[RFC] Unnumbered BGP automatic peering based on IPv6 ND

[Molly Miller]

Hi Matteo,

There's some prior art in other routing implementations which you might
be interested in looking at. FRR already has similar functionality for
automatic peering via IPv6 link-local addresses, as does Nokia's SR
Linux platform (there may be others, but those are the ones I know).

FRR uses IPv6 router advertisements for peer discovery, by periodically
sending RAs to announce itself on a given interface and listening for
RAs from other hosts on the same link to discover other peers. BGP
sessions are then established via link-local addresses automatically
when a new peer on the link becomes known. Off the top of my head I'm
not sure how FRR handles established sessions with peers which stop
sending RAs (I'm on holiday and don't have access to my work notes right
now, I might have looked at that when I last worked on automatic peer
discovery in the lab).

This scheme in FRR appears to have been developed for uses cases like
routed datacentre fabrics with point-to-point connections between
routers rather than shared L2 segments where multiple routers are
present. I'm not sure if the implementation in FRR would handle multiple
peers on the same L2 correctly. Nokia SR Linux implements the same
scheme using IPv6 RAs and is compatible with FRR -- an FRR instance and
a Nokia device on opposite sides of the same link will recognise each
other's presence and attempt to establish sessions with each other.

As far as I'm aware this is a non-standard feature of FRR which has been
copied and reimplemented by at least Nokia, if not other vendors. It
would be practical if Bird were also compatible with this scheme of
course, though in an ideal world there would be an RFC specifying this
behaviour rather than implementers all reverse engineering FRR
independently.

Best wishes,
--mm.