Alcatel-Lucent 7750 Configuration Manual page 566

Interfaces
policer is set to prevent overrunning the downstream bandwidth. The burst tolerance of the parent policer
defines how much more traffic may be sent beyond the downstream scheduling capacity. In the worst case
scenario, when the downstream buffering is insufficient to handle the total possible burst from the parent
policer, downstream discards based on lack of buffering may occur. However, in all likelihood, this is not the
case.
In most cases, lower priority traffic in the policer will be responsible for the greater part of congestion above
the parent policer rate. Since this traffic is discarded with a lower threshold, this lowers the effective burst
tolerance even while the highest priority traffic is present.
Configuring a Priority Level's MBS Contribution Value
In the most conservative case, a priority level's mbs-contribution value may be set to be greater than the
sum of child policer's mbs and one max-size-frame per child policer. This ensures that even in the absolute
worst case where all the lower priority levels are simultaneously bursting to the maximum capacity of each
child, enough burst tolerance for the priority's children will exist if they also burst to their maximum capac-
ity.
Since simply adding up all the child policer's PIR MBS values may result in large overall burst tolerances
that are not ever likely to be needed, you should consider some level of burst oversubscription when config-
uring the mbs-contribution value for each priority level. The amount of oversubscription should be deter-
mined based on the needs of each priority level.
Using the Fixed Keyword to Create Deterministic Parent Policer Discard Thresholds
In the default behavior, the system ignores the mbs-contribution values for a priority level on a subscriber
or SAP parent policer when a child policer is not currently associated with the level. This prevents additional
burst tolerance from being added to higher priority traffic within the parent policer.
This does cause fluctuations in the defined threshold values when child policers are added or removed from
a parent policer instance. If this behavior is undesirable, the fixed keyword may be used which causes the
mbs-contribution value to always be included in the calculation of parent policer's discard thresholds. The
defined mbs-contribution value may be overridden on a subscriber sla-profile or on a SAP instance, but the
fixed nature of the contribution cannot be overridden.
If the defined mbs-contribution value for the priority level is zero, the priority level will have no effect on
the parent policer's defined discard thresholds. A packet associated with the priority level will use the next
lower priority level's discard-unfair and discard-all thresholds.
Parameters size [bytes | kilobytes] — The size parameter is required when executing the mbs-contribution command.
It is expressed as an integer and specifies the priority's specific portion amount of accumulative MBS for the
priority level in bytes or kilobytes which is selected by the trailing bytes or kilobytes keywords. If both
bytes and kilobytes are missing, kilobytes is assumed. Setting this value has no effect on parent policer
instances where the priority level's mbs-contribution value has been overridden. Clearing an override on
parent policer instance causes this value to be enforced.
Values
Default
bytes | kilobytes: — The bytes keyword is optional and is mutually exclusive with the kilobytes keyword.
When specified, size is interpreted as specifying the size of min-thresh-separation in bytes.
The kilobytes keyword is optional and is mutually exclusive with the bytes keyword. When specified, size
is interpreted as specifying the size of min-thresh-separation in kilobytes.
Default
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0 — 16777216
none
kilobytes
7750 SR Interface Configuration Guide