Operation Manual – Multicast ProtocolH3C S3610&S5510 Series Ethernet Switches Chapter 6 PIM Configuration6-1Chapter 6 PIM Configuration6.1 PIM OverviewProtocol Independent Multicast (PIM) provides IP multicast forwarding by leveragingstatic routes or unicast routing tables generated by any unicast routing protocol, suchas routing information protocol (RIP), open shortest path first (OSPF), intermediatesystem to intermediate system (IS-IS), or border gateway protocol (BGP). PIM uses aunicast routing table to perform reverse path forwarding (RPF) check to implementmulticast forwarding. For more information about RPF, refer to ” RPF mechanism”.Based on the forwarding mechanism, PIM falls into two modes:z Protocol Independent Multicast–Dense Mode (PIM-DM), andz Protocol Independent Multicast–Sparse Mode (PIM-SM).Note:To facilitate description, a network comprising PIM-capable routers is referred to as a“PIM domain” in this document.6.1.1 Introduction to PIM-DMPIM-DM is a type of dense mode multicast protocol. It uses the “push mode” formulticast forwarding, and is suitable for small-sized networks with densely distributedmulticast members.PIM-DM has the following features:z PIM-DM assumes that at least one multicast group member exists on each subnetof a network, and therefore multicast data is flooded to all nodes on the network.Then, branches without multicast forwarding are pruned from the forwarding tree,leaving only those branches that contain receivers. This “flood and prune” processtakes place periodically, that is, pruned branches resume multicast forwardingwhen the pruned state times out and then data is re-flooded down these branches,and then are pruned again.z When a new receiver on a previously pruned branch joins a multicast group, toreduce the join latency, PIM-DM uses a graft mechanism to resume dataforwarding to that branch.Generally speaking, the multicast forwarding path is a source tree, namely a forwardingtree with the multicast source as its “root” and multicast group members as its “leaves”.
