Virtual Scheduling Unused Bandwidth Distribution - Alcatel-Lucent 7450 Quality Of Service Manual

Overview
Similarly, the rates defined in the scheduling policy with frame based accounting enabled will
automatically be interpreted as frame based rates.
The port based scheduler aggregate rate limit command always interprets its configured rate
limit value as a frame based rate. Setting the frame based accounting parameter on the
aggregate rate limit command only affects the queues managed by the aggregate rate limit and
converts them from packet based to frame based accounting mode.

Virtual Scheduling Unused Bandwidth Distribution

The Hierarchical QoS (HQoS) mechanism is designed to enforce a user definable hierarchical
shaping behavior on an arbitrary set of queues. The mechanism accomplishes this by
monitoring the offered rate of each queue and using the result as an input to a virtual scheduler
hierarchy defined by the user. The hierarchy consists of a number of virtual scheduler with
configurable maximum rates per scheduler and attachment parameters between each. The
parameters consist of weights and priority levels used to distribute the available bandwidth in
a top down fashion through the hierarchy with the queues at the bottom. The resulting
bandwidth provided to each member queue by the virtual schedulers is then configured as an
operational PIR on the corresponding hardware queue, which prevents that queue from
receiving more hardware scheduler bandwidth than dictated by the virtual scheduler.
Default Unused Bandwidth Distribution
The default behavior of HQoS is to only throttle active queues currently exceeding their
allocated bandwidth by the virtual schedulers controlling the active queue. A queue that is
currently operating below its share of bandwidth is allowed an operational PIR greater than
its current rate, this includes inactive queues. The operational PIR for a queue is capped by
its admin PIR and set to the queue's fair-share of the available bandwidth based on its priority
level in the HQoS hierarchy and its weight within that priority level. The result is that between
HQoS iterations, a queue below its share of bandwidth may burst to a higher rate and
momentarily overrun the prescribed aggregate rate.
This default behavior works well in situations where an aggregate rate is being applied as a
customer capping function to limit excessive use of network resources. However, in certain
circumstances where an aggregate rate must be maintained due to limited downstream QoS
abilities or due to downstream priority unaware aggregate policing, a more conservative
behavior is required. The following functions can be used to control the unused bandwidth
distribution:
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