Brocade FastIron Ethernet Switch Traffic Management Guide - Manual (Page 79)

ICX 7750 Flow control and buffer management
● Priority flow control .........................................................................................................77
● Configuring priority flow control.......................................................................................78
● Packet buffer management............................................................................................. 78
Priority flow control
PFC prevents frame loss from congestion by pausing traffic based on the congested priority without
affecting the traffic of uncongested priorities.
Flow control enables feedback from a receiver to its sender to communicate buffer availability.
Brocade's priority flow control (PFC) feature implements IEEE 802.1Qbb PFC on the Brocade ICX 7750
device, supporting eight priorities and four priority groups (PGs) that can be subject to flow control
independently. You can configure PGs for priority flow control and ingress buffer management.
Because multiple priorities can be mapped to a single PG, congestion on one priority in a PG might
generate a pause, stopping transmission of all priorities in the PG. Therefore, it is important to create a
custom priority-to-PG map to meet your application needs, using either PFC pause honoring or PFC
pause transmission.
PFC pause honoring
• The MAC decodes the class enable vector field to extract the priorities and pause timer value from
the packet.
• The per priority XOFF/XON status is passed to the pausing logic to pause/resume packet
scheduling to the corresponding queue of the egress port.
PFC pause transmission
• Each priority can be mapped to a PG. The mapping is configurable.
• When buffer threshold of a PG exceeds XOFF value, a PFC pause frame is sent. The pause frame
is encoded with all priorities that belong to the PG in class enable vector.
A receiver using PFC must predict the potential for buffer exhaustion for a PG and respond by
generating an explicit PAUSE frame for that class when that condition arises. At any time the receiver
must have enough ingress buffers available to store any packet that might be in flight while the PAUSE
frame travels back to the sender and gets processed there. In ICX 7750 the number of ingress buffers is
set automatically according to the port speed when PFC is enabled.
NOTE
Configuring PFC commands interrupts traffic temporarily.
You can configure the qos priority-to-pg command to change the default priority to PG mapping. See
the description of the qos priority-to-pg command for more information on the default mapping.
By default, the Brocade ICX 7750 device boots up with tail-drop mode, which means that packets are
dropped at the egress queues during congestion. By default, all ports honor IEEE 802.3X pause.
However, when transmission of the 802.3x pause is disabled, PFC is also disabled. You can configure
the symmetrical-flow-control enable command to enable the transmission of the 802.3x pause. See
the description of the symmetrical-flow-control enable command for more information on symmetrical
flow control.
FastIron Ethernet Switch Traffic Management Guide 77
53-1003093-03