Administrator Guide

PIM Source-Specific Mode (PIM-

SSM)

Dell Networking OS supports PIM source-specific mode (PIM-SSM).

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.

Topics:

• Configure PIM-SMM

• Implementation Information

• Enabling PIM-SSM

• Use PIM-SSM with IGMP Version 2 Hosts

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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