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

An enterprise router connects to an Internet Service Provider over a 1 Gbps physical Ethernet interface, but the WAN contract strictly enforces a Committed Information Rate (CIR) of 50 Mbps. The network engineer notices that bursty corporate traffic periodically exceeds 50 Mbps, causing the service provider to immediately drop the non-conforming packets. Which Quality of Service (QoS) mechanism should be applied to the router's WAN egress interface to buffer excess packets during spikes and release them at a rate matching the 50 Mbps contract?

  1. Traffic shapingAnswer
  2. B
    Traffic policing
  3. C
    Class-Based Weighted Fair Queuing (CBWFQ)
  4. D
    Expedited Forwarding (EF) DSCP marking

Answer

Traffic shaping
Traffic shaping is designed to rate-limit outbound traffic by holding excess packets in a queue and scheduling them for delayed transmission. This smoothes out bursty traffic profiles so that egress rates do not exceed a service provider's sub-rate Committed Information Rate (CIR), preventing ISP packet drops.

Step-by-Step Solution

1
Analyze the network requirement and problem statement
The physical interface speed is 1 Gbps, but the ISP enforces a lower sub-rate CIR of 50 Mbps and drops bursts exceeding this limit.
When a physical link speed exceeds the purchased CIR, unconditioned egress traffic will burst at the line rate of 1 Gbps, triggering rate-limiting drops at the provider edge.
2
Compare traffic conditioning mechanisms (Policing vs Shaping)
Traffic policing discards or remarks out-of-profile traffic without buffering. Traffic shaping uses memory buffers to queue excess packets and transmit them at a metered pace matching the target rate.
To prevent tail drop at the provider edge during traffic spikes, the customer edge device must smooth output using queue buffering.
3
Select the mechanism that buffers bursts to match a CIR
Traffic shaping is the correct QoS technique designed specifically to buffer micro-bursts on egress and pace packet output to fit sub-rate WAN contracts.
Traffic shaping aligns egress throughput with the 50 Mbps CIR, preventing packet loss at the ISP's policing boundary.

Key Concept

Difference between Traffic Shaping (buffering traffic bursts to a CIR) and Traffic Policing (dropping/remarking excess traffic immediately)
Estimated Time:1m 15s
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Topics

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

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