Operation Manual – MulticastH3C S7500 Series Ethernet Switches Chapter 7 PIM Configuration7-1Chapter 7 PIM Configuration7.1 PIM OverviewProtocol independent multicast (PIM) means that the unicast routing protocolsproviding routes for IP multicast could be static routes, RIP, OSPF, IS-IS, or BGP. Themulticast routing protocol is independent of unicast routing protocols only if unicastrouting protocols can generate route entries.With the help of the reverse path forwarding (RPF), PIM can transmit multicastinformation in the network. For the convenience of description, the network consisted ofPIM-enabled multicast routers is called PIM multicast domain.7.1.1 Introduction to PIM-DMProtocol independent multicast dense mode (PIM-DM) is a dense mode multicastprotocol. It is suitable for small networks.The features of such networks are:z Members in a multicast group are densely distributed.z PIM-DM assumes that in each subnet of the network there is at least one receiverinterested in the multicast source.z Multicast packets are flooded to all the nodes in the network, and the relatedresources (bandwidth and the CPU of the router) are consumed at the same time.In order to reduce the network resource consumption, PIM-DM prunes the brancheswhich do not forward multicast data and keeps only the branches including receivers. Inorder that the pruned branches which are demanded to forward multicast data canreceive multicast data flows again, the pruned branches can be restored to theforwarding status periodically.In order to reduce the delay time for a pruned branch to resume the forwarding status,PIM-DM uses the graft mechanism to resume multicast packet forwarding automatically.Such periodical floods and prunes are the features of PIM-DM, which is suitable forsmall LANs only. The flood-prune technology adopted in PIM-DM is unacceptable inWAN.Generally, the packet forwarding path in PIM-DM is a shortest path tree (SPT) with themulticast source as the root and multicast members as the leaves. The SPT uses theshortest path from the multicast source to the receiver.7.1.2 Work Mechanism of PIM-DMThe working procedure of PIM-DM is summarized as follows:
