The PIM protocol is used to implement multicast routing
and data forwarding inside an AS. The PIM-DM protocol is a multicast
routing protocol of dense node. It is applicable to a small-scale
network with densely-distributed members.
This section describes the multicast data forwarding process in a PIM-DM domain, PIM control message types, and security functions required for the multicast service.
Ensure that unicast routes are reachable before enabling IPv4 multicast routing, and enable PIM-DM on each interface of the multicast device. In this manner, the PIM-DM network can work normally.
A multicast device can control the forwarding of multicast data based on multicast sources. This helps to control multicast data flows and limit information that can be obtained by downstream receivers to enhance security.
PIM devices exchange Hello messages to set up neighbor relationships and negotiate various control parameters for controlling the neighbor relationships.
When the last member leaves a group, the multicast device sends a Prune message upstream, requesting the upstream device to execute the prune action. If other downstream devices on the same network segment need the multicast data for this group, they need to send Join messages to override the prune action.
In a PIM-DM network, the periodic flooding-pruning wastes lots of network resources. To prevent the pruned interface from forwarding messages because the prune timer times out, you can enable the State-Refresh function. The multicast device then sends State-Refresh messages periodically to refresh the prune state of the interface and maintain the shortest path tree (SPT).
To make new members in a network to quickly receive multicast data, a multicast device actively sends a Graft message through an upstream interface, requesting the upstream device to forward multicast data to this network segment.
If a multicast device can receive multicast data through a downstream interface, this indicates that other upstream devices exist in this network segment. The device then sends an Assert message through the downstream interface to take part in the election of the unique upstream device.
After the interface is configured with PIM silent, do not receive or forward any PIM protocol packet. All PIM neighbors and PIM state machines on this interface are deleted. Then, this interface automatically becomes the Designated router (DR). IGMP on the interface is not affected.
You can run related commands to check the PIM-DM running status. If necessary, you can clear statistics on PIM-DM control messages or enable the PIM-DM debugging.