Multicast VPN Implementation Modes

At present, multicast VPN can be implemented in multicast domain (MD) or BGP auto-discovery (A-D) mode.

The greatest advantage of MD VPN is that only Provider Edges (PEs) are required to support multi-instance. Customer Edges (CEs) and Providers (Ps) do not need to be upgraded, and the existing protocol independent multicast (PIM) configurations on CEs and Ps do not need to be modified, which means the MD VPN solution is transparent to CEs and Ps. However, in MD mode, multicast VPN services can be transmitted only over the public network tunnel based on the PIM-ASM MDT (PIM-ASM is short for protocol independent multicast - any-source multicast).

The advantage of BGP A-D multicast VPN is that peers in a multicast VPN can be discovered automatically. Therefore, multicast VPN services can be transmitted over the public network tunnel based on the PIM-SSM MDT (PIM-SSM is short for protocol independent multicast - source-specific multicast).

Fixed switches only support MD VPN.

MD VPN

Multicast VPN that is implemented in MD mode is called MD VPN.

In MD VPN implementation, all VPN instances on the PEs in an MD must join a same multicast group called the Share-Group.

A Share-MDT, short for Share-Multicast Distribution Tree, is set up when a private network PIM instance joins the Share-Group on each of the PEs. The Share-MDT transmits PIM protocol packets and data packets between PEs in the same VPN. A Share-Group address uniquely identifies a Share-MDT on a VPN.

VPN data is transparently transmitted through the multicast tunnel (MT) over the public network. A multicast tunnel interface (MTI) is configured on each PE to seamlessly connect to the MT. A VPN instance only knows that private network data is sent out from an MTI and will be received by another MTI on the remote end. As virtual interfaces, MTIs transmit data between private and public networks.

If the VPN spans several autonomous systems (ASs), VPN sites in different ASs need to be connected. In MD VPN, there are two connection modes:

VPN instance-to-VPN instance connection
Multi-hop External BGP (EBGP) connection

BGP A-D Multicast VPN

In BGP A-D multicast VPN implementation, Share-MDTs are also set up using Share-Groups. Different from MD VPN, BGP A-D multicast VPN does not require manual configuration of a public network RP. This is because BGP can automatically discover peers in a multicast VPN using its A-D function. Therefore, in BGP A-D mode, multicast VPN services can run on the public network tunnel based on the PIM-SSM MDT.