[j-nsp] Cut through and buffer questions

[Saku Ytti]

On Fri, 19 Nov 2021 at 17:12, Thomas Bellman via juniper-nsp
<juniper-nsp at puck.nether.net> wrote:

> Cut-through actually *can* help a little bit.  The buffer space in
> the Trident and Tomahawk chips is mostly shared between all ports;
> only a small portion of it is dedicated per port[1].  If you have
> lots of traffic on some ports, with little or no congestion,
> enabling cut-through will leave more buffer space available for
> the congested ports, as the packets will leave the switch/router
> quicker.

Correct, you can save packetSize * egressInts of buffer with
cut-through. So if you have 48 ports and we assume 1500B frames, you
can save 72kB of buffer space.

> One should note though that these chips will fall back to store-
> and-forward if the ingress port and egress port run at different

I had hoped this was obvious, when I mentioned the percentage of
frames getting cut-through. And strictly speaking, it is not 'these
chips', you cannot implement cut-through without store-and-forward.
You'd end up dropping most of the traffic in all but very esoteric
topology/scenario.

--
  ++ytti