Can someone clarify this to me? It is a theory question, so maybe too in-depth 
but... :)

It is about PIM-SM Register process. When a router directly connected to a 
multicast source wants to be able to forward traffic, it sends a register 
message to the RP. This an unicast message, and within it there's a multicast 
packet encapsulated.

Now, if there is no parties interested for this traffic, RP will send back 
Register-Stop message. Then sending router will start Register Suppression 
Timer and after it times out, it will send again PIM register. And so on.

If there are interested parties, then RP will unpack the encapsulated multicast 
packet and forward it immediately. Then it should notify the sending router to 
stop sending unicast packets (which are for registration purposes, basically), 
because it can send normal multicast ones instead.

My question is - how does RP notify the sending router to stop sending unicast 
PIM Register messages? Is it also PIM Register-stop? But how will the sending 
router differentiate these two PIM Register messages? How will it know when to 
start Register Suppression Timer (and keep on sending unicasts), and when to 
start sending normal multicast packets?

Thanks in advance. :)

Best Regards,

Bojan Zivancevic
Network Engineer
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