IBM PowerPRS Q-64G
Packet Routing Switch
Preliminary
3.4.6.1 Best-Effort Discard Counters
Operation of the lossy switch is based on a set of five counters (see Table 3-24) per output port that are incre-
mented when the port’s output queues receive specific packet types.
Table 3-24. Best-Effort Discard Counters
Counter
Counter 1
Description
The main counter. Counts each incoming packet.
Counts each incoming packet that is either guaranteed bandwidth of any priority or best-effort bandwidth of
priority 0, 1, or 2.
Counter 2
Counter 3
Counts each incoming packet that is either guaranteed bandwidth of any priority or best-effort bandwidth of
priority 0 or 1.
Counts each incoming packet that is either guaranteed bandwidth of any priority or best-effort bandwidth of
priority 0.
Counter 4
Counter 5
Counts each incoming packet that is guaranteed bandwidth of any priority.
Best-effort discard counters are decremented at a packet speed equivalent to the throughput of the port (the
line rate is provided by a register defined at port initialization). The counters are accessible via the Best-Effort
Resources Access Register (page 118).
3.4.6.2 Best-Effort Discard Thresholds
Three thresholds govern the best-effort discard process:
• Enable discard threshold. When counter 1 reaches this threshold, the discard process starts.
• Halt discard threshold. When counter 1 reaches this threshold, the discard process stops.
• Priority discard threshold. This threshold, in comparison to counters 1, 2, 3, and 4, determines which
packets are discarded:
- When counter 1 is above the priority discard threshold, best-effort packets of priority 3 are discarded.
- When counter 2 is above the priority discard threshold, best-effort packets of priority 2 are discarded.
- When counter 3 is above the priority discard threshold, best-effort packets of priority 1 are discarded.
- When counter 4 is above the priority discard threshold, best-effort packets of priority 0 are discarded.
The priority discard threshold only determines which packets are discarded; the halt discard threshold deter-
mines when the discard process stops. The best-effort discard thresholds are accessed via the Best-Effort
Resources Access Register.
Figure 3-8 illustrates how the best-effort discard thresholds operate. In the figure, best-effort discard starts
because counter 1 for the output port exceeds the enable discard threshold. Counters 1, 2, and 3 are all
above the priority discard threshold and, consequently, the best-effort packets of priority 1, 2, and 3 destined
for that output port are discarded at the input controller. Discard continues until counter 1 falls below the halt
discard threshold.
Functional Description
Page 50 of 199
prsq-64g.01.fm
December 20, 2001
IBM [ IBM ]
