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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 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 EMC 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.
Topics:
Implementation Information
Configure PIM-SSM
Enabling PIM-SSM
Use PIM-SSM with IGMP Version 2 Hosts
Electing an RP using the BSR Mechanism
Implementation Information
The Dell EMC Networking implementation of PIM-SSM is based on RFC 3569.
The Dell EMC Networking OS 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-SSM
Configuring PIM-SSM is a two-step process.
1. Configure PIM-SSM.
2. Enable PIM-SSM for a range of addresses.
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