I can't seem to grasp the sheer concept of pim sparse dense mode. Documentation says that an enabled interface in pim sparse dense mode will use dense mode for dense groups and sparse mode for sparse group. It says that ip pim sparse dense mode can be used in situations such as Auto-RP where candidate RPs and mapping agents need to communicate with multicast traffic; however the RP isnt known which results in the catch 22. I understand the the two groups used by auto-rp in sparse dense mode will use dense mode for the 220.127.116.11, 40 and sparse mode for client traffic. That part I understand and understand the reasoning for ip pim sparse dense mode.
My question is why is an RP always needed in sparse-dense-mode whether through Auto-RP or statically. It seems like regular client multicast traffic would fall back to dense mode if there wasnt an RP enabled. Although, I know its not considered as efficient, multicast traffic should still be forwarded through normal dense mode operation.