Understanding Source-Specific Multicast VPNs
A draft-rosen MVPN with service provider tunnels operating in SSM mode uses BGP signaling for autodiscovery of the PE routers. These MVPNs are also referred to as Draft Rosen 7.
Each PE sends an MDT subsequent address family identifier (MDT-SAFI) BGP network layer reachability information (NLRI) advertisement. The advertisement contains the following information:
- Route distinguisher
- Unicast address of the PE router to which the source site is attached (usually the loopback)
- Multicast group address
- Route target extended community attribute
Each remote PE router imports the MDT-SAFI advertisements from each of the other PE routers if the route target matches. Each PE router then joins the (S,G) tree rooted at each of the other PE routers.
After a PE router discovers the other PE routers, the source and group are bound to the VPN routing and forwarding (VRF) through the multicast tunnel de-encapsulation interface.
A draft-rosen MVPN with service provider tunnels operating in any-source multicast sparse-mode uses a shared tree and rendezvous point (RP) for autodiscovery of the PE routers. The PE that is the source of the multicast group encapsulates multicast data packets into a PIM register message and sends them by means of unicast to the RP router. The RP then builds a shortest-path tree (SPT) toward the source PE. The remote PE that acts as a receiver for the MDT multicast group sends (*,G) join messages toward the RP and joins the distribution tree for that group.
