3Com Switch 8800 Advanced Software V5 Configuration Guide

564 CHAPTER 42: PIM CONFIGURATION
Introduction to 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 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
re-flooded down these branches, and then 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, 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 shortest path tree (SPT).
How PIM-DM Works The working mechanism of PIM-DM is summarized as follows:
■ Neighbor discovery
■ SPT building
■ Graft
■ Assert
Neighbor discovery
In a PIM domain, a PIM router discovers PIM neighbors, maintains PIM neighboring
relationships with other routers, and builds and maintains SPTs by periodically
multicasting hello messages to all other PIM routers (224.0.0.13).
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Every activated interface on a router sends hello messages periodically, and thus
learns the PIM neighboring information pertinent to the interface.
SPT building
The process of building an SPT is the process of "flood and prune".
1 In a PIM-DM domain, when a multicast source S sends multicast data to a
multicast group G, the multicast packet is first flooded throughout the
domain: The router first performs RPF check on the multicast packet. If the
packet passes the RPF check, the router creates an (S, G) entry and
forwards the data to all downstream nodes in the network. In the
flooding process, an (S, G) entry is created on all the routers in the PIM-DM
domain.
2 Then, nodes without receivers downstream are pruned: A router having
no receivers downstream sends a prune message to the upstream node to
notify the upstream node to delete the corresponding interface from the
outgoing interface list in the (S, G) entry and stop forwarding subsequent
packets addressed to that multicast group down to this node.