
QoS Scheduler Policies
7950 XRS Quality of Service Guide
Page 455
Frame-Based Accounting
The standard accounting mechanism uses ‘packet based’ rules that account for the DLC header,
any existing tags, Ethernet payload and the 4 byte CRC. The Ethernet framing overhead which
includes the Inter-Frame Gap (IFG) and preamble (20 bytes total) are not included in packet based
accounting. When frame based accounting is enabled, the 20 byte framing overhead is included in
the queue CIR, PIR and scheduling operations allowing the operations to take into consideration
on-wire bandwidth consumed by each Ethernet packet.
Since the native queue accounting functions (stats, CIR and PIR) are based on packet sizes and do
not include Ethernet frame encapsulation overhead, the system must manage the conversion
between packet based and frame based accounting. To accomplish this, the system requires that a
queue operates in frame based accounting mode, and must be managed by a virtual scheduler
policy or by a port virtual scheduler policy. Egress queues can use either port or service schedulers
to accomplish frame based accounting, but ingress queues are limited to service based scheduling
policies.
Turning on frame based accounting for a queue is accomplished through a frame based accounting
command defined on the scheduling policy level associated with the queue or through a queue
frame based accounting parameter on the aggregate rate limit command associated with the queues
SAP, multi-service site
or
context.
Operational Modifications
To add frame overhead to the existing QoS Ethernet packet handling functions, the system uses the
already existing virtual scheduling capability of the system. The system currently monitors each
queue included in a virtual scheduler to determine its offered load. This offered load value is
interpreted based on the queues defined CIR and PIR threshold rates to determine bandwidth
offerings from the queues virtual scheduler. When egress port based virtual scheduling was added,
frame based usage on the wire was added to allow for the port bandwidth to be accurately
allocated to each child queue on the port.
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