HP A8800 Configuration Manual page 130

Figure 42 Multicast source registration
As shown in
1. When the multicast source S sends the first multicast packet to multicast group G, the DR directly
connected with the multicast source, upon 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 towards 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 DR at the multicast source side is the root, while 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 an SPT switchover as described in this section. Otherwise, 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 the SPT switchover takes
place, the DR at the multicast source side encapsulates all multicast data destined to the multicast group
in register messages and sends these messages to the RP. Upon receiving these register messages, the RP
abstracts 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 three issues:
• The DR at the source side and the RP need to implement complicated encapsulation and
de-encapsulation of multicast packets.
Multicast packets are delivered along a path that may not be the shortest one.
•
When the multicast traffic increases, a great burden is added to the RP, increasing the risk of failure.
•
Figure 42, the multicast source registers with the RP as follows:
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