Re: LAG/ECMP and 'exact-route'

Arista does have a command that does exactly this (?)



*Pedro Martins Prado*
[email protected] / +353 83 036 1875 (FaceTime & WhatsApp)

On Mon 22 Jun 2026 at 16:49, James Bensley via NANOG <[email protected]>
wrote:

> On Tuesday, August 12th, 2025 at 16:59, Saku Ytti via NANOG <
> [email protected]> wrote:
>
> > Hey-o,
> >
>
> >
>
> > Which platform/software has a command to show which interface will be
> > used for forwarding with given keys?
> >
>
> > ASR9k has a cef exec-route, and I see references to this in c-nsp,
> > reddit and cisco.com forums, stressing how useful debugging tool it
> > has been. Despite it not actually working, since it's just RE
> > software, it doesn't talk to the EZchip/lightspeed, unless it has been
> > fixed in the past couple of years, certainly hasn't worked in the
> > timeline of various forums finding it useful.
> >
>
> > MX has 'jsim'
> https://www.juniper.net/documentation/en_US/day-one-books/TW_MX3D_PacketWalkthrough.pdf
> > which I think actually works, but it is quite involved. I have some
> > (false?) memory that I saw in some release note this being a bit more
> > productised into CLI command, but I'm failing to find anything to
> > support this memory.
> >
>
> > There is also RFC5837, which is actually implemented in QFX5k, but not
> > for TTL exceeded, we've opened ER to get it supported on MX and PTX
> > and for TTL exceeded. This RFC will allow programmatic platform
> > agnostic discovery of the actual interface used, without relying on
> > platform specific magic. So please do ask your vendors to implement
> > it.
>
>
> After much a-do I managed to get this working on Jericho2 + Arista EOS
> boxes by using the Broadcom debug tools i.e. craft any packet you want and
> inject it into the ASIC, and then capture the ASIC lookup result.
>
> These are my notes for version 1.0 so it's a bit rough, but works. I'm
> sure it can be improved upon:
> https://null.53bits.co.uk/index.php?page=7280r3-diag-packet-tx
>
> Cheers,
> James._______________________________________________
> NANOG mailing list
>
> https://lists.nanog.org/archives/list/[email protected]/message/YB5IW53XGT2HUCIETTTJER6T23TAPTGI/
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