H3C Technologies H3C S7500E Series Switches - Manual (Page 148)

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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:
z The DR at the source side and the RP need to implement complicated encapsulation and
decapsulation of multicast packets.
z Multicast packets are delivered along a path that may not be the shortest one.
z 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 Multicast source registration .
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:
z 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.
z 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