The outgoing port is to a shared Ethernet in the scenario, as stated
in the original message. A collision could certainly occur. To
understand the issue, you have to think outside the box, so to speak.
;-) Remember Ethernet says listen before you send, but two stations
listening and hearing nothings and sending at the same time is
perfectly normal. It's csMA/CD afterall.
Bottom line: I got the info I needed about buffering. The
descriptions of cut-through that make it sound like the frame
disappears out the port as soon as the dest address is read are
oversimplified. Thanks for the info on the buffering techniques.
Priscilla
>Priscilla,
>I think we missing the issue of a switch. Remember that for a frame to move
>from one port to another...
>If it uses the backplane... the (MCP SAMBA or one of those memorization
>thingys for CLSC) arbitrators for the backplane.... allows that port access
>to the backplane... THEN sends a signal to the port(s) that is NOT to drop
>the frame... or something like that... every port has a HUGE buffer for
>incoming packets from the backplane that need to egress out the port..
>(small buffer for the ingress from the port) buy and large across the
>backplane no collision and at that point the egress on the port is already
>doing CSMA/CD... and jamming the port to prepare for transmission
> Late collisions can occur... but timing will not allow a normal collision
>
>If no backplane in some of the newer cards where it just passes from one
>side of the card to another ... I understand the same sequence...
>
>Geez I wish I had not sold my study guides for the CLSC...
>
>
>Lou Nelson, CCNP, CCDA
>
>
>----- Original Message -----
>From: "Priscilla Oppenheimer" <[EMAIL PROTECTED]>
>To: <[EMAIL PROTECTED]>
>Sent: Friday, June 16, 2000 10:09 PM
>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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--
__________________________________
Priscilla Oppenheimer
Phone 541-482-5685
Fax 541-488-1708
Web http://www.priscilla.com
___________________________________
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