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Configuring IP Multicast Routing
Use the ip pim ssm global configuration command to configure the SSM range and to enable SSM. This
configuration has the following effects:
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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 ISM and 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.
Configuration Guidelines
This section contains the guidelines for configuring SSM.
Legacy Applications Within the SSM Range Restrictions
Existing applications in a network predating SSM do not work within the SSM range unless they are
modified to support (S, G) channel subscriptions. Therefore, enabling SSM in a network can cause
problems for existing applications if they use addresses within the designated SSM range.
Address Management Restrictions
Address management is still necessary to some degree when SSM is used with Layer 2 switching
mechanisms. Cisco Group Management Protocol (CGMP), IGMP snooping, or Router-Port Group
Management Protocol (RGMP) support only group-specific filtering, not (S, G) channel-specific
filtering. If different receivers in a switched network request different (S, G) channels sharing the same
group, they do not benefit from these existing mechanisms. Instead, both receivers receive all (S, G)
channel traffic and filter out the unwanted traffic on input. Because SSM can re-use the group addresses
in the SSM range for many independent applications, this situation can lead to decreased traffic filtering
in a switched network. For this reason, it is important to use random IP addresses from the SSM range
for an application to minimize the chance for re-use of a single address within the SSM range between
different applications. For example, an application service providing a set of television channels should,
even with SSM, use a different group for each television (S, G) channel. This setup guarantees that
multiple receivers to different channels within the same application service never experience traffic
aliasing in networks that include Layer 2 switches.
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
44-16
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.
Chapter 44
Configuring IP Multicast Routing
OL-12247-04
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