Reference Guide

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PIM Source-Specific Mode (PIM-SSM)
PIM source-specific mode (PIM-SSM) is supported on the Z9000 platform.
PIM-SSM is a multicast protocol that forwards multicast traffic 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 traffic 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 traffic.
PIM-SSM is more efficient than PIM-SM because it immediately creates shortest path trees (SPT) to the source rather
than first 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 traffic 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 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 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 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 operating system (FTOS) reduces the number of control messages sent between multicast
routers by bundling Join and Prune requests 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 range and 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-SMM
Configuring PIM-SSM is a two-step process.
1. Configure PIM-SMM.
2. Enable PIM-SSM for a range of addresses.
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