3Com Switch 7750 Configuration Guide

466 CHAPTER 47: PIM CONFIGURATION

Figure 114 Diagram for DR election

Each router on the shared network sends Hello messages with the DR priority

option to each other. The router with the highest DR priority is elected as the DR in

the network. If the priority is the same, the router with the highest IP address is

elected as the DR. When DR fails, the received Hello messages will time out. A

new DR election procedure will be triggered among neighboring routers.

In PIM-SM network, DR mainly serves as the querier of IGMPv1.

RP discovery

RP is the core router in the PIM-SM domain. The shared tree established based on

the multicast routing information is rooted in RP. There is a mapping relationship

between the multicast group and RP. One multicast group is mapped to one RP,

and multiple multicast groups can be mapped to the same RP.

In a small and simple network, there is only little multicast information. One RP is

enough for information forwarding. In this case, you can statically specify the

position of RP in each router in the SM domain.

However, PIM-SM network is of very large scale. RP forwards a lot of multicast

information. In order to reduce the workload of RP and optimize the topology of

the shared tree, different multicast groups must have different RPs. In this case, RP

must be elected dynamically through the auto-election mechanism and BootStrap

router (BSR) must be configured.

BSR is the core management device in PIM-SM network, which is responsible for:

■ Collecting the Advertisement messages sent by the Candidate-RP (C-RP) in the

network.

■ Selecting part of the C-RP information to constitute the RP-set, namely, the

mapping database between the multicast group and RP.

■ Advertising the RP-set to the whole network in order that all the router

(including DR) in the network knows the position of RP.

Join message

Ethernet

RPDR

Hello message

Source

Receiver