Multicast Source Discovery Protocol (MSDP) is a Protocol Independent Multicast (PIM) family multicast routing protocol defined by Experimental RFC 3618.
[1] Despite becoming the IPv4 de facto standard for inter-domain multicast, development of the protocol stopped in 2006 and it was decided by the authors not to proceed with making it a proposed standard.
[2] MSDP interconnects multiple IPv4 PIM Sparse-Mode (PIM-SM) domains which enables PIM-SM to have Rendezvous Point (RP) redundancy and inter-domain multicasting RFC 4611.
If an RP on its own domain receives a message, it determines if there are listeners for this group.
MSDP is deliberately unspecified for IPv6,[2][3] for a number of reasons, including that protocols such as Source-Specific Multicast (SSM), Bi-directional Multicast and IPv6 Embedded Rendezvous Points would mean there was less requirement for MSDP.