Introduction To Pim-Dm; How Pim-Dm Works - HP 4800G Series Configuration Manual

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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).

The working mechanism of PIM-DM is summarized as follows:

Neighbor discovery

SPT building

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.

SPT establishment

The process of building an SPT is the process of "flood and prune".

In a PIM-DM domain, when a multicast source S sends multicast data to 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.

Then, nodes without receivers downstream are pruned: A router having no receivers downstream

sends a prune message to the upstream node to "tell" the upstream node to delete the

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