What Is Multicast VPN?

Multicast VPN (MVPN) is a networking architecture that enables IP multicast traffic to be delivered across a provider's MPLS or IP backbone to customers connected through Layer 3 Virtual Private Networks. Standard BGP/MPLS L3VPN services carry unicast traffic efficiently using label-switched paths, but extending multicast delivery to those VPNs requires additional mechanisms: a framework for establishing per-VPN multicast distribution trees over the shared provider core and a set of protocols for advertising group membership and source information between provider edge routers. MVPN addresses both requirements while preserving the traffic isolation and routing separation that make VPNs useful.

The architecture separates the customer's multicast topology from the provider's transport mechanism by introducing the concept of a Provider Multicast Service Interface (PMSI). The PMSI is a conceptual overlay on the provider network through which provider edge (PE) routers exchange multicast data and control information for a specific customer VPN. The actual data-plane tunnels that realize the PMSI are called P-tunnels and can use several underlying transport technologies.

MVPN Architecture and P-Tunnels

The foundational MVPN specification is defined in RFC 6513, Multicast in MPLS/BGP IP VPNs, published by the IETF in 2012. RFC 6513 defines two categories of PMSI: the Inclusive PMSI (I-PMSI), which carries traffic to all PE routers in a VPN by default, and the Selective PMSI (S-PMSI), which creates a separate tunnel for high-bandwidth sources to avoid flooding low-traffic PE routers with unwanted traffic. P-tunnels that realize these PMSIs include RSVP-TE point-to-multipoint label-switched paths, PIM trees built over the provider core, and Ingress Replication, which converts multicast at the ingress PE into separate unicast copies for each remote PE.

BGP Auto-Discovery and Signaling

BGP carries the control plane for MVPN, advertising the existence of each VPN's multicast topology and coordinating P-tunnel assignments without requiring operators to manually configure PE-to-PE associations. RFC 6514, BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs, specifies the BGP NLRI types and attributes used for MVPN auto-discovery, including the encodings that identify I-PMSI and S-PMSI tunnel assignments. A PE router that receives multicast traffic from a customer VPN site uses these advertisements to determine which P-tunnel to use and which remote PEs to reach. Source-active routes carry (S,G) state from PE routers that have active sources, allowing remote PEs with interested receivers to join the correct selective tree.

Extranet Multicast

MVPN supports scenarios in which multicast sources in one VPN must deliver traffic to receivers in a different VPN, a configuration known as extranet multicast. RFC 7900 extends the base MVPN architecture to handle this by specifying how PE routers advertise multicast sources and groups across VPN boundaries while maintaining the access control and routing separation that the VPN model requires. The RFC 7900 extranet multicast specification updates the BGP encoding procedures of RFC 6513 and RFC 6514 to support inter-VPN source reachability, allowing, for example, a centralized content source in one VPN to serve receivers in multiple separate customer VPNs over the same provider core.

Applications

Multicast VPN has applications across carrier and enterprise environments, including:

• IPTV and live video distribution over service provider managed VPN networks
• Financial market data delivery to geographically distributed trading firm sites
• Enterprise WAN multicast for video conferencing and collaborative applications
• Software update distribution across large multi-site enterprise networks
• Government and defense agency communications requiring traffic separation between organizations