Support for L3VPN Inter AS Option C

Contrail Release 5.1 and later supports L3VPN inter AS Option C, which is used to interconnect multi-AS backbones as described in RFC 4364. Inter-AS option C uses BGP as the label distribution protocol. Labeled IPv4 routes are redistributed by eBGP between the neighboring autonomous systems. A new address family inet-labeled is added to maintain labeled unicast routes. This table is used for resolving L3VPN routes which are encapsulated using MPLS. Figure 1 shows the connectivity and the roles of different components in this architecture.

Figure 1: L3VPN Inter AS Option C Architecture

The controller maintains an eBGP session with the SDN-GW router and an iBGP session with the ASBR router. The controller exchanges labeled routes with the vRouters over XMPP. The vRouter uses MPLSoUDP or MPLSoGRE to reach the ASBR and encapsulates two labels within it - the inner VPN label and outer BGP-LU label. For the opposite direction, the vRouter advertises a labeled unicast route for its vhost address with a label 3 (implicit null), so the traffic from the ASBR delivered through the fabric to the vRouter over a UDP/GRE tunnel contains only the VPN label.