PIM dense mode constructs the multicast tree by flooding traffic out of every interface from the source to every dense mode router across the network.
PIM dense mode is a suitable configuration where receivers of the multicast group are located across every subnet on the network. It is applicable to small networks, but not recommended for larger production environments.
As each router receives traffic for the multicast group, at which point the router must decide whether to forward the traffic on to any receivers further downstream or to send a prune message towards the source.
When a prune message is sent towards the source, the branch of that tree is cut off so unnecessary multicast traffic is not send towards that router.
The multicast tree grows from the root towards the leaves (routers). Each router receives the multicast traffic from their upstream neighbour through its reverse path forwarding (RPF) interface. That router forwards the multicast traffic to all its PIM dense mode neighbours.
It can mean that multicast traffic arrives on non RPF interfaces and is discarded. Traffic arriving on non RPF interfaces is normal for the initial flooding of multicast traffic, and is gradually corrected with the destination router sending pruning messages back towards the source.
Pruning is sent out via RPF interfaces to indicate upstream that there are no hosts that are interested in receiving the multicast source. Non-RPF interfaces can sent out pruning messages too to indicate that they are interfaces not meant to be receiving multicast traffic.
Pruning expires after three minutes in PIM dense mode. This will cause multicast traffic to be flooded back down the same link, the same as when the initial set-up was established. A new pruning message be sent in response if there are still no interested hosts in receiving the stream.
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