How to avoid late collisions ?
thanks
Anil
On Tue, 20 Jun 2000, Heskett, Tony wrote:
>
> 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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