3Com Switch 8800 Advanced Software V5 Configuration Guide

IPv6 PIM Overview 677
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, each IPv6 multicast group should have its own RP. Therefore, a bootstrap
mechanism is needed for dynamic RP election. For this purpose, a bootstrap router
(BSR) should be configured.
As the administrative core of an IPv6 PIM-SM domain, the BSR collects
advertisement messages (C-RP-Adv messages) from candidate-RPs (C-RPs) and
chooses the appropriate C-RP information for each IPv6 multicast group to form
an RP-Set, which is a database of mappings between IPv6 multicast groups and
RPs. The BSR then floods the RP-Set to the entire IPv6 PIM-SM domain. Based on
the information in these RP-Sets, all routers (including the DRs) in the network can
calculate the location of the corresponding RPs.
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 through the bootstrap mechanism to avoid service interruption.
Similarly, multiple C-RPs can be configured in an IPv6 PIM-SM domain, and the
position of the RP corresponding to each IPv6 multicast group is calculated
through the BSR mechanism.
Figure 204 shows the positions of C-RPs and the BSR in the network.
Figure 204 Communication between the BSR and C-RPs
Embedded RP
The Embedded RP mechanism allows a router to resolve the RP address from an
IPv6 multicast address so that the IPv6 multicast group is mapped to an RP, which
can take the place of the statically configured RP or the RP dynamically calculated
Advertisement message
BSR message
IPv6 PIM-SM
BSR
C-RP
C-RP
C-RP
C-BSR