Users 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.

PIM Source-Specic Mode (PIM-SSM) 619