3Com Switch 8800 Advanced Software V5 Configuration Guide

PIM Overview 569
A PIM-SM domain (or an administratively scoped region) can have only one BSR,
but can have multiple candidate-BSRs (C-BSRs). Once the BSR fails, a new BSR is
automatically elected from the C-BSRs through the bootstrap mechanism to avoid
service interruption. Similarly, multiple C-RPs can be configured in a PIM-SM
domain, and the position of the RP corresponding to each multicast group is
calculated through the BSR mechanism.
Figure 177 shows the positions of C-RPs and the BSR in the network.
Figure 177 BSR and C-RPs
RPT formation
Figure 178 Build an RPT in PIM-SM
Advertisement message
BSR message
PIM-SM
BSR
C-RP
C-RP
C-RP
C-BSR
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
RPT
Join message
RP DR
DR