Alcatel-Lucent 7750 Configuration Manual page 565

at the priority level should receive relative to the other children at the same level according to the policer
control policy instance managing the child policers. This fair rate is applied as the decrement rate for each
child's FIR bucket. Changing a child's FIR rate does not modify the amount of packets forwarded by the par-
ent policer for the child's priority level. It simply modifies the forwarded ratio between the children on that
priority level. Since each child FIR bucket has some level 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 cal-
culation, 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 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:
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.
• 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.
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
The highest in-use priority level's discard-all threshold is the total burst tolerance of the parent policer. In
some cases the parent policer represents downstream bandwidth capacity and the max-rate of the parent
7750 SR Interface Configuration Guide
FIR Rate
Child 1 4 Mbps
Child 2 3 Mbps
Child 3 1 Mbps
Interface Configuration
FIR MBS
10 Kbytes
10 Kbytes
10 Kbytes
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