What is MSDP?

Understanding MSDP

MSDP (Multicast Source Discovery Protocol) is a protocol defined in RFC 3618 that lets PIM Sparse-Mode (PIM-SM) domains learn about active multicast sources in other domains. Rendezvous Points (RPs) in different domains peer with each other and exchange Source-Active (SA) messages, enabling interdomain multicast without sharing a single RP.

MSDP runs over TCP port 639. Each RP that learns of a local source originates an SA message describing the source and group (S,G) and floods it to its MSDP peers using peer-RPF (reverse path forwarding) checks to prevent loops. A remote RP receiving an SA for a group that has interested receivers can then trigger an (S,G) join back toward the source, building an interdomain shortest-path tree. MSDP is commonly paired with MBGP for the RPF routing information and is also used for Anycast RP within a single domain to provide RP redundancy and load sharing.

From a security standpoint, MSDP matters because uncontrolled SA flooding can be abused for resource exhaustion and amplification. Operators apply SA filters, RPF checks, and TCP MD5 authentication (per RFC 3618) on peerings, and rate-limit SA messages to defend against spoofed or excessive source advertisements that could overwhelm RPs or leak internal source information across trust boundaries.

For example, two service providers that each run PIM-SM internally configure MSDP peering between their RPs over MBGP-derived paths. When a video source becomes active in Provider A, its RP sends an SA message; Provider B's RP, seeing local subscribers for that group, joins the source directly, so the multicast stream flows between the two autonomous systems without either provider exposing or relying on the other's rendezvous point.