AS4 at BGP4 session open?

[Laurent GUERBY]

On Wed, 2011-06-15 at 08:10 +0200, Laurent GUERBY wrote:
> On Wed, 2011-06-15 at 01:06 +0200, Ondrej Zajicek wrote:
> > On Sat, Jun 11, 2011 at 11:57:54AM +0200, Laurent GUERBY wrote:
> > > Hi,
> > > 
> > > 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)
> > > 	    goto err;
> > > 	  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
> > > request.
> > 
> > I am not sure what exactly is requested. 32 bit ASNs cannot be sent
> > directly in an OPEN message in its ASN field, they are always sent as a
> > part of an AS4 capability. You could check that (that an AS4 capability
> > is attached in your OPEN message) using tcpdump. If your peer does not
> > send AS4 capability that probably means (unless it is misconfigured)
> > that it does not support AS4 and you cannot establish AS4-style session,
> > but old-style BGP session should be established.
> 
> Hi,
> 
> Thanks for the clarification, I reached the same conclusion reading
> the RFCs. tcpdump shows that BIRD is sending OPEN with the AS4
> capability so I will contact our peer again.

Hi,

FYI our peer has now enabled AS4 support and all works fine with BIRD.

Sincerely,

Laurent