H3C Technologies H3C S7500E Series Switches - Manual (Page 353)

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To facilitate description, a network comprising IPv6 PIM–supporting routers is referred to as an
“IPv6 PIM domain” in this document.
Introduction to IPv6 PIM-DM
IPv6 PIM-DM is a type of dense mode IPv6 multicast protocol. It uses the “push mode” for IPv6
multicast forwarding, and is suitable for small-sized networks with densely distributed IPv6
multicast members.
The basic implementation of IPv6 PIM-DM is as follows:
z IPv6 PIM-DM assumes that at least one IPv6 multicast group member exists on each
subnet of a network, and therefore IPv6 multicast data is flooded to all nodes on the
network. Then, branches without IPv6 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 IPv6 multicast forwarding when
the pruned state times out and then data is re-flooded down these branches, and then are
pruned again.
z When a new receiver on a previously pruned branch joins an IPv6 multicast group, to
reduce the join latency, IPv6 PIM-DM uses the graft mechanism to resume IPv6 multicast
data forwarding to that branch.
Generally speaking, the IPv6 multicast forwarding path is a source tree, namely a forwarding
tree with the IPv6 multicast source as its “root” and IPv6 multicast group members as its
“leaves”. Because the source tree is the shortest path from the IPv6 multicast source to the
receivers, it is also called shortest path tree (SPT).
How IPv6 PIM-DM Works
The working mechanism of IPv6 PIM-DM is summarized as follows:
z Neighbor discovery
z SPT establishment
z Graft
z Assert
Neighbor discovery
In an IPv6 PIM domain, a PIM router discovers IPv6 PIM neighbors, maintains IPv6 PIM
neighboring relationships with other routers, and builds and maintains SPTs by periodically
multicasting IPv6 PIM hello messages (hereinafter referred to as “hello messages”) to all other
IPv6 PIM routers.