On Nov 9, 8:23am, Priscilla Oppenheimer wrote:
}
} 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.
I would be really surprised if it worked this way. If it did,
then you logically you would have an overpriced hub, not a switch and
there would be no point in using a switch (ignoring the fact that a
switch can extend the radius of the network). This simply doesn't make
sense. Also, with full-duplex ethernet, there would be no way to send
back a jam signal, so the only option would be to just drop the
packet. Personally, if I paid good money for a switch and it behaved
in the described manner, I would be quite perturbed.
} 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.
Personally, I would expect that the frame would simply be buffered
and not transmitted if the destination port is busy, so that no
collision occurs. That is, after all, the whole purpose of having a
switch in the first place.
} Who do you think is right? Can you point me to any white papers that would
} prove who is right?
Given the equipment needed to teach a sniffing class, I would expect
that you and/or the other instructor could easily setup an experiment to
test the theory. I would be interested in the results, but don't have an
easy way to test it at the moment.
}-- End of excerpt from Priscilla Oppenheimer
___________________________________
UPDATED Posting Guidelines: http://www.groupstudy.com/list/guide.html
FAQ, list archives, and subscription info: http://www.groupstudy.com
Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
