[j-nsp] Cut through and buffer questions

[Saku Ytti]

On Thu, 18 Nov 2021 at 23:20, james list via juniper-nsp
<juniper-nsp at puck.nether.net> wrote:

> 1) is MX family switching by default in cut through or store and forward
> mode? I was not able to find a clear information

Store and forward.

> 2) is in general (on MX or QFX) jeopardizing the traffic the action to
> enable cut through or change buffer allocation?

I don't understand the question.

> I have some output discard on an interface (class best effort) and some UDP
> packets are lost hence I am tuning to find a solution.

I don't think how this relates to cut-through at all.

Cut-through works when ingress can start writing frame to egress while
still reading it, this is ~never the case in multistage ingress+egress
buffered devices. And even in devices where it is the case, it only
works if egress interface happens to be not serialising the packet at
that time, so the percentage of frames actually getting cut-through
behaviour in cut-through devices is low in typical applications,
applications where it is high likely could have been replaced by a
direct connection.
Modern multistage devices have low single digit microseconds internal
latency and nanoseconds jitter.  One microsecond is about 200m in
fiber, so that gives you the scale of how much distance you can reduce
by reducing the delay incurred by multistage device.

Now having said that, what actually is the problem. What are 'output
discards', which counter are you looking at? Have you modified QoS
configuration, can you share it? By default JNPR is 95% BE, 5% NC
(unlike Cisco, which is 100% BE, which I think is better default), and
buffer allocation is same, so if you are actually QoS tail-dropping in
default JNPR configuration, you're creating massive delays, because
the buffer allocation us huge and your problem is rather simply that
you're offering too much to the egress, and best you can do is reduce
buffer allocation to have lower collateral damage.

-- 
  ++ytti