Re: Token Buckets-Maybe one for the QOS Gurus.

From: Paul Cosgrove (paul.cosgrove@heanet.ie)
Date: Tue Jul 08 2008 - 09:44:44 ART

Hi Si,

My understanding is that policing adds tokens when a new packet arrives,
whereas shaping adds tokens at regular time intervals. The following
link compares the two approaches:

http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a00800a3a25.shtml

Perhaps a different mechanism is used by router shaping because its use
of queues means that the output rate is not just based on the current
incoming traffic rate. Although different shaping options are available
on switches I would expect that token allocation is similar on those as
well.

Does the 3550/3560s material you are reading indicate that policing and
shaping use the same mechanism for token allocation on switches?

It maybe that some of the QoS docs suffer from over simplification, CAR
is explained a couple of different ways.

Paul.

CCIE3000 wrote:
> Hi Guys,
>
> Something silly but I wonder if anyone else has noticed this and have any
> idea why it is so.
>
> Crudely cut down versions:
>
> If I'm correct (please correct me if not). When reading about QOS with
> 3550/60's it mentions the token bucket. We are told that every time a frame
> hits the switch a token is ADDED to the bucket and these are leaked at the
> average traffic rate. If a frame will 'equate' to more tokens than can be
> FITTED in the bucket the drop/mark etc actions are taken.
>
> With QOS on the routers tokens are ADDED to the bucket at the average rate
> and when packets need to be sent, the equivalent amount of tokens are
> REMOVED from the bucket. If there aren't enough tokens available
> dropping/marking etc takes place.
>
> I think I'm right that both methods are doing the same thing but why the
> difference in the way that they achieve their goal?
> One adding as frames come in, one removing as packets are sent?
>
> If I've got completely the wrong end of the stick...sorry about that :)
>
> Thanks in advance,
>
> Si
>
>
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