I was playing with the eBGP SDN fabric and noticed that there was no mention of peering with stuff that isn’t PVE nodes. Say if I wanted to announce a default route from a different router, how would I do that? All I really control is the individual nodes ASN not any details of the peering relationships.
It is utilizing BGP unnumbered, which is BGP but with a few underlying assumptions. BGP unnumbered assumes that every interface added is a point-to-point link and eBGP. So peering happens via the specified interfaces, with one peer per interface. It automatically establishes peering relationships by utilizing the link-local address of the interface. So you'd peer with external nodes by discovering them on the specified interfaces - analogous to how it works in routing protocols like OSPF / OpenFabric.