Multicast VPN implementation involves the following processes:
Share-MDT establishment on the public network
After multicast VPN is configured, a Share-MDT is automatically established. VPN multicast packets are transmitted along this Share-MDT.
Encapsulation and decapsulation of VPN multicast packets on PE devices
Upon receiving VPN multicast packets from a CE device, a PE device uses GRE to encapsulate the packets into public network data packets and forwards the encapsulated packet along the Share-MDT. Other PE devices decapsulate the packets and determine whether to forward these packets to connected CE devices.
Reverse path forwarding (RPF) checks on PE devices
For CE and P devices, the RFP check mechanism does not change after multicast VPN is configured. That is, the RPF interface is the outbound interface of the unicast route to the multicast source address, and the RPF neighbor is the next hop of the unicast route. For PE devices, the RPF check mechanism used in the public network instance remains the same after multicast VPN is configured. In a VPN instance, the RPF interface and neighbor are defined based on the outbound interface type of the unicast route to the source address.
VPN multicast packet transmission on the public network
The transmission procedure of VPN multicast packets varies depending on the PIM protocol used on the public network.
Switch-MDT switchover
When multicast data packets sent from a multicast source are transmitted along a Share-MDT, all PE devices receive these packets regardless of whether there are receivers at the connected sites. When a Switch-MDT is used, only PE devices with receivers attached can receive data packets from the multicast source. This lowers burdens on PE devices.