IETF RFC 7024 PDF

With BGP/MPLS Virtual Private Networks (VPNs), providing any-to-any connectivity among sites of a given VPN would require each Provider Edge (PE) router connected to one or more of these sites to hold all the routes of that VPN. The approach described in this document allows the VPN service provider to reduce the number of PE routers that have to maintain all these routes by requiring only a subset of these routers to maintain all these routes.

Furthermore, when PE routers use ingress replication to carry the multicast traffic of VPN customers, the approach described in this document may, under certain circumstances, reduce bandwidth inefficiency associated with ingress replication and redistribute the replication load among PE routers.