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Difficulty: Very hardQoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

A network operations team is troubleshooting performance discrepancies across a Cisco enterprise switch interface connected to a media server. An ingress policy map containing a traffic policing engine is configured on the switch interface to enforce rate limits. During bursts of high bandwidth utilization, real-time video traffic exceeding the configured rate limit is dropped directly at the ingress interface. Conversely, a WAN edge router interface configured with traffic shaping handles similar bandwidth bursts above the rate limit without packet loss, introducing only minor queueing latency. Which operational difference between QoS mechanisms explains this outcome?

  1. Traffic policing meters traffic and immediately drops or remarks packets exceeding the rate limit without buffering, whereas traffic shaping meters traffic and buffers excess packets in memory to smooth out bursts.Answer
  2. B
    Traffic policing buffers non-conforming packets into an internal queue until interface bandwidth becomes available, whereas traffic shaping drops all excess packets unconditionally.
  3. C
    Traffic policing modifies TCP window sizes to dynamically throttle host transmission speeds, whereas traffic shaping relies on Random Early Detection (RED) to drop packets randomly before buffers fill up.
  4. D
    Traffic policing re-marks Layer 2 Class of Service (CoS) bits to DSCP 0 and queues them in Low Latency Queuing (LLQ), whereas traffic shaping drops untagged Ethernet frames.

Answer

Traffic policing meters traffic and immediately drops or remarks packets exceeding the rate limit without buffering, whereas traffic shaping meters traffic and buffers excess packets in memory to smooth out bursts.
The correct answer identifies the fundamental difference between policing and shaping: Traffic policing meters traffic against a token bucket and immediately drops or remarks out-of-profile traffic without storing it in memory. In contrast, traffic shaping retains excess packets in a buffer memory queue and releases them smoothed over time, causing latency rather than immediate packet drops.

Step-by-Step Solution

1
Analyze the operational mechanics of Traffic Policing
Policing measures incoming or outgoing traffic rate against a configured Committed Information Rate (CIR). When traffic exceeds CIR, excess packets are immediately dropped or remarked. Policing does NOT use a queue/buffer.
Policing is designed for hard rate-limiting at ingress or egress interfaces without adding latency/jitter.
2
Analyze the operational mechanics of Traffic Shaping
Shaping measures traffic against a target CIR and holds non-conforming (excess) traffic in a queue/buffer, transmitting it later as bandwidth becomes available.
Shaping is designed for egress interfaces to smooth out traffic micro-bursts and prevent downstream policing drops, at the cost of queueing delay.
3
Compare the observed scenario against QoS mechanics
The ingress switch policy dropped packets because policing has no buffer. The egress WAN interface delayed packets without dropping them because shaping buffered the excess burst.
The difference in packet loss vs latency directly reflects the presence of a shaping buffer versus the bufferless action of a policer.

Key Concept

QoS Traffic Policing vs. Traffic Shaping Operational Mechanics
Estimated Time:2m 0s
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Topics

QoS Per-Hop Behaviors, Classification, Marking, Queuing, and Traffic Shaping/Policing

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