Gentle correction - late collisions occur after the first 64 bytes of a
frame have been placed on the wire.
Essentially, a station listens, perceives the wire as clear, starts sending,
only to find that another station has already begun to transmit.
Can happen for a couple of reasons. Network too long. Faulty hardware. Real
busy network, possibly with lots of very small packets ( as might happen in
a brokerage firm, where I used to see my fair share of late collisions. )
Interesting point about the duplex settings. Having made that mistake once
or twice, my recollection is that connectivity is not established, but then
memory may not serve on this one. Does that show up as 100% collisions?
Chuck
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
[EMAIL PROTECTED]
Sent: Wednesday, June 28, 2000 4:57 AM
To: ANIL.YADAV
Cc: Heskett, Tony; [EMAIL PROTECTED]
Subject: RE: collision on cut-through switch
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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