Unfortunately, this number of 100 BGP communities has no basis, at least not yet, in the RFC.
It is just a recommendation (a very healthy one, by the way) from the NLNOG team.

As far as I remember, neither the maximum mask limit of /24 for IPv4 nor /48 for IPv6 in a BGP session are defined in RFC as either must or should.

It would be good if it were in the RFCs, but it is not.

Em sex., 12 de jul. de 2024 às 06:43, Jeroen Massar via Bird-users <bird-users@network.cz> escreveu:

> On 12 Jul 2024, at 04:30, Ponikierski, Grzegorz via Bird-users <bird-users@network.cz> wrote:
>
> Hello all!
> How many standard communities, extended communities and large communities, user can apply on a route using Bird? BGP UPDATE message has finite size so I guess that there must be a limit somewhere.

The limit is that ISPs are limiting it to a 100:

https://bgpfilterguide.nlnog.net/guides/many_communities/

Above that, it will be emptied completely in many networks.

Greets,
Jeroen