H3C Technologies H3C S7500E Series Switches - Manual (Page 139)

6-2
To facilitate description, a network comprising PIM-capable routers is referred to as a “PIM
domain” in this document.
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:
z 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.
z 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:
z Neighbor discovery
z SPT building
z Graft
z 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).
Every PIM-enabled interface on a router sends hello messages periodically, and thus learns the
PIM neighboring information pertinent to the interface.