If there are parallel paths to a source, duplicate packets can travel downstream through different routers to the network. If a forwarding router receives a multicast packet on its outgoing interface, the router identifies that the packet is a duplicate and notifies the upstream routers. See Figure 10.
Figure 10: Detecting Duplication
The upstream routers responsible for the duplication send assert messages to determine which router becomes the forwarder. Downstream routers listen to the assert messages to discover which router becomes the forwarder.
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