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Catalyst 3560 Switch Software Configuration Guide
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Chapter 44 Configuring IP Multicast Routing
Configuring IP Multicast Routing
How SSM Differs from Internet Standard Multicast
The current 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 the
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.
The ISM service consists of the delivery of IP datagrams from any source to a group of receivers called
the multicast host group. The datagram traffic for the multicast host group consists of datagrams with an
arbitrary IP unicast source address S and the multicast group address G as the IP destination address.
Systems receive this traffic by becoming members of the host group.
Membership in a host group simply requires signalling the host group through IGMP version 1, 2, or 3.
In SSM, delivery of datagrams is based on (S, G) channels. In both SSM and ISM, no signalling is
required to become a source. However, in SSM, receivers must subscribe or unsubscribe to (S, G)
channels to receive or not receive traffic from specific sources. In other words, receivers can receive
traffic only from (S, G) channels to which they are subscribed, whereas in ISM, receivers need not know
the IP addresses of sources from which they receive their traffic. The proposed standard approach for
channel subscription signalling use IGMP include mode membership reports, which are supported only
in IGMP version 3.
SSM IP Address Range
SSM can coexist with the ISM service by applying the SSM delivery model to a configured subset of the
IP multicast group address range. Cisco IOS software allows SSM configuration for the IP multicast
address range of 224.0.0.0 through 239.255.255.255. When an SSM range is defined, existing IP
multicast receiver applications do not receive any traffic when they try to use an address in the SSM
range (unless the application is modified to use an explicit (S, G) channel subscription).
SSM Operations
An established network, in which IP multicast service is based on PIM-SM, can support SSM services.
SSM can also be deployed alone in a network without the full range of protocols that are required for
interdomain PIM-SM (for example, MSDP, Auto-RP, or bootstrap router [BSR]) if only SSM service is
needed.
If SSM is deployed in a network already configured for PIM-SM, only the last-hop routers support SSM.
Routers that are not directly connected to receivers do not require support for SSM. In general, these
not-last-hop routers must only run PIM-SM in the SSM range and might need additional access control
configuration to suppress MSDP signalling, registering, or PIM-SM shared tree operations from
occurring within the SSM range.
Use the ip pim ssm global configuration command to configure the SSM range and to enable SSM. This
configuration has the following effects:
• For groups within the SSM range, (S, G) channel subscriptions are accepted through IGMPv3
include-mode membership reports.
• PIM operations within the SSM range of addresses change to PIM-SSM, a mode derived from
PIM-SM. In this mode, only PIM (S, G) join and prune messages are generated by the router, and
no (S, G) rendezvous point tree (RPT) or (*, G) RPT messages are generated. Incoming messages
related to RPT operations are ignored or rejected, and incoming PIM register messages are
immediately answered with register-stop messages. PIM-SSM is backward-compatible with
PIM-SM unless a router is a last-hop router. Therefore, routers that are not last-hop routers can run
PIM-SM for SSM groups (for example, if they do not yet support SSM).