Alcatel-Lucent 7450 Quality Of Service Manual page 879

of burst tolerance before marking its packets as unfair, the current parent policer bucket depth may at
times rise above the discard-unfair threshold. The mbs-contribution value provides a means to define
how much separation is provided between the priority level's discard-unfair and discard-all threshold
to allow the parent policer to absorb some amount of FIR burst before reaching the priority's discard-
all threshold.
This level of fair aggregate burst tolerance is based on the decrement rate of the parent policer's PIR
bucket while the individual fair bursts making up the aggregate are based on each child's FIR
decrement rate. The aggregate fair rate of the priority level is managed by the system with
consideration of the current rate of traffic in higher priority levels. In essence, the system ensures that
for each iteration of the child FIR rate calculation, the sum of the child FIR decrement rates plus the
sum of the higher priority traffic increment rates equals the parent policers decrement rate. This means
that dynamic amounts of higher priority traffic can be ignored when sizing a lower priority's fair
aggregate burst tolerance. Consider the following:
The 12 Mbps of the higher priority traffic and the 8 Mbps of fair traffic equal the 20 Mbps decrement
rate of the parent policer.
It is clear that the higher priority traffic is consuming 12 Mbps of the parent policer's decrement rate,
leaving 8 Mbps of decrement rate for the lower priority's fair traffic.
If all three children burst simultaneously (unlikely), they will consume 30 Kbytes above 8 Mbps. This
is the same as the remaining decrement rate after the higher priority traffic.
Parent Policer Total Burst Tolerance and Downstream Buffering
Quality of Service Guide
• The parent policer decrement rate is set to 20 Mbps (max-rate 20,000).
• A priority level's fair burst size is set to 30 Kbytes (mbs-contribution 30 kilobytes).
• Higher priority traffic is currently taking 12 Mbps.
• The priority level has three child policers attached.
• Each child's PIR MBS is set to 10 Kbytes, which makes each child's FIR MBS 10 Kbytes.
• The children want 10 Mbps, but only 8 Mbps is available,
• Based on weights, the children's FIR rates are set as follows:
Child 1
Child 2
Child 3
• The burst tolerance of child 1 is based on 10 Kbytes above 4 Mbps,
• The burst tolerance of child 2 is based on 10 Kbytes above 3 Mbps,
• The burst tolerance of child 3 is based on 10 Kbytes above 1 Mbps.
Class Fair Hierarchical Policing (CFHP)
FIR Rate FIR MBS
4 Mbps 10 Kbytes
3 Mbps 10 Kbytes
1 Mbps 10 Kbytes
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