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Figure 45 Multicast source registration
As shown in Figure 45, the multicast source registers with the RP as follows:
1. The multicast source S sends the first multicast packet to multicast group G. After receiving the
multicast packet, the DR that directly connects to the multicast source encapsulates the packet in a
PIM register message. Then it sends the message to the corresponding RP by unicast.
2. When the RP receives the register message, it extracts the multicast packet from the register
message and forwards the multicast packet down the RPT, and sends an (S, G) join message hop
by hop toward the multicast source. Thus, the routers along the path from the RP to the multicast
source constitute an SPT branch. Each router on this branch generates an (S, G) entry in its
forwarding table. The source-side DR is the root of the SPT, and the RP is the leaf of the SPT.
3. The subsequent multicast data from the multicast source travels along the established SPT to the RP.
Then the RP forwards the data along the RPT to the receivers. When the multicast traffic arrives at
the RP along the SPT, the RP sends a register-stop message to the source-side DR by unicast to stop
the source registration process.
This section assumes that the RP is allowed to initiates the switchover to SPT. If the RP is not allowed to do
so, the source-side DR keeps encapsulating multicast data in register messages, and the registration
process will not stop unless no outgoing interfaces exist in the (S, G) entry on the RP.
Switchover to SPT
In a PIM-SM domain, a multicast group corresponds to one RP and RPT. Before the switchover to SPT
occurs, the source-side DR encapsulates all multicast data destined to the multicast group in register
messages and sends these messages to the RP. After receiving these register messages, the RP extracts 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 the following issues:
• The source-side DR and the RP need to implement complicated encapsulation and de-encapsulation
of multicast packets.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Join message
Register message
RPDR
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