634 CHAPTER 47: PIM CONFIGURATION
Figure 189 DR election
As shown in Figure 189, 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, each multicast group should
have its own RP. Therefore, a bootstrap mechanism is needed for dynamic RP
election. For this purpose, a bootstrap router (BSR) should be configured.
As the administrative core of a PIM-SM domain, the BSR collects advertisement
messages (C-RP-Adv messages) from candidate-RPs (C-RPs) and chooses the
appropriate C-RP information for each multicast group to form an RP-set, which is
a database of mappings between multicast groups and RPs. The BSR then floods
the RP-set to the entire PIM-SM domain. Based on the information in these
RP-sets, all routers (including the DRs) in the network can calculate the location of
the corresponding RPs.
A PIM-SM domain (or an administratively scoped region) 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 through the bootstrap mechanism to avoid
service interruption. Similarly, multiple C-RPs can be configured in a PIM-SM
Join message
Ethernet
Ethernet
RPDR
DR
Hello message
Register message
Source
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