3Com S7906E Configuration Manual page 1028

Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For details, refer to
discovery.
DR election
PIM-SM also uses hello messages to elect a DR for a multi-access network (such as Ethernet). The
elected DR will be the only multicast forwarder on this multi-access network.
A DR must be elected in a multi-access network, no matter this network connects to multicast sources
or to receivers. The DR at the receiver side sends join messages to the RP; the DR at the multicast
source side sends register messages to the RP.
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 PIM-SM.
PIM-DM itself does not require a DR. However, if IGMPv1 runs on any multi-access network in a
PIM-DM domain, a DR must be elected to act as the IGMPv1 querier on that multi-access network.
IGMP must be enabled on a device that acts as a receiver-side DR before receivers attached to
this device can join multicast groups through this DR.
For details about IGMP, refer to IGMP Configuration in the IP Multicast Volume.
Figure 1-3 DR election
DR
Source
Hello message
Register message
Join message
As shown in Figure 1-3, 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 IP address will win the DR
election.
DR
RP
1-6
Receiver
Receiver
Neighbor