Source Specific Multicast (SSM)

Recall that networks enabled with multicast protocols from the Internet Standard Multicast (ISM) model must maintain knowledge of both sources and groups (S, G) to enable the receiver to subscribe to (*, G) groups without any knowledge of what senders they are receiving the multicast traffic from. With the SSM model however, the receivers must explicitly subscribe and unsubscribe to the (S, G) multicast channels from which they want to receive traffic. Your receivers must specify both the source unicast IP address that the stream is originating from and the multicast group address that the stream is destined to. Receivers must learn of the sources by a method external to the SSM model, such as through a user selection from a list of sources on a webpage. Internet-based multicast applications, such as audio, video, and stock quote streaming, fit the one-to-many SSM model.

Benefits of SSM over ISM are

• Reduced source address state maintenance by routers SSM enables routers to maintain only the group IP address in their routing tables and not both source and group IP addresses, greatly reducing the amount of memory and processing that your routers require to distribute multicast traffic. SSM no longer requires the source discovery and distribution protocols ISM uses, including PIM-SM, Auto-RP, BSR, and MSRP.

• Denial of service protection The SSM network only forwards traffic if last-hop routers explicitly receive IGMPv3 (S, G) join messages. In contrast, ISM forwards multicast traffic from any active source to all receivers requesting traffic from group G's.

• Reduced group address management Because SSM indexes streams by unicast source IP address in addition to the group IP address, you can configure sources of different applications to use the same group address without the receivers worrying about receiving traffic from both sources. Traffic from each source is forwarded through the network independently of one another. As a result, you do not have to manage your multicast address allocation to avoid group IP address overlap as you must with ISM multicast.

The drawbacks to SSM are

• SSM state maintenance As long as receivers send IGMPv3 joins for a source, the routers within the SSM network maintain (*, G) state for the source of the multicast, even if the source is no longer sending data. In contrast, ISM routers detect sources that discontinue sending data and tear down the (S, G) state.

• IGMP snooping and CGMP support IGMP Snooping and CGMP may not recognize IGMPv3 Include/Exclude messages. Ensure that the version of Cisco IOS you are running supports compatibility between these protocols before deploying multicast in your network.

Recall that IGMPv3, IGMP v3lite, and URD enable receivers to send join messages that contain the sources and group (S, G) pairs they want to join. Once first-hop routers join the RP shared tree, they switch over to the SPT by sending PIM (S, G) joins directly to the source, creating a SPT from the source to the receivers. The source multicast stream is then forwarded through the SPT from the source directly to the receivers using the PIM (S, G) state tables and RPF.

The PIM-SSM multicast protocol is derived from PIM-SM and includes only the generation and sending of PIM join (S, G) messages destined to sources. PIM (S, G) and (*, G) join messages are no longer generated and sent to RPs by PIM-SSM last-hop routers. First-hop routers no longer generate PIM-SM register messages. Furthermore, routers enabled with PIM-SSM ignore all Auto-RP, BSR, and MSDP-related messages.

You can run PIM-SSM concurrently with PIM-SM in your network. To enable SSM in a network already enabled for PIM-SM, you need only enable the last-hop routers with the SSM ip pim ssm interface configuration command on select interfaces. The last-hop routers process PIM messages for groups within the SSM IP address range with PIM-SSM functionality. PIM routers treat all PIM messages for groups outside the SSM range with PIM-SM functionality.

To enable PIM-SSM alone in your network, use the ip pim ssm command on all router interfaces.