Why cannot a multicast distribution tree be set up correctly if PIM is not enabled on an RPF interface

The PIM protocol does not maintain its own unicast routing table. Instead, it uses the existing unicast routing table for RPF checks. Generally, the object of an RPF check is the multicast source. On a PIM-SM network, the object of an RPF check can be the multicast source, RP, or BSR. A PIM device searches the unicast routing table for an optimal route to an object. The outbound interface of the route is an RPF interface. That is, whether an interface is an RPF interface depends on unicast routing and is independent of the PIM protocol. The PIM protocol requires that PIM be enabled on an RPF interface. If PIM is not enabled, the RPF interface cannot process received PIM protocol packets, and therefore the multicast distribution tree cannot be set up.

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