is not supported on the Cisco Nexus 9300 ALE 40G ports. For more information on ALE
Cisco Nexus 9000 Series NX-OS Quality of Service Configuration Guide, Release 7.x
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configure the queuing policy.
period of traffic loss. All ports of the specified type are affected.
applied that defines the behavior for the new queue, the traffic mapping to that queue might experience
performance degradation.
store-and-forward mode when packets are queued.
40G uplink ports, see the Limitations for ALE 40G Uplink Ports on the Cisco Nexus 9000 Series
Configuring Queuing and Scheduling
Switches.