AS4 at BGP4 session open?
laurent at guerby.net
Sat Jun 11 11:57:54 CEST 2011
One of our BPG4 peer is asking us to send directly an AS4 open message
instead of going through transitional AS23456. We run a 32 bits ASN
and our peer has a 16 bits ASN.
If my reading of documentation and BIRD source code (1.2.5 and 1.3.1) is
correct BIRD only starts using AS4 if the peer advertise the AS4
capability in its open message.
p->as4_session = conn->want_as4_support && conn->peer_as4_support;
conn->want_as4_support = conn->bgp->cf->enable_as4 && (conn->start_state != BSS_CONNECT_NOCAP);
conn->peer_as4_support = 0; // Default value, possibly changed by receiving capability.
case 65: /* AS4 capability, RFC 4893 */
if (cl != 4)
conn->peer_as4_support = 1;
Our peer open doesn't currently send the AS4 capability in its open
message so we're stuck for now.
I don't know if hardware and software BGP4 implementations other than
BIRD have this capability and what are the true implications of this
We're using successfully so far BIRD 1.2.5 on debian squeeze to connect
to our transit providers, IX route servers, and peers, thanks for
providing BIRD as free software.
More information about the Bird-users