Multicast Source Discovery Protocol (MSDP)

Originally written in May 2019 while studying and working through multicast design concepts. Revisited in 2025 to add perspective gained from real-world network design and operations.

During one of my Incredibly fun projects with BGP, I had to quickly and efficiently research, learn, and implement a new ( to me ) technology. While setting up my side of a BGP peer, I also had to set up a Multicast Source Discovery Protocol (MSDP) peer. Of course, the service provider was like Oh, and not forget the MSDP config. Because the service provider uses Juniper equipment, I started my search on the Juniper help pages.

The Multicast Source Discovery Protocol (MSDP) is used to connect multicast routing domains. It typically runs on the same router as the Protocol Independent Multicast (PIM) sparse-mode rendezvous point (RP). Each MSDP router establishes adjacencies with internal and external MSDP peers similar to the way BGP establishes peers. These peer routers inform each other about active sources within the domain. When they detect active sources, the routers can send PIM sparse-mode explicit join messages to the active source. 

The Juniper TechLibrary did a really great job of spelling it out for me. After that, the configuration was pretty smooth, except for a mistyped password. luckily, a quick look in the log showed an MSDP authentication error :)

blog.xuite.net/flytw1/

Looking Back

Looking back at MSDP, the problem it solved was a real headache at the time: sharing multicast source information across different domains without turning every hop into a mess. It was a very specific tool for a very specific problem, and it taught me two lessons that still apply today.

Design over features.
You didn’t use MSDP because it was “cool”; you used it because the architecture demanded it. It was never a silver bullet, just the right tool when the design called for it.

The “social” side of networking.
The hardest part of MSDP, and of modern SD-WAN and overlay designs, isn’t the configuration. It’s the coordination between teams and domains to make sure everyone shares the same understanding of how the network is supposed to behave.

MSDP isn’t the “hot new thing” anymore, but the core ideas behind it, source discovery and a clear separation between the control plane and the data plane, are the same ones showing up in modern fabrics and application-aware routing. Knowing why you’re using a protocol has always mattered more than memorizing the CLI.