Information About Ssm - Cisco Catalyst 3650 Configuration Manual

Configuring SSM
• In PIM-SSM, the last hop router continues to periodically send (S, G) join messages if appropriate (S,

Information About SSM

The source-specific multicast (SSM) feature is an extension of IP multicast in which datagram traffic is
forwarded to receivers from only those multicast sources that the receivers have explicitly joined. For multicast
groups configured for SSM, only SSM distribution trees (no shared trees) are created.
This section describes how to configure source-specific multicast (SSM). For a complete description of the
SSM commands in this section, refer to the IP Multicast Command Reference. To locate documentation for
other commands that appear in this chapter, use the command reference master index, or search online.
SSM Components Overview
SSM is a datagram delivery model that best supports one-to-many applications, also known as broadcast
applications. SSM is a core networking technology for the Cisco implementation of IP multicast solutions
targeted for audio and video broadcast application environments. The device supports the following components
that support SSM implementation:
• Protocol independent multicast source-specific mode (PIM-SSM)
• Internet Group Management Protocol version 3 (IGMPv3)
Related Topics
Configuring Source Specific Multicast, on page 237
SSM with IGMPv3 Example, on page 246
How SSM Differs from Internet Standard Multicast
The standard IP multicast infrastructure in the Internet and many enterprise intranets is based on the PIM-SM
protocol and Multicast Source Discovery Protocol (MSDP). These protocols have proved to be reliable,
extensive, and efficient. However, they are bound to the complexity and functionality limitations of the Internet
Standard Multicast (ISM) service model. For example, with ISM, the network must maintain knowledge about
which hosts in the network are actively sending multicast traffic. With SSM, this information is provided by
receivers through the source addresses relayed to the last-hop devices by IGMPv3. SSM is an incremental
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G) subscriptions are on the interfaces. Therefore, as long as receivers send (S, G) subscriptions, the
shortest path tree (SPT) state from the receivers to the source is maintained, even if the source does not
send traffic for longer periods of time (or even never).
The opposite situation occurs with PIM-SM, where (S, G) state is maintained only if the source is sending
traffic and receivers are joining the group. If a source stops sending traffic for more than 3 minutes in
PIM-SM, the (S, G) state is deleted and only reestablished after packets from the source arrive again
through the RPT (rendezvous point tree). Because no mechanism in PIM-SSM notifies a receiver that
a source is active, the network must maintain the (S, G) state in PIM-SSM as long as receivers are
requesting receipt of that channel.
PIM-SSM is the routing protocol that supports the implementation of SSM and is derived from PIM
sparse mode (PIM-SM).
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
Information About SSM
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