Multicast Source Discovery Protocol (MSDP) overview

Multicast Source Discovery Protocol (MSDP) is a mechanism to connect multiple Protocol Independent Multicast sparse mode (PIM-SM) domains. MSDP allows multicast sources for a group to be known to all rendezvous points (RPs) in different domains. An RP runs MSDP over TCP to discover multicast sources in other domains. The main advantage of MSDP is that it reduces the complexity of interconnecting multiple PIM-SM domains by allowing PIM-SM domains to use an interdomain source tree (rather than a common shared tree).

When MSDP is configured in a network, RPs running MSDP exchange source information with MSDP enabled RPs in other domains. An RP can join the interdomain source tree for sources that are sending to groups for which it has receivers. The RP can do that because it is the root of the shared tree within its domain, which has branches to all points in the domain where there are active receivers. When a last-hop device learns of a new source outside the PIM-SM domain (through the arrival of a multicast packet from the source down the shared tree), it then can send a join toward the source and join the interdomain source tree, which behaves similar to a local PIM register packet.

Benefits of Using MSDP to Interconnect Multiple PIM-SM Domains:
  • Allows a rendezvous point (RP) to dynamically discover active sources outside of its domain.

  • Introduces a more manageable approach for building multicast distribution trees between multiple domains and thus provides administrative independence.

  • Allows filtering.

PIM Anycast RP is supported with the help of MSDP mesh groups. The main purpose of an Anycast RP implementation is that the downstream multicast routers will see just one address for an RP.


Currently, only intra-domain MSDP deployments are supported; inter-domain MSDP deployments are not supported.