Frame Relay Traffic Policing and Shaping - Page 3

By Cisco Press | Posted Feb 11, 2002
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Traffic Policing with CAR
CAR is a rate-limiting feature for policing traffic, in addition to its packet classification feature. The rate-limiting feature of CAR manages the access bandwidth policy for a network by ensuring that traffic that falls within specified rate parameters is sent, while dropping packets that exceed the acceptable amount of traffic or sending them with a different priority. CAR's exceed action is to either drop or mark down the packet's priority.

The CAR rate-limiting function performs the following:

  • Controls the maximum rate of traffic sent or received on an interface.
  • Defines Layer 3 aggregate or granular incoming or outgoing (ingress or egress) bandwidth rate limits and specifies traffic-handling policies when the traffic either conforms to or exceeds the specified rate limits.
CAR bandwidth rate limits perform one of two functions:
  • Aggregate -- Aggregate bandwidth rate limits match all of the packets on an interface or subinterface.
  • Granular -- Granular bandwidth rate limits match a particular type of traffic based on precedence, MAC address, or other parameters.
CAR is often configured on interfaces at the edge of a network to limit traffic into or out of a network.

CAR Operation
CAR examines traffic received on an interface or a subset of that traffic selected by access list criteria. CAR compares the rate of the traffic to a configured token bucket and takes action based on the result. For example, CAR will drop the packet or rewrite the IP precedence by resetting the type of service (ToS) bits.

CAR can be configured to send, drop, or set precedence.

Aspects of CAR rate limiting include the following:

  • Matching criteria
  • Rate limits
  • Conform and exceed actions
  • Multiple rate policies
CAR utilizes a token bucket measurement, operating as follows:
  • Tokens are inserted into the bucket at the committed rate.
  • The depth of the bucket is the burst size.
  • Traffic arriving at the bucket when sufficient tokens are available is said to conform, and the corresponding number of tokens is removed from the bucket.
  • If a sufficient number of tokens is not available, then the traffic is said to exceed.

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