TP-Link T3700G-28TQ User Manual page 288

The device working as DR should be enabled with the IGMP function; otherwise the

receivers connected to it would be unable to join the multicast group via this DR.
As shown in Figure 11-15, the DR election process is illustrated below:
(1) Routers in the shared network sends Hello message carrying DR-election priority to each
other, and the router with the highest priority will be elected as the DR;
(2) If the routers have the same priorities, or at least one route in the network doesn't support
carrying the DR-election priority in the Hello packet, the routers with the highest IP
address will be elected as the DR.
When the DR fails, a new DR election process will be triggered if the other routers haven't
received the hello packet from the DR before they time out.
RP Discovering
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RP is the core device in the PIM SM domain. In a small network with simple structure, the
multicast data is so little that merely one RP is enough to forward it. In this network an RP can
be statically designated among the routers in the PIM SM domain; in more circumstances, the
PIM SM domain is of large scale and the forwarding data for the RP is huge. To release the
burden of the RP and optimize the RPT topology, each multicast group should have its own RP.
Thus the bootstrapping mechanism is needed to elect the RP dynamically. The BSR (BootStrap
Router) should be configured in this mechanism.
BSR is the administrative core in the PIM SM. It collects the Advertisement Messages sent from
the C-RP (Candidate-RP) in the network and selects certain C-RP information to compose a
RP-Set (which is the mapping relationship database between the multicast group and the RP).
The RP-Set is published to the whole PIM SM domain and all the routers (including DR) can
calculate the required RP location according to the information offered by the RP-Set.
In a PIM SM domain (or administrative domain), there is only one BSR (for more details about
BSR administrative domain, please refer to
(Candidate-BSR). Once the BSR fails, a new BSR will be elected among the other C-BSRs to
Figure 11-13 DR Elect
BSR Adminsitrative
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Domain) and several C-BSRs