Re: LAG/ECMP and 'exact-route'

[Saku Ytti via NANOG <nanog () lists nanog org>]

Thanks Nitzan, that was what I was thinking, that is quite recent (to
me) and I suspect it is syntactical sugar for 'jsim'?

Unfortunately the 'show forwarding-options load-balance' doesn't allow
giving MPLS label stack to it which greatly limits utility for SP
networks.

Steinar, in your experience does the bundle-hash give correct results?
Is it actually injecting packets to ezchip/lightspeed and getting
results from the HW (cef exact-route is not doing this at least).

Thanks to Pedro Prado for sharing that Arista has a command for this,
and indeed in Arista like in Juniper packet is actually injected to
the hardware to get the result.


I think none of them allow giving MPLS stack though? So mostly useful
for cloudy people, not SP people. RFC5837 would more reliably give us
the correct answer.

On Thu, 14 Aug 2025 at 09:10, Nitzan Tzelniker via NANOG
<nanog () lists nanog org> wrote:
> For JUNOS I think that you are looking for
> user@lab> show forwarding-options load-balance ?
> Possible completions:
>   destination-address  Destination IP address
>   destination-port     Destination port
>   family               Layer 3 family
>   ingress-interface    Ingress Logical Interface
>   packet-dump          Raw packet dump in hex without '0x'
>   source-address       Source IP address
>   source-port          Source port
>   tos                  Type of Service field
>   transport-protocol   Transport layer protocol
>
> Nitzan
>
> On Tue, Aug 12, 2025 at 5:58 PM Saku Ytti via NANOG <nanog () lists nanog org>
> 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.
> >
> > --
> >   ++ytti
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