Administrator Guide

1516 IPv4 and IPv6 Multicast
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 and where bandwidth is constrained. PIM-SM
uses shared trees by default and implements source-based trees for efficiency.
PIM-SM assumes that no hosts want the multicast traffic unless they
specifically ask for it. It initially creates a shared distribution tree centered on
a defined “rendezvous point” (RP) through which source traffic is relayed to
the ultimate receiver. Multicast traffic sources 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 or
most optimal path. In such cases, a Dell Networking PIM-SM router adjacent
to the host switches to the shortest path upon seeing the very first multicast
data packet.
Many IP multicast applications, such as those that handle real-time
dissemination of financial information, require high performance. Multicast
group membership management (IGMP), unicast routing protocols (OSPF,
RIP), and multicast routing protocols are all required to enable end-to-end
multicast capabilities. The RP is a critical function for PIM-SM deployments.
RP redundancy is always recommended. In a shared-tree model, multicast
traffic from the multicast source is routed via the RP. If the RP goes down, the
multicast receivers do not receive traffic until the RP comes up again. In
general, more than one RP is configured (for a group range) to provide RP
redundancy. The PIM-SM router acting as a BSR advertises the list of
candidate RPs to all the PIM routers in the network. Each PIM router then
runs the RP selection algorithm to determine an RP for the given group
range. All the interested PIMSM routers then initiate re-reception of traffic
through this new RP, and the multicast traffic is rerouted via the new RP. This
is to provide high availability to the multicast applications and help ensure
that the multicast traffic is recovered quickly in such scenarios.