Released
Datasheet
PM73487 QRT
PMC-Sierra, Inc.
PMC-980618
Issue 3
622 Mbps ATM Traffic Management Device
2.6.7 Transmit Multicast Congestion Management
The transmit multicast can have congestion management applied to it. Three of the five conges-
tion measurements apply: the device, the SC, and the channel. The VO and the SC queue limits do
not apply to multicast cells as they do not make sense. This is because only one copy of the cell is
kept in the DRAM, regardless of the number of destinations to which the cell is headed. Those
counts contain only the number of unicast cells present.
The QRT can be configured to either generate or not generate backpressure on a per-SC basis. If
no
backpressure
is
desired,
configure
TX_EXP_MAX_SC_QD
(refer
to
“TX_EXP_MAX_SC_QD” on page 163) to one-half of the input pointer FIFO depth for that
AL_RAM_CONFIG (refer to “AL_RAM_CONFIG” on page 105). This will drop all cells at a
depth deeper than this, preventing backpressure from reaching back into the switch fabric. The
setting of this is a system-level decision. Preventing backpressure prevents a failure or congestion
on one card from affecting the performance of the fabric as a whole. On the other hand, using the
backpressure allows more multicast cells to be passed without the fear of dropping in the egress
QRT.
The high priority backpressure bit is derived from the near-fullness of queues 8 and 9. The
medium priority backpressure bit is derived from the near-fullness of queue 10 and 11. The low
priority backpressure bit is derived from the OR of the near-fullness of queues 12 to 15.
EPD, CLP-based dropping, and EFCI are all valid for multicast cells and are configured in the
TX_CH_CONFIG word (refer to section 9.3.1.7 “TX_CH_CONFIG” starting on page 189) using
the same bits as for unicast connections.
42
PMC [ PMC-SIERRA, INC ]
