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After receiving the first multicast packet, the RP sends an (S, G) join message hop by hop
toward the multicast source to establish an SPT between the DR at the source side and the RP.
Subsequent 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.
After receiving the first multicast packet, the receiver-side DR initiates a switchover to SPT as
follows:
a.
The receiver-side DR sends an (S, G) join message, hop by hop, toward the 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.
b.
When the 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 multicast source (supposing only one receiver exists). Thus, the
switchover to SPT process is completed.
c.
Multicast data is directly sent from the source to the receivers along the SPT.
PIM-SM builds SPTs through the switchover to SPT more economically than PIM-DM does through
the flood-and-prune mechanism.
Assert
PIM-SM uses a similar assert mechanism as PIM-DM does. For more information, see "
BIDIR-PIM
In some many-to-many applications, such as multi-side video conference, there might be multiple
receivers interested in multiple multicast sources simultaneously. With PIM-DM or PIM-SM, each
router along the SPT must create an (S, G) entry for each multicast source, consuming a lot of
system resources.
BIDIR-PIM addresses the problem. Derived from PIM-SM, BIDIR-PIM builds and maintains
bidirectional RPTs. Each RPT is rooted at an RP and connects multiple multicast sources with
multiple receivers. Traffic from the multicast sources is forwarded through the RPs to the receivers
along the bidirectional RPTs. Each router needs to maintain only one (*, G) multicast routing entry,
saving system resources.
BIDIR-PIM is suitable for networks with dense multicast sources and dense receivers.
The operating mechanism of BIDIR-PIM is summarized as follows:
•
•
•
•
Neighbor discovery
BIDIR-PIM uses the same neighbor discovery mechanism as PIM-SM does. For more information,
see "
RP discovery
BIDIR-PIM uses the same RP discovery mechanism as PIM-SM does. For more information, see
"
In PIM-SM, an RP must be specified with a real IP address. In BIDIR-PIM, however, an RP can be
specified with a virtual IP address, which is called the rendezvous point address (RPA). The link
corresponding to the RPA's subnet is called the "rendezvous point link (RPL)." All interfaces
connected to the RPL can act as the RP, and they back up one another.
