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PIM Source-Specific Multicast

PIM SSM is an extension of the PIM protocol. Using SSM, a client can receive multicast traffic directly from the source. PIM SSM uses PIM sparse mode functionality to create an SPT between the client and the source, but builds the SPT without using an RP.

By default, the SSM group multicast address is limited to the IP address range 232.0.0.0 to 232.255.255.255. You can use the ip pim ssm command to extend SSM operations into another Class D range. (See Configuring PIM SSM .)

An SSM-configured network has the following advantages over a traditionally configured PIM sparse mode network include the following:

No need for shared trees or RP mapping (no RP is required).
No need for RP-to-RP source discovery through Multicast Source Discovery Protocol (MSDP).
Simplified administrative deployment; you need only configure PIM sparse mode on all router interfaces and issue the necessary SSM commands (including specifying IGMPv3 on the receiver local area network).
Support for source lists; you can use source lists, supported in IGMPv3, where only specified sources send traffic to the SSM group.

In a PIM SSM–configured network, an E-series router subscribes to an SSM channel (by means of IGMPv3 or by means of IGMP ssm-mapping for IGMPv2/v1 joins), requesting to join group G and source S. The directly connected PIM sparse mode router, the designated router of the receiver, sends an (S,G) join message to its RPF neighbor for the source. For PIM SSM, the RP is not contacted in this process by the receiver (as happens in normal PIM sparse mode operations).

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