Data Center Bridging; Pfc (Priority Flow Control); Cn (Congestion Notification) - Fujitsu PSWITCH 2048T User Manual

FUJITSU PSWITCH
- Data Center Bridging(DCB)
- FIP Snooping,
- Edge Virtual Bridging(EVB)
- DCVPN Gateway/Layer2 Overlay

3.1.8.1. Data Center Bridging

3.1.8.1.1. PFC (Priority Flow Control)

Priority Flow Control (PFC) provides a means of pausing individual priorities within
a single physical link. By pausing the congested priority or priorities independently,
protocols that are highly loss-sensitive can share the same link with traffic that has
different loss tolerances. The priorities are differentiated by the 802.1p priority
field of the 802.1Q VLAN header. PFC is available only on certain silicon.
PFC is intended to eliminate frame loss due to congestion on a link. This is
achieved by a mechanism that is similar to the IEEE 802.3x pause mechanism, but
operates on individual priorities. This mechanism, in conjunction with other data
center bridging (DCB) technologies, enables support for higher layer protocols that
are highly loss-sensitive while not affecting the operation of traditional LAN
protocols that utilize other priorities.
PFC uses a unique control packet defined in IEEE 802.1Qbb; therefore, PFC is not
compatible with 802.3x Flow Control (FC). An interface that is configured for PFC
automatically disables FC. When PFC is disabled on an interface, the FC
configuration for the interface becomes active. Any FC frames received on a
PFC-configured interface are ignored.

3.1.8.1.2. CN (Congestion Notification)

Congestion Notification (CN) is defined in the 802.1Qau standard. A consequence
of link level pausing (i.e. 802.1Qbb) is "congestion spreading". This is the domino
effect of buffer congestion propagating upstream causing secondary bottlenecks. A
layer two congestion control algorithm allows a primary bottleneck to directly
reduce the rates of those sources whose packets pass through it, thereby
preventing secondary bottlenecks. Congestion notification is broken up into two
algorithms: CP and RP. CP, Switch or Congestion Point Dynamics is the mechanism
in which a switch buffer samples incoming packets and generates a feedback
message addressed to the source of the sampled packets with the extent of the
congestion. RP, Rate Limiter or Reaction Point Dynamics is the mechanism by
which a Rate Limiter (RL) decreases its sending rate based on feedback and
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User's Guide
December/2018