R3303-HP HSR6800 Routers IP Multicast Configuration Guide

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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 need to 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 as
follows:
• The RP initiates a switchover to SPT:
When the RP receives the first multicast packet, it sends an (S, G) join message 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 SPT switchover initiated by the RP, see "Multicast source
regi
stration."
• The receiver-side DR initiates a switchover to SPT:
When the receiver-side DR receives the first multicast packet, it initiates a switchover to SPT as
follows:
a. The receiver-side DR sends an (S, G) join message toward the multicast source. When the join
message reaches the source-side DR, all the routers on the path have installed the (S, G) entry
in their forwarding table, establishing an SPT branch.
b. When the multicast packets travel to the router where the RPT and the SPT deviate, the router
drops the multicast packets that travel along the RPT and sends an RP-bit prune message to the
RP. After receiving this prune message, the RP sends a prune message toward the multicast
source (suppose only one receiver exists). Thus, SPT switchover is completed.
c. Multicast data is directly sent from the source to the receivers along the SPT.
PIM-SM builds SPTs through SPT switchover 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 "Assert."
BIDIR-PIM overview
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
• RP discovery