The Nokia SR OS egress port scheduler uses a sophisticated hierarchical scheduling algorithm to manage bandwidth. The process operates in two distinct passes. First, it allocates the guaranteed bandwidth (Committed Information Rate, CIR) to all child queues and schedulers. After the CIR commitments are met, any remaining port bandwidth is distributed as excess bandwidth in a second pass. This excess bandwidth distribution is based on a Weighted Round Robin (WRR) mechanism, where queues and schedulers can receive additional bandwidth up to their Peak Information Rate (PIR). The entire process is constrained by the maximum configured speed of the egress port.

A. This is an oversimplification. The scheduler does not use a simple ratio; it uses a distinct two-pass mechanism for CIR (guaranteed) and PIR (excess) bandwidth, which is more complex than a single fair allocation.

C. The scheduler is not "first come, first served" (FIFO). It is a work-conserving, weighted, and prioritized scheduler that allocates resources based on configured QoS policies (CIR/PIR), not packet arrival time.

D. The over-subscription factor is a provisioning tool to manage the aggregate CIR on a port, not the real-time scheduling algorithm itself. The scheduler's function is to distribute the available physical bandwidth.

E. The "Russian Dolls Method" is a conceptual analogy for hierarchical QoS, not the name of the scheduling algorithm. High-Priority-Only (HPO) is a buffer management feature, not a bandwidth allocation mechanism.

1. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1, Section: Hierarchical Scheduling.

Page 103, "Scheduler Operation": "The SR OS schedulers are two-pass schedulers. In the first pass (the CIR pass), the scheduler goes through all queues and schedulers and provides bandwidth up to their configured CIR. In the second pass (the PIR pass), the scheduler goes through all queues and schedulers and provides bandwidth up to their configured PIR."

Page 104, "Within-PIR Scheduling": "The within-PIR scheduler distributes the available excess bandwidth (bandwidth not used by the CIR loop) to the queues and schedulers based on their configured weights." This describes the WRR-like behavior for excess bandwidth.

2. Nokia SRC Self-Paced Training, Module: SR-OS Hierarchical Quality of Service (HQoS).

Section: Egress Port Scheduler: This module details the two-pass (CIR and PIR) operation of the egress port scheduler. It explains that the first pass guarantees the CIR, and the second pass distributes remaining bandwidth up to the PIR using a weighted algorithm, effectively a WRR distribution based on configured rates. The total output is always capped by the port's physical or configured rate.