IBM PowerPRS Q-64G
Preliminary
Packet Routing Switch
3.4 Ingress Flow Control
Note: In the flow control sections (that is, Sections 3.4 through 3.7), references to 32 ports and port ranges
of 0 to 15 and 16 to 31 are specific to the PowerPRS Q-64G 512-Gbps configuration. Substitute 16 ports and
port ranges of 0 to 7 and 8 to 15 for the 256-Gbps configuration.
Ingress traffic flow to the PowerPRS Q-64G is controlled by a variety of mechanisms, primarily:
• Output queue grants that reflect the output queue occupancy. There is one output queue grant issued per
subswitch element pair, per output, and per priority.
• Memory grants that reflect the shared memory occupancy. There is one memory grant issued for input
ports 0 to 15 and one issued for input ports 16 to 31.
• Multicast grants that reflect the multicast packet use of shared memory. There is one multicast grant
issued per subswitch element and per priority.
As discussed in Section 3.3, the PowerPRS Q-64G inserts the grant status into egress data, control, service,
and idle packet headers to convey these grants to the ingress side of the attached devices. These grants
allow the devices to transmit packets to the PowerPRS Q-64G. Ingress traffic flow is also controlled by the
flow control latency and best-effort discard functions. Each of these flow control mechanisms is discussed
below.
Note: An attached device transmits a unicast packet to the PowerPRS Q-64G only when it has received both
the memory grant and the output queue grant for the destination output. An attached device transmits a mul-
ticast packet to the PowerPRS Q-64G when it has received the memory grant, regardless of the output queue
grant status and the multicast grant status. The PowerPRS Q-64G discards ingress packets destined for an
output that lacks the correct output queue grant status (see Section 3.4.5 Flow Control Latency on page 49).
3.4.1 Output Queue Grants
Output queue grants:
• Prevent packets of a single priority destined for the same output from filling the shared memory.
• Prevent low-priority packets from occupying too much shared memory.
The PowerPRS Q-64G issues one output queue grant per subswitch element pair (A and B, and C and D),
per output port (0 to 31), and per priority (0 to 3). For a subswitch element, the PowerPRS Q-64G issues an
output queue grant for an output priority when the total number of packets in the output queue is below the
output queue threshold for that priority. The PowerPRS Q-64G inserts the output queue grant status into
egress data, control, service, and idle packet headers. Ports 0 to 15 receive the status of the output queues in
subswitch elements A and B; ports 16 to 31 receive the status of the output queues in subswitch elements C
and D. The PowerPRS Q-64G removes the grant when the number of packets is equal to or greater than the
threshold (there is no hysteresis). The four programmable output queue thresholds, one for each output
priority, are accessed through the Threshold Access Register (page 114).
To generate output queue grants, the PowerPRS Q-64G continuously compares the total packet count in
each output queue to the four output queue thresholds. However, the PowerPRS Q-64G output queue grant
table is refreshed only once per packet cycle. This guarantees that all the attached devices receive the same
flow control information.
prsq-64g.01.fm
December 20, 2001
Functional Description
Page 47 of 199
IBM [ IBM ]
