TCP/IP Tutorial and Technical Overview - IBM Redbooks

RP selection
An RP is selected as part of standard PIM-SM operations. An RP is mapped to
each specific multicast group. To perform this mapping, a router configured to
support PIM-SM distributes a list of candidate RPs to all other routers in the
environment. When a mapping needs to be performed, each router hashes the
multicast group address into an IP address that represents the RP.
PIM-SM benefits
PIM-SM is optimized for environments containing a large number of multicast
data streams. Each stream should flow to a relatively small number of the LAN
segments. For these groups, the flooding and pruning associated with PIM-DM
and DVMRP is an inefficient use of network bandwidth. PIM-SM is also
appropriate when:
► There are few receivers in a group.
► Senders and receivers are separated by WAN links.
► The stream of multicast traffic is intermittent.
Like PIM-DM, PIM-SM assumes that the route obtained through the unicast
routing protocol supports multicast routing. Therefore, the network administrator
must ensure that each device connected to the end-to-end path is
multicast-enabled.
6.8 Interconnecting multicast domains
Early multicast development focused on a flat network topology. This differed
from the hierarchical topology deployed in the Internet. Like the Internet,
multicast developers soon realized the need to perform inter-domain routing.
New classes of protocol have been proposed to address this deficiency. There
are currently two approaches to interconnecting multicast domains.
6.8.1 Multicast Source Discovery Protocol (MSDP)
MSDP, defined in RFC 3618, is a protocol to logically connect multiple PIM-SM
domains. It is used to find active multicast sources in other domains. RPs in
separate autonomous systems communicate through MSDP to exchange
information about these multicast sources. Each domain continues to use its own
RP. There is no direct dependence on other RPs for intra-domain
communication.
266 TCP/IP Tutorial and Technical Overview