Dell N1100-ON, Reference Manual

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Data Center Technology Commands
1179
Priority Flow Control Commands
Dell EMC Networking N4000 Series Switches
Priority Flow Control (PFC) provides a means of pausing frames based on
individual priorities on 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 with less
congestion spreading than standard flow control. The priorities are
differentiated by the priority field of the 802.1Q VLAN header. PFC is
standardized by the IEEE 802.1Qbb specification.
PFC uses a new control packet defined in 802.1Qbb and therefore is not
compatible with standard flow control. An interface that is configured for
PFC will be automatically disabled for 802.3x flow control. When PFC is
disabled on an interface, the flow control configuration for the interface
becomes active. Any flow control frames received on a PFC configured
interface are ignored.
Each priority is configured as either drop or no-drop. If a priority that is
designated as no-drop is congested, the priority is paused. Drop priorities do
not participate in pause. By default there are no priority classifications
configured and PFC is not enabled.
While several no-drop priorities may be configured on a supporting system,
the actual number of lossless priorities supported on a given system is a
function of the switch chips packet buffer, the maximum supported MTU
size, pause delay, the media type and the total number of ports enabled for
lossless behavior. In order to guarantee lossless behavior, the switch chip must
send a pause message prior to exhausting its available packet buffer and have
sufficient buffer to absorb the delay. In order to accomplish this, it must
reserve enough memory (headroom) to handle the max delay in processing
the pause packet.
The maximum number of lossless priorities per interface is two. The
headroom is only used for guaranteeing lossless behavior. There must be
enough dynamic memory to handle the typical work load of the switch in
addition to the headroom. With two no-drop priorities per interface and
static allocations, there is only about 30 percent of the buffer space available
for normal forwarding behavior.