Frame Relay Traffic Policing and Shaping - Page 4

By Cisco Press | Posted Feb 11, 2002
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CAR Traffic-Matching Criteria
Traffic matching involves the identification of interesting traffic for rate limiting, precedence setting, or both. Rate policies can be associated with one of the following qualities:

  • Incoming interface
  • All IP traffic
  • IP precedence (defined by a rate-limit access list)
  • MAC address (defined by a rate-limit access list)
  • IP access list (standard and extended)
CAR provides configurable actions, such as send, drop, or set precedence when traffic conforms to or exceeds the rate limit.
NOTE:   Matching to IP access lists is more processor intensive than matching based on other criteria.

Rate Limits
CAR propagates bursts and performs no smoothing or shaping of traffic; therefore, it performs no buffering and adds no delay. CAR is optimized (but not limited) to run on high-speed links, such as DS3 or higher, and in distributed mode on Versatile Interface Processors (VIPs) on the Cisco 7500 series.

CAR rate limits can be implemented either on input or output interfaces or subinterfaces, including those found with Frame Relay and ATM implementations.

Rate limits define which packets conform to or exceed the defined rate based on the following three parameters:

  • Average rate -- Determines the long-term average transmission rate. Traffic that falls under this rate will always conform.
  • Normal burst size -- Determines how large traffic bursts can be before some traffic exceeds the rate limit.
  • Excess Burst size (BE) -- Determines how large traffic bursts can be before all traffic exceeds the rate limit. Traffic that falls between the normal burst size and the Excess Burst size exceeds the rate limit with a probability that increases as the burst size increases.
The tokens in a token bucket are replenished at regular intervals, in accordance with the configured committed rate. The maximum number of tokens that a bucket can contain is determined by the token bucket's normal burst size configuration.

When the CAR rate limit is applied to a packet, CAR removes from the bucket tokens that are equivalent in number to the byte size of the packet. If a packet arrives and the byte size of the packet is greater than the number of tokens available in the token bucket, extended burst capability is engaged (if it is configured).

Setting the extended burst value greater than the normal burst value configures extended burst. Setting the extended burst value equal to the normal burst value excludes the extended burst capability. If extended burst is not configured, the CAR exceed action takes effect because a sufficient number of tokens are not available.

When extended burst is configured, the flow is allowed to borrow the needed tokens to allow the packet to be sent. This capability exists to avoid tail-drop behavior, and, instead, engage behavior like that of random early detection (RED).

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