Service Manual

36
PIM Source-Specic Mode (PIM-SSM)
PIM source-specic mode (PIM-SSM) is supported on Dell Networking OS.
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.
Implementation 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
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 eective even when the default range is not added to the SSM ACL.
• Extended ACLs cannot be used for conguring SSM range. Be sure to create the ACL rst and then apply it to the SSM range.
• The default range is always supported, so range can never be smaller than the default.
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
PIM Source-Specic Mode (PIM-SSM)
549