Protocol Independent Multicast-Source Specific Multicast

Note

PIM is supported in Global Routing Table (GRT) only.

Source Specific Multicast optimizes PIM-SM by simplifying the many-to-many model. Because most multicast applications distribute content to a group in one direction, SSM uses a one-to-many model that uses only a subset of the PIM-SM features. This model is more efficient and reduces the load on multicast routing devices.

SSM only builds source-based SPTs. Whereas PIM-SM always joins a shared tree first, and then switches to the source tree, SSM eliminates the need to start with a shared tree by immediately joining a source through the SPT. SSM avoids using an RP and RP-based shared trees, which can be a potential problem.

Until now only one channel for one group was allowed to exist in ssm map. From now on multiple channels for the members of the SSM group are allowed to be configured in this map.

This configuration is ideal for applications like television channel distribution and other content-distribution businesses. Banking and trade applications can also use SSM as it provides more control over the hosts receiving and sending data over their networks.

When a v2 report in SSM range is received it is translated to an igmpv3 report message with one group record with type ALLOW and the source lists copied from the igmp ssm map static entries and passed to igmpv3 module. When a v2 leave in SSM range is received it is translated to an igmpv3 report message with one group record with type BLOCK and the source lists copied from the igmp ssm map static entries and passed to igmpv3 module. This behaviour is displayed only when PIM-SSM mode is enabled.

SSM applications use IP addresses reserved by the Internet Assigned Numbers Authority (IANA) in the 232/8 range (232.0.0.0 to 232.255.255.255). SSM recognizes packets in this range and controls the behavior of multicast routing devices and hosts that use these addresses. When a source (S) transmits IP datagrams to an SSM destination address (G), a receiver can receive these datagrams by subscribing to the (S,G) channel.

A channel is a source-group (S,G) pair where S is the source that sends to the multicast group and G is an SSM group address. SSM defines channels on an individual or multiple source basis, which enforces the one-to-many concept of SSM applications. In an SSM channel, each group is associated with multiple sources.