The 1900 and 2820 Catalyst switches do cut-through. I did find a good white
paper at:
http://www.cisco.com/warp/public/cc/cisco/mkt/switch/cat/c1928/tech/nwgen_wp.htm
But it never comes right out and says what happens upon a collision when
forwarding. I can guess based on the architecture, but I'm looking for some
more definitive answers to prove my point with the other instructor. ;-)
Priscilla
At 04:30 PM 6/19/00, Daniel Cotts wrote:
>Are any current Cisco switches using cut through? The 5000s use Store and
>Forward. The old Kalpana switches aka Catalyst 3000 used cut through.
>Looking at an old EtherSwitch PRO16 manual (same same cat3k) it mentions
>on-board buffering. "If the destination port is receiving a packet from
>another EtherSwitch PRO16 port or if the output segment is busy, the
>EtherSwitch PRO16 stores the packet in one of its on-board buffers. Each
>EtherSwitch PRO16 buffer can hold up to 384 packets in each direction
>(incoming and outgoing). This helps balance throughput when networks are
>operating near peak load and more than one packet may be directed to the
>same port at the same time."
>Hope this helps.
> > -----Original Message-----
> > From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED]]
> > Sent: Monday, June 19, 2000 3:47 PM
> > To: [EMAIL PROTECTED]
> > Subject: collision on cut-through switch
> >
> >
> > Hi Group Study,
> >
> > 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?
> >
> > Thanks
> >
> > Priscilla
> >
> > ________________________
> >
> > Priscilla Oppenheimer
> > http://www.priscilla.com
> >
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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