What occurs in PIM Sparse-Dense Mode when there is no RP?

In PIM Sparse-Dense Mode, when there is no Rendezvous Point (RP) available, the behavior of the protocol can be understood better by recognizing how multicast traffic is typically handled in the absence of a designated RP.

In this situation, the network defaults to its dense mode behavior. Dense mode assumes a more traditional, broadcast approach to multicast traffic, where the multicast packets are sent out on all interfaces of a router except those that explicitly prune the traffic. This is particularly applicable when the network as a whole is not set to intricate sparse mode configurations due to the missing RP. Thus, the multicast traffic will be forwarded as if dense mode was specified, which allows for the transmission of multicast packets over the network instead of outright blocking the traffic.

This behavior highlights the adaptable nature of PIM, where it can fall back on dense mode characteristics in the absence of the necessary control structure (the RP). It allows for the multicast sessions to continue functioning, albeit with potential inefficiencies typical of dense mode, rather than entirely blocking multicast traffic.