Committed Access Rate (CAR) in Cisco QoS
Short answer
A traffic policing
Question
Key Takeaways
Tests the classification of CAR as a policing mechanism rather than shaping, with the common trap being confusion between rate limiting (policing) and buffering/delaying (shaping).
Committed Access Rate (CAR) is a legacy Cisco feature used for traffic policing. The community consensus confirms it limits transmission rates by dropping or marking excess traffic, distinguishing it from shaping.
Expert Analysis
Why the Answer Is Correct
CAR (Committed Access Rate) is explicitly defined by Cisco as a traffic policing tool. Policing operates at the edge of the network to enforce rate limits by discarding or remarking packets that exceed the committed rate, ensuring immediate enforcement without queuing delays.
Why the Other Options Are Wrong
Traffic shaping (Option D) works by buffering excess traffic to smooth out bursts, whereas CAR drops excess packets immediately. Traffic signaling (Option B) refers to protocols like RSVP, not rate limiting. Traffic conditioning (Option C) is a broader category that includes both policing and shaping, but CAR is specifically a policing implementation.
Community Comment Notes
Comment [1] provides an official Cisco document clarifying CAR as the 'legacy traffic policing feature.' Comment [2] highlights that CAR can mark and send excess traffic but remains a policing mechanism due to its non-buffered nature. Comment [3] reinforces that CAR is listed under 'Policing tools' in official documentation.
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 4 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 4
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pcp84 👍 4 Answered AFirst line "This document clarifies the differences between committed access rate (CAR), which is the Cisco legacy traffic policing feature, and class-based policing, which is the newer Cisco traffic policer." https://www.cisco.com/c/en/us/support/docs/quality-of-service-qos/qos-policing/10118-cbpcar.html
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G0y0 👍 1 Answered APolicing mechanisms include Class-Based Policing and Committed Access Rate (CAR). Traffic policing mechanisms such as Class-based policing or Committed Access Rate (CAR) also have marking capabilities in addition to rate-limiting capabilities. Instead of dropping the excess traffic, traffic policing can alternatively mark and then send the excess traffic. This allows the excess traffic to be re-marked with a lower priority before the excess traffic is sent out. Traffic shapers, on the other hand, do not re-mark traffic; these only delay excess traffic bursts to conform to a specified rate.
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b3532e4 👍 1The correct answer to the question "CAR is an example of which technique?" is A. traffic policing. This is because CAR (Committed Access Rate) is a traffic policing tool used to limit the input or output transmission rate on an interface based on certain criteria, as mentioned in your document on page 318 under the section on Policing: "Policing tools include class-based policing and committed access rate (CAR). CAR services limit the input or output transmission rate on an interface or subinterface based on a flexible set of criteria
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Komy 👍 1 Answered ABy Definition: "Committed access rate (CAR) is a metric used Internet quality of service (QoS) agreements to classify and limit customer traffic" So it is used to "Limit" traffic
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