H3C Technologies H3C S12500 Series Switches - Manual (Page 125)

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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.
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 includes the following categories:
• Protocol Independent Multicast–Dense Mode (PIM-DM)
• Protocol Independent Multicast–Sparse Mode (PIM-SM)
• Bidirectional Protocol Independent Multicast (BIDIR-PIM)
• 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, 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 process takes place periodically. That is, pruned branches resume multicast
forwarding when the pruned state times out and then data is 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, also known as 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 operating mechanism of PIM-DM is summarized as follows:
• Neighbor discovery
• SPT building
• Graft
• Assert