Hi Ankit,
PIM-DM is based on "flood and prune" mechanism. It doesn't work on explicit Join mechanism.
Explicit join messages are used in Sparse mode implementations only.
The "Join" messages in dense-mode implementations are used for different purpose. Mainly it is used for "Prune Override" feature in multi-access network (Ethernet).
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R1
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________|_______
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R2 R3
In the above diagram, imagine that R1 is getting a multicast stream and R2 & R3 are having recievers of that multicast group connected to them.
If recievers connected to R2 leave the group ( by sending IGMP leave) , then R2 sends a Prune message through the RPF interface ( LAN segment) and R1 and R3 will see that Prune message. R1 will think that it should stop sending the multicast traffif through the LAN segment by removing the LAN interface from OIL.
If R3, still has recievers connected to it, R1 shouldn't stop sending multicast traffic to the LAN. In order to make this happen, whenever R3 sees a prune message from anyone in the LAN , It will generate a Join message to negate that Prune message and R1 will not prune the LAN interface from sending traffic.
As you mentioned, graft message is specific for re-joining to a group which was pruned by the router earlier.
Hope this helps.
Regards
Moses N
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