Can someone please just clear this up for me so I can move on with my life. I'm trying to understand the point of the assert message with PIM, and why it's necessary if we have a DR for a shared segment. As I understand it the DR is responsible for sending PIM joins/prunes as well as PIM registers. The point being, that if (for example) 2 last hop routers are connected to the same segment as a receiver, only 1 router sends PIM joins upstream for any receiver who wants a feed. In my mind, this would imply that since the DR is the only one sending PIM joins for a particular multicast group, that it is the only one building the shared tree (lets assume we are running PIM-SM). Now the assert message is to elect a forwarder for the segment. Now, why would we need that? If for example, the designated forwarder for the segment was the other router (i.e. not the DR), then the tree is not built on this device, and the outgoing interface lists upstream would be pointing to the DR for any multicast feeds being sent from the source downstream to the receivers. So my question is, what is the point of the PIM forwarder/assert message if we have a DR on the segment?
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