Versatile Routing and Services with BGP: Understanding and Implementing BGP in SR-OS

Chapter 7

Multicast

Delivery of multicast traffic from one or more sources to potentially many receivers requires that the correct multicast forwarding state is established on all routers in the path from source to receiver. This includes performing packet replication where necessary (for example, receive traffic for group (S¹, G¹) on interface i, and forward that traffic on interface i¹ and i²). Primarily, Protocol Independent Multicast (PIM) has been used to create this multicast state, which, unlike some of its lesser-deployed predecessors, is abstract from the underlying protocols used to exchange unicast reachability. However, PIM needs that unicast reachability information to determine the Reverse Path Forwarding (RPF) interface toward the source (or next-hop toward the source), and this is where BGP first became important in multicast environments. In intra-AS environments this information could be obtained from the IGP, but in inter-AS environments this information was exchanged in BGP, and so the IPv4 Multicast Address Family was introduced.

As extensions for multicast in BGP-MPLS IP-VPNs were defined, originally using the draft-rosen architecture, BGP was again used as an Auto-Discovery mechanism for PIM neighbors of the same multicast domain. More recent developments in Multicast VPN technology have subsumed the draft-rosen architecture as a subset of its capabilities and have extended the role of BGP so that it can be used to create multicast forwarding state, thereby ...