The Multicast Source Discovery Protocol (MSDP) is designed to connect multiple Protocol Independent Multicast - Sparse Mode (PIM-SM) domains, enabling PIM routers to discover multicast sources in other PIM-SM domains.
MSDP can only be deployed on an IPv4 network, and the multicast routing protocol used in each PIM domain must be PIM-SM. MSDP is valid only for the Any-Source Multicast (ASM) model.
On a PIM-SM network, a source designated router (DR) registers to a rendezvous point (RP) and a receiver DR sends Join messages to the RP. Therefore, the RP can obtain information about all multicast sources and group members. A large-scale PIM-SM network can be divided into multiple PIM-SM domains to facilitate control of multicast resources. In this case, the RP in one PIM-SM domain cannot obtain multicast source information in other PIM-SM domains. MSDP is addresses this problem.
MSDP enables routers (usually RPs) in different PIM-SM domains to set up MSDP peers. The MSDP peers exchange Source-Active (SA) messages to share multicast source information, so that multicast users in one PIM-SM domain can receive multicast data from multicast sources in other domains.
MSDP sets up peer relationships between Internet service provider (ISP) networks. In general, ISPs do not want to rely on other ISPs to provide services, because this may bring security risks, and failures of RPs on other ISP networks may affect satisfaction of their own users. Using MSDP, ISPs can use their own RPs to send multicast data to or receive multicast data from the Internet.
In addition to inter-domain multicast transmission, MSDP also allows Anycast RP within a PIM-SM domain, that is, multiple RPs with the same address. These RPs set up MSDP peers to back up each other and share multicast traffic.