Priority-Based Flow Control - Lenovo Flex System Fabric CN4093 Application Manual

Priority-Based Flow Control

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CN4093 Application Guide for N/OS 8.2
Priority‐based Flow Control (PFC) is defined in IEEE 802.1Qbb. PFC extends the
IEEE 802.3x standard flow control mechanism. Under standard flow control, when
a port becomes busy, the switch manages congestion by pausing all the traffic on
the 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 other
traffic on the port continues.
PFC pauses traffic based on 802.1p priority values in the VLAN tag. The
administrator can assign different priority values to different types of traffic and
then enable PFC for up to two specific priority values: priority value 3, and one
other. The configuration can be applied on a port‐by‐port basis, or globally for all
ports on the switch. Then, when traffic congestion occurs on a port (caused when
ingress traffic exceeds internal buffer thresholds), only traffic with priority values
where PFC is enabled is paused. Traffic with priority values where PFC is disabled
proceeds without interruption 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 FCoE
implementation where storage (SAN) and networking (LAN) traffic are converged
on the same Ethernet links. Typical LAN traffic tolerates Ethernet packet loss that
can occur from congestion or other factors, but SAN traffic must be lossless and
requires flow control.
For FCoE, standard flow control would pause both SAN and LAN traffic during
congestion. While this approach would limit SAN traffic loss, it could degrade the
performance of some LAN applications that expect to handle congestion by
dropping traffic. PFC resolves these FCoE flow control issues. Different types of
SAN and LAN traffic can be assigned different IEEE 802.1p priority values. PFC
can then be enabled for priority values that represent SAN and LAN traffic that
must be paused during congestion, and disabled for priority values that represent
LAN traffic that is more loss‐tolerant.
PFC requires CEE to be turned on ("Turning CEE On or Off" on page
CEE is turned on, PFC is enabled on priority value 3 by default. Optionally, the
administrator can also enable PFC on one other priority value, providing lossless
handling 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 at any
given time: either PFC or standard IEEE 802.3x flow control.
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