•
Before you configure static SSM mapping, you must configure access control lists (ACLs) that define
the group ranges to be mapped to source addresses.
•
Before you can configure and use SSM mapping with DNS lookups, you need to add records to a running
DNS server. If you do not already have a DNS server running, you need to install one.
You can use a product such as Cisco Network Registrar to add records to a running
DNS server.
Note
Related Topics
Configuring Basic IP Multicast Routing, on page 290
Configuring Source Specific Multicast, on page 267
Restrictions for Configuring SSM
The following are the restrictions for configuring SSM:
•
To run SSM with IGMPv3, SSM must be supported in the Cisco IOS router, the host where the application
is running, and the application itself.
•
Existing applications in a network predating SSM will not work within the SSM range unless they are
modified to support (S, G) channel subscriptions. Therefore, enabling SSM in a network may cause
problems for existing applications if they use addresses within the designated SSM range.
• IGMP Snooping—IGMPv3 uses new membership report messages that might not be correctly recognized
by older IGMP snooping devices.
•
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 devices.
•
In PIM-SSM, the last hop router will continue 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 will be maintained, even if the source is
not sending traffic for longer periods of time (or even never).
The opposite situation occurs with 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 reestablished after packets from the source arrive again
through the RPT (rendezvous point tree). Because no mechanism in PIM-SSM notifies a receiver that
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
260 OL-32598-01
Configuring SSM
Restrictions for Configuring SSM
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