HP MSR2003 Configuration Manual page 95

Figure 35 Multicast source registration
As shown in
1. The multicast source S sends the first multicast packet to the multicast group G. When receiving the
multicast packet, the source-side DR encapsulates the packet into a PIM register message and
unicasts the message to the RP.
2. After the RP receives the register message, it decapsulates the register message and forwards the
register message down to the RPT. Meanwhile, it sends an (S, G) source-specific join message
toward the multicast source. The routers along the path from the RP to the multicast source
constitute an SPT branch. Each router on this branch creates an (S, G) entry in its forwarding table.
3. The subsequent multicast data from the multicast source are forwarded to the RP along the
established SPT. When the multicast data reaches the RP along the SPT, the RP forwards the data
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.
Multicast forwarding along the RPT 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 when the traffic rate exceeds a specific threshold.
• The RP initiates a switchover to SPT:
The RP periodically checks the multicast packet forwarding rate. If the RP finds that the traffic rate
exceeds the specified threshold, it sends an (S, G) source-specific join message toward the
multicast source. The routers along the path from the RP to the multicast source constitute an SPT.
Figure 35, the multicast source registers with the RP as follows:
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