F3726, F3211, F3174, R5135, R3816-HP Firewalls and UTM Devices Appendix Protocol Reference-6PW100

SPT building

The decision to build an RPT for PIM-SM or an SPT for PIM-SSM depends on whether the multicast group

that the receiver will join falls into the SSM group range. (SSM group range reserved by IANA is

232.0.0.0/8.)

Figure 60 SPT building in PIM-SSM

As shown in Figure 60, Host B and Host C are multicast information receivers. They send IGMPv3 report

messages to the respective DRs to express their interest in the information about the specific multicast

source S.

After receiving a report message, the DR first examines whether the group address in this message falls

into the SSM group range and does the following:

• If the group address falls into the SSM group range, the DR sends a subscribe message for channel

subscription hop-by-hop toward the multicast source S.

An (S, G) entry is created on all routers on the path from the DR to the source. An SPT is thereby

built in the network, with the source S as its root and receivers as its leaves. This SPT is the

transmission channel in PIM-SSM.

• If the group address does not fall into the SSM group range, the receiver-side DR sends a (*, G) join

message to the RP, and the source-side DR registers the multicast source.

In PIM-SSM, the term "channel" refers to a multicast group, and the term "channel subscription" refers to

a join message.

Relationship among PIM protocols

In a PIM network, PIM-DM cannot run together with PIM-SM or PIM-SSM. For more information about

IGMP SSM mapping, see "IGMP SSM mapping."

Protocols and standards

• RFC 3973, Protocol Independent Multicast-Dense Mode (PIM-DM): Protocol Specification(Revised)

• RFC 4601, Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification (Revised)

Source

Server

Host A

Host B

Host C

Receiver

Multicast packets

SPT

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