HP 6125XLG R2306-HP 6125XLG Blade Switch IP Multicast Configuration Guide - Page 67
Configuring PIM, Overview, PIM-DM overview
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Configuring 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 RIP, OSPF, IS-IS, or BGP. PIM is not dependent on any particular unicast routing protocol, and it uses the underlying unicast routing to generate a routing table with routes. PIM uses the 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 a multicast 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) In this document, a PIM domain refers to a network composed of PIM routers. PIM-DM overview PIM-DM uses the push mode for multicast forwarding, and is suitable for small-sized networks with densely distributed multicast members. The following describes the basic implementation of PIM-DM: • PIM-DM assumes that all downstream nodes want to receive multicast data when a source starts sending, so multicast data is flooded to all downstream nodes on the network. • Branches without downstream receivers are pruned from the forwarding trees, leaving only those branches that contain receivers. • The pruned state of a branch has a finite holdtime timer. When the timer expires, multicast data is again forwarded to the pruned branch. This flood-and-prune cycle takes place periodically to maintain the forwarding branches. • To reduce join latency when a new receiver on a previously pruned branch joins a multicast group, PIM-DM uses a graft mechanism to turn the pruned branch into a forwarding branch. In PIM-DM, the multicast forwarding paths for a multicast group constitutes a source tree, which is rooted at the multicast source and has multicast group members as its "leaves." Because the source tree consists of the shortest paths from the multicast source to the receivers, it is also called a "shortest path tree (SPT)." The operating mechanism of PIM-DM is summarized as follows: • Neighbor discovery • SPT building • Graft • Assert 60