Configuring Port Scheduler Policies - Alcatel-Lucent 7950 Quality Of Service Manual

Configuring Port Scheduler Policies

Port Scheduler Structure
Every port scheduler supports eight strict priority levels with a two pass bandwidth allocation
mechanism for each priority level. Priority levels 8 through 1 (level 8 is the highest priority) are
available for port-parent association for child queues and schedulers. Each priority level supports a
maximum rate limit parameter that limits the amount of bandwidth that may be allocated to that
level. A CIR parameter is also supported that limits the amount of bandwidth allocated to the
priority level for the child queue's offered load, within their defined CIR. An overall maximum
rate parameter defines the total bandwidth that will be allocated to all priority levels.
Special Orphan Queue and Scheduler Behavior
When a port scheduler is present on an egress port, the system ensures that all queues and
schedulers receive bandwidth from that scheduler to prevent free-running queues which can cause
the aggregate operational PIR of the port to oversubscribe the bandwidth available. When the
aggregate maximum rate for the queues on a port operate above the available line rate, the
forwarding ratio between the queues will be affected by the hardware schedulers on the port and
may not reflect the scheduling defined on the port or intermediate schedulers. Queues and
schedulers that are either explicitly attached to the port scheduler using the port-parent command
or are attached to an intermediate scheduler hierarchy that is ultimately attached to the port
scheduler are managed through the normal eight priority levels. Queues and schedulers that are not
attached directly to the port scheduler and are not attached to an intermediate scheduler that itself
is attached to the port scheduler are considered orphaned queues and, by default, are tied to
priority 1 with a weight of 0. All weight 0 queues and schedulers at priority level 1 are allocated
bandwidth after all other children and each weight 0 child is given an equal share of the remaining
bandwidth. This default orphan behavior may be overridden at the port scheduler policy by using
the orphan-override command. The orphan-override command accepts the same parameters as the
port-parent command. When the orphan-override command is executed, the parameters will be
used as the port parent parameters for all orphans associated with a port using the port scheduler
policy.
Packet to Frame Bandwidth Conversion
Another difference between the service level scheduler-policy and the port level port-scheduler-
policy is in bandwidth allocation behavior. The port scheduler is designed to offer on-the-wire
bandwidth. For Ethernet ports, this includes the IFG and the preamble for each frame and
represents 20 bytes total per frame. The queues and intermediate service level schedulers (a
7950 XRS Quality of Service Guide
QoS Scheduler Policies
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