Virtual Scheduling Unused Bandwidth Distribution - Alcatel-Lucent 7950 Quality Of Service Manual

Virtual Scheduling Unused Bandwidth Distribution

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:
•
•
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.
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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.
7950 XRS Quality of Service Guide