PIM Source Specific Multicast

PIM-SM works well in many-to-many multicasting situations. For example, in video conferencing, each participating site multicasts a stream that is sent to all the other participating sites. However, PIM-SM is overly complex for one-to-many multicast situations, such as multimedia content distribution or streaming stock quotes. In these and similar applications, the listener is silent and can know the source of the multicast in advance, or can obtain it. In these situations, there is no need to join an RP, as the join request can be made directly towards the source.

PIM Source Specific Multicast (PIM-SSM) is a special case of PIM-SM, in which a host explicitly sends a request to receive a stream from a specific source, rather than from any source.

IGMPv3 hosts can use PIM SSM directly, because the ability to request a stream from a specific source first became available with IGMPv3. The PIM-SSM capable router interprets the IGMPv3 message to initiate a PIM-SM join towards the source.

Using PIM-SSM While Sending IGMPV3 Joins (IPv4) describes PIM-SSM behavior while sending IGMPV3 joins in the SSM range and outside the SSM range for IPv4:

Table 1. Using PIM-SSM While Sending IGMPV3 Joins (IPv4)

				ExtremeXOS 15.4		ExtremeXOS 15.5
SSM Enabled	SSM range	Mode	Include Src	Action	Observation	Action	Observation
No	Yes	Incl	Yes	Send IGMPv3 join in SSM range	-the group is learned -(*;G) is not created	Send IGMPv3 join in SSM range	-the group is learned - (*;G) is created
No	Yes	Incl	Yes	Send IGMPv3 out of SSM range	-the group is learned -no (*;G) is created	Send IGMPv3 out of SSM range	-the group is learned - (*;G) is created
No	Yes	Excl	No	Send IGMPv3 join in SSM range	-the group is not learned (PD4-3138792131) -no (*;G) is created	Send IGMPv3 join in SSM range	-the group is not learned (PD4-3138792131) -no (*;G) is created
No	Yes	Excl	No	Send IGMPv3 join out of SSM range	-the group is learned - (*;G) is created	Send IGMPv3 join out of SSM range	-the group is learned - (*;G) is created
No	Yes	Excl	Yes	Send IGMPv3 join in SSM range	-the group is not learned -no (*;G) is created	Send IGMPv3 join in SSM range	-the group is not learned -no (*;G) is created
No	Yes	Excl	Yes	Send IGMPv3 join out SSM range	-the group is learned -no (*;G) is created	Send IGMPv3 join out SSM range	-the group is learned -no (*;G) is created
No	No	Incl	Yes	Send IGMPv3 join	-the group is learned -no (*;G) is created	Send IGMPv3 join	-the group is learned -(*;G) is created
No	No	Excl	No	Send IGMPv3 join	-the group is learnt - (*;G) is created	Send IGMPv3 join	-the group is learnt - (*;G) is created
No	No	Excl	Yes	Send IGMPv3 join	-the group is learned -no (*;G) is created	Send IGMPv3 join	-the group is learned -no (*;G) is created
Yes	Yes	Incl	Yes	Send IGMPv3 join in SSM range	-the group is learned -(S;G) is created	Send IGMPv3 join in SSM range	-the group is learned -(S;G) is created
Yes	Yes	Incl	Yes	Send IGMPv3 out of SSM range	-the group is learned -no (*;G) is created	Send IGMPv3 out of SSM range	-the group is learned - (*;G) is created
Yes	Yes	Excl	No	Send IGMPv3 join in SSM range	-the group is not learned -no (*;G) is created	Send IGMPv3 join in SSM range	-the group is not learned -no (*;G) is created
Yes	Yes	Excl	No	Send IGMPv3 join out of SSM range	-the group is learned - (*;G) is created	Send IGMPv3 join out of SSM range	-the group is learned - (*;G) is created
Yes	Yes	Excl	Yes	Send IGMPv3 join in SSM range	-the group is not learned -no (*;G) is created	Send IGMPv3 join in SSM range	-the group is not learned -no (*;G) is created
Yes	Yes	Excl	Yes	Send IGMPv3 join out SSM range	-the group is learned -no (*;G) is created	Send IGMPv3 join out SSM range	-the group is learned -no (*;G) is created
Yes	No	Incl	Yes	Send IGMPv3 join	-the group is learned -no (*;G) is created	Send IGMPv3 join	-the group is learned -(*;G) is created
Yes	No	Excl	No	Send IGMPv3 join	-the group is learned - (*;G) is created	Send IGMPv3 join	-the group is learned - (*;G) is created
Yes	No	Excl	Yes	Send IGMPv3 join	-the group is learned -no (*;G) is created	Send IGMPv3 join	-the group is learned -no (*;G) is created

Using PIM-SSM While Sending MLDV2 Joins (IPv6) describes PIM-SSM behavior while sending MLDV2 joins in the SSM range and outside the SSM range for IPv6:

Table 2. Using PIM-SSM While Sending MLDV2 Joins (IPv6)

				ExtremeXOS 15.4		ExtremeXOS 15.5
SSM Enabled	SSM range	Mode	Include Src	Action	Observation	Action	Observation
No	Yes	Incl	Yes	Send MLDv2 join in SSM range	-the group is learned - no (*;G) is created	Send MLDv2 join in SSM range	-the group is learned -(*;G) is created
No	Yes	Incl	Yes	Send MLDv2 out of SSM range	-the group is learned - no (*;G) is created - (S;G) is created	Send MLDv2 out of SSM range	-the group is learned -(*;G) is created
No	Yes	Excl	No	Send MLDv2 join in SSM range	-the group is learned -(*;G) is created	Send MLDv2 join in SSM range	-the group is not learned - no (*;G) is created
No	Yes	Excl	No	Send MLDv2 join out of SSM range	-the group is learned -(*;G) is created	Send MLDv2 join out of SSM range	-the group is learned -(*;G) is created
No	Yes	Excl	Yes	Send MLDv2 join in SSM range	-the group is learned -(*;G) is created	Send MLDv2 join in SSM range	-the group is not learned - no (*;G) is created
No	Yes	Excl	Yes	Send MLDv2 join out SSM range	-the group is learned -(*;G) is created	Send MLDv2 join out SSM range	-the group is learned -(*;G) is created
No	No	Incl	Yes	Send MLDv2 join	-the group is learned - (S;G) is created	Send MLDv2 join	-the group is learned - (*;G) is created
No	No	Excl	No	Send MLDv2 join	-the group is learned -(*;G) is created	Send MLDv2 join	-the group is learned -(*;G) is created
No	No	Excl	Yes	Send MLDv2 join	-the group is learned -(*;G) is created	Send MLDv2 join	-the group is learned -(*;G) is created
Yes	Yes	Incl	Yes	Send MLDv2 join in SSM range	-the group is learned - (S;G) is created	Send MLDv2 join in SSM range	-the group is learned - (S;G) is created
Yes	Yes	Incl	Yes	Send MLDv2 out of SSM range	-the group is learned - no (*;G) is created - (S;G) is created	Send MLDv2 out of SSM range	-the group is learned -(*;G) is created
Yes	Yes	Excl	No	Send MLDv2 join in SSM range	-the group is learned -(*;G) is created	Send MLDv2 join in SSM range	-the group is not learned - no (*;G) is created
Yes	Yes	Excl	No	Send MLDv2 join out of SSM range	-the group is learned -(*;G) is created	Send MLDv2 join out of SSM range	-the group is learned -(*;G) is created
Yes	Yes	Excl	Yes	Send MLDv2 join in SSM range	-the group is learned -(*;G) is created	Send MLDv2 join in SSM range	-the group is not learned - no (*;G) is created
Yes	Yes	Excl	Yes	Send MLDv2 join out SSM range	-the group is learned -(*;G) is created	Send MLDv2 join out SSM range	-the group is learned -(*;G) is created
Yes	No	Incl	Yes	Send MLDv2 join	-the group is learned - (S;G) is created	Send MLDv2 join	-the group is learned - (*;G) is created
Yes	No	Excl	No	Send MLDv2 join	-the group is learned -(*;G) is created	Send MLDv2 join	-the group is learned -(*;G) is created
Yes	No	Excl	Yes	Send MLDv2 join	-the group is learned -(*;G) is created	Send MLDv2 join	-the group is learned -(*;G) is created

PIM-SSM has the following advantages:

• No overhead of switching to the source-specific tree and waiting for the first packet to arrive
• No need to learn and maintain an RP
• Fewer states to maintain on each router
• No need for the complex register mechanism from the source to the RP
• Better security, as each stream is forwarded from sources known in advance

PIM-SSM has the following requirements:

• Any host that participates directly in PIM-SSM must use IGMPv3. For PIM IPv6 host must use MLDv2.
• To support IGMPv1 and IGMPv2 hosts, IGMP-SSM mapping must be enabled and configured. To support MLDv1 hosts, MLD-SSM mapping must be enabled and configured.

PIM-SSM is designed as a subset of PIM-SM and all messages are compliant with PIM-SM. PIM-SSM and PIM-SM can coexist in a PIM network; only the last hop router need to be configured for PIM-SSM if both source and receivers are present all the time. However, to avoid any JOIN delay, it is recommended that you enable all routers along the (s,g) path for PIM-SSM.