MPLS-CE Connected to PE
The VPC-SI functions as an MPLS-CE (Customer Edge) network element connected to a Provider Edge (PE) Label Edge Router (LER), which in turn connects to the MPLS core as per RFC 4364.
The following figure illustrates the MPLS-CE to PE connection:

The MPLS-CE functions like a PE router within its own Autonomous System (AS). It maintains Virtual Routing and Forwarding (VRF) routes and exchanges VPN route information with the PE through an MP-eBGP (Multi Protocol external BGP) session.
The PE is also configured with VRFs and exchanges VPN routes with other PEs in its AS through MP-iBGP (Multi Protocol internal BGP) connection and MPLS-CE through an MP-eBGP connection.
The EBGP connection allows the PE to change next-hop IP addresses and labels in the routes learnt from IBGP peers before advertising them to the MPLS-CE. The MPLS-CE uses only MP-eBGP to advertise and learn routes. Label Distribution Protocol (LDP) and Resource Reservation Protocol (RSVP) are not required because of direct-connect EBGP peering. The MPLS-CE pushes or pops a single label (learnt over the MP-eBGP connection) to or from the PE.