H3C Technologies H3C S12500 Series Switches - Manual (Page 129)

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A DR must be elected in a multi-access network, no matter this network connects to multicast sources or
to receivers. The receiver-side DR sends join messages to the RP, and the source-side DR sends register
messages to the RP.
A DR is elected on a multi-access subnet by means of comparison of the priorities and IP addresses
carried in hello messages. An elected DR is substantially meaningful to PIM-SM. PIM-DM itself does not
require a DR. However, if IGMPv1 runs on any multi-access network in a PIM-DM domain, a DR must be
elected to act as the IGMPv1 querier on that multi-access network.
IGMP must be enabled on a device that acts as a receiver-side DR before receivers attached to this device
can join multicast groups through this DR. For more information about IGMP, see "Configuring IGMP."
Figure 39 DR election
As shown in Figure 39 , 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 you can statically specify the position of the RP 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 must be forwarded through the RP. To lessen the RP burden and
optimize the topological structure of the RPT, you can configure multiple candidate RPs (C-RPs) in a
PIM-SM domain, among which an RP is dynamically elected through the bootstrap mechanism. Each
elected RP provides services for a different multicast group range. For this purpose, you must configure a
bootstrap router (BSR). 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). If the BSR fails, a new
BSR is automatically elected from the C-BSRs to avoid service interruption.