Because multiple priorities can be mapped to a single PG, congestion on one priority in a PG may generate a pause, stopping transmission
of all priorities in that 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 the timer value from the packet.
•
The per-priority XOFF/XON status is passed to the pausing logic to pause or resume packet scheduling to the corresponding
queue of the egress port.
PFC pause transmission
•
Priorities 0 through 6 can be mapped to a PG; Priority 7 can not be mapped.
•
The mapping is configurable.
•
When the buffer threshold of a PG exceeds the XOFF value, a PFC pause frame is sent. The pause frame is encoded with all
priorities that belong to the PG in the 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 Ruckus ICX 7250, ICX 7450, and ICX 7750 devices,
the number of ingress buffers is set automatically according to the port speed when PFC is enabled.
NOTE
Configuring PFC commands may temporarily interrupt traffic.
You can configure the
qos priority-to-pg
command to change the default priority to PG mapping.
By default, the Ruckus ICX 7250, ICX 7450, and ICX 7750 devices boot 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.
NOTE
Enabling flow control on ports that have auto-neg enabled causes flap because the port pause capabilities must be advertised
and negotiated again with the peer.
Ports that have auto-neg disabled do not experience flap.
Conditions
Ruckus ICX 7150 devices—PFC is not supported.
Ruckus ICX 7250 devices—Symmetrical flow control (SFC) is not supported for ports across stack units.
Ruckus ICX 7450 devices—SFC is not supported for ports across stack units or for ports across master and slave packet processor (pp)
devices in Ruckus ICX 7450-48 units.
Ruckus ICX 7650 devices—PFC is not supported.
Ruckus ICX 7750 devices—PFC and SFC are not supported for ports across stack units.
Quality of Service
Flow control and buffer management
Ruckus FastIron QoS and Traffic Management Configuration Guide, 08.0.70
26
Part Number: 53-1005294-01
