HP VSR1000 Configuration Manual page 89

Figure 35 Multicast source registration
As shown in
The multicast source S sends the first multicast packet to the multicast group G. When
1.
receiving the multicast packet, the source-side DR encapsulates the packet into a PIM register
message and unicasts the message to the RP.
After the RP receives the register message, it decapsulates the register message and forwards
2.
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.
The subsequent multicast data from the multicast source are forwarded to the RP along the
3.
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
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 in register
messages and sends them to the RP. After receiving these register messages, the RP decapsulates
them and forwards them to the receiver-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 the
switchover to SPT when the traffic rate exceeds a specific threshold.
•
The RP initiates the switchover to SPT:
Figure 35, the multicast source registers with the RP as follows:
81