Dell PowerEdge M420 Configuration Manual page 693

This feature is used in networks where the traffic has differing loss tolerances.
For example, iSCSI and VoIP traffic are highly sensitive to traffic loss. If a link
contains both loss-sensitive data and other less loss-sensitive data, the loss-
sensitive data should use a no-drop priority that is enabled for flow control.
Priorities are differentiated by the priority field of the IEEE 802.1Q VLAN
header, which identifies an IEEE 802.1p priority value. These priority values
must be mapped to internal class-of-service (CoS) values.
The PFC feature allows you to specify the CoS values that should be paused
(due to greater loss sensitivity) instead of dropped. when congestion occurs
on a link. Unless configured as no-drop, all CoS priorities are considered non-
pausable ("drop") when priority-based flow control is enabled.
PFC Operation and Behavior
PFC uses a new control packet defined in IEEE 802.1Qbb and therefore is
not compatible with 802.3x flow control (FC). An interface that is configured
for PFC will be automatically disabled for 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.
Each priority is configured as either
designated as no-drop is congested, the priority is paused. Drop priorities do
not participate in pause.
When an interface is enabled for PFC, a manual configuration is applied.
Even when interoperating with equipment that supports PFC negotiation,
the connecting equipment should be configured in similar fashion. Interfaces
that are not enabled for PFC do not act on PFC control frames.
NOTE:
This feature is configurable on physical full duplex interfaces only. If PFC
is configured on a LAG interface, the member interfaces must have the same
configuration.
When PFC is disabled, the interface defaults to the IEEE 802.3x flow control
setting for the interface. PFC is disabled by default.
If you enable priority-based flow control for a particular priority value on an
interface, ensure that Voice VLAN tagging is enabled on the interface so that
the 802.1p priority values are carried through the network (see "Voice VLAN"
on page 581). Additionally, make sure 802.1p priority values are mapped to
CoS values (see "Configuring Class-of-Service" on page 1127).
drop no-drop
or
Configuring Port-Based Traffic Control
. If a priority that is
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