Overview - Alcatel-Lucent 7950 Quality Of Service Manual

Scheduler Policies

Overview

Scheduler Policies
Virtual schedulers are created within the context of a scheduler policy that is used to define the
hierarchy and parameters for each scheduler. A scheduler is defined in the context of a tier which
is used to place the scheduler within the hierarchy. Three tiers of virtual schedulers are supported.
Root schedulers are defined without a parent scheduler meaning it is not subject to obtaining
bandwidth from a higher tier scheduler. A scheduler has the option of enforcing a maximum rate of
operation for all child queues and schedulers associated with it.
Because a scheduler is designed to arbitrate bandwidth between many inputs, a metric must be
assigned to each child queue or scheduler vying for transmit bandwidth. This metric indicates
whether the child is to be scheduled in a strict or weighted fashion and the level or weight the child
has to other children.
Egress Port-Based Schedulers
In previous releases, HQoS root (top tier) schedulers always assumed that the configured rate was
available, regardless of egress port level oversubscription and congestion. This resulted in the
possibility that the aggregate bandwidth assigned to queues was not actually available at the port
level. When the HQoS algorithm configures queues with more bandwidth than available on an
egress port, actual bandwidth distribution to queues on the port will be solely based on the action
of the hardware scheduler. This can result in a forwarding rate at each queue that is very different
than the desired rate.
The port-based scheduler feature was introduced to allow HQoS bandwidth allocation based on
available bandwidth at the egress port level. The port-based scheduler works at the egress line rate
of the port to which it is attached. Port-based scheduling bandwidth allocation automatically
includes the Inter-Frame Gap (IFG) and preamble for packets forwarded on queues servicing
egress Ethernet ports.
A special port scheduler policy can be configured to define the virtual scheduling behavior for an
egress port. The port scheduler is a software-based state machine managing a bandwidth allocation
algorithm that represents the scheduling hierarchy shown in
The first tier of the scheduling hierarchy manages the total frame based bandwidth that the port
scheduler will allocate to the eight priority levels.
The second tier receives bandwidth from the first tier in two priorities, a "within-cir" loop and an
"above-cir" loop. The second tier "within-cir" loop provides bandwidth to the third tier "within-
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Figure 15 on page
7950 XRS Quality of Service Guide
445.