3Com Switch 8800 Configuration Guide Chapter 29 PIM-SM Configuration
29-2
I. Build the RP shared tree (RPT)
When hosts join a multicast group G, the leaf routers that directly connect with the hosts
send IGMP messages to learn the receivers of multicast group G. In this way, the leaf
routers calculate the corresponding rendezvous point (RP) for multicast group G and
then send join messages to the node of the next level toward the rendezvous point (RP).
Each router along the path between the leaf routers and the RP will generate (*, G)
entries in the forwarding table, indicating that all packets sent to multicast group G are
applicable to the entries no matter from which source they are sent. When the RP
receives the packets sent to multicast group G, the packets will be sent to leaf routers
along the path built and then reach the hosts. In this way, an RP-rooted tree (RPT) is
built as shown in the following figure.
Multicast Source S
RPT
join
Multicast source registration
RP
Receiver
Figure 29-1 RPT schematic diagram
II. 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.
29.1.3 Preparations before Configuring PIM-SM I. 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.