3Com SuperStack 4 Configuration Manual page 183

Preparations before
Configuring PIM-SM
Figure 45 RPT schematic diagram
Multicast Source S
RPT
join
Multicast source registration
Multicast Source Registration
When multicast source S sends a multicast packet to the multicast group G, the
PIM-SM multicast router directly connected to S will encapsulate the received
packet into a registration packet and send it to the corresponding RP in unicast
form. If there are multiple PIM-SM multicast routers on a network segment, the
Designated Router (DR) will be responsible for sending the multicast packet.
Configuring Candidate RPs
In a PIM-SM network, multiple RPs (candidate-RPs) can be configured. Each
Candidate-RP (C-RP) is responsible for forwarding multicast packets with the
destination addresses in a certain range. Configuring multiple C-RPs is to
implement load balancing of the RP. These C-RPs are equal. All multicast routers
calculate the RPs corresponding to multicast groups according to the same
algorithm after receiving the C-RP messages that the BSR advertises.
It should be noted that one RP can serve multiple multicast groups or all multicast
groups. Each multicast group can only be uniquely correspondent to one RP at a
time rather than multiple RPs.
Configuring BSRs
The BSR is the management core in a PIM-SM network. Candidate-RPs send
announcement to the BSR, which is responsible for collecting and advertising the
information about all candidate-RPs.
It should be noted that there can be only one BSR in a network but you can
configure multiple candidate-BSRs. In this case, once a BSR fails, you can switch
over to another BSR. A BSR is elected among the C-BSRs automatically. The C-BSR
with the highest priority is elected as the BSR. If the priority is the same, the C-BSR
with the largest IP address is elected as the BSR.
PIM-SM Overview 195
RP
Receiver