Multicast forwarding and PIM
In a multicast network, hosts are responsible for informing routers that they want to receive a particular multicast stream. In IPv4 networks, they do this by using IGMP. In IPv6 networks, they do this by using MLD, which is part of Internet Control Message Protocol for Internet Protocol version 6 (ICMPv6).
The multicast-capable routers are responsible for replicating messages and forwarding them to the appropriate recipients. To do this replication and forwarding, multicast routers create distribution trees that control the path that IP multicast traffic takes through the network to deliver traffic to all receivers. The vRouter supports the use of Protocol Independent Multicast (PIM) to manage the communication between multicast routers.
Prior to PIM routing protocols, such as DVMRP, included a unicast routing protocol and operated without any dependency on other unicast routing protocols. The independent nature of PIM means that it performs multicast routing independent of a dependency on any specific unicast routing protocol. PIM makes use of whatever unicast routing mechanism (static routing, RIP, OSPF, etc) that is used.