Dell PowerConnect 8024, User Configuration Manual

Page: 811 / 1294

Configuring Data Center Bridging Features
811
Priority Flow Control
Ordinarily, when flow control is enabled on a physical link, it applies to all
traffic on the link. When congestion occurs, the hardware sends pause frames
that temporarily suspend traffic flow to help prevent buffer overflow and
dropped frames.
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.
This feature is used in networks where the traffic has differing loss tolerances.
For example, Fibre Channel traffic is 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 until no-drop is
specifically turned on.
PFC Operation and Behavior
PFC uses a control packet newly defined in IEEE 802.1Qbb and, therefore, is
not compatible with IEEE 802.3 Annex 31B flow control. An interface that is
configured for PFC is automatically disabled for flow control. When PFC is
disabled on an interface, the flow control configuration for the interface
becomes active. Any IEEE 802.3 Annex 31B link-layer 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. You must configure the same no-drop priorities and
enable VLAN tagging for the no-drop priorities across the network to ensure
end-to-end lossless behavior.