H3C S7500E Series Operation Manual page 696

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches
I. Neighbor discovery
PIM-SM uses exactly the same neighbor discovery mechanism as PIM-DM does. Refer
to Neighbor
II. DR election
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.
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.
Note:
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 DR before receivers attached to
this device can join multicast groups through this DR.
Source
Figure 5-3 DR election
As shown in
discovery.
DR
Hello message
Register message
Join message
Figure 5-3, the DR election process is as follows:
DR
RP
5-7
Chapter 5 PIM Configuration
Receiver
Receiver