Limit of numbers of BGP communities

[Ondrej Zajicek]

On Fri, Jul 12, 2024 at 02:30:10AM +0000, Ponikierski, Grzegorz via Bird-users 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.

Hello

There is no specific limit on communities, just a limit on overall BGP
attribute length on export, which is 1 kB less than BGP message size
(there is 1 kB reserved for prefixes). So with regular BGP messages,
there can be at most 3 kB of attributes, which is ~768 standard, ~384
extended, or ~256 large communities. In fact, it is a slightly less due
to attribute framing and presence of other attributes than communities.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."