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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 41 IPv4 Multicast
Feature Overview

PIM SSM Mapping

PIM SSM mapping supports SSM transition in cases where neither the URD nor IGMP v3lite is
available, or when supporting SSM on the end system is not feasible. SSM mapping enables you to
leverage SSM for video delivery to legacy set-top boxes (STBs) that do not support IGMPv3 or for
applications that do not take advantage of the IGMPv3 host stack. URD and IGMPv3lite are applications
used on receivers which do not have SSM support.
SSM mapping introduces a means for the last hop router to discover sources sending to groups. When
SSM mapping is configured, if a router receives an IGMPv1 or IGMPv2 membership report for a
particular group G, the router translates this report into one or more (S, G) channel memberships for the
well-known sources associated with this group.
SSM mapping only needs to be configured on the last hop router connected to receivers. No support is
needed on any other routers in the network. When the router receives an IGMPv1 or IGMPv2
membership report for a group G, the router uses SSM mapping to determine one or more source IP
addresses for the group G. SSM mapping then translates the membership report as an IGMPv3 report
INCLUDE (G, [S1, G], [S2, G]...[Sn, G] and continues as if it had received an IGMPv3 report.

Static SSM Mapping

SSM static mapping enables you to configure the last hop router to use a static map to determine the
sources sending to groups. Static SSM mapping requires that you configure access lists (ACLs) to define
group ranges. The groups permitted by those ACLs then can be mapped to sources using the
ip igmp static ssm-map command.
For more information on SSM Mapping, see the IP Multicast: IGMP Configuration Guide at:
http://www.cisco.com/en/US/docs/ios-xml/ios/ipmulti_igmp/configuration/xe-3s/imc_ssm_map.html

Reverse Path Forwarding

Reverse-path forwarding is used for forwarding multicast datagrams. It functions as follows:
•If a router receives a datagram on an interface it uses to send unicast packets to the source, it means
the packet has arrived on the RPF interface.
•If the packet arrives on the RPF interface, a router forwards the packet out the interfaces present in
the outgoing interface list of a multicast routing table entry.
•If the packet does not arrive on the RPF interface, the packet is silently discarded to prevent loops.
PIM SSM uses source trees to forward datagrams; the RPF check is performed as follows:
•If a PIM router has source-tree state (that is, an [S, G] entry is present in the multicast routing table),
the router performs the RPF check against the IPv4 address of the source of the multicast packet.
•Sparse-mode PIM uses the RPF lookup function to determine where it needs to send joins and
prunes. (S, G) joins (which are source-tree states) are sent toward the source.
For more information on Reverse Path Forwarding, see the Configuring Unicast Reverse Path
Forwarding document at:
http://www.cisco.com/en/US/docs/ios/12_2/security/configuration/guide/scfrpf.html