Not a white paper, but let's see whether *I* understand
how it's supposed to work <dons flameproof underwear ;->
Cut-thru waits for the dest addy, then starts forwarding.
Frag-free waits for 64bytes, then starts forwarding.
Store'n'forward waits for the whole packet and CRC's it,
so will only forward truly valid (tm) packets.
Sooo... cut-thru will forward runts, and you'll get
those if there's a collision after the dest addy but
within the first 64.
Frag-free will /not/ forward runts, so will protect
you from collisions that really should have happened.
It won't protect you from 'late collisions' (that
shouldn't have happened), but you're gonna fix your
cable plant to remedy those (aren't you? :-)
Or am I lying ? :-)
(big quote for context, below; sorry)
Tone
> From: Barry Hofland [mailto:[EMAIL PROTECTED]]
> Hi, Priscilla,
>
> I think the best way to find this out is to just get yourself
> a network
> tester to connect between the transmitting host on port 1 and
> the CISCO box.
> ( like a FLUKE ) You will be able to see ( or not ) the JAMS
> on port 1. When
> you connect the host in that segment only and there's a jam during a
> collision on port 2 you know enough...
>
> In my humble opinion you are right, but that's male intuition ;-)
> Priscilla Oppenheimer <[EMAIL PROTECTED]> wrote in message
> [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > I got into a discussion with a knowledgeable Sniffer
> instructor recently.
> > When he teaches cut-through-switching theory, he warns his
> students that a
> > cut-through switch does not really isolate collision
> domains. Consider this example:
> >
> > * The switch is receiving a frame from port 1 destined for
> a station out
> > port 2.
> > * The switch recognizes the destination address and starts
> forwarding the
> > frame to port 2 ASAP.
> > * There is a collision on port 2. (It's a shared and/or half-duplex
> Ethernet.)
> >
> > According to the instructor, the Switch sends a jam signal
> back to port 1
> > to let the initial sender know that the frame experienced a
> collision.
> This
> > allows the sender to retransmit.
> >
> > If you read some of the books on switching, you would think
> that this is
> > true. The books make it sound like the frame is passing
> through the switch
> > and disappearing out the destination port as soon as the destination
> > address is recognized.
> >
> > I don't think the Sniffer instructor's conclusion is true,
> however. I
> > believe that a Cisco cut-through switch buffers the frame
> and hence has
> the
> > ability to retransmit. There is no requirement to send a jam to the
> > original sender because port 2 in our example retransmits
> after sensing
> the
> > collision.
> >
> > I believe that Cisco switches store frames, even when doing
> cut-through,
> > whereas the instructor assumed that the frame has passed
> through and out
> > the port and is no longer available for retransmission by
> the switch.
> >
> > Cisco positions cut-through as reducing delay, not reducing
> the need for
> > buffering, so I'm contending that I'm right.
> >
> > Who do you think is right? Can you point me to any white
> papers that would
> > prove who is right?
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