You can use the Multicast Source Discovery Protocol (MSDP) to exchange multicast source information between multiple BGP-enabled Protocol Independent Multicast (PIM) sparse-mode domains. In addition, MSDP can be used to create an Anycast-RP configuration to provide RP redundancy and load sharing. For information about PIM, see Configuring PIM and PIM6. For information about BGP, see the Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide.
When a receiver joins a group that is transmitted by a source in another domain, the rendezvous point (RP) sends PIM join messages in the direction of the source to build a shortest path tree. The designated router (DR) sends packets on the source-tree within the source domain, which may travel through the RP in the source domain and along the branches of the source-tree to other domains. In domains where there are receivers, RPs in those domains can be on the source-tree. The peering relationship is conducted over a TCP connection.
The following figure shows four PIM domains. The connected RPs (routers) are called MSDP peers because they are exchanging active source information with each other. Each MSDP peer advertises its own set of multicast source information to the other peers. Source Host 2 sends the multicast data to group 188.8.131.52. On RP 6, the MSDP process learns about the source through PIM register messages and generates Source-Active (SA) messages to its MSDP peers that contain information about the sources in its domain. When RP 3 and RP 5 receive the SA messages, they forward them to their MSDP peers. When RP 5 receives the request from Host 1 for the multicast data on group 184.108.40.206, it builds a shortest path tree to the source by sending a PIM join message in the direction of Host 2 at 220.127.116.11.
Figure 1. MSDP Peering Between RPs in Different PIM Domains
When you configure MSDP peering between each RP, you create a full mesh. Full MSDP meshing is typically done within an autonomous system, as shown between RPs 1, 2, and 3, but not across autonomous systems. You use BGP to do loop suppression and MSDP peer-RPF to suppress looping SA messages.
You do not need to configure BGP in order to use Anycast-RP (a set of RPs that can perform load balancing and failover) within a PIM domain.
You can use PIM Anycast (RFC 4610) to provide the Anycast-RP function instead of MSDP. For information about PIM, see Configuring PIM and PIM6.
For detailed information about MSDP, see RFC 3618.