HP 6125G HP 6125G & 6125G/XG Blade Switches IP Multicast Configuration - Page 113

Configuring PIM PIM overview Protocol Independent Multicast (PIM) provides IP multicast forwarding by leveraging unicast static routes or unicast routing tables generated by any unicast routing protocol, such as routing information protocol (RIP), open shortest path first (OSPF), or border gateway protocol (BGP). Independent of the unicast routing protocols running on the device, multicast routing can be implemented as long as the corresponding multicast routing entries are created through unicast routes. PIM uses the reverse path forwarding (RPF) mechanism to implement multicast forwarding. When a multicast packet arrives on an interface of the device, it undergoes an RPF check. If the RPF check succeeds, the device creates the corresponding routing entry and forwards the packet. If the RPF check fails, the device discards the packet. For more information about RPF, see "Configuring multicast routing and forwarding." Based on the implementation mechanism, PIM falls into the following categories: • Protocol Independent Multicast-Dense Mode (PIM-DM) • Protocol Independent Multicast-Sparse Mode (PIM-SM) • Protocol Independent Multicast Source-Specific Multicast (PIM-SSM) The term "router" in this document refers to both routers and Layer 3 switches. PIM-DM overview PIM-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: • PIM-DM assumes that at least one multicast group member exists on each subnet of a network. 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 process takes place periodically. Pruned branches resume multicast forwarding when the pruned state times out. Data is then flooded again down these branches, and then the branches are pruned again. • When a new receiver on a previously pruned branch joins a multicast group, to reduce the join latency, PIM-DM uses a graft mechanism to resume data forwarding to that branch. Generally speaking, the multicast forwarding path is a source tree. That is, it is a forwarding tree with the multicast source as its "root" and multicast group members as its "leaves." Because the source tree is the shortest path from the multicast source to the receivers, it is also called a shortest path tree (SPT). The working mechanism of PIM-DM is summarized as follows: • Neighbor discovery • SPT building • Graft • Assert 102