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Figure 88 DR election

Join message

Ethernet

RPDR

Hello message

Source

Receiver

As shown in Figure 88, 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 IPv6 link-local address will win

the DR election.

When the DR works abnormally, a timeout in receiving hello message triggers a new DR election

process among the other routers.

RP discovery

The RP is the core of an IPv6 PIM-SM domain. For a small-sized, simple network, one RP is enough for

forwarding IPv6 multicast information throughout the network, and the position of the RP can be statically

specified on each router in the IPv6 PIM-SM domain. In most cases, however, an IPv6 PIM-SM network

covers a wide area and a huge amount of IPv6 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 an IPv6 PIM-SM domain. Among them, an RP is dynamically elected through the

bootstrap mechanism. Each elected RP serves a different multicast group range. For this purpose, you

must configure a bootstrap router (BSR). The BSR serves as the administrative core of the IPv6 PIM-SM

domain. An IPv6 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 serve IPv6 multiple multicast groups or all IPv6 multicast groups. Only one RP can serve a

given IPv6 multicast group at a time.

• A device can serve as a C-RP and a C-BSR at the same time.

As shown in Figure 89, each C-RP periodically unicasts its advertisement messages—C-RP-Adv

messages—to the BSR. A C-RP-Adv message contains the address of the advertising C-RP and the IPv6

multicast group range that it serves. The BSR collects these advertisement messages and chooses the

appropriate C-RP information for each multicast group to form an RP-set, which is a database of

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