IGMPv3 and Source-Specific Multicast Protocols

When IGMPv3 and PIM Sparse (PIM-SM) are enabled, the source specific multicast service (SSM) can be configured. SSM simplifies PIM-SM by eliminating the RP and all protocols related to the RP. IGMPv3 and PIM-SM must be enabled on any ports where you want SSM to operate.

PIM Source-Specific Multicast (SSM) is a subset of the PIM SM protocol. In PIM SSM mode, the shortest path tree (SPT) is created at the source. The SPT is created between the receiver and source, but the SPT is built without the help of the RP. The router closest to the interested receiver host is notified of the unicast IP address of the source for the multicast traffic. PIM SSM goes directly to the source-based distribution tree without the need of the RP connection. PIM SSM is different from PIM SM because it forms its own SPT, without forming a shared tree.

You can configure a single SSM group address, the default multicast address group range is 232.0.0.0/8.

You can configure multiple SSM group ranges using an ACL. When you are using an ACL, the following configuration considerations apply:

  • The existing ssm-enable range command with the group-address/address-mask options is used to identify a single SSM group address.
  • The ACL must be configured with the SSM group address in the permit clause of the ssm-enable range command using the ACL ID or ACL name. If the ssm-enable range command with the group-address/address-mask options permits a clause, then that group will also operate in the PIM-SM mode.
  • If the ssm-enable range command with an ACL ID or ACL name is configured with a non-existent or empty ACL, then the SSM group will operate in PIM-SM mode (non PIM-SSM mode). However when an ACL is added or updated, then the group will exist in a PIM-SSM mode. By default, an empty ACL will deny all.
  • By default, the group address mentioned in the IGMPv2 ssm-mapping ACL will decide if the group address is a PIM-SSM group or non PIM-SSM group. Therefore, if a user wants to prevent a group from operating in PIM-SSM mode, then the user’s configuration must consistently deny the group in all configuration options for PIM-SSM range.
  • ACL of any type (named or unnamed, standard or extended) can be used to specify the SSM group range. If an extended ACL is used, then the destination ip address should be used to specify the group address. Any configuration in the source address of an extended ACL is ignored. Only permit statements are considered in the ACL configuration. Any deny statements in the ACL clause are also ignored.
Parent topic: Source-Specific Multicast