48
Figure 23
Multicast source registration
As shown in
, the multicast source registers with the RP as follows:
1.
The multicast source S sends the first multicast packet to multicast group G.
2.
After receiving the multicast packet, the DR that directly connects to the multicast source
encapsulates the packet in a PIM register message, and then sends the message to the
corresponding RP by unicast.
3.
When the RP receives the register message, it does the following:
a.
Extracts the multicast packet from the register message.
b.
Forwards the multicast packet down the RPT.
c.
Sends an (S, G) join message hop-by-hop 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 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.
4.
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.
NOTE:
In this section, the RP is configured to initiate a switchover to SPT. If the RP is not configured with switchover
to SPT, the DR at the multicast source side 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 a 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
