From: Brian McGahan (brian@cyscoexpert.com)
Date: Thu Jul 10 2003 - 16:31:54 GMT-3
Jim & Jonathan,
Configuring the burst values to correspond to the formula *IS*
necessary. Test it with actual traffic and you will see that the rate
does not correctly conform unless you use the recommended burst values.
I'm not sure where these authors are getting their burst values from,
but they're not correct.
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
Toll Free: 866.CyscoXP
Fax: 847.674.2625
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Jonathan V Hays
> Sent: Wednesday, July 09, 2003 6:37 PM
> To: 'Jim Phillipo'; ccielab@groupstudy.com
> Subject: RE: More QOS CAR example
>
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> > Behalf Of Jim Phillipo
> > Sent: Wednesday, July 09, 2003 6:48 PM
> > To: ccielab@groupstudy.com
> > Subject: More QOS CAR example
> >
> >
> > This is from VEGESNA's book
> >
> > All traffic except HTTP over a rate of 15 Mbps is marked with
> > IP Prec 4
> > HTTP over 15 Mbps is marked with 0
> > This is a 30 Mbps pipe from provider
> >
> > There answer:
> >
> > rate-limit input 30000000 15000 15000 conform-action continue
> > exceed-action-drop
> > rate-limit input access-group 101 15000000 10000 10000
conform-action
> > set-prec-transmit 4 exceed-action set-prec-transmit 0
> > rate-limit input 30000000 15000 15000 conform-action
> > set-prec-transmit 4
> > exceed-action set-prec-transmit 4
> >
> > access-list 101 permit tcp any any eq www
> > access-list 101 permit tcp any eq www any <This is for web
> > traffic leaving
> > the site correct ?>
> >
> > How do they arrive at 15000 and 10000 respectively ?
>
> [snip]
>
> Good observation.
>
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
> fqos_c/fqcprt4/qcfpolsh.htm#1000977
>
> The above reference on the Doc CD gives the CAR formula as:
> <quote>
> Recommended Burst Values
> 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
> </quote>
>
> Here are the calculations (commas added for readability - my apologies
> to Europeans and others who don't use commas to break up the thousands
> ;-)
>
> configured rate = 30,000,000 bps
> normal burst = 30000000/8 * 1.5 = 5,625,000 bytes
> extended burst = 2 * 5625000 = 11,250,000 bytes
>
> configured rate = 10,000,000 bps
> normal burst = 10000000/8 * 1.5 = 1,875,000 bytes
> extended burst = 2 * 1875000 = 3,750,000 bytes
>
> Vegesna is obviously *not* using this formula. Maybe there is a
> saturation point or ceiling for burst size? (This is no more than a
> SWAG.)
>
> Perhaps the "experts" will chime in ...
>
> Jonathan
>
>
>
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