QoS Scheduler Policies
Quality of Service Guide 527
•The above-offered-cap command within the adv-config-policy provides control of
each queue’s operational PIR to prevent aggregate rate overrun. This is accomplished
by defining how much a queue’s operational PIR is allowed to exceed the queue’s
current allocated bandwidth.
•The limit-unused-bandwidth (LUB) command.
Limit Unused Bandwidth
The limit-unused-bandwidth (LUB) command protects against exceeding the aggregated
bandwidth by adding a LUB second-pass to the HQoS function, which ensures that the
aggregate fair-share bandwidth does not exceed the aggregate rate.
The command can be applied on any tier 1 scheduler within an egress scheduler policy or
within any agg-rate node (except when using the HS-MDA) and affects all queues controlled
by the object.
When LUB is enabled, the LUB second pass is performed as part of the HQoS algorithm The
order of operation between HQoS and LUB is as follows:
• Queue offered rate calculation.
• Offered rate modifications based on adv-config-policy offered-measurement
parameters.
• HQoS Bandwidth determination based on modified offered-rates.
• LUB second pass to ensure aggregate rates are not exceeded where LUB enabled.
• Bandwidth distribution modification based on adv-config-policy bandwidth-
distribution parameters.
• Each queue’s operational PIR is then modified.
When LUB is enabled on a scheduler rate or aggregate rate, a LUB context is created
containing the rate and the associated queues the rate controls. Because a queue may be
controlled by multiple LUB enabled rates in a hierarchy, a queue may be associated with
multiple LUB contexts.
LUB is applied to the contexts where it is enabled. LUB first considers how much of the
aggregate rate is unused by the aggregate rates of each member queue after the first pass of
the HQoS algorithm. This represents the current bandwidth that may be distributed between
the member queues. LUB then distributes the available bandwidth to its member queues
based on each queue’s LUB-weight. A queue’s LUB-weight is determined as follows:
• If a queue is using all of its default H-QoS assigned rate then its LUB-weight is 0. It
is not participating in the bandwidth distribution as it cannot accept more bandwidth.
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