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Outbound interfaces for RPT once SPT is joined

I'm studying PIM Sparse mode at the moment.

I understand the principle that, upon receiving traffic from a multicast source, the Group Members DR will build an SPT direct to the source (rather than going through the RP). However it is also stating the DR will send a prune (or more accurately a PIM Join/Prune with the referenced group in the prune list) to the RP.

Is this correct? Assuming the Cisco default behavior of PIM threshold 0 (e.g. "always create SPT"), won't that mean that once all group member have joined the multicast group, the RP will have all its outgoing (*,G) interfaces pruned (but perhaps not the (S,G) interfaces if the SPT passes through the RP).

If this is right then the RP will only ever have outgoing (*,G) interfaces during the period when a new port is added by the receipt of the PIM Join/Prune and the DR that originated the Join/Prune creates the SPT... am I understanding that correctly?

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