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Since multicast data packets could end up going in circles, RPF [Reverse Path Forwarding] enables routers to forward multicast data down the distribution tree.

RPF uses existing multicasting routing protocols to determine up/downstream sources.

RPF only forwards multicasting packets only if on the upstream port interface.

MSDP [Multicast Source Discovery Protocol] is a process that connects PIM-SP [Protocol Independent Multicast-Sparse Mode] domains and allows RP [Rendezvous Points] or Shared Root to share information about active sources or shared trees.

RP in distant AS [Autonomous System] domains can pass/forward route information to their internal systems. Each AS can maintain its own Rendezvous Points as well as forward traffic to other AS via peering points.

When new multicast traffic arrives, the RP encapsulates the first data packet in a SA [Source Active] message and sends to other MSDP RPs. Each RP forwards the SA to other RPs throughout the entire multicast network. RPs may also send SA messages to maintain and test multicast traffic connections.

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[tags]msdp,multicast source discovery protocol,rpf,reverse path forwarding,pim-sp,protocol independent multicast-sparse mode[/tags]