H3C S7500 Series Operation Manual page 480

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches
Source
Figure 7-3 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 the DR fails, the received Hello messages will time out. A new DR election
procedure will be triggered among neighboring routers.
Note:
In a PIM-SM network, the DR mainly serves as the querier of IGMPv1.
III. RP discovery
The RP is the core router in a 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 only 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, a PIM-SM network is of very large scale generally. The RP forwards a lot of
multicast information. In order to reduce the workload of the RP and optimize the
topology of the shared tree, different multicast groups must have different RPs. In this
case, RPs must be elected dynamically through the Bootstrap mechanism and
Bootstrap router (BSR) must be configured.
DR
Hello message
Register message
Join message
DR
RP
7-6
Chapter 7 PIM Configuration
Receiver
Receiver