Cisco IE 4000, Software Configuration Manual

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Configuring IP Multicast Routing
Information About Cisco’s Implementation of IP Multicast Routing
To run SSM with IGMPv3, SSM must be supported in the Cisco IOS router, the host where the application is running,
and the application itself.
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).

No MSDP source-active (SA) messages within the SSM range are accepted, generated, or forwarded.