Configuring Pim; Understanding Pim-Dm - HP A6600 Configuration Manual

Configuring PIM

PIM provides IP multicast forwarding by leveraging 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 router, multicast routing can be implemented as long as the corresponding
multicast routing entries are created through unicast routes. PIM uses the RPF mechanism to implement
multicast forwarding. When a multicast packet arrives on an interface of the router, it undergoes an RPF
check. If the RPF check succeeds, the router creates the corresponding routing entry and forwards the
packet. If the RPF check fails, the router discards the packet. For more information about RPF, see
"Configuring multicast routing and
Based on the implementation mechanism, PIM falls into the following categories:
PIM-DM
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PIM-SM
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BIDIR-PIM
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PIM-SSM
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Understanding PIM-DM

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
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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
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latency, PIM-DM uses a graft mechanism to resume data forwarding to that branch.
Generally speaking, the multicast forwarding path is a source tree, namely, 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 SPT.
The working mechanism of PIM-DM is summarized as follows:
• Neighbor discovery
• SPT building
• Graft
• Assert
forwarding."
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