BGP communities attached to prefix issue

[Ondrej Zajicek]

On Thu, Jun 30, 2016 at 04:28:00PM +0300, Javor Kliachev wrote:
> Hello,
> 
> We use BIRD as route server with the well-known "BGP Community policy based
> filtering" as in most of all other IXP with the following type:
> 
...
> But recently we came across the following case:
> 
> A member of our IXP is sending his prefixes with attached a huge count of
> BGP communities ( ~ 750 pcs )
> 
> Each of the attached community has following type: 0:ASN ( peer-as ).
> 
> We see that BIRD accepts these prefixes from member but it return the
> following error notification:
> 
> 2016-06-30 10:06:59 <ERR> R0_248: Attribute list too long, skipping


Hi

By RFC 4271, BGP message is limited to 4 kB of length. BIRD internally
limits attribute part of a transmitted message to 2 kB. This is
equivalent of 1024 (4 kB) or 512 (2 kB) communities.

You could replace constants 3072 [1] and 2048 [2] with say 3840 and 3584,
that would allow 3.5 kB of attributes (~ 900 communities).

[1] proto/bgp/packets.c:372 and proto/bgp/packets.c:465
[2] proto/bgp/packets.c:385 and proto/bgp/packets.c:481


Or, as these communities are destinated for your IXP, you could simply
strip 0:* and IPX_AS:* communities before prefixes are forwarded to
your clients. This is probably a good idea anyways as having too large
attribute list may cause problems later.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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