Concept Guide

PIM Source-Specic Mode (PIM-SSM)
Dell Networking OS supports PIM source-specic mode (PIM-SSM).
PIM-SSM is a multicast protocol that forwards multicast trac from a single source to a subnet. In the other versions of protocol
independent multicast (PIM), a receiver subscribes to a group only. The receiver receives trac not just from the source in which it is
interested but from all sources sending to that group. PIM-SSM requires that receivers specify the sources in which they are interested
using IGMPv3 include messages to avoid receiving unwanted trac.
PIM-SSM is more ecient than PIM-SM because it immediately creates shortest path trees (SPT) to the source rather than rst using
shared trees. PIM-SM requires a shared tree rooted at the RP because IGMPv2 receivers do not know about the source sending multicast
data. Multicast trac passes from the source to the receiver through the RP, until the receiver learns the source address, at which point it
switches 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-SSM
eliminates the unnecessary protocol overhead.
PIM-SSM also solves the multicast address allocation problem. Applications must use unique multicast addresses because if multiple
applications use the same address, receivers receive unwanted trac. 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 multicast
addresses. For short-term applications, an address could be leased, but no global dynamic multicast address allocation scheme 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 trac when multiple applications use the same address.
Topics:
• Congure PIM-SMM
• Implementation Information
• Enabling PIM-SSM
• Use PIM-SSM with IGMP Version 2 Hosts
• Electing an RP using the BSR Mechanism
Congure PIM-SMM
Conguring PIM-SSM is a two-step process.
1 Congure PIM-SMM.
2 Enable PIM-SSM for a range of addresses.
Related Conguration Tasks
• Use PIM-SSM with IGMP Version 2 Hosts
Implementation Information
• The Dell Networking implementation of PIM-SSM is based on RFC 3569.
• The Dell Networking operating system (OS) reduces the number of control messages sent between multicast routers by bundling Join
and Prune requests in the same message.
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624 PIM Source-Specic Mode (PIM-SSM)