A service provider determines whether a particular VPN is multicast-enabled. If it is, it corresponds to a "multicast domain". A PE that attaches to a particular multicast-enabled VPN is said to belong to the corresponding multicast domain. For each multicast domain, there is a default "Multicast Distribution Tree (MDT)" through the backbone, connecting all of the PEs that belong to that multicast domain. A given PE may be in as many multicast domains as there are VPNs attached to that PE. However, each multicast domain has its own MDT. The MDTs are created by running PIM in the backbone, and in general an MDT also includes P routers on the paths between the PE routers.
The default MDT for a multicast domain is constructed automatically as the PEs in the domain come up. Construction of the default MDT does not depend on the existence of multicast traffic in the domain; it will exist before any such multicast traffic is seen. Default MDTs correspond to the "MI-PMSIs" of MVPN.
In MVPNs, each CE router is a unicast routing adjacency of a PE router, but CE routers at different sites do not become unicast routing adjacencies of each other. This important characteristic is retained for multicast routing—a CE router becomes a PIM adjacency of a PE router, but CE routers at different sites do not become PIM adjacencies of each other. Multicast packets from within a VPN are received from a CE router by an ingress PE router. The ingress PE encapsulates the multicast packets and forwards them along the default MDT tree to all the PE routers connected to sites of the given VPN. Every PE router attached to a site of the given VPN thus receives all multicast packets from within that VPN. If a particular PE routers is not on the path to any receiver of that multicast group, the PE simply discards that packet.
If a large amount of traffic is being sent to a particular multicast group, but that group does not have receivers at all the VPN sites, it may not be needed to forward that group's traffic along the default MDT. Therefore, a method is specified for establishing individual MDTs for specific multicast groups called as "Data MDTs". A data MDT delivers VPN data traffic for a particular multicast group only to those PE routers that are on the path to receivers of that multicast group. Using a data MDT has the benefit of reducing the amount of multicast traffic on the backbone, as well reducing the load on some of the PEs; it has the disadvantage of increasing the amount of state that must be maintained by the P routers. The service provider has complete control over this tradeoff. Data MDTs correspond to the S-PMSIs of MVPN.
An enterprise that uses PIM-based multicasting in its network can migrate from a private network to a BGP/MPLS IP VPN service, while continuing to use whatever multicast router configurations it was previously using; no changes need be made to CE routers or to other routers at customer sites. For instance, any dynamic RP-discovery procedures that are already in use may be left in place.
The BGP MVPN signaling mechanism does not use the MDT tunnel for signaling. The BGP signaling separates the control-plane and the data-plane for MVPN multicast c-route exchange. The MVPN BGP C-Route Full SM Support feature helps the service providers to leverage the existing BGP operation experience for deploying MVPN services.
For more details see Cisco Systems' Solution for Multicast in MPLS/BGP IP VPNs.