406
A DR is elected on a multi-access subnet by means of comparison of the priorities and IPv6 link-local
addresses carried in hello messages.
NOTE:
MLD must be enabled on a device that acts as a receiver-side DR before receivers attached to this
device can join IPv6 multicast groups through this DR. For more information about MLD, see
"
Figure 118 DR election
As shown in
, 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 becomes the
DR.
2.
The router with the highest IPv6 link-local address wins the DR election under one of the
following conditions:
a.
All the routers have the same DR election priority.
b.
A router does not support carrying the DR-election priority in hello messages.
When the DR works abnormally, a timeout in receiving a 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. 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 C-RPs in an IPv6 PIM-SM domain. Among them, an RP is dynamically elected
through the bootstrap mechanism. Each elected RP is designated to a different multicast group
range. For this purpose, you must configure a BSR. The BSR acts as the administrative core of the
IPv6 PIM-SM domain. An IPv6 PIM-SM domain can have only one BSR, but can have multiple
C-BSRs. If the BSR fails, a new BSR is automatically elected from the C-BSRs to avoid service
interruption.
