3Com S7902E, Configuration Manual

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1-7
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 the 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.
z
An RP can serve multiple multicast groups or all multicast groups. Only one RP can serve a given
multicast group at a time.
z
A device can serve as a C-RP and a C-BSR at the same time.
As shown in Figure 1-4 , 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
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 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 PIM-SM domain.
Figure 1-4 BSR and C-RPs
Based on the information in the RP-sets, all routers in the network can calculate the location of the
corresponding RPs based on the following rules:
1) The C-RP with the highest priority wins.