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Configuring IP Multicast Routing
Information About Cisco's Implementation of IP Multicast Routing
IGMPv3 Host Signalling
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.
Information About Source Specific Multicast Mapping
The Source Specific Multicast (SSM) mapping feature supports SSM transition when supporting SSM on the end system
is impossible or unwanted due to administrative or technical reasons. You can use SSM mapping to leverage SSM for
video delivery to legacy STBs that do not support IGMPv3 or for applications that do not use the IGMPv3 host stack.
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.
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.
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 (see
Figure 88 on page
683
684).
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