для Cisco Cisco IOS Software Release 12.0(15)S

through 239.255.255.255. When an SSM range is defined, existing IP multicast receiver applications 
will not receive any traffic when they try to use addresses in the SSM range (unless the application is 
modified to use explicit (S, G) channel subscription or is SSM enabled through URD).

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 (Cisco IOS Release 12.0 or later 
releases is recommended), then only the last hop routers must be upgraded to a Cisco IOS 
Release 12.0(15)S or later release image that supports SSM. Routers that are not directly connected to 
receivers can run Cisco IOS Release 12.0 or later releases. In general, these nonlast hop routers must 
only run PIM-SM in the SSM range, and may need additional access control configuration to suppress 
MSDP signalling, registering, or PIM-SM shared tree operations from occurring within the SSM range.

In Cisco IOS Release 12.0(15)S and later releases, the SSM mode of operation is enabled by configuring 
the SSM range through the ip pim ssm command. This configuration has the following effects:

For groups within the SSM range, (S, G) channel subscriptions are accepted through IGMPv3 
INCLUDE mode membership reports, IGMP v3lite, or URD (each of these methods must be 
configured on a per-interface basis). IGMP v3lite and URD (S, G) channel subscriptions are 
ignored for groups outside the SSM range.

Both IGMP v3lite and URD are based on utilizing existing application IGMP group membership 
and extending it with their respective (S, G) channel subscription mechanism, which is ignored by 
Cisco IOS software outside the SSM range of addresses. Within the SSM range, IGMP Version 1 
(IGMPv1) or Version 2 (IGMPv2) group membership reports or IGMPv3 EXCLUDE mode 
membership reports are acted upon only in conjunction with an (S, G) specific membership report 
from URD or IGMP v3lite.

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 will be accepted, generated, or 
forwarded.

IGMPv3 is the third version of the IETF standards track protocol in which hosts signal membership to 
last hop routers of multicast groups. IGMPv3 introduces the ability for hosts to signal group 
membership with filtering capabilities with respect to sources. A host can either signal that it wants to 
receive traffic from all sources sending to a group except for some specific sources (called EXCLUDE 
mode), or that it wants to receive traffic only from some specific sources sending to the group (called 
INCLUDE mode).