IBM PowerPRS Q-64G
Preliminary
Packet Routing Switch
Figure 3-8. Best-Effort Discard Counters and Thresholds
Incoming Packet
Counter 1
Counter 2
Counter 3
Counter 4
Counter 5
Enable Discard
Threshold
P3-BE
P2-BE
P1-BE
P2-BE
P1-BE
Compare
Priority to
Priority Discard
Threshold
Be Discarded
No
P1-BE
Wraparound
Counters
P0-BE
P3-G
P2-G
P0-BE
P3-G
P2-G
P0-BE
P3-G
P2-G
P0-BE
P3-G
P2-G
Halt Discard
Threshold
P3-G
P2-G
P3 = priority 3 packet
P2 = priority 2 packet
P1 = priority 1 packet
P1-G
P0-G
P1-G
P0-G
P1-G
P0-G
P1-G
P0-G
P1-G
P0-G
P0 = priority 0 packet
BE = best-effort bandwidth packet
G = guaranteed bandwidth packet
Output Line Port
Dequeue Rate
The assumption is that guaranteed bandwidth should be engineered so that it never exceeds the priority
discard threshold—it dequeues traffic without the need for flow control. However, situations exist in which the
traffic pattern changes before the halt discard threshold is reached. In this case, if another counter exceeds
the priority discard threshold, the input controllers discard additional packets.
3.4.6.3 Best-Effort Discard Filters
Best-effort discard occurs within the input controllers, which filter the destination bitmaps of incoming packets
(see Figure 3-9). There is one best-effort discard filter per priority. The best-effort discard filter is used as a
destination bitmap mask, with the discard set to ‘0’. For each combination of destination output port and
priority, there is a 20-bit counter that provides discard quantity and rate information. These drop counters are
enabled with the best-effort drop counters enable bit in the Configuration 0 Register (page 110) and are
accessible via the Best-Effort Resources Access Register (page 118).
When an incoming packet arrives at the PowerPRS Q-64G, its packet priority field, best-effort bit, and desti-
nation bitmap are provided to the best-effort filter logic. If the packet is read as best-effort, the best-effort
discard filter for the packet priority is applied to the destination bitmap. If the combination is ‘0’, the packet is
discarded rather than enqueued, and the corresponding drop counter (or counters for a multicast packet) is
updated.
prsq-64g.01.fm
December 20, 2001
Functional Description
Page 51 of 199
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