Alcatel-Lucent 7750 Configuration Manual page 564

Interfaces
The priority level command contains the mbs-contribution configuration command for a given strict prior-
Description
ity level. Eight levels are supported numbered 1 through 8 with 8 being the highest strict priority.
Each of the eight priority CLI nodes always exists and do not need to be created. While parameters exist for
each priority level, the parameters are only applied when the priority level within a parent policer instance is
currently supporting child policers.
Default
None.
mbs-contribution
Syntax mbs-contribution size [bytes | kilobytes] [fixed]
no mbs-contribution
Context config>card>fp>ingress>access>queue-group>policer-control-override>priority-mbs-thresholds
config>card>fp>ingress>network>queue-group>policer-control-override>priority-mbs-thresholds
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 subscriber context. 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 thresh-
old.
The value for a priority level's mbs-contribution within the policer-control-policy may be overridden on
the SAP or subscriber sub-profile 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 pri-
ority packets eventually experience discard based on each priority's discard-unfair and discard-all thresh-
olds. 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
Page 564 7750 SR Interface Configuration Guide