[Xorp-hackers] Peering problem with mixed 2- and 4-byte ASN

[Canini Marco]

Hello,

I believe I've hit a bug in the BGP implementation of Xorp but before reporting it through bugzilla I want to double check it here.
In essence, the problem lies in route advertisements propagation from a 2-byte ASN Xorp instance (old-bgp speaker) to a 4-byte ASN Xorp instance (new-bgp speaker).
The simplest way to see it is to configure 3 bgp speakers in the following way:
New-bgp (AS 1.0) +----+ Old-bgp (AS 2) +----+ New-bgp (AS 3.0)
The way Old-bgp needs to be configured involves two peerings with AS_TRANS (23456) because both AS 1.0 and 3.0 use the AS_TRANS in their open message.
Then, suppose that AS 1.0 advertises the prefix 10.100.0.0/16.
The route advertisement reaches AS 2 which installs it correctly in its routing table with AS_PATH = 23456.
However, AS 3.0 never receives this advertisement from AS 2.
In theory, AS 3.0 should get an update message from AS 2 with AS_PATH = 2 23456 and AS4_PATH = 1.0 but this is simply not happening.
I've tried to use Quagga as the old-bgp speaker and in this case AS 3.0 receives the update.
Unless I've missed some important configuration detail, I believe there is a bug in the way Xorp (doesn't) forward the advertisements for AS_TRANS peers.
Both AS 1.0 and 3.0 have enable-4byte-as-numbers: true while AS 2 doesn't.

I can provide more details if necessary.
Does it appear as a bug to you?
Thanks

Marco Canini, Ph.D.
EPFL, Networked Systems Laboratory