© Copyright Lenovo 2016 Chapter 21: FCoE and CEE 377Priority-Based Flow ControlPriority‐based Flow Control (PFC) is defined in IEEE 802.1Qbb. PFC extends theIEEE 802.3x standard flow control mechanism. Under standard flow control, whena port becomes busy, the switch manages congestion by pausing all the traffic onthe port, regardless of the traffic type. PFC provides more granular flow control,allowing the switch to pause specified types of traffic on the port, while othertraffic on the port continues.PFC pauses traffic based on 802.1p priority values in the VLAN tag. Theadministrator can assign different priority values to different types of traffic andthen enable PFC for up to two specific priority values: priority value 3, and oneother. The configuration can be applied globally for all ports on the switch. Then,when traffic congestion occurs on a port (caused when ingress traffic exceedsinternal buffer thresholds), only traffic with priority values where PFC is enabled ispaused. Traffic with priority values where PFC is disabled proceeds withoutinterruption but may be subject to loss if port ingress buffers become full.Although PFC is useful for a variety of applications, it is required for FCoEimplementation where storage (SAN) and networking (LAN) traffic are convergedon the same Ethernet links. Typical LAN traffic tolerates Ethernet packet loss thatcan occur from congestion or other factors, but SAN traffic must be lossless andrequires flow control.For FCoE, standard flow control would pause both SAN and LAN traffic duringcongestion. While this approach would limit SAN traffic loss, it could degrade theperformance of some LAN applications that expect to handle congestion bydropping traffic. PFC resolves these FCoE flow control issues. Different types ofSAN and LAN traffic can be assigned different IEEE 802.1p priority values. PFCcan then be enabled for priority values that represent SAN and LAN traffic thatmust be paused during congestion, and disabled for priority values that representLAN traffic that is more loss‐tolerant.PFC requires CEE to be turned on (“Turning CEE On or Off” on page 368). WhenCEE is turned on, PFC is enabled on priority value 3 by default. Optionally, theadministrator can also enable PFC on one other priority value, providing losslesshandling for another traffic type, such as for a business‐critical LAN application.Note: For any given port, only one flow control method can be implemented atany given time: either PFC or standard IEEE 802.3x flow control.
