47 PIM CONFIGURATIONPIM Overview Protocol independent multicast (PIM) means that the unicast routing protocolsproviding routes for the multicast could be static routes, RIP, OSPF, IS-IS, or BGP.The multicast routing protocol is independent of unicast routing protocols only ifunicast routing protocols can generate route entries.With the help of the reverse path forwarding, PIM can transmit multicastinformation in the network. For the convenience of description, the networkconsisted of PIM-enabled multicast routers is called PIM multicast domain.Introduction to PIM-DM Protocol independent multicast dense mode (PIM-DM) is a dense mode multicastprotocol. It is suitable for small networks.The features of such network are:■ Members in a multicast group are dense.■ PIM-DM assumes that in each subnet of the network there is at least onereceiver interested in the multicast source.■ Multicast packets are flooded to all the points in the network, and the relatedresources (bandwidth and the CPU of the router) are consumed at the sametime.In order to reduce the network resource consumption, PIM-DM prunes thebranches which do not forward multicast data and keeps only the branchesincluding receivers. In order that the pruned branches which are demanded toforward multicast data can receive multicast data flows again, the prunedbranches can be restored to the forwarding status periodically.In order to reduce the delay time for a pruned branch to be restored to theforwarding status, PIM-DM uses the graft mechanism to restore the multicastpacket forwarding automatically. Such periodical floods and prunes are thefeatures of PIM-DM, which is suitable for small LANs only. The flood-prune”technology adopted in PIM-DM is unacceptable in WAN.Generally, the packet forwarding path in PIM-DM is a shortest path tree (SPT) withthe multicast source as the root and multicast members as the leaves. The SPT usesthe shortest path from the multicast source to the receiver.Work Mechanism ofPIM-DMThe working procedure of PIM-DM is summarized as follows:■ Neighbor discovery■ SPT establishing■ Graft
