GTM with BGP Multicast VPN (BGP-MVPN), as specified in RFC 7716, allows a Service Provider (SP) to use the same multicast architecture that was originally developed for VPNs to distribute multicast routing information that is not specific to VPNs. Instead of storing the routing information in VRFs, multicast routing information is maintained in a global table for the router.
The architecture can be logically divided into a core network and non-core (attachment) networks. The multicast routing protocol used in the core network may not be the same as the protocol used in the attachment networks. As there is a protocol boundary between the core and attachment networks, the term Protocol Boundary Router (PBR) refers to the core routers that are at the boundary. A PBR is not necessarily an edge router in the PE sense; however, a PBR in the SP network marks the border of any tunnels that are used to transport multicast traffic across the core network. Routers that are attached to the PBRs but that are not part of the core network are referred to as Attachment Routers (ARs). See Figure: GTM network topology example.
Multicast data traffic from an AR is tunneled through the core network from an ingress PBR to one or more egress PBRs, using multicast routing information stored in the PBR’s global table. The global table learns the PBR’s multicast routing information from the ARs attached to the PBR and distributes the information among the PBRs using BGP. PBRs use the same BGP-MVPN procedures used by PE routers to route multicast VPN traffic, with some adaptations to the procedures to use the global table instead of a VRF.
By using the BGP procedures designed for MVPN to support GTM, a single control plane is available to govern the use of both VPN and non-VPN multicast. The features and characteristics of MVPN carry over automatically to GTM, including, but not limited to:
scaling
aggregation
transport over RSVP tunnels in the SP network
support for non-segmented intra-autonomous systems (ASs) tunnels
support for PIM-SSM outside of the core
support for both IPv4 and IPv6 multicast flows over an IPv4 SP infrastructure
support for unsolicited flooded data (including support for BSR as an RP-to-group mapping protocol