Limit of numbers of BGP communities

[Ponikierski, Grzegorz]

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?

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

Regards,
Grzegorz

From: Ondrej Zajicek <santiago at crfreenet.org>
Date: Friday, 12 July 2024 at 05:40
To: "Ponikierski, Grzegorz" <gponikie at akamai.com>
Cc: "bird-users at network.cz" <bird-users at network.cz>
Subject: Re: Limit of numbers of BGP communities

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<mailto:santiago at crfreenet.org>)
"To err is human -- to blame it on a computer is even more so."

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