HP VPN Firewall Appliances Appendix Protocol Reference

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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 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 when the
traffic rate exceeds the threshold.
• The RP initiates a switchover to SPT:
The RP can periodically examine the passing-by multicast packets. If it finds that the traffic rate
exceeds a configurable threshold, 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 SPT switchover initiated by the RP, see "Multicast source
registration."
• The rece
iver-side DR initiates a switchover to SPT:
After discovering that the traffic rate exceeds a configurable threshold, 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 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 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 (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."
Administrative scoping overview
Typically, a PIM-SM domain contains only one BSR. The BSR advertises RP-set information within the
entire PIM-SM domain. The information for all multicast groups is forwarded within the network scope
that the BSR administers. This is called the "non-scoped BSR mechanism."
To implement refined management, you can divide a PIM-SM domain into one global-scoped zone and
multiple administratively scoped zones (admin-scoped zones). This is called the "administrative scoping
mechanism."
The administrative scoping mechanism effectively releases stress on the management in a single-BSR
domain and enables provision of zone-specific services through private group addresses.