[j-nsp] QFX5100 buffer allocation

Thomas Bellman bellman at nsc.liu.se
Thu May 17 02:58:36 EDT 2018

On 2018-05-17 02:41, Brian Rak wrote:

> We're not even doing 10gbit of traffic, so the buffers should last at
> least a little bit.

And you're not hitting 10 Gbit/s even under very short bursts of a few
milliseconds?  Microbursts like that don't show up in "normal" usage
graphs where you only poll your switches/routers every minute or so.

> Thanks for the tip about cut-through, we didn't have that enabled.
> Do you happen to know if it works from a 10g port to a broken out
> 4x10g port?

Should do.  From the perspective of the Trident II chip, they are
not any different from normal 10G ports.  Cut-through doesn't work
between ports of different speed, and the ports involved must not
have any rate limiting or shaping, but other than that I don't know
of any limitations.  (And if you receive broken packets, those will
be forwarded instead of thrown away; that is the only disadvantage
of cut-through mode that I have heard of.)

> It's annoying to be dropping packets with a bunch of unused buffer
> space.

Just make sure you don't fill your buffers so much that you get a long
(measured in time), standing queue, since that will just turn into a
long delay for the packets without helping anything (search for "buffer-
bloat" for mor information).  Not a big problem on Trident II-based
hardware, but if you have equipment that boasts about gigabytes of buffer
space, you may need to watch out.

Oh, and I believe both changing buffer allocation and enabling/disabling
cut-through mode resets the Trident chip, causing a short period (less
than one second, I belive) where traffic is lost.


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