3Com Switch 8800 Advanced Software V5 Configuration Guide

PIM Overview 571
receiving the multicast packet, encapsulates the packet in a PIM register
message, and sends the message to the corresponding RP by unicast.
2 When the RP receives the register message, on one hand, it extracts the
multicast packet from the register message and forwards the multicast
packet down the RPT, and, on the other hand, it sends an (S, G) join
message hop by hop toward the multicast source. Thus, the routers along
the path from the RP to the multicast source constitute an SPT branch.
Each router on this branch generates a (S, G) entry in its forwarding table.
The multicast source is the root, while the RP is the leaf, of the SPT.
3 The subsequent multicast data from the multicast source travels along the
established SPT to the RP, and then the RP forwards the data along the RPT
to the receivers. When the multicast traffic arrives at the RP along the SPT,
the RP sends a register-stop message to the source-side DR by unicast to
stop the source registration process.
Switchover from RPT to SPT
Initially, multicast traffic flows along an RPT from the RP to the receivers. Because
the RPT is not necessarily the tree that has the shortest path, the receiver-side DR
initiates an RPT-to-SPT switchover process upon receiving the first multicast packet
along the RPT by default.
1 First, the receiver-side DR sends an (S, G) join message hop by hop to 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, and thus an SPT branch is established.
2 Subsequently, the receiver-side DR sends a prune message hop by hop to
the RP. Upon receiving this prune message, the RP forwards it towards the
multicast source, thus to implement RPT-to-SPT switchover.
After the RPT-to-SPT switchover, multicast data can be directly sent from the
source to the receivers. PIM-SM builds SPTs through RPT-to-SPT switchover more
economically than PIM-DM does through the "flood and prune" mechanism.
Assert
PIM-SM uses exactly the same assert mechanism as PIM-DM does. Refer to
“Assert” on page 566 described above.
Introduction to BSR
Admin-scope Regions in
PIM-SM
Division of PIM-SM domains
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
for all multicast groups is forwarded within the network scope administered by the
BSR.
To implement refined management and provide group-specific services, a PIM-SM
domain can be divided into one global scope zone and multiple BSR
administratively scoped regions (BSR admin-scope regions), like the division of
subnets.
Specific to particular multicast groups, the BSR administrative scoping mechanism
effectively lessens the management workload of a single-BSR domain and provides
group-specific services.