Cisco Systems 3560 Manual De Usuario

No MSDP source-active (SA) messages within the SSM range are accepted, generated, or 
forwarded.

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.

This section contains the guidelines for configuring SSM.

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 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.

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