There can be scenarios in a multicast network where duplicate multicast packets can flow onto the network. There is a mechanism in PIM called the ‘PIM assert mechanism’ that can stop these duplicate flows. This mostly happens in PIM Dense Mode networks due its flooding properties, and rarely happens in PIM Sparse Mode.
A PIM assert message is sent when multiple routers receive a packet into their outgoing interface that is meant to be going out of the same interface for the same (S, G).
When this occurs, a PIM assert message is sent into the LAN. It contains the administrative distance and metric back towards the source, and it determines which router should be forwarding multicast traffic to that network segment.
Preference is given to the router with the lower route metric. If metrics are tied, the router with the higher IP address is used.
The losing router prunes its interface to prevent further duplicate multicast packets from arriving on that interface. The prune lasts for around three minutes before the process will start again.