Multicast allows information to be efficiently distributed between a single multicast source and many receivers. Multicast has many uses, including financial applications, software downloads, and audio and video streaming.
Initially, MPLS VPNs did not support IP multicast traffic. In early deployments, support for multicast traffic was provided through GRE tunnels. GRE tunnels were built between CE routers, and all multicast traffic between VPN sites was encapsulated using GRE. However, in this scenario, optimal multicast routing requires a full mesh of GRE tunnels, which is not scalable or manageable with a large number of VPN sites.
mVPN allows service providers to offer multicast services in a very similar manner to the way they offer MPLS VPN services for unicast. In an MPLS VPN environment, the P routers maintain routing information and labels for the global routing table only; they do not maintain any VPN information. The CE routers maintain a routing relationship with the PE router. The CE router does not peer with other CE routers but it can still, indirectly, exchange routing information with other CE routers
http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/L3VPNCon.html#wp309778
