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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