HP 6125XLG Configuration Manual page 74

multicast data reaches the RP along the SPT, the RP unicasts a register-stop message to the
source-side DR to prevent the DR from unnecessarily encapsulating the data.
Switchover to SPT
CAUTION:
If the switch is an RP, disabling switchover to SPT might cause multicast traffic forwarding failures on the
source-side DR. When disabling switchover to SPT, be sure you fully understand its impact on your
network.
In a PIM-SM domain, only one RP and one RPT provide services for a specific multicast group. Before the
switchover to SPT occurs, the source-side DR encapsulates all multicast data addressed to the multicast
group in register messages and sends them to the RP. After receiving these register messages, the RP
decapsulates them and forwards them to the receivers-side DR along the RPT.
Switchover to SPT has the following weaknesses:
Encapsulation and decapsulation are complex on the source-side DR and the RP.
•
The path for an multicast packet might not be the shortest one.
•
• The RP might be overloaded by multicast traffic bursts.
To eliminate these weaknesses, PIM-SM allows an RP or the receiver-side DR to initiate a switchover to
SPT.
• The RP initiates a switchover to SPT:
After receiving the first (S, G) multicast packet, the RP immediately sends an (S, G) source-specific
join message hop by hop toward the multicast source. The routers along the path from the RP to the
multicast source constitute an SPT branch. The subsequent multicast data for the multicast group
can be forwarded to the RP along the branch without being encapsulated.
For more information about the switchover to SPT initiated by the RP, see
registration."
The receiver-side DR initiates a switchover to SPT:
•
After receiving the first (S, G) multicast packet, the receiver-side DR immediately initiates a
switchover to SPT as follows:
a. The receiver-side DR sends an (S, G) source-specific join message hop by hop toward the
multicast source. The routers along the path from the RP to the multicast source create an (S, G)
entry in their forwarding table to constitute an SPT branch.
b. When the multicast packets for the multicast group are forwarded to the router where the RPT
and the SPT branches, the router drops the multicast packets that reach it along the RPT and
sends a prune message with the RP bit hop by hop to the RP. After receiving the prune message,
the RP forwards it toward the multicast source (supposed only one receiver exists). Thus, the
switchover to SPT is completed.
c. Finally, the multicast source sends the multicast packets for the multicast group to the receiver
along the SPT.
With the switchover to SPT, PIM-SM builds SPTs more economically than PIM-DM does.
Assert
PIM-SM uses a similar assert mechanism as PIM-DM does. For more information, see "Assert."
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"Multicast source