Hello,

I'm reading about some network switch and most of them work only on store-and-forward mode, like the linux kernel for example.

There are some switch that work also on cut-through mode, that for larger ethernet networks really is more efficient.

At buffer level In store-and-forward NICs only trigger an interrupt after the packet has been completely DMAed into memory.

But how cut-through is implement at buffer level?

Someone knows if it is possible to implement cut-through on today's linux kernel?

I don´t know if something like that was been already implemented.

Thanks in advanced,
Regards,

store and forward check each packet to verify it's destination
cut-through, checks the first packet and thereafter something like the first 64 bytes of the packet,
if the headier information is the same, same source, same destination, source port, dest port,
or considered part of the same session, the packets are sent on their way without further a due.
or something to that effect.

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"If you're good at anticipating the human mind. It leaves nothing to chance."
-Jigsaw

That was useful in the old days, but don't bother now. I'd be surprised if you gained more than 100µs. Also, every NIC these days does store-and-forward in hardware (except virtual machines), so it may very well be that cut-through in software results in a slower connection.

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You're thinking of fast switching in your description (e.g. vs CEF).

The simplest description of cut-through is a switch that starts spewing bits egress as soon as it has figured out where to spew them to (I.e. after receiving the first 6B of the frame and finding the dest Mac).

There are variations on this. Fragment free is a variation which doesn't begin forwarding until a sufficient period has elapsed in which a receiving station in the collision domain could have sensed a collision and responded with a jam signal. If a jam signal isn't received in that time period, then it begins forwarding. Some switches may also want to listen past the dest Mac so they can extract other details from the frame than just the dest Mac, which the switch may need for other features.

back to CCNA school for me....

_________________
"If you're good at anticipating the human mind. It leaves nothing to chance."
-Jigsaw