R21xx-HP FlexFabric 11900 Layer 2 LAN Switching Command Reference

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Usage guidelines
PFC performs flow control based on 802.1p priorities. With PFC enabled, an interface requires its peer
to suspend sending packets with certain 802.1p priorities when congestion occurs. By decreasing the
transmission rate, PFC helps avoid packet loss.
You can enable PFC for certain 802.1p priorities at the two ends of a link. When network congestion
occurs, the local switch checks the PFC status for the 802.1p priority carried in each arriving packet.
• If PFC is enabled for the 802.1p priority, the local switch accepts the packet and sends a PFC pause
frame to the peer. The peer stops sending packets carrying this 802.1p priority for an interval as
specified in the PFC pause frame. This process repeats until the congestion is removed.
• If PFC is disabled for the 802.1p priority, the local port drops the packet.
• The relationship between the PFC function and the generic flow control function is shown in Table
8.
Table 8 The relationship between the PFC function and the generic flow control function
flow-control
priority-flow-control
enable
priority-flow-control
no-dro
p
dot1
p
Remarks
Unconfigurable Configured Configured
You cannot enable generic flow
control by using the flow-control
command on a port where PFC is
enabled and PFC is enabled for the
specified 802.1p priority values.
Configured Configurable Unconfigurable
• On a port configured with the
flow-control command, you can
enable PFC, but cannot enable
PFC for specific 802.1p
priorities.
• Enabling both generic flow
control and PFC disables a port
from sending common or PFC
pause frames to inform its peer
of congestion conditions.
However, the port can still
handle common and PFC pause
frames from its peer.
When you configure PFC, follow these guidelines:
To make PFC take effect, you must configure PFC in both system view and Ethernet interface view,
and make sure the 802.1p priority list specified in Ethernet interface view is within the 802.1p
priority list specified in system view.
To avoid affecting the normal operations of the IRF function and other protocols, do not configure
PFC for 802.1p priority value 0, 6, or 7.
To avoid packet loss caused by congestion, you must perform the same PFC configuration on all
ports that the packets pass through.
For more information about the 802.1p priority, priority trust mode, and port priority, see ACL and
QoS Configuration Guide. Suppose the local end is enabled with PFC and configured with the
priority-flow-control no-drop dot1p dot1p-list command. When congestion occurs, the local end
first maps the 802.1p priority of the received packets to a local precedence value according to the
802.1p-local priority mapping table, which is configurable. If the local precedence value is within