HP A8800 Configuration Manual page 127

Figure 39 DR election
As shown in
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 the position of the RP can be statically specified 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 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 a
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 a PIM-SM domain. A 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.
NOTE:
An RP can serve multiple multicast groups or all multicast groups. Only one RP can serve a given
•
multicast group at a time.
A device can serve as a C-RP and a C-BSR at the same time.
•
As shown in
to the BSR. A C-RP-Adv message contains the address and priority of the advertising C-RP and the
multicast group range it servers. 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
Figure 39, the DR election process is as follows:
Figure 40, each C-RP periodically unicasts its advertisement messages (C-RP-Adv messages)
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