BGP communities attached to prefix issue
santiago at crfreenet.org
Mon Jul 4 12:02:59 CEST 2016
On Thu, Jun 30, 2016 at 04:28:00PM +0300, Javor Kliachev wrote:
> 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
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  and 2048  with say 3840 and 3584,
that would allow 3.5 kB of attributes (~ 900 communities).
 proto/bgp/packets.c:372 and proto/bgp/packets.c:465
 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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