Which of the following statements about the token buckets of a policer is TRUE?

In the Nokia SR OS dual token bucket policer model, an arriving packet is first checked against the Peak Information Rate (PIR) bucket. This bucket has a capacity defined by the Maximum Burst Size (MBS). If there are insufficient tokens in the PIR bucket to accommodate the size of the arriving packet, the packet is considered non-conforming to the peak rate. Such packets are colored "red" and are immediately discarded. Therefore, the PIR bucket enforces the absolute maximum rate, and any traffic exceeding this limit is dropped.

B. The Nokia SR OS uses a dual-bucket policer where the CIR and PIR buckets are separate and operate independently to meter traffic against the committed and peak rates, respectively.

C. The CIR bucket is primarily defined by two parameters: the Committed Information Rate (CIR) and the Committed Burst Size (CBS). HPO (High Priority Only) is a feature, not a bucket threshold.

D. Packets exceeding the PIR are colored "red" and discarded. Packets that conform to PIR but exceed the CIR are forwarded as "out-of-profile" (colored "yellow").

1. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1, Section: "Policer Control Plane".

Page 289, "Dual Token Bucket Model": This section details the two-bucket algorithm. It states, "When a packet is received, the policer first checks for sufficient tokens in the PIR bucket... If there are not enough tokens in the PIR bucket to accommodate the packet, the packet is immediately colored red." The guide further clarifies that the default action for red packets is to discard them. This directly supports answer A and refutes D.

2. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1, Section: "Policer Control Plane".

Page 289, "Dual Token Bucket Model": The description of the algorithm clearly shows two distinct buckets, one for PIR/MBS and one for CIR/CBS, which are evaluated sequentially. This refutes the claim in option B that one is a portion of the other.

3. Nokia 4A0-107 Courseware, Module 3: Quality of Service, Section: "Policing and Shaping".

The official course material explains that the CIR bucket's size is the CBS, which determines the burst of traffic that can be sent at the CIR. It describes HPO as a mechanism to reserve a portion of the CIR for high-priority traffic, not as a fundamental threshold of the bucket itself, thus refuting option C.