Bidir-Pim Overview - HP FlexNetwork 7500 Series Configuration Manual

Switchover to SPT
In a PIM-SM domain, only one RP and one RPT provide services for a specific multicast group.
Before the switchover to SPT occurs, the source-side DR encapsulates all multicast data in register
messages and sends them to the RP. After receiving these register messages, the RP decapsulates
them and forwards them to the receiver-side DR along the RPT.
Multicast forwarding along the RPT has the following weaknesses:
•
Encapsulation and decapsulation are complex on the source-side DR and the RP.
•
The path for a multicast packet might not be the shortest one.
•
The RP might be overloaded by multicast traffic bursts.
To eliminate these weaknesses, PIM-SM allows an RP or the receiver-side DR to initiate the
switchover to SPT when the traffic rate exceeds a specific threshold.
•
The RP initiates the switchover to SPT:
The RP periodically checks the multicast packet forwarding rate. If the RP finds that the traffic
rate exceeds the specified threshold, it sends an (S, G) source-specific join message toward
the multicast source. The routers along the path from the RP to the multicast source constitute
an SPT. The subsequent multicast data is forwarded to the RP along the SPT without being
encapsulated into register messages.
For more information about the switchover to SPT initiated by the RP, see
registration."
•
The receiver-side DR initiates the switchover to SPT:
The receiver-side DR periodically checks the forwarding rate of the multicast packets that the
multicast source S sends to the multicast group G. If the forwarding rate exceeds the specified
threshold, the DR initiates the switchover to SPT as follows:
a. The receiver-side DR sends an (S, G) source-specific join message toward the multicast
source. The routers along the path create an (S, G) entry in their forwarding table to
constitute an SPT branch.
b. When the multicast packets reach the router where the RPT and the SPT branches, the
router drops the multicast packets that travel along the RPT. It then sends a prune message
with the RP bit toward the RP.
c. After receiving the prune message, the RP forwards it toward the multicast source
(supposed only one receiver exists). Thus, the switchover to SPT is completed. The
subsequent multicast packets travel along the SPT from the multicast source to the receiver
hosts.
With the switchover to SPT, PIM-SM builds SPTs more economically than PIM-DM does.
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 a multi-side video conference, multiple receivers of a
multicast group might be interested in the multicast data from multiple multicast sources. 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 a
bidirectional RPT, which is rooted at the RP and connects the multicast sources and the receivers.
Along the bidirectional RPT, the multicast sources send multicast data to the RP, and the RP
forwards the data to the receivers. Each router along the bidirectional RPT needs to maintain only
one (*, G) entry, saving system resources.
BIDIR-PIM is suitable for a network with dense multicast sources and receivers.
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"Multicast source