A receivers DR first performs the RPF check: It searches for a unicast route to the RP. The outbound interface of the searched route acts as an upstream interface and the next hop of the searched route acts as an RPF neighbor. Then, the receivers DR sends a Join message to the RPF neighbor. After receiving the Join message, the previous-hop device performs the RPF check and forwards the Join message upstream at the same time. The Join message is sent upstream hop by hop until it reaches the RP.


In the preceding process, if there is one device that has no route to the RP, the RPT cannot be established.

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