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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Configuring IP Multicast Routing
Configuring IP Multicast Routing
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.
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), see the
“Configuring IGMP Version 3” section of the “Configuring IP Multicast Routing” chapter.
State Maintenance Limitations
In PIM-SSM, the last hop router continues to periodically send (S, G) join messages if appropriate (S,
G) subscriptions are on the interfaces. Therefore, as long as receivers send (S, G) subscriptions, the
shortest path tree (SPT) state from the receivers to the source is maintained, even if the source does not
send traffic for longer periods of time (or even never).
This case is opposite to PIM-SM, where (S, G) state is maintained only if the source is sending traffic
and receivers are joining the group. If a source stops sending traffic for more than 3 minutes in PIM-SM,
the (S, G) state is deleted and only re-established after packets from the source arrive again through the
RPT. Because no mechanism in PIM-SSM notifies a receiver that a source is active, the network must
maintain the (S, G) state in PIM-SSM as long as receivers are requesting receipt of that channel.
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