Priority flow control (PFC) as defined in the IEEE 802.1Qbb standard is an extension of IEEE 802.3x flow control, which is discussed in IEEE 802.3x Flow Control.
When buffer congestion is detected, IEEE 802.3x flow control allows the communicating device to pause all traffic on the port, whereas IEEE 802.1Qbb allows the device to pause just a portion of the traffic while allowing other traffic on the same port to continue.
For PFC, when an ingress port detects congestion, it generates a MAC control packet to the connected partner with an indication of which traffic priority to pause and an associated time for the pause to remain in effect. The recipient of the PFC packet then stops transmission on the priority indicated in the control packet and starts a timer indicating when traffic can resume.
To verify that your switch supports PFC, use the show version command. If you attempt to enable PFC on unsupported ports, an error message is displayed. (See Abnormal Configuration Examples.)
Priority is established for reception of PFC packets with a QoS profile value on the ExtremeXOS switch and for transmission with a priority value added to the PFC packet.
QoS profile—Ingress traffic is associated with a QoS profile for assignment to one of eight hardware queues in the system that define how the traffic flows with respect to bandwidth, priority, and other parameters. By default, there are two QoS profiles (QP1 and QP8) defined in these supported platforms and PFC works with this default. To segregate the ingress traffic with more granularity, you will want to define other QoS profiles. The traffic that will be paused on reception of the PFC packet is associated with the hardware queue of the QoS profile that you specify.
The QoS profile is also used to configure the fabric ports.
It is suggested that the priority in the VLAN header match the QoS profile priority when traffic ingresses at the edge of the network so that the traffic can be more easily controlled as it traverses through the network.