PIM Source-Specific Mode | 77735PIM Source-Specific ModePIM Source-Specific Mode is supported on platforms: c e sPIM-SSM is supported on the E-Series ExaScale platform with FTOS 8.1.1.0 and later.PIM-Source-Specific Mode (PIM-SSM) is a multicast protocol that forwards multicast traffic from a singlesource to a subnet. In the other versions of Protocol Independent Multicast (PIM), a receiver subscribes toa group only. The receiver receives traffic not just from the source in which it is interested but from allsources sending to that group. PIM-SSM requires that receivers specify the sources in which they areinterested 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 thesource rather than first using shared trees. PIM-SM requires a shared tree rooted at the RP becauseIGMPv2 receivers do not know about the source sending multicast data. Multicast traffic passes from thesource to the receiver through the RP, until the receiver learns the source address, at which point it switchesto the SPT. PIM-SSM uses IGMPv3. Since receivers subscribe to a source and group, the RP and sharedtree is unnecessary, so only SPTs are used. On Dell Force10 systems, it is possible to use PIM-SM withIGMPv3 to achieve the same result, but PIM-SSM eliminates the unnecessary protocol overhead.PIM-SSM also solves the multicast address allocation problem. Applications should use unique multicastaddresses because if multiple applications use the same address, receivers receive unwanted traffic.However, global multicast address space is limited. Currently GLOP/EGLOP is used to statically assignInternet-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 multicastaddress allocation scheme has been accepted yet. PIM-SSM eliminates the need for unique multicastaddresses because routing decisions for (S1, G1) are independent from (S2, G1). As a result, subnets do notreceive unwanted traffic when multiple applications use the same address.In Figure 35-1, Receiver 1 is an IGMPv2 host. The packets for group 239.0.0.2 travel to it first via the RP,then by the SPT. Receiver 2 is an IGMPv3 host. The packets for group 239.0.0.1 travel only via the STP.
