Re: LAG/ECMP and 'exact-route'

James Bensley via NANOG <[email protected]> Mon, 22 Jun 2026 15:49:15 +0000

Newsgroups	gmane.org.operators.nanog
Message-ID	<osk3qYULu8WJrK2YtBdxELa3MqGlKt-o4aNTC3CnC7ckTkVfNDgF2-VBkBw5ebdjBTs1fZBvt7vzchg0gTo6vUkKllQ_PacfIKTx5AVjHoI=@bensley.me>

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.

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