2Introduction to PIM-DMPIM-DM is a type of dense mode multicast protocol. It uses the “push mode” for multicast forwarding,and is suitable for small-sized networks with densely distributed multicast members.The basic implementation of PIM-DM is as follows:z PIM-DM assumes that at least one multicast group member exists on each subnet of a network,and therefore multicast data is flooded to all nodes on the network. Then, branches withoutmulticast forwarding are pruned from the forwarding tree, leaving only those branches that containreceivers. This “flood and prune” process takes place periodically, that is, pruned branches resumemulticast forwarding when the pruned state times out and then data is re-flooded down thesebranches, and then are pruned again.z When a new receiver on a previously pruned branch joins a multicast group, to reduce the joinlatency, PIM-DM uses a graft mechanism to resume data forwarding to that branch.Generally speaking, the multicast forwarding path is a source tree, namely a forwarding tree with themulticast source as its “root” and multicast group members as its “leaves”. Because the source tree isthe shortest path from the multicast source to the receivers, it is also called shortest path tree (SPT).How PIM-DM WorksThe working mechanism of PIM-DM is summarized as follows:z Neighbor discoveryz SPT buildingz Graftz AssertNeighbor discoveryIn a PIM domain, a PIM router discovers PIM neighbors, maintains PIM neighboring relationships withother routers, and builds and maintains SPTs by periodically multicasting hello messages to all otherPIM routers (224.0.0.13).Every activated interface on a router sends hello messages periodically, and thus learns the PIMneighboring information pertinent to the interface.SPT buildingThe process of building an SPT is the process of “flood and prune”.1) In a PIM-DM domain, when a multicast source S sends multicast data to a multicast group G, themulticast packet is first flooded throughout the domain: The router first performs RPF check on themulticast packet. If the packet passes the RPF check, the router creates an (S, G) entry andforwards the data to all downstream nodes in the network. In the flooding process, an (S, G) entry iscreated on all the routers in the PIM-DM domain.2) Then, nodes without receivers downstream are pruned: A router having no receivers downstreamsends a prune message to the upstream node to “tell” the upstream node to delete the
