Cisco Systems 3560 Manual De Usuario

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 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).

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).