BGP multicast (MBGP) is an extension of the BGP unicast routing protocol. Many of the functions available for BGP unicasting are also available for MBGP.
The MBGP extensions specify that BGP can exchange information within different types of address families. The address families available are unicast IPv4, multicast IPv4, VPN-IPv4, IPv6, and multicast IPv6. When you enable BGP, the router employs unicast IPv4 addresses by default.
You should be thoroughly familiar with BGP before configuring MBGP. See Configuring BGP Routing in the JUNOSe BGP and MPLS Configuration Guide for detailed information about BGP and MBGP.