H3C Technologies H3C S12500-X Series Switches - Manual (Page 71)

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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 includes 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 "PIM domain" in this chapter refers to a network composed of PIM routers.
The term "interface" in this chapter collectively refers to VLAN interfaces and Layer 3 Ethernet interfaces.
You can set an Ethernet port as a Layer 3 interface by using the port link-mode route command (see
Layer 2—LAN Switching Configuration Guide).
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