Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

Protocol Overview

MSDP is a Layer 3 protocol that connects IPv4 protocol-independent multicast-sparse mode (PIM-SM) domains. A domain in the context of MSDP is a contiguous set of routers operating PIM within a common boundary defined by an exterior gateway protocol, such as border gateway protocol (BGP).

Each rendezvous point (RP) peers with every other RP via the transmission control protocol (TCP). Through this connection, peers advertise the sources in their domain.

1. When an RP in a PIM-SM domain receives a PIM register message from a source, it sends a source-active (SA) message to MSDP peers, as shown in the following illustration.
2. Each MSDP peer receives and forwards the message to its peers away from the originating RP.
3. When an MSDP peer receives an SA message, it determines if there are any group members within the domain interested in any of the advertised sources. If there are, the receiving RP sends a join message to the originating RP, creating a shortest path tree (SPT) to the source.

Figure 1. Multicast Source Discovery Protocol (MSDP)

RPs advertise each (S,G) in its domain in type, length, value (TLV) format. The total number of TLVs contained in the SA is indicated in the “Entry Count” field. SA messages are transmitted every 60 seconds, and immediately when a new source is detected.

Figure 2. MSDP SA Message Format