Unfortunately the 'show forwarding-options load-balance' doesn't allow giving MPLS label stack to it which greatly limits utility for SP networks.
I *think* that you can use the packet-dump option to paste a packet in hex and it will give the proper result with the label stack considered. It was a couple years ago I tried this, and my memory is fuzzy if it did work correctly or not. Even if it did work it's obvious clunky as hell to have to slap a hex decode in there. I did find a note that I asked for an ER to allow label IDs on the CLI, but I can't find anything further if they said yes/no. I'll ask. On Thu, Aug 14, 2025 at 2:26 AM Saku Ytti via NANOG <nanog@lists.nanog.org> wrote:
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_PacketWalk...
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.
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