Priority-based Flow Control

Priority flow control (PFC) is defined in the IEEE 802.1Qbb standard as an extension of the IEEE 802.3x flow control standard. When buffer congestion is detected, IEEE 802.3x flow control allows the communicating device to pause all traffic on the port, whereas PFC allows the device to pause just a portion of the traffic and allow other traffic on the same port to continue.

The rest of this section provides general guidelines for configuring the ExtremeXOS PFC feature for DCB (Data Center Bridging) operation. After you configure PFC, DCBX advertises the PFC compatible configuration to DCBX peers on all DCBX enabled ports.

PFC configuration is controlled per-port using the following command:

enable flow-control [tx-pause {priority priority} | rx-pause {qosprofile qosprofile}] ports [all | port_list]

The rx-pause option is configured on the QoS (Quality of Service) profile.

The PFC priority to which a QoS profile responds is fixed and is determined by the QoS profile number such that qpN responds to a PFC frame for priority N-1.

For example, the following command enables PFC priority 4 for qp5 on ports 1-24:

enable flow-control rx-pause qosprofile qp5 ports 1-24

After the above command is entered, if a PFC frame is received indicating that priority 4 should be paused, then qp5 will be paused. Note that qp5 is paused regardless of whether the packets mapped to qp5 have priority 4 or other priorities. For example, if we enter the command configure dot1p type 3 qosprofile qp5, priority 3 packets are queued in qp5, and a PFC pause frame for priority 4 pauses priority 3 frames, which might not be desired. For this reason, you should be careful about mapping multiple priorities to the same QoS profile when PFC is enabled for that profile.

The tx-pause option is configured on the priority itself. For example, the following command enables the transmittal of PFC Pause frames for priority 4 when frames with priority 4 are congested:

enable flow-control tx-pause priority 4 ports 1-24

The tx-pause configuration determines what is advertised in the DCBX PFC TLV. In order for PFC to work correctly, it is important to ensure that all switches in the DCB network are receiving and transmitting PFC consistently for each priority on all ports.

In summary, the following three commands ensure that PFC is enabled for priority 4 traffic on ports 1-24:

configure dot1p type 4 qosprofile qp5
enable flow-control rx-pause qosprofile qp5 ports 1-24 
enable flow-control tx-pause priority 4 ports 1-24