3Com Switch 8800 Advanced Software V5 Configuration Guide

676 CHAPTER 46: IPV6 PIM CONFIGURATION

Neighbor discovery

IPv6 PIM-SM uses exactly the same neighbor discovery mechanism as IPv6 PIM-DM

does. Refer to

“Neighbor discovery” on page 672.

DR election

IPv6 PIM-SM also uses hello messages to elect a designated router (DR) for a

multi-access network. The elected DR will be the only multicast forwarder on this

multi-access network.

In the case of a multi-access network, a DR must be elected, no matter this

network connects to IPv6 multicast sources or to receivers. The DR at the receiver

side sends join messages to the RP; the DR at the IPv6 multicast source side sends

■ A DR is elected on a multi-access subnet by means of comparison of the

priorities and IP addresses carried in hello messages. An elected DR is

substantially meaningful to IPv6 PIM-SM. IPv6 PIM-DM itself does not require a

DR. However, if MLDv1 runs on any multi-access network in an IPv6 PIM-DM

domain, a DR must be elected to act as the MLDv1 querier on that multi-access

network.

■ MLD must be enabled on a device that acts as a DR before receivers attached

to this device can join IPv6 multicast groups through this DR.

For details about MLD, refer to “MLD Overview” on page 637.

Figure 203 DR election

As shown in Figure 203, 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.

Join message

Ethernet

RPDR

Hello message

Source

Receiver