Egress Port-Based Schedulers - Alcatel-Lucent 7450 Quality Of Service Manual

Overview

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. However, on PoS and SDH based ports, the HDLC
encapsulation overhead and other framing overhead per packet is not known by the system.
Instead of automatically determining the encapsulation overhead for SDH or SONET queues,
the system provides a configurable frame encapsulation efficiency parameter that allows the
user to select the average encapsulation efficiency for all packets forwarded out the egress
queue.
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
Virtual Scheduler Bandwidth Allocation Based on Priority and
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-cir" loops, one for each of the eight priority levels. The second tier "above-cir" loop
provides bandwidth to the third tier "above-cir" loops for each of the eight priority levels.
The "within-cir" loop for each priority level on the third tier supports an optional rate limiter
used to restrict the maximum amount of "within-cir" bandwidth the priority level can receive.
A maximum priority level rate limit is also supported that restricts the total amount of
bandwidth the level can receive for both "within-cir" and "above-cir". The amount of
bandwidth consumed by each priority level for "within-cir" and "above-cir" is predicated on
the rate limits described and the ability for each child queue or scheduler attached to the
priority level to use the bandwidth.
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Port Level
CIR.
Quality of Service Guide