H3C Technologies H3C S7500E Series Switches - Manual (Page 144)

6-7
Figure 6-3 DR election
Join message
RP DR
DR
Hello message
Register message
Source
Receiver
Receiver
As shown in Figure 6-3 , the DR election process is as follows:
1) Routers on the multi-access network send hello messages to one another. The hello
messages contain the router priority for DR election. The router with the highest DR priority
will become the DR.
2) In the case of a tie in the router priority, or if any router in the network does not support
carrying the DR-election priority in hello messages, the router with the highest IP address
will win the DR election.
When the DR fails, a timeout in receiving hello message triggers a new DR election process
among the other routers.
RP discovery
The RP is the core of a PIM-SM domain. For a small-sized, simple network, one RP is enough
for forwarding information throughout the network, and the position of the RP can be statically
specified on each router in the PIM-SM domain. In most cases, however, a PIM-SM network
covers a wide area and a huge amount of multicast traffic needs to be forwarded through the RP.
To lessen the RP burden and optimize the topological structure of the RPT, multiple candidate
RPs (C-RPs) can be configured in a PIM-SM domain, among which an RP is dynamically
elected through the bootstrap mechanism. Each elected RP serves a different multicast group
range. For this purpose, a bootstrap router (BSR) must be configured. The BSR serves as the
administrative core of the PIM-SM domain. A PIM-SM domain 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 to avoid service interruption.