Operation Manual – Multicast
H3C S5600 Series Ethernet Switches
Chapter 4 PIM Configuration
4-2
Note:
To facilitate description, a network comprising PIM-capable routers is referred to as a
“PIM domain” in this document.
4.1.1 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:
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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.
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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).
4.1.2 How PIM-DM Works
The working mechanism of PIM-DM is summarized as follows:
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Neighbor discovery
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SPT building
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Graft
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Assert
I. 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).
