Class Fair Hierarchical Policing (CFHP) Policy Command Reference
880 Quality of Service Guide
mbs-contribution
Syntax mbs-contribution size [bytes | kilobytes] [fixed]
no mbs-contribution
Context config>qos>policer-control-policy>root>priority-mbs-thresholds>priority
Description The mbs-contribution command is used to configure the policy-based burst tolerance for a parent
policer instance created when the policy is applied to a SAP, or a subscriber context for the 7450 ESS
and 7750 SR, or a 7950 XRS multi-service site. The system uses the parent policer’s min-thresh-
separation value, the priority level’s mbs-contribution value and the number of child policers
currently attached to the priority level to derive the priority level’s shared-portion and fair-portion of
burst tolerance within the local priority level. The shared-portion and fair-portions for each priority
level are then used by the system to calculate each priority level’s discard-unfair threshold and discard-
all threshold. The mbs-contribution is the minimum separation between two adjacent active discard-
all thresholds.
The value for a priority level’s mbs-contribution within the policer-control-policy may be overridden
on the SAP, or subscriber sub-profile (applies to the 7450 ESS and 7750 SR) or multi-service site (for
7950 XRS) where the policy is applied in order to allow fine tuning of the discard-unfair and discard-
all thresholds relevant to the needs of the local child policers on the object.
Accumulative Nature of Burst Tolerance for a Parent Policer Priority Level
When defining mbs-contribution, the specified size may only be a portion of the burst tolerance
associated with the priority level. The packets associated with the priority level share the burst
tolerance of lower within the parent policer. As the parent policer PIR bucket depth increases during
congestion, the lower priority packets eventually experience discard based on each priority’s discard-
unfair and discard-all thresholds. Assuming congestion continues once all the lower priority packets
have been prevented from consuming bucket depth, the burst tolerance for the priority level will be
consumed by its own packets and any packets associated with higher priorities.
The Effect of Fair and Unfair Child Policer Traffic at a Parent Policer Priority Level
The system continually monitors the offered rate of each child policer on each parent policer priority
level and detects when the policer is in a congested state (the aggregate offered load is greater than the
decrement rate defined on the parent policer). As previously stated, the result of congestion is that the
parent policer's bucket depth will increase until it eventually hovers around either a discard-unfair or
discard-all threshold belonging to one of the priority levels. This threshold is the point where enough
packets are being discarded that the increment rate and decrement rate begin to even out. If only a
single child policer is associated to the priority level, the discard-unfair threshold is not used since
fairness is only applicable when multiple child policers are competing at the same priority level.
When multiple child policers are sharing the congested priority level, the system uses the offered rates
and the parenting parameters of each child to determine the fair rate per child when the parent policer
is unable to meet the bandwidth needs of each child. The fair rate represents the amount of bandwidth
that each child 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 parent 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
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