H3C Technologies H3C S7500E Series Switches - Manual (Page 358)

13-7
z 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.
z 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 details about MLD, refer to MLD Configuration in the IP Multicast Configuration Guide.
Figure 13-3 DR election
Join message
Ether
net
Ether
net
RP DR
DR
Hello message
Register message
Source
Receiver
Receiver
As shown in Figure 13-3 , 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
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