A late collision usually occurs when one end of a switched ethernet link
is set to full duplex but the other end of the link is set to half-duplex.
Darren
On Thu, 22 Jun 2000, ANIL.YADAV wrote:
>
>
>
> 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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