3Com Switch 7754, Configuration Manual

Page: 466 / 940

466 C
HAPTER
47: PIM C
ONFIGURATION
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.
n
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
E
ther
net
E
ther
net
RP DR
DR
Hello message
Register message
Source
Receiver
Receiver