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Configuring SSM Mapping for Dynamic IGMP and MLD

Source-specific multicast (SSM) is a service model that identifies session traffic by both source and group address. SSM builds shortest-path trees (SPTs) directly represented by (S,G) pairs. The “S” refers to the source's unicast IP address, and the “G” refers to the specific multicast group address. The SSM (S,G) pairs are called channels to differentiate them from any-source multicast (ASM) groups. SSM is ideal for one-to-many multicast services such as network entertainment channels. Although ASM supports one-to-many, its method of source discovery is less efficient than SSM. For example, if you click a link in a browser, ASM notifies the receiver about the group information, but not the source information. With SSM, the client receives both source and group information.

To deploy SSM successfully, you need an end-to-end multicast-enabled network and applications that use an Internet Group Management Protocol version 3 (IGMPv3) or Multicast Listener Discovery version 2 (MLDv2) stack, or you need to configure SSM mapping from IGMPv1 or IGMPv2 to IGMPv3. An IGMPv3 stack provides the capability of a host operating system to use the IGMPv3 protocol.

You can accommodate hosts that do not support IGMPv3 or MLDv1 by using SSM mapping. SSM mapping translates IGMPv1 or IGMPv2 membership reports to an IGMPv3 report, and MLDv1 reports to MLDv2. SSM mapping applies to all group addresses that match the policy, not just those that conform to SSM addressing conventions (232/8 for IPv4, ff30::/32 through ff3F::/32 for IPv6).

Best Practice:

Create separate SSM maps for the IPv4 and IPv6 address families when both families require SSM support.

If you apply an SSM map policy containing both IPv4 and IPv6 addresses to an interface in an IPv4 context (using IGMP), only the IPv4 addresses in the list are used. If there are no such addresses, no action is taken. Similarly, if you apply an SSM map policy containing both IPv4 and IPv6 addresses to an interface in an IPv6 context (using MLD), only the IPv6 addresses in the list are used. If there are no such addresses, no action is taken.

To configure SSM mapping for dynamic IGMP:

  1. Create an SSM policy to match the desired IPv4, IPv6, or both group addresses.
  2. Configure terms for the policy to identify and accept group addresses
  3. Apply the SSM map policy to the dynamic interface in a dynamic profile.

For example, the following configuration creates SSM policy ssm-1. The policy term v4 exactly matches the IPv4 SSM group address 233.252.1.1/32. The policy rejects all other addresses. The policy ssm-1 is then applied to dynamic interfaces created when the igmp-prof dynamic profile is instantiated.

For example, the following configuration creates SSM policy ssm-2. Policy term v6 exactly matches the IPv6 group address ff35::1/128. The policy rejects all other addresses. The policy ssm-2 is then applied to dynamic interfaces created when the mld-prof dynamic profile is instantiated.

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