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NOTE:
The RP is configured to initiate a switchover to SPT in the above description. Otherwise, the DR at
the IPv6 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 an IPv6 PIM-SM domain, an IPv6 multicast group corresponds to one RP and one RPT. Before the
switchover to SPT occurs, the DR at the IPv6 multicast source side 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 IPv6 multicast
packets. The whole process involves the following issues:
•
The DR at the source side and the RP need to implement complicated encapsulation and
de-encapsulation of IPv6 multicast packets.
•
IPv6 multicast packets are delivered along a path that might not be the shortest one.
•
An increase in IPv6 multicast traffic heavily burdens the RP, increasing the risk of failure.
To solve these issues, IPv6 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:
After receiving the first IPv6 multicast packet, the RP sends an (S, G) join message hop by hop
toward the IPv6 multicast source to establish an SPT between the DR at the source side and
the RP. Subsequent IPv6 multicast data travels along the established SPT to the RP. For more
information about the switchover to SPT initiated by the RP, see "
2.
The receiver-side DR initiates a switchover to SPT process:
After receiving the first IPv6 multicast packet, the receiver-side DR initiates a switchover to SPT,
as follows:
{
The receiver-side DR sends an (S, G) join message hop by hop toward the IPv6 multicast
source. When the join message reaches the source-side DR, all routers on the path have
installed the (S, G) entry in their forwarding table, and thus an SPT branch is established.
{
When the IPv6 multicast packets travel to the router where the RPT and the SPT deviate,
the router drops the multicast packets received from the RPT and sends an RP-bit prune
message hop by hop to the RP. After receiving this prune message, the RP sends a prune
message toward the IPv6 multicast source (supposing only one receiver exists) to
implement switchover to SPT.
{
IPv6 multicast data is directly sent from the source to the receivers along the SPT.
IPv6 PIM-SM builds SPTs through switchover to SPT more economically than IPv6 PIM-DM does
through the flood-and-prune mechanism.
Assert
IPv6 PIM-SM uses a similar assert mechanism as IPv6 PIM-DM does. For more information, see
"
IPv6 BIDIR-PIM
In some many-to-many applications, such as multi-side video conference, there might be multiple
receivers interested in multiple IPv6 multicast sources simultaneously. With IPv6 PIM-DM or IPv6
PIM-SM, each router along the SPT must create an (S, G) entry for each IPv6 multicast source,
consuming a lot of system resources. IPv6 BIDIR-PIM is introduced
to address this problem. Derived
from IPv6 PIM-SM, IPv6 BIDIR-PIM builds and maintains bidirectional RPTs, each of which is rooted
at an RP and connects IPv6 multiple multicast sources with multiple receivers. Traffic from the IPv6
multicast sources is forwarded through the RP to the receivers along the bidirectional RPT. In this
case, each router needs to maintain only a (*, G) multicast routing entry, saving system resources.
