Dell PowerConnect 7024, Manual

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Managing IPv4 and IPv6 Multicast
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What Is PIM?
The Protocol Independent Multicast protocol is a simple, protocol-
independent multicast routing protocol. PIM uses an existing unicast routing
table and a Join/Prune/Graft mechanism to build a tree. PIM PowerConnect
7000 Series switches support two types of PIM: sparse mode (PIM-SM) and
dense mode (PIM-DM).
PIM-SM is most effective in networks with a sparse population of multicast
receivers. In contrast, PIM-DM is most effective in networks with densely
populated multicast receivers. In other words, PIM-DM can be used if the
majority of network hosts request to receive a multicast stream, while PIM-
SM might be a better choice in networks in which a small percentage of
network hosts, located throughout the network, wish to receive the multicast
stream.
Using PIM-SM as the Multicast Routing Protocol
PIM-SM is used to efficiently route multicast traffic to multicast groups that
may span wide area networks where bandwidth is a constraint.
PIM-SM uses shared trees by default and implements source-based trees for
efficiency; it assumes that no hosts want the multicast traffic unless they
specifically ask for it. It creates a shared distribution tree centered on a
defined rendezvous point (RP) from which source traffic is relayed to the
receivers. Senders first send the multicast data to the RP, which in turn sends
the data down the shared tree to the receivers.
Shared trees centered on an RP do not necessarily provide the shortest, most
optimal path. In such cases, PIM-SM provides a means to switch to more
efficient source-specific trees. A data threshold rate is configured to
determine when to switch from shared-tree to source-tree.
PIM-SM uses a Bootstrap Router (BSR), which advertises information to
other multicast routers about the RP. In a given network, a set of routers can
be administratively enabled as candidate bootstrap routers. If it is not
apparent which router should be the BSR, the candidates flood the domain
with advertisements. The router with the highest priority is elected. If all the
priorities are equal, then the candidate with the highest IP address becomes
the BSR.