Class Fair Hierarchical Policing (CFHP) Policy Command Reference
878 Quality of Service Guide
• When a parent policer instance's priority level has two or more child policers associated, the
fair-portion will be equal to the maximum of the following:
→ min-thresh-separation value
→ The priority level’s mbs-contribution value less min-thresh-separation value
When the mbs-contribution command’s optional fixed keyword is defined for a priority level within
the policy, the system will treat the defined mbs-contribution value as an explicit definition of the
priority level’s MBS. While the system will continue to track child policer associations with the parent
policer priority levels, the association counters will have no effect. Instead the following rules will be
used to determine a fixed priority level’s shared-portion and fair-portion:
• If a fixed priority level’s mbs-contribution value is set to zero, both the shared-portion and
fair-portion will be set to zero
•If the mbs-contribution value is not set to zero:
→ The shared-portion will be set to the current min-thresh-separation value
→ The fair-portion will be set to the maximum of the following:
min-thresh-separation value
mbs-contribution value less min-thresh-separation value
Each time the min-thresh-separation value is modified, the thresholds for all instances of the parent
policer created through association with this policer-control-policy are reevaluated
Determining the Correct Value for the Minimum Threshold Separation Value
The minimum value for min-thresh-separation should be set equal to the maximum size packet that
will be handled by the parent policer. This ensures that when a lower priority packet is incrementing
the bucket, the size of the increment will not cause the bucket's depth to equal or exceed a higher
priority threshold. It also ensures that an unfair packet within a priority level cannot cause the PIR
bucket to increment to the discard-all threshold within the priority.
When evaluating maximum packet size, each child policer’s per-packet-offset setting should be taken
into consideration. If the maximum size packet is 1518 bytes and a per-packet-offset parameter is
configured to add 20 bytes per packet, min-thresh-separation should be set to 1538 due to the fact that
the parent policer will increment its PIR bucket using the extra 20 bytes.
In most circumstances, a value larger than the maximum packet size is not necessary. Management of
priority level aggregate burst tolerance is intended to be implemented using the priority level mbs-
contribution command. Setting a value larger than the maximum packet size will not adversely affect
the policer performance, but it may increase the aggregate burst tolerance for each priority level.
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