PIM 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 the otherversions of protocol independent multicast (PIM), a receiver subscribes to a group only. The receiver receives traffic not just from thesource in which it is interested but from all sources sending to that group. PIM-SSM requires that receivers specify the sources in whichthey 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 first usingshared trees. PIM-SM requires a shared tree rooted at the RP because IGMPv2 receivers do not know about the source sending multicastdata. Multicast traffic passes from the source to the receiver through the RP, until the receiver learns the source address, at which point itswitches to the SPT. PIM-SSM uses IGMPv3. Because receivers subscribe to a source and group, the RP and shared tree is unnecessary;only SPTs are used. On Dell Networking systems, it is possible to use PIM-SM with IGMPv3 to achieve the same result, but PIM-SSMeliminates 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. Currently GLOP/EGLOP is used to statically assign Internet-routable multicast addresses, but each autonomous system number yields only 255 multicastaddresses. For short-term applications, an address could be leased, but no global dynamic multicast address allocation scheme has beenaccepted 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.Topics:• Implementation Information• Configure PIM-SSM• Enabling PIM-SSM• Use PIM-SSM with IGMP Version 2 Hosts• Electing an RP using the BSR MechanismImplementation 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 Prune requestsin 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 range and SSMrange 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.39688 PIM Source-Specific Mode (PIM-SSM)
