Class-Based Policing - Recommended Burst Sizes

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Oct 12th, 2009
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Can anyone clarify what the recommended burst vaules are for Class-Based Policing? The documentation for the rate-limit command suggests a normal burst of CIR [BPS] * (1 byte)/(8 bits) * 1.5 seconds and a extended burst of 2 * normal burst. For example:

rate-limit 1000000 187500 375000

The documentation for the police command is not as helpful and only provides the following example:

police 8000 1000 1000 conform-action transmit exceed-action set-qos-transmit 1 violate-action drop

If the burst size is not specified with Class-Based Policing the IOS defaults to a Bc of CIR/32 (250ms Tc) which is very different to the 1.5 seconds value Cisco recommends for the rate-limit command.

The documentation for the rate-limit command also states “Setting the extended burst value equal to the normal burst value excludes the extended burst capability.” I take it this is not the case with Class-Based Policing as the Cisco example has 1000 bytes for both normal and excess burst sizes?

If anyone can explain the differences between Class-Based Policing and Committed Access Rate burst values it would be appreciated.



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Giuseppe Larosa Tue, 10/13/2009 - 11:03
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Hello Paul,

CAR has only two possible states:

conforming and exceeding.

Class based policing is different because it has three possible state for a policed packet:

conforming, exceeding, violating.

both features use a token bucket algorythm to "count" traffic against the configured rates.

Class based policing uses Be when Be <> 0, token rate fills first Bc and then spills down to Be until it fills it.

In this way unused tokens in previous intervals can be accumulated up to Bc+Be and are available at next interval.

CAR implements its token bucket differently.

in CAR extended burst size has effect if Be > Bc and actually what counts is Be / Bc ratio at least this was our impression when we tested CAR some (well 10 ) years ago

A useful document is enterprise QoS Solution reference network design

Hope to help



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