AS4 at BGP4 session open?

Ondrej Zajicek santiago at
Wed Jun 15 01:06:51 CEST 2011

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.

Elen sila lumenn' omentielvo

Ondrej 'SanTiago' Zajicek (email: santiago at
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3,
"To err is human -- to blame it on a computer is even more so."
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