
PRS28.4G
IBM Packet Routing Switch
Functional Description
Page 36 of 131
prs28.03.fm
August 31, 2000
ControlPacketLocations represents the 16 locations that are reserved for Control Packet reception, if
necessary.
3.4.3 Output Queue Threshold Exceeded Condition
There are four programmable output queue-full thresholds, one for each packet priority. All output queues
use the same threshold per priority. These thresholds can be used to:
Prevent packets of a certain priority and destined to a specific output from using the entire packet
memory.
Prevent packets of a certain priority from consuming too much output queue memory space (in relation to
packets of a higher priority).
When the total number of addresses, regardless of priorities, in an output queue exceeds a priority threshold,
the corresponding output queue grant is cleared. It is set whenever the total number of addresses in the
output queue is below the threshold value.
3.4.4 Packet Reception Fairness
Two fairness mechanisms are implemented to guarantee that, on the average, each input has an equal
chance of receiving a packet:
Output queue-full fairness
. When multiple inputs receive packets destined for the same set of outputs,
then on the average, each input has the same chance to receive its packet.
Memory-full fairness
. When multiple inputs receive packets with the same priority and the output
queue(s) are not full, then on the average, each input has the same chance to receive its packet.
This is accomplished by updating the memory grant and output queue grant values once per sequencer
cycle.