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Figure 45 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. When 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. 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 one 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 receiver-side
DRs. 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 must 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.
To solve these issues, PIM-SM allows an RP or the receiver-side DR to initiate a switchover to SPT
process.
1.
The RP initiates a switchover to SPT process.
