This implementation of PIM supports PIM sparse mode and PIM source-specific multicast (PIM SSM) for IPv6 multicast.
SSM is an extension to the Any Source Multicast (ASM) service model and facilitates the deployment of broadcast (one-to-many) applications, such as Internet TV and radio where large receiver audiences require traffic from a few well-known sources.
Figure 21 represents how PIM builds an (S,G) entry in an SRT. When multiple routers are connected to a multiaccess network, one router is assigned the role of the designated router. The designated router receives data from the source on interface 1/0 and multicasts the data to its downstream neighbors on interfaces 1/1, 2/0, and 2/1. In the designated router routing table, the entry for this operation lists the source as the IP address of the source and the group as the IP address of the multicast group.
Neighbors exchange hello messages periodically to determine the designated router. The router with the highest network layer address becomes the designated router. If the designated router subsequently receives a hello message from a neighbor with a higher network layer address, that neighbor becomes the designated router.
Figure 21: Source-Rooted Tree
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