H3C Technologies H3C S12500 Series Switches - Manual (Page 132)

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Figure 42 Multicast source registration
As shown in Figure 42 , the multicast source registers with the RP as follows:
1. When the multicast source S sends the first multicast packet to multicast group G, the DR directly
connected with the multicast source —after receiving the multicast packet, encapsulates the packet
in a PIM register message, and 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, 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,
and 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.
NOTE:
The RP is configured to initiate a switchover to SPT as described in this section. Otherwise, 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.
• Multicast packets are delivered along a path that might not be the shortest one.
• An increase in multicast traffic adds a great burden on the RP, increasing the risk of failure.