Customize PFC X-ON Threshold and Per-Queue Alpha Values

date_range 12-Nov-24

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Overview

When you configure a congestion notification profile on an ingress port, lossless traffic is mapped to lossless priority groups. You configure these priority groups with priority-based flow control (PFC) X-OFF and X-ON thresholds. In case of congestion at an egress port, these priority groups ensure that the ingress port generates the PFC frames toward the peer based on the configured thresholds. When the shared occupancy or receive buffer on an ingress priority group reaches its PFC X-OFF limit, the corresponding priority group transmits the PFC pause frame to the egress peer. The peer temporarily stops transmitting packets to give the device time to resolve the traffic congestion.

The X-ON threshold is a buffer limit that is shared by the priority group. When the buffer usage on the ingress priority group drops below this PFC X-ON limit, the priority group sends a PFC message to the peer so it can resume packet transmission. Make sure the device has enough time to resolve the congestion. You must also ensure that traffic is not paused for long enough to cause disruption to your network. To optimize the downtime during a PFC pause storm, adjust the X-ON threshold through the congestion notification profile (CNP).

You can globally adjust the limit of buffers that each queue can consume from the shared buffer pool. The shared buffer pool is based on a dynamic threshold setting called the alpha value. You can configure a scheduler with different dynamic buffer threshold values for different queues, thereby controlling the shared buffer access by individual queues.

Benefits

Ensure that the device has enough time to resolve traffic congestion without disrupting your network.
Customize device responses to PFC pause storms.
Globally adjust the limits of buffers for ease of configuration.

Configuration

Enable PFC.

Map the code-point configuration to no-loss queues.

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set class-of-service congestion-notification-profile profile-name input dscp code-point code-point-bits pfc

Specify the number of X-ON threshold offset cells before the peer resumes transmission from the dynamic shared buffer.
The range is 0 through 100000:
```
set class-of-service congestion-notification-profile profile-name input dscp code-point code-point-bits xon value
```
Configure the per-queue alpha value through a scheduler.
Per-queue alpha values are not supported for lossless queues. The range of the threshold for the maximum buffer share for a queue at the egress buffer partition is 0 through 10:
```
set class-of-service schedulers scheduler-name buffer-dynamic-threshold value
```
Verify the shared buffer configuration.
```
show class-of-service shared-buffer
```
Verify the buffer profile configuration.
```
show class-of-service dedicated-buffer-profile
```
Verify the configuration on the interface.
```
show class-of-service interface interface-name
```

Platform Support

See Feature Explorer for platform and release support.

AI-ML Data Center Feature Guide

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Customize PFC X-ON Threshold and Per-Queue Alpha Values

Overview

Benefits

Configuration

Platform Support

Related Documentation

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