[c-nsp] Rant: ASR1000 MPLS (not) load-balancing

adamv0025 at netconsultings.com adamv0025 at netconsultings.com
Fri Jan 3 06:30:37 EST 2020

> From: Saku Ytti <saku at ytti.fi>
> Sent: Thursday, January 2, 2020 4:07 PM
> On Thu, 2 Jan 2020 at 17:42, Robert Raszuk <robert at raszuk.net> wrote:
> > First most of the ASICs or NPs you get do just fine LEM or LPM at their line
> rate. And while processing wise they may be more cycles to do LPM your
> invoice amount for box is still the same :) I guess one vendor tried to do pure
> LEM box ... but you know the story there.
> No box does line rate, there is no market for it and it's expensive.
> Only way to do line rate is to stop using half or more of the ports.
Well there are a few routers/line-cards that can deliver line-rate with a following disclaimer, 

On run to completion NPUs there's really no such thing as just line-rate (it's always line-rate with what combination and scale of features enabled).
One can make any run to completion NPU fail miserably revealing interesting facts about the NPU or whole box design choices.
So the "line-rate-ness" is just a good indication of how much headroom one has in terms of enabling additional features while expecting a certain throughput -which with advent of 100G very little line-card variants deliver good headroom for additional features (with "additional" I mean above basic IP/MPLS forwarding).   

On pipeline NPUs there is such a thing as line-rate.
There’s a max time (number of clock ticks) the packet can spend in the pipeline  - and if my u-code is too complex that the packet would spend more, then the compiler won’t allow me to compile such a u-code.
That's why they claim you get line-rate for any combination of features on these chips. (obviously there's a limited number of supported feature combinations compared to run to completion chips where one get carte la blanche in terms of time the packet can spend in the assigned packet processing engine).

But there's a big HOWEVER, 
Line-rate is a very ambiguous term these days, so the question is what packet size ergo packet rate did they use as line-rate baseline.
In my book it's lowest possible packet size 64B at L2 hence fastest possible packet rate is line-rate, but some conveniently use 128B (half the pps rate compared to 64B) and even higher. 

And of course in both cases it's about how many ports a vendor hooks up to an NPU  -and as Saku mentioned they tend to overdo it in majority of the cases and that's perfectly fine as we need holes first and foremost the more the better on the bases that all these pipes will be half empty anyways.
Unfortunately the modern licensing is screwing us big time in this regard.

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