The IPv4 implementation of PIM supports PIM dense mode, PIM sparse mode, PIM sparse-dense mode, and PIM source-specific multicast (PIM SSM).
Figure 8 represents how PIM builds a source, group (S,G) entry in a source-rooted tree (SRT). When multiple routers are connected to a multiaccess network, one router becomes 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.
Figure 8: Source-Rooted Tree
- 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.
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