bird 3.2.0 - Malformed AS_PATH attribute - AS_SET segment (1) at 26

[Maria Matejka]

Hello Benjamin,

On Wed, Feb 04, 2026 at 03:52:27PM -0500, Benjamin LaHaise wrote:

> With Fedora 43's bird 3.2.0-1 rpm installed on an x86_64 system, I'm
> seeing that several routes are being dropped from a full feed with the log
> messages below.  Using Fedora's bird 3.1.4-1 does not seem to have any
> issues with the same BGP config and peer.  Is this a known bug?  What
> additional information can I provide to aid in the debugging?  This is a
> backup transit connection that is not yet in full production use (but will
> be soon), so I can help with debugging and testing for now.

> Feb  4 14:42:49 router bird[24317]: upstream: Malformed AS_PATH attribute - AS_SET segment (1) at 14
> Feb  4 14:42:49 router bird[24317]: upstream: Invalid route 76.165.0.0/16 withdrawn

This is a known feature, mandated by RFC 9774, Section 3:

> Unless explicitly configured by a network operator to do otherwise (e.g., during a transition phase), BGP speakers:
> 
> - MUST NOT advertise BGP UPDATE messages containing AS_SETs or AS_CONFED_SETs and
> - MUST use the "treat-as-withdraw" error handling behavior per [RFC7606] upon reception of BGP UPDATE messages containing AS_SETs or AS_CONFED_SETs in the AS_PATH or AS4_PATH [RFC6793].Per the above specifications, this document updates [RFC4271] and [RFC5065] by deprecating AS_SET (see [RFC4271], Section 4.3) and AS_CONFED_SET (see [RFC5065], Section 3), respectively.

It is still possible to `allow as sets` explicitly, if you really need
to keep them.

Maria

-- 
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.
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