When SAP-based scheduling is enabled, the following figure shows the scheduling for access port, with multiple SAPs configured and when the port-scheduler-mode is disabled. There are 8 egress queues per SAP, a per SAP scheduler and a per access port scheduler.
Each FC/queue for the port can be shaped to configured rates (CIR/PIR). This is used to control the amount of bandwidth allocated to the FC/queue.
FC-to-queue mapping is system-defined and not user configurable.
The queue number determines the priority of the queue, which is used only when the queues are configured as ‟strict.” Queue "8" is the highest priority, and queue "1" is the lowest priority.
On the 7210 SAS-Mxp, only unicast traffic sent out of RVPLS SAPs uses per-SAP egress queues. BUM traffic sent out of RVPLS SAPs uses per-port egress queues. Per-port egress queues are not depicted in the preceding diagram. There are eight per-port queues, and they contend with per-SAP queues for bandwidth.
A queue can be defined to operate in strict mode or weighted mode. The queue mode determines the order of scheduling by the port scheduler.
In a CIR loop, scheduling is packet-based round-robin with a weight of 1.
The behavior of the scheduler for an access port is as follows:
Per port scheduler is available and works at line-rate or configured port egress rate.
Port scheduler distributes the bandwidth available to all the SAPs using WDRR scheduling mechanism (that is, all SAPs have equal weights assigned by the system).
The port scheduler uses the following two passes to distribute bandwidth across SAPs.
CIR loop
The port scheduler distributes the available bandwidth to all the SAP in a round-robin order up to the configured CIR rate (CIR is configured in the aggregate shaper rates for the SAP).
PIR loop
The port scheduler distributes the remaining bandwidth (the bandwidth available after the CIR loop) to all SAPs in a round-robin order (all SAPs are assigned equal weights by the system) up to the configured PIR rate (PIR is configured in the aggregate shaper rates for the SAP).
Each SAP has a per SAP scheduler which operates in SP + WDRR mode and an aggregate per SAP shaper (CIR/PIR). The per SAP scheduler distributes the available bandwidth to the configured strict and weighted SAP queues, using the configured mode and rates, in 2 passes - CIR loop and PIR loop
The CIR loop distributes the available bandwidth (from the bandwidth allocated to it by the port scheduler) to all the queues is in the following order:
higher priority strict queues get the bandwidth up to the configured CIR
any remaining bandwidth, if available, is distributed among the lower priority strict queues up to the configured CIR
any remaining bandwidth, if available, is distributed among the weighted queues (in the CIR loop, weights are not used and therefore the bandwidth is distributed in equal proportion irrespective of weights configured)
The PIR loop distributes the remaining bandwidth (the bandwidth remaining after CIR loop) to all the queues in the following order:
higher priority strict queues get the bandwidth, up to the configured PIR
any remaining bandwidth, if available, is distributed among the lower priority strict queues, up to the configured PIR
any remaining bandwidth, if available, is distributed among the weighted queues in proportion to their configured weights
Each queue can be configured with a queue-mode (strict or weighted) and is associated with a shaper (which allows for configuration of CIR/PIR). The queue mode determines the order of scheduling by the SAP scheduler and shaper rate controls the amount of bandwidth used by the queue.