Overview of MSDP

Multicast Source Discovery Protocol (MSDP) functions to set up an MSDP peer relationship between Rendezvous Points (RPs) in different PIM-SM domains. MSDP peers exchange (S, G) information by sending Source Active (SA) messages. In this manner, MSDP peers share multicast source information and hosts can receive multicast data from the multicast sources in another PIM-SM domain.

In the general PIM-SM mode, a multicast source registers only with the local RP. The information on the inter-domain multicast sources is isolated. The RP knows only the source in its domain, establishes a multicast distribution tree (MDT) in its domain, and distributes the data sent by the source to the local users.

A mechanism is required to enable the local RP to share the information on the multicast sources of other domains. With the mechanism, the local RP can send Join messages to the multicast sources of other domains and establish MDTs. Therefore, multicast packets can be transmitted across domains, and hosts in the local domain can receive data sent by multicast sources in other domains.

The MSDP is an inter-area multicast solution based on multiple interconnected PIM-SM domains, and can solve the preceding problem.

MSDP achieves this objective by setting up the MSDP peer relationship between RPs of different domains. MSDP peers share the information on multicast sources by sending SA messages. They transmit the (S, G) information from the RP that the source S registers with to other RPs connected to members of G.

MSDP peers are connected through the TCP connection. MSDP peers perform the Reverse Path Forwarding (RPF) check on received SA messages.

MSDP is applicable only to PIM-SM domains, and useful only for the Any-Source Multicast (ASM) mode.