39PIM Source-Specific Mode (PIM-SSM)PIM source-specific mode (PIM-SSM) is a multicast protocol that forwards multicast traffic from a single source to a subnet. In theother versions of protocol independent multicast (PIM), a receiver subscribes to a group only. The receiver receives traffic not justfrom the source in which it is interested but from all sources sending to that group. PIM-SSM requires that receivers specify thesources in which they are interested using IGMPv3 include messages to avoid receiving unwanted traffic.PIM-SSM is more efficient than PIM-SM because it immediately creates shortest path trees (SPT) to the source rather than firstusing shared trees. PIM-SM requires a shared tree rooted at the RP because IGMPv2 receivers do not know about the sourcesending multicast data. Multicast traffic passes from the source to the receiver through the RP, until the receiver learns the sourceaddress, at which point it switches to the SPT. PIM-SSM uses IGMPv3. Because receivers subscribe to a source and group, the RPand shared tree is unnecessary; only SPTs are used. On Dell Networking systems, it is possible to use PIM-SM with IGMPv3 toachieve the same result, but PIM-SSM eliminates the unnecessary protocol overhead.PIM-SSM also solves the multicast address allocation problem. Applications must use unique multicast addresses because if multipleapplications use the same address, receivers receive unwanted traffic. However, global multicast address space is limited. CurrentlyGLOP/EGLOP is used to statically assign Internet-routable multicast addresses, but each autonomous system number yields only255 multicast addresses. For short-term applications, an address could be leased, but no global dynamic multicast address allocationscheme has been accepted yet. PIM-SSM eliminates the need for unique multicast addresses because routing decisions for (S1, G1)are independent from (S2, G1). As a result, subnets do not receive unwanted traffic when multiple applications use the same address.Implementation Information• The Dell Networking implementation of PIM-SSM is based on RFC 3569.• The Dell Networking OS reduces the number of control messages sent between multicast routers by bundling Join and Prunerequests in the same message.Important Points to Remember• The default SSM range is 232/8 always. Applying an SSM range does not overwrite the default range. Both the default rangeand SSM range are effective even when the default range is not added to the SSM ACL.• Extended ACLs cannot be used for configuring SSM range. Be sure to create the ACL first and then apply it to the SSM range.• The default range is always supported, so range can never be smaller than the default.Configure PIM-SSMConfiguring PIM-SSM is a two-step process.1. Configure PIM-SSM.2. Enable PIM-SSM for a range of addresses.Related Configuration Tasks• Use PIM-SSM with IGMP Version 2 Hosts640 PIM Source-Specific Mode (PIM-SSM)
