3Com S7902E, Configuration Manual

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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 needs to be forwarded
through the RP. To lessen the RP burden and optimize the topological structure of the RPT, multiple
candidate RPs (C-RPs) can be configured in an IPv6 PIM-SM domain, among which an RP is
dynamically elected through the bootstrap mechanism. Each elected RP serves a different multicast
group range. For this purpose, a bootstrap router (BSR) must be configured. 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). Once the BSR fails, a new BSR is automatically elected
from the C-BSRs to avoid service interruption.
z
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.
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A device can server as a C-RP and a C-BSR at the same time.
As shown in the figure below, 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 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
mappings between IPv6 multicast groups and RPs. The BSR then encapsulates the RP-set in the
bootstrap messages it periodically originates and floods the bootstrap messages to the entire IPv6
PIM-SM domain.