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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 48 Configuring IP Multicast Routing
Configuring IP Multicast Routing
• Before you can configure and use SSM mapping with DNS lookups, you must be able 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. Go to this URL for more information:
http://www.cisco.com/warp/public/cc/pd/nemnsw/nerr/index.shtml
These are the SSM mapping restrictions:
• The SSM mapping feature does not have all the benefits of full SSM. Because SSM mapping takes
a group join from a host and identifies this group with an application associated with one or more
sources, it can only support one such application per group. Full SSM applications can still share
the same group as in SSM mapping.
• Enable IGMPv3 with care on the last hop router when you rely solely on SSM mapping as a
transition solution for full SSM. When you enable both SSM mapping and IGMPv3 and the hosts
already support IGMPv3 (but not SSM), the hosts send IGMPv3 group reports. SSM mapping does
not support these IGMPv3 group reports, and the router does not correctly associate sources with
these reports.
SSM Mapping Overview
In a typical STB deployment, each TV channel uses one separate IP multicast group and has one active
server host sending the TV channel. A single server can send multiple TV channels, but each to a
different group. In this network environment, if a router receives an IGMPv1 or IGMPv2 membership
report for a particular group, the report addresses the well-known TV server for the TV channel
associated with the multicast group.
When SSM mapping is configured, if a router receives an IGMPv1 or IGMPv2 membership report for a
particular group, the router translates this report into one or more channel memberships for the
well-known sources associated with this group.
When the router receives an IGMPv1 or IGMPv2 membership report for a group, the router uses SSM
mapping to determine one or more source IP addresses for the group. SSM mapping then translates the
membership report as an IGMPv3 report and continues as if it had received an IGMPv3 report. The
router then sends PIM joins and continues to be joined to these groups as long as it continues to receive
the IGMPv1 or IGMPv2 membership reports, and the SSM mapping for the group remains the same.
SSM mapping enables the last hop router to determine the source addresses either by a statically
configured table on the router or through a DNS server. When the statically configured table or the DNS
mapping changes, the router leaves the current sources associated with the joined groups.
Go to this URL for additional information on SSM mapping:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801a6d6f.
html

Static SSM Mapping

With static SSM mapping, you can configure the last hop router to use a static map to determine the
sources that are sending to groups. Static SSM mapping requires that you configure ACLs to define
group ranges. Then you can map the groups permitted by those ACLs to sources by using the ip igmp
static ssm-map global configuration command.
You can configure static SSM mapping in smaller networks when a DNS is not needed or to locally
override DNS mappings. When configured, static SSM mappings take precedence over DNS mappings.