CCNP Practice Question: The purpose of the 'police' command in a QoS…

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  To shape traffic to a specific rate by buffering excess packets.

  Why it's wrong here

  Shaping (GTS or class-based shaping) intentionally buffers excess packets in a queue and transmits them later using a token bucket, which enforces the rate while adding latency and jitter. Policing, in contrast, does not queue or delay traffic; it immediately drops or remarks packets that exceed the configured rate. Therefore, this answer describes shaping, not the policing mechanism the question asks about.

• ✓

  To limit the rate of traffic and take action (drop or remark) on packets that exceed the rate.

  Why this is correct

  Policing uses a token bucket (or single/two-rate policer) to measure traffic arrival against a configured committed information rate (CIR) and burst size. Packets that conform are forwarded unchanged, while excess packets are either dropped or have their precedence/DSCP marked down (e.g., set to a lower drop probability). This fits the classic definition of policing, which is a rate-limiting action that takes immediate action on nonconforming packets without buffering.

• ✗

  To prioritize traffic by assigning it to a strict priority queue.

  Why it's wrong here

  Strict priority queuing is a scheduling mechanism used in QoS designs such as Low Latency Queuing (LLQ), where a designated class is always served before other queues until its queue becomes empty. This controls latency ordering, not the rate of traffic, and it does not drop or mark packets based on exceeding a rate. Assigning a class to a priority queue is orthogonal to policing, which is a separate traffic conditioning operation.

• ✗

  To classify traffic based on IP precedence or DSCP values.

  Why it's wrong here

  Classification is the process of identifying traffic by matching fields such as IP precedence, DSCP, or ACLs to assign it to a class, typically using class-maps with match commands in a policy-map. Classification alone does not enforce any rate or shape; it merely labels packets for later QoS actions like marking, queuing, or policing. While policing often relies on classification to select the traffic to police, the classification step is not the policing action itself.