•
The prevention against DoS attacks is an important factor for Internet broadcast services because, with
their exposure to a large number of receivers, they are the most common targets for such attacks.
•
The ease of installation and operation of SSM makes it ideal for network operators, especially in those
cases where content needs to be forwarded between multiple independent PIM domains (because there
is no need to manage MSDP for SSM between PIM domains).
SSM Mapping
In a typical set-top box (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.
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.
After configuring the ACLs to define group ranges, you can then map the groups permitted by those ACLs
to sources by using the
ip igmp ssm-map static
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.
DNS-Based SSM Mapping
You can use DNS-based SSM mapping to configure the last hop router to perform a reverse DNS lookup to
determine sources sending to groups. When DNS-based SSM mapping is configured, the router constructs a
domain name that includes the group address and performs a reverse lookup into the DNS. The router looks
up IP address resource records and uses them as the source addresses associated with this group. SSM mapping
supports up to 20 sources for each group. The router joins all sources configured for a group.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
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Configuring SSM
SSM Mapping
