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Dell EMC SmartFabric OS10 User Guide Release 10.5.1

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Priority flow control

In a converged data-center network, to ensure that no frames are lost due to congestion, use PFC. PFC uses the 802.1p priority in the Ethernet header to pause priority-specific traffic that is sent from a transmitting device. The 802.1p priority is also known as the class of service (CoS) or dot1p priority value.

When PFC detects congestion of a dot1p traffic class, it sends a pause frame for the priority traffic to the transmitting device. In this way, PFC ensures that the switch does not drop specified priority traffic.

PFC enhances the existing 802.3x pause capability to enable flow control based on 802.1p priorities. Instead of stopping all traffic on a link, as performed by the 802.3x pause mechanism, PFC pauses traffic for 802.1p traffic types. For example, when LAN traffic congestion occurs on an interface, PFC ensures lossless flows of storage and server traffic while allowing for lossy best-effort transmission of other traffic.

NOTE: Ethernet traffic ingressing from ETS and PFC enabled interfaces will always queue through queue 0, irrespective of the network type configured on VLAN.

PFC handles traffic congestion by pausing prioritized dot1p traffic on an ingress interface and allowing other dot1p traffic best-effort, also known as lossy data transmission.

Priority flow control

Configuration notes

Dell EMC PowerSwitch S4200-ON Series:
  • Provisioning PFC is not supported when deep buffer mode is enabled.
  • Configure the traffic class ID to queue mapping policy on egress interfaces.
  • You cannot enable PFC on all the physical interfaces, when you have split the ports to multiple breakout interfaces. For more information, see the 'PFC configuration notes' section in the Dell EMC SmartFabric OS10 User Guide.
  • When you add or remove the PFC configuration on an interface, the interface gets flapped. Stop the traffic before applying or modifying the PFC configuration.

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