QoS and QoS Policies
78
Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
The Expedited scheduler has precedence over the Best Effort scheduler. Thus, at
access ingress, CoS queues that are marked with an Expedited priority are serviced
first. Then, the Best Effort marked queues are serviced. In a default configuration, the
Expedited scheduler services the following CoS queues before the Best Effort
scheduler services the rest:
• Expedited scheduler: NC, H1, EF, H2
• Best-Effort scheduler: L1, AF, L2, BE
If a packet with an Expedited forwarding class arrives while a Best Effort marked
queue is being serviced, then the Expedited Scheduler will take over and service the
Expedited marked CoS queue, as soon as the packet from the Best Effort scheduler
is serviced.
The schedulers at access ingress in the 7705 SAR service the group of all Expedited
queues exhaustively ahead of the group of all Best Effort queues. This means that
all expedited queues have to be empty before any packet from a Best Effort queue
is serviced.
The following basic rules apply to the queue-type scheduling of CoS queues.
1. Queues marked for Expedited scheduling are serviced in a round-robin fashion
before any queues marked as Best Effort (in a default configuration, these would
be queues CoS-8 through CoS-5).
2. These Expedited queues are serviced exhaustively within the round robin. For
example, if in a default configuration there are two packets scheduled for service
in both CoS-8 and CoS-5, one packet from CoS-8 is serviced, then one packet
from CoS-5 is serviced, and then the scheduler returns back to CoS-8, until all
the packets are serviced.
3. Once the Expedited scheduler has serviced all the packets in the queues
marked for Expedited scheduling, the Best Effort scheduler starts serving the
queues marked as Best Effort. The same principle described in step 2 is
followed, until all the packets in the Best Effort queues are serviced.
Note: There is no user configuration for the schedulers. The operation of the schedulers is
described for informational purposes. A user can control the mapping of traffic flows based
on classification controls, that is, by mapping forwarding classes to as many as eight CoS
queues.
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