The Nokia Service Router (SR) egress port scheduler operates on a strict priority basis across eight priority levels (1-lowest, 8-highest). Network traffic is classified into Forwarding Classes (FCs); for example, voice traffic is typically mapped to the Expedited Forwarding (EF) FC, while general internet traffic is mapped to the Best-Effort (BE) FC. These FCs are then mapped to egress queues with specific priority levels. By convention and design, expedited queues are assigned a higher priority level than best-effort queues. The scheduler will always service all packets in higher-priority queues before servicing any packets from lower-priority queues, making this statement fundamentally true for the scheduler's operation.

A. The scheduler uses a combination of strict priority (between priority levels) and weighted mechanisms (within a priority level), not just strict priority.

C. Orphaned queues (queues not explicitly associated with a scheduler) are serviced at the lowest priority level, which is level 1, not the highest (level 8).

D. A child's configured CIR dictates the maximum bandwidth for that specific child during the within-CIR pass, not the "maximum port bandwidth". The wording is imprecise.

1. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1

Reference for Correct Answer (B) and Incorrect Answer (A): In the chapter "Quality of Service," section "Hierarchical Schedulers," it is stated: "The schedulers service the queues based on the queue’s priority, with priority 8 being the highest and 1 being the lowest. A scheduler will not serve a lower-priority queue until all higher-priority queues are empty." This confirms the strict priority nature that underpins option B. The same section also details the use of weights for scheduling within a priority level, which invalidates the "strict priority only" claim in option A.

Reference for Incorrect Answer (C): In the chapter "Quality of Service," section "Orphan Queues," the guide specifies: "By default, an orphaned queue is serviced at priority level 1." This directly contradicts the claim that they are serviced at priority level 8.

Reference for Incorrect Answer (D): In the chapter "Quality of Service," section "Hierarchical Schedulers," subsection "Scheduler Operation," the guide describes the "Within-CIR Pass": "During the CIR pass, the scheduler limits the amount of traffic serviced by a child to its configured CIR." This shows that the CIR is a limit for the child entity, not a determinant of the "maximum port bandwidth," making the statement in option D inaccurate.