From: Chris Lewis (chrlewiscsco@gmail.com)
Date: Tue May 16 2006 - 12:17:10 ART
Hi Petr and Julius,
I believe the link quoted by Petr is not a very well written piece of Cisco
documentation.
As far as policing goes, tokens get added to a bucket and tokens get taken
out of a bucket, whether a packet gets passed or policed depends on whether
there are enough tokens.
Policing works on the formula of adding tokens based upon the following
CIR*(t-t1) where t is the time a packet arrived at and t1 is the time the
previous packet arrived. Tokens are added and subtracted to the bucket and
the decision to pass or police a packet is all taken at the same time.
What the tech note referenced says is that the decision to remove tokens
from the bucket happens 8000 times a second and you can't change it. This is
really an irrelevant piece of information. If it is correct, all it is
saying is that there are 8000 times a second when the decision to pass or
police a packet is taken. This does not affect the calculation for whether
the packet is policed or not, just when that decision is executed.
The other unfortunate thing is that it quotes numbers that are reminiscent
of the shaping algorithm, however the figure quoted here of 125 is actually
.125 msec, not the 125msec used in common shaping situations.
Chris
On 5/16/06, Julius Kinsler <jkinsler@harbortech.com> wrote:
>
> This is the same article I was reading yesterday. It so happen to be that
> I was trying to do policing on a 3550.
> For example I created a policy-map and under the policy map, for the
> default class I put in the keyword
> police 1000000 <Normal Burst bytes> exceed action drop. I didnt completely
> understand the normal burst bytes I just wanted to police at 1Mbps.
> When I looked it up I came across the link below and was trying to
> interpret this Interval to come up with the normal burst bytes. I came to
> believe that this can be an arbitrary number based on the specifications in
> a practice lab.
> I was doing an IPExpert lab and the answer looked like this:
>
> mls qos
> policy-map MyPolice
> class class-default
> police 1000000 187500 exceed drop
>
> Now I was racking my brain trying to come up with the logic behind the
> number "187500" but I believe the number was made up after everything I
read
> about rate/intervals/and burst as stated below.
> Please tell me if I am wrong
>
> Julius
>
> ------------------------------
> *From:* Petr Lapukhov [mailto:petrsoft@gmail.com]
> *Sent:* Tuesday, May 16, 2006 1:17 AM
> *To:* Chris Lewis
> *Cc:* Julius Kinsler; ccielab@groupstudy.com
> *Subject:* Re: Policing
>
>
> Chris,
>
> There is an interesting thing they say about 3550 policing:
>
>
>
http://www.cisco.com/en/US/products/hw/switches/ps646/products_tech_note09186
a00800feff5.shtml
>
> Specifically:
>
> ---- quote
>
> These parameters control the operation of policing:
>
> - Rate - defines how many tokens are removed at each interval. This
> effectively sets
> the policing rate. All traffic below the rate is considered in profile.
> Supported rates
> range from 8 Kbps to 2 Gbps, and increment by 8 Kbps.
>
> - Intervaldefines how often tokens are removed from the bucket. The
> interval is fixed
> at 0.125 milliseconds (or 8000 times per second). This interval cannot be
> changed.
>
> - Burstdefines the maximum amount of tokens the bucket can hold at any
> time.
> Supported bursts range from 8000 bytes to to 2000000 bytes, and increment
> by 64 bytes.
> ---- quote
>
> I wonder if they do actually use *leaky* bucket with 3550 policer and
> *token*
> bucket (metering) with CAR/IOS Policer..
>
> Petr
>
> 2006/5/16, Chris Lewis <chrlewiscsco@gmail.com>:
> >
> > Julius,
> >
> > You are mixing two concwpts here. There is no Tc in policing that
> > adheres to
> > the shaping formula quoted. Policing does not calculate things at
> > regular
> > intervals, it calculates tokens to be credited and removed from the
> > bucket
> > based off packet arrival times.
> >
> > Chris
> >
> >
> > On 5/15/06, Julius Kinsler <jkinsler@harbortech.com > wrote:
> > >
> > > Using the standard equation CIR = Bc / Tc where can I find the Tc when
> > > trying to complete this formula?
> > >
> > > TIA
> > > Julius
> > >
> > >
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