HP VPN Firewall Appliances Appendix Protocol Reference

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PIM
Overview
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 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 "RPF check mechanism."
Ba
sed 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 "router" in this document refers to both routers and routing-capable firewalls.
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 following describes the basic implementation of PIM-DM:
• PIM-DM assumes that at least one multicast group member exists on each subnet of a network.
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. Pruned branches resume multicast
forwarding when the pruned state times out. 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. That is, it is 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 an SPT.
The working mechanism of PIM-DM is summarized as follows:
• Neighbor discovery
• SPT building
• Graft
• Assert