H3C Technologies H3C S12500-X Series Switches - Manual (Page 78)

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to the receivers along the RPT. Meanwhile, it unicasts a register-stop message to the source-side DR
to prevent the DR from unnecessarily encapsulating the data.
Switchover to SPT
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 a 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 sends an (S, G) source-specific join message
toward the multicast source immediately. The routers along the path from the RP to the multicast
source constitute an SPT. The subsequent multicast packets are forwarded to the RP along the SPT
without being encapsulated into register messages.
For more information about the switchover to SPT initiated by the RP, see "
Multicast source
registration ."
• The receiver-side DR initiates a switchover to SPT:
After receiving the first (S, G) multicast packet, the receiver-side DR initiates a switchover to SPT
immediately, as follows:
a. The receiver-side DR sends an (S, G) source-specific join message toward the multicast source.
The routers along the path create an (S, G) entry in their forwarding table to constitute an SPT
branch.
b. When the multicast packets reach the router where the RPT and the SPT branches, the router
drops the multicast packets that travel along the RPT. It then sends a prune message with the RP
bit to the RP.
c. 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. The subsequent multicast
packets travel along the SPT from the multicast source to the receiver hosts.
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 ."
Administrative scoping overview
Typically, a PIM-SM domain contains only one BSR, which is responsible for advertising RP-set
information within the entire PIM-SM domain. The information about all multicast groups is forwarded
within the network that the BSR administers. This is called the "non-scoped BSR mechanism."