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Re: CAR configuration parameters - IE-LAB17 posted 07/26/2007
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I know this doesn't answer your question, but from command reference, there
is a formula for configuring the normal and extend burst. I've found in
"real life" it to be true. Using the 'default' bytes for normal and extended
burst like the solution shows will seriously degrade the performance and it
will police at a much lower rate. Using the formula though works well. Since
this is the lab, I guess it doesn't matter.

I have no idea how they got 496000 either...based on the question and the
formula, I would have put in:

int e 1/0
rate-limit output access-group 191  500000 93750 187500 conform-action
transmit exceed-action drop


Usage Guidelines

Use this command to configure your CAR policy on an interface. To specify
multiple policies, enter this command once for each policy.

CAR and DCAR can be configured on an interface or subinterface.

Policing Traffic with CAR

CAR embodies a rate-limiting feature for policing traffic. When policing
traffic with CAR, Cisco recommends the following values for the normal and
extended burst parameters:

normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds

extended burst = 2 * normal burst

With the listed choices for parameters, extensive test results have shown
CAR to achieve the configured rate. If the burst values are too low, then
the achieved rate is often much lower than the configured rate.

On 7/26/07, Alan Ewer <acewer64@xxxxxxxxx> wrote:
> Hi All
> apologies for asking this here if it has already been answered elsewhere (
> i
> didnt find it if it has .)
> I am struggling with CAR. Question reads like this... Configure CAR on an
> interface to not exceed 500kbps outbound..
> cool i think to myself !!
> The command syntax is :
> rate-limit input|output access-group <name> <sustained-rate in bps>
> <normal
> burst  BYTES> <excess burst BYTES> conform-action transmit exceed-action
> drop
> OK this is applied to an ethernet interface on a 3640 and has nothing to
> do
> with time-intervals <true/false ???>
> the answer in the workbook is
> int e 1/0
> rate-limit out put access-group 191  496000 4000 4000 conform-action
> transmit exceed-action drop
> Question 1  How does the 496000 get derived
> Question 2 what and where does 4000 bytes normal burst get derived.. what
> are the considering factors here ??
> Question 3 what difference does the excess burst have if the exceed-action
> is "drop" ..... i
> If anyone could point me in the right direction.. i would me grateful...
> I have already read the QOS guide and the configuration guide and they
> dont
> seem to be helping me with this concept !
> I dont get how 496000 bits per second + 4000 bytes per second = 500000
> bits
> per second
> Regds
> AlanE
> Brisbane Oztralia
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