As well as profiled scheduling described above, queue-type scheduling is supported at access ingress. Queues are divided into two categories, those that are serviced by the Expedited scheduler and those that are serviced by the Best Effort scheduler.
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.
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.
The following basic rules apply to the queue-type scheduling of CoS queues.
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).
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.
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.
If a packet arrives at any of the queues marked for Expedited scheduling while the scheduler is servicing a packet from a Best Effort queue, the Best Effort scheduler finishes servicing the current packet and then the Expedited scheduler immediately activates to service the packet in the Expedited queue. If there are no other packets to be serviced in any of the Expedited queues, the Best Effort scheduler resumes servicing the packets in the Best Effort queues. If the queues are configured according to the tables and defaults described in this guide, CoS-4 will be scheduled prior to CoS-1 among queues marked as Best Effort.
After one cycle is completed across all the queues marked as Best Effort, the next pass is started until all the packets in all the queues marked as Best Effort are serviced, or a packet arrives to a queue marked as Expedited and is serviced as described in step 2.