R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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• 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 a switchover to
SPT.
• The RP initiates a switchover to SPT:
When the RP receives the first (S, G) multicast packet, it sends an (S, G) source-specific join
message hop by hop toward the multicast source. The routers along the path from the RP to the
multicast source constitute an SPT branch. The subsequent multicast data for the multicast group
can be forwarded to the RP along the branch without being encapsulated.
For more information about the switchover to SPT initiated by the RP, see "Multicast source
regi
stration."
• The receiver-side DR initiates a switchover to SPT:
The receiver-side DR receives the first (S, G) multicast packet, it initiates a switchover to SPT as
follows:
a. The receiver-side DR sends an (S, G) source-specific join message hop by hop toward the
multicast source. The routers along the path from the RP to the source-side DR create an (S, G)
entry in their forwarding table to constitute an SPT branch.
b. When the multicast packets for the multicast group are forwarded to the router where the RPT
and the SPT branches, the router drops the multicast packets that reach it along the RPT and
sends a prune message with the RP bit hop by hop to the RP. 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.
c. Finally, the multicast source sends the multicast packets for the multicast group to the receiver
along the SPT.
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."
Administrative scoping overview
Typically, a PIM-SM domain contains only one BSR, which is responsible for advertising RP-set
information within the entire PIM-SM domain. The information about all multicast groups is forwarded
within the network that the BSR administers. This is called the "non-scoped BSR mechanism."
To implement refined management, you can divide a PIM-SM domain into a global-scoped zone and
multiple administratively-scoped zones (admin-scoped zones). This is called the "administrative scoping
mechanism."