From: Price, Jamie (jprice@xxxxxxxxxxx)
Date: Sat Jul 22 2000 - 23:17:35 GMT-3
Title: RE: cut-through or store-and-forward
I was under the belief that Cat's use cut-through until the error
rates reach a certain level (I dont know what the predefined level is)
and at that point they cut over to store-and-forward. They remain in
store-and-forward until error rates drop below the cutover level at
which point the catalyst switches (no pun intended) back to
cut-through.
I believe (I will look into this more and get back to everyone) that
the error level that causes the switch from cut through to
store-and-forward is configurable. Then again I could be dreaming :)
As I said I'll check into it more and get back to you.
Jamie
-----Original Message-----
From: mark salmon
To: Ccielab
Sent: 7/22/00 10:29 PM
Subject: Re: cut-through or store-and-forward
Excellent explanation but a bit misleading. The processor you are
referring to is an ASIC not a central CPU and it is on the SUP card.
THe 6xxx uses a crossbar matrix so it eliminates the problem you
mentioned.
"Jason T. Rohm" wrote:
>
> I can't speed for the others specifically, but the 5xxx series (and
> presumably the others) use a hybrid switching type.
>
> Explain:
>
> A pure cut through switch switches all packets w/o error checking
(other
> than runt).
> Pure Store-and-forward, copies the entire frame to the switch
processor,
> checks it out, then copies the frame to the outbound port(s).
>
> A catalyst 5xxx, will immediately copy the frame to ALL active ports
and the
> switch processor. The switch processor will check the frame for
errors
and
> destination, then inform all the non-destination ports to drop the
frame
> from their send queues.
>
> The result is a hybrid switching type that gives you all the
benefits
of a
> pure store and forward, but incurs less switching delay because the
second
> frame copy is eliminated.
>
> The disadvantage of this method is that it requires a lot of
back-plane
> bandwidth and large buffers on the individual ports.
>
> -Jason T. Rohm
> jtrohm@athenet.net
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf
Of
> alfred zhang
> Sent: Saturday, July 22, 2000 3:25 AM
> To: ccielab@groupstudy.com
> Subject: cut-through or store-and-forward
>
> Hi
>
> Can anybody help me?Does a Catalyst switch(for example
> 6509,4003,2948G) use cut-through switching by default and
> store-and-forward based on errors?
>
> Thank you and best regards
> alfred
>
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