400G forwarding - how does it work?

Saku Ytti saku at ytti.fi
Mon Jul 25 19:02:50 UTC 2022

On Mon, 25 Jul 2022 at 21:51, James Bensley <jwbensley+nanog at gmail.com> wrote:

> I have no frame of reference here, but in comparison to Gen 6 Trio of
> NP5, that seems very high to me (to the point where I assume I am
> wrong).

No you are right, FP has much much more PPEs than Trio.

For fair calculation, you compare how many lines FP has to PPEs in
Trio. Because in Trio single PPE handles entire packet, and all PPEs
run identical ucode, performing same work.

In FP each PPE in line has its own function, like first PPE in line
could be parsing the packet and extracting keys from it, second could
be doing ingressACL, 3rd ingressQoS, 4th ingress lookup and so forth.

Why choose this NP design instead of Trio design, I don't know. I
don't understand the upsides.

Downside is easy to understand, picture yourself as ucode developer,
and you get task to 'add this magic feature in the ucode'.
Implementing it in Trio seems trivial, add the code in ucode, rock on.
On FP, you might have to go 'aww shit, I need to do this before PPE5
but after PPE3 in the pipeline, but the instruction cost it adds isn't
in the budget that I have in the PPE4, crap, now I need to shuffle
around and figure out which PPE in line runs what function to keep the
PPS we promise to customer.

Let's look it from another vantage point, let's cook-up IPv6 header
with crapton of EH, in Trio, PPE keeps churning it out, taking long
time, but eventually it gets there or raises exception and gives up.
Every other PPE in the box is fully available to perform work.
Same thing in FP? You have HOLB, the PPEs in the line after thisPPE
are not doing anything and can't do anything, until the PPE before in
line is done.

Today Cisco and Juniper do 'proper' CoPP, that is, they do ingressACL
before and after lookup, before is normally needed for ingressACL but
after lookup ingressACL is needed for CoPP (we only know after lookup
if it is control-plane packet). Nokia doesn't do this at all, and I
bet they can't do it, because if they'd add it in the core where it
needs to be in line, total PPS would go down. as there is no budget
for additional ACL. Instead all control-plane packets from ingressFP
are sent to control plane FP, and inshallah we don't congest the
connection there or it.

> Cheers,
> James.


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