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
>
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