Miroslav,

This is called a cut-through, and is not the default. The reason it isn’t the 
default is because it propagates invalid frames (runts, run-ons, crc errors, 
etc.) beyond the source port. Store and forward is the default. Cut-though 
generally doesn’t work with port speed mismatch and I’ve never seen it used 
with 100Mb. This really isn’t something you have to worry about. 

FWIW, assuming two 1Gb ports on the same switch, the one-way transmission time 
with cut-through would go from 1504ns to 800ns. However the round trip would 
still be symmetrical.

Denny


> On Nov 21, 2016, at 09:52, Miroslav Lichvar <mlich...@redhat.com> wrote:
> 
> As I understand it, modern switches don't wait until they have whole
> packet before forwading it to another port. They just need to wait for
> the part that contains the destination MAC address. So the total time
> should be closer to 7540 ns.

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