Limit of numbers of BGP communities

[Ondrej Zajicek]

On Fri, Jul 12, 2024 at 07:34:28AM +0000, Ponikierski, Grzegorz wrote:
> It assumes that Extended Length bit of Attribute Type is properly implemented so community attribute can use attribute length field as 2 octet field, right?

Yes, that is a standard part of BGP.

> How situation changes with RFC 8654<http://www.rfc-editor.org/info/rfc8654> - Extended Message Support for BGP?

With that enabled on both sides, there is still 1 kB reserved for
prefixes, so you can have 63 kB for attributes, i.e. ~16128 standard,
~8064 extended, or ~5376 large communities. But propagation of such
route will break on the first non-RFC8654 session.

> 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."