Command Reference Guide

Priority-Based Flow Control Commands
CLI Command Reference
September 2014 Page 495
HP Moonshot Switch Module CLI Command Reference
Priority-Based Flow Control Commands
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. Pausing traffic helps prevent
buffer overflow and dropped frames.
Priority-based flow control (PFC) provides a way to distinguish which traffic on physical link is paused when
congestion occurs, based on the priority of the traffic. An interface can be configured to pause only high priority
(i.e., loss-sensitive) traffic when necessary prevent dropped frames, while allowing traffic that has greater loss
tolerance to continue to flow on the interface.
Priorities are differentiated by the priority field of the IEEE 802.1Q VLAN header, which identifies an IEEE
802.1p priority value. In the HP Moonshot Switch Module, these priority values must be mapped to internal
class-of-service (CoS) values.
To enable priority-based flow control for a particular CoS value on an interface:
1. Ensure that VLAN tagging is enabled on the interface so that the 802.1p priority values are carried through
the network (see “Provisioning (IEEE 802.1p) Commands” on page 320).
2. Ensure that 802.1p priority values are mapped to HP Moonshot Switch Module CoS values (see
“classofservice dot1p-mapping” on page 619).
3. Use the
datacenter-bridging priority-flow-control mode on command to enable priority-based flow
control on the interface.
4. Use the
datacenter-bridging priority-flow-control priority command to specify the CoS values that
should be paused (“no-drop”) due to greater loss sensitivity. Unless configured as “no-drop”, all CoS
priorities are considered non-pausable (“drop”) when priority-based flow control is enabled.
When priority-flow-control is disabled, the interface defaults to the IEEE 802.3x flow control setting for the
interface. When priority-based flow control is enabled, the interface will not pause any CoS unless there is at
least one no-drop priority.
priority-flow-control mode
Use the priority-flow-control mode on command in Datacenter-Bridging Config mode to enable Priority-Flow-
Control (PFC) on the given interface.
PFC must be enabled before FIP snooping can operate over the interface. Use the no form of the command to
return the mode to the default (off). VLAN tagging (trunk or general mode) must be enabled on the interface
in order to carry the dot1p value through the network. Additionally, the dot1mapping to class-of-service must
be set to one-to-one.
When PFC is enabled on an interface, the normal PAUSE control mechanism is operationally disabled.
Default Priority-flow-control mode is off (disabled) by default.
Format
priority-flow-control mode { on | off }
Mode Datacenter-Bridging Config mode