[j-nsp] ex4500 best-effort drops nowhere near congested

[Michael Loftis]

On Thursday, May 2, 2013, Benny Amorsen wrote:

> joel jaeggli <joelja at bogus.com <javascript:;>> writes:
>
> > There's literally no options in between. so a 1/10Gb/s TOR like the
> > force10 s60 might have 2GB of shared packet buffer, while an like an
> > arista 7050s-64 would have 9MB for all the ports, assuming you run it
> > as all 10Gb/s rather than 100/1000/10000/40000 mixes of ports it can
> > cut-through-forward to every port which goes a long way toward
> > ameliorating your exposure to shallow buffers.
>
> Why does cut-through help so much? In theory it should save precisely
> one packets worth of memory, i.e. around 9kB per port. 500kB extra
> buffer for the whole 50-port switch does not seem like a lot.
>
> Lots of people say that cut-through helps prevents packet loss due to
> lack of buffer, so something more complicated must be happening.


My guess is it just hides the problem better...putting the burden of a
retransmit on the host NIC or similar by causing an old hub style collision.


>
>
> /Benny
>
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