PIM-DM routers flood multicast traffic to all possible downstream neighbors and then prunes downstream interfaces from the distribution tree as requested by downstream routers. This “flood and prune” approach allows for lighter weight configuration on each router. However, this approach causes some unnecessary packet forwarding.

If a router receives a multicast packet but has no downstream receivers for that group it sends a PIM prune message to the upstream router asking it to stop forwarding traffic for that group. The path remains pruned for about three minutes, after which the router with a pruned interface sends a PIM source refresh message downstream. If the downstream router still has no downstream listeners, it sends a refreshing PIM prune upstream. The downstream router may also, at any time, send a PIM join message to effectively cancel a previous prune.

PIM-DM is recommended for networks where some multicast flooding is an acceptable trade off for reduced configuration complexity. Dense networks, such as LANs, are good candidates for PIM-DM.

The PIM-DM protocol is specified in RFC 3973, Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised).