HP 4800G Series Configuration Manual page 826

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 as follows:
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 is not necessarily 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 travel 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:
First, 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 subsequent multicast packets arrive at the router at the junction of the RPT and SPT, the
router drops those transmitted along 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 to implement the SPT switchover.
Finally, multicast data is directly sent from the source to the receivers along the SPT.
PIM-SM builds SPTs through SPT switchover more economically than PIM-DM does through the "flood
and prune" mechanism.
Assert
PIM-SM uses a similar assert mechanism as PIM-DM does. For details, refer to Assert.
Multicast source
1-10
registration.