[c-nsp] Cisco N540-ACC-SYS ipv4 routes

[adamv0025 at netconsultings.com]

> From: Saku Ytti <saku at ytti.fi>
> Sent: Monday, July 13, 2020 11:06 AM
> 
> On Mon, 13 Jul 2020 at 12:42, <adamv0025 at netconsultings.com> wrote:
> 
> > On J2 you can pretty much enable all the bells and whistles you can
> > and still get the same pps rate out of it as with no features enabled.
> 
> I don't think this is strictly true, maybe it's true for several cases. But even
> without recirculation, I think you can take variable time through J2 and if you
> wanted, you could make it perform pathologically poor, it is flexible enough
> for that. You can do a lot of stuff on the lookup pipeline for the first 144B in
> J2. Then there is the programmable elements matrix for future functions, can
> I put every frame there with no cost? You can use system headers to skip
> devices inside the pipeline.
> It's hard for me to imagine with all this flexibility we'd still always guarantee
> constant time.
> 
> J2 is much more flexible than what J1 was. But of course it's still very much a
> pipeline of different types of silicons, some more some less programmable.
> So I don't know.
> 
I think the idea behind it is that the whole thing is clocked to offer "line-rate" (whatever the vendor deems as line-rate -my definition is per 10Gbps at L2(64B)@L1(84B) is 29.761Mpps(two directions) )
So if you guarantee that no matter what features operator enables, the packet headers will leave the pipeline in frequency no less that << line-rate >>, you have an ASIC. 
Also since the feature set is very narrow on these things (compared to NPUs) the deviation from the mean is probably very low.
And of course then there's the recirc. - but that's usually a known caveat. 

adam