Overview
530 Quality of Service Guide
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 service level scheduler is a scheduler instance at the SAP, multi-service site or
subscriber or multi-service site profile level) operate based on packet overhead which does
not include the IFG or preamble on Ethernet packets. In order for the port based virtual
scheduling algorithm to function, it must convert the queue and service scheduler packet
based required bandwidth and bandwidth limiters (CIR and rate PIR) to frame based values.
This is accomplished by adding 20 bytes to each Ethernet frame offered at the queue level to
calculate a frame based offered load. Then the algorithm calculates the ratio increase between
the packet based offered load and the frame based offered load and uses this ratio to adapt the
CIR and rate PIR values for the queue to frame-CIR and frame-PIR values. When a service
level scheduler hierarchy is between the queues and the port based schedulers, the ratio
between the average frame-offered-load and the average packet-offered-load is used to adapt
the scheduler’s packet based CIR and rate PIR to frame based values. The frame based values
are then used to distribute the port based bandwidth down to the queue level.
Packet over SONET (PoS) and SDH queues on the 7450 ESS and 7750 SR also operate based
on packet sizes and do not include on-the-wire frame overhead. Unfortunately, the port based
virtual scheduler algorithm does not have access to all the frame encapsulation overhead
occurring at the framer level. Instead of automatically calculating the difference between
packet-offered-load and frame-offered-load, the system relies on a provisioned value at the
queue level. This avg-frame-overhead parameter is used to calculate the difference between
the packet-offered-load and the frame-offered-load. This difference is added to the packet-
offered-load to derive the frame-offered-load. Proper setting of this percentage value is
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