136
For more information about IGMP, see "Configuring IGMP (available only on the S5500-EI)."
Figure 42 DR election
As shown in Figure 42, 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 a 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 is designated to a different multicast group range. For this purpose, you must configure a
bootstrap router (BSR). The BSR acts 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.
NOTE:
• An RP can provide services for multiple multicast groups, but a multicast group only uses one RP.
• A device can act as a C-RP and a C-BSR at the same time.
Join message
Ethernet
Ethernet
RPDR
DR
Hello message
Register message
Source
Receiver
Receiver
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