A router receives multicast packets for (S,G) but its unicast routing table has no route toward S, causing an RPF check failure. By default how does the router handle those multicast packets?

When a router receives a multicast packet, it performs a Reverse Path Forwarding (RPF) check to prevent loops and ensure the packet arrived on the correct interface. This check verifies that the incoming interface is the one that would be used to send unicast traffic back to the source (S) of the multicast group (G). If the router's unicast routing table lacks a route to the source (S), the RPF check fails. By default, a router will drop multicast packets that fail the RPF check. Encapsulating and sending to the RP, forwarding out all downstream interfaces, or generating a PIM Register are actions taken under different multicast scenarios, not for a fundamental RPF failure due to a missing unicast route to the source.