Router(config)# interface virtual-interface
Defines a virtual interface for multicast fast switching. Routers joining the same session must have their virtual interfaces on different subnets. Otherwise packets are not switched to the IP network.
Router(config-if)# ip address address subnet-mask
Assigns the IP address and subnet mask for the virtual interface.
Router(config-if)# ip pim { sparse mode | dense-mode | sparse-dense-mode }
Specifies Protocol Independent Multicast (PIM). Whatever mode you choose should match all the interfaces in all the routers of your network.