Configuring Pim; Overview; Pim-Dm Overview - HP MSR2003 Configuration Manual

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
uses the underlying unicast routing to generate a multicast routing table without relying on any particular
unicast routing protocol.
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)
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• Protocol Independent Multicast–Sparse Mode (PIM-SM)
Bidirectional Protocol Independent Multicast (BIDIR-PIM)
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Protocol Independent Multicast Source-Specific Multicast (PIM-SSM)
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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 from a source, so
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multicast data is flooded to all downstream nodes on the network.
Branches without downstream receivers are pruned from the forwarding trees, leaving only those
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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.
The graft mechanism is used to reduce the latency for resuming the forwarding capability of a
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previously pruned branch.
In PIM-DM, the multicast forwarding paths for a multicast group constitutes a source tree. The source tree
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)."
Neighbor discovery
In a PIM domain, each PIM interface on a router periodically multicasts PIM hello messages to all other
PIM routers (identified by the address 224.0.0.13) on the local subnet. Through the exchanging of hello
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