3Com S7906E Configuration Manual page 1032

The RP is configured to initiate an SPT switchover as described in this section. Otherwise, the DR at the
multicast source side keeps encapsulating multicast data in register messages and the registration
process will not stop unless no outgoing interfaces exist in the (S, G) entry on the RP.
Switchover to SPT
In a PIM-SM domain, a multicast group corresponds to one RP and RPT. Before the SPT switchover
takes place, the DR at the multicast source side encapsulates all multicast data destined to the
multicast group in register messages and sends these messages to the RP. Upon receiving these
register messages, the RP abstracts the multicast data and sends the multicast data down the RPT to
the DRs at the receiver side. The RP acts as a transfer station for all multicast packets. The whole
process involves three issues:
The DR at the source side and the RP need to implement complicated encapsulation and
decapsulation of multicast packets.
Multicast packets are delivered along a path that may not be the shortest one.
When the multicast traffic increases, a great burden is added to the RP, increasing the risk of
failure.
To solve the issues, PIM-SM allows an RP or the DR at the receiver side to initiate an SPT switchover
process:
1) The RP initiates an SPT switchover process
Upon receiving the first multicast packet, the RP sends an (S, G) join message hop by hop toward the
multicast source to establish an SPT between the DR at the source side and the RP. The subsequent
multicast data from the multicast source travels along the established SPT to the RP.
For details about the SPT switchover initiated by the RP, refer to
2) The receiver-side DR initiates an SPT switchover process
Upon receiving the first multicast packet, the receiver-side DR initiates an SPT switchover process, as
follows:
The receiver-side DR sends an (S, G) join message hop by hop toward the multicast source. When
the join message reaches the source-side DR, all the routers on the path have installed the (S, G)
entry in their forwarding table, and thus an SPT branch is established.
When the multicast packets travel to the router where the RPT and the SPT deviate, the router
drops the multicast packets received from the RPT and sends an RP-bit prune message hop by
hop to the RP. Upon receiving this prune message, the RP sends a prune message toward the
multicast source (suppose only one receiver exists). Thus, SPT switchover is completed.
Finally, multicast data is directly sent from the source to the receivers along the SPT.
Multicast source
1-10
registration.