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Cisco ME 3800X and 3600X Switch Software Configuration Guide
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Chapter 33 Configuring IP Multicast Routing
Configuring IP Multicast Routing
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).
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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 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
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.
IGMP Snooping and CGMP Limitations
IGMPv3 uses new membership report messages that might not be correctly recognized by older IGMP
snooping switches.
For more information about switching issues related to IGMP (especially with CGMP), refer to the
“Configuring IGMP Version 3” section of the “Configuring IP Multicast Routing” chapter.
