Frame Relay Traffic Policing and Shaping - Page 6

By Cisco Press | Posted Feb 11, 2002
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Conform and Exceed Actions
Because CAR utilizes a token bucket, CAR can pass temporary bursts that exceed the rate limit as long as tokens are available.

After a packet has been classified as conforming to or exceeding a particular rate limit, the router performs one of the following actions:

  • Transmit -- The packet is sent.
  • Drop -- The packet is discarded.
  • Set precedence and transmit -- The IP precedence (ToS) bits in the packet header are rewritten. The packet is then sent. You can use this action to either color (set precedence) or recolor (modify existing packet precedence) the packet.
  • Continue -- The packet is evaluated using the next rate policy in a chain of rate limits. If another rate policy does not exist, the packet is sent.
  • Set precedence and continue -- Set the IP precedence bits to a specified value and then evaluate the next rate policy in the chain of rate limits.
For VIP-based platforms, two more actions are possible:
  • Set QoS group and transmit -- The packet is assigned to a QoS group and sent.
  • Set QoS group and continue -- The packet is assigned to a QoS group and then evaluated using the next rate policy. If another rate policy does not exist, the packet is sent.

Multiple Rate Policies
A single CAR rate policy includes information about the rate limit, conform actions, and exceed actions. Each interface can have multiple CAR rate policies corresponding to different types of traffic. For example, low-priority traffic might be limited to a lower rate than high-priority traffic. When multiple rate policies exist, the router examines each policy in the order entered until the packet matches. If no match is found, the default action is to send.

Rate policies can be independent or cascading:

  • Independent -- Each rate policy deals with a different type of traffic.
  • Cascading -- A packet might be compared to multiple different rate policies in succession.
Cascading of rate policies supports a series of rate limits to be applied to packets to specify more granular policies. For example, total traffic could be rate limited on an access link to a specified subrate bandwidth and then rate limit World Wide Web traffic on the same link to a given proportion of the subrate limit.

Match packets could be rate limited against an ordered sequence of policies until an applicable rate limit is encountered. For example, as the sequence of policies is applied, the rate limiting of several MAC addresses with different bandwidth allocations can occur at an exchange point.

Up to 100 rate policies can be configured on a subinterface.

CAR Restrictions
CAR and VIP-distributed CAR can only be used with IP traffic. Non-IP traffic is not rate limited. CAR or VIP-distributed CAR can be configured on an interface or subinterface, with the exception of the following interface types:

  • Fast EtherChannel
  • Tunnel
  • PRI
  • Any interface that does not support Cisco Express Forwarding (CEF)
CAR is supported only on ATM subinterfaces with the following encapsulations: aal5snap, aal5mux, and aal5nlpid.


NOTE:   CAR provides rate limiting and does not guarantee bandwidth. CAR should be used with other QoS features, such as distributed weighted fair queuing (DWFQ), if premium bandwidth assurances are required.

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Our final segment from Cisco Press' Network Consultants Handbook will summarize Frame Relay.

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