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 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.
IGMPv3 Host Signalling
In IGMPv3, hosts signal membership to last hop routers of multicast groups. Hosts can 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).
IGMPv3 can operate with both Internet Standard Multicast (ISM) and Source Specific Multicast (SSM). In
ISM, both exclude and include mode reports are applicable. In SSM, only include mode reports are accepted
by the last-hop router. Exclude mode reports are ignored.
Catalyst 2960-XR Switch IP Multicast Routing Configuration Guide, Cisco IOS Release 15.0(2)EX1
80 OL-29426-01
Configuring SSM
SSM IP Address Range
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