Routing - address-family
An address-family is a configuration mode found under routing protocol configurations such as BGP, OSPF, EIGRP, that can be used to configure routing sessions to use specific protocols.
For example, for BGP, you can configure address family sessions for IPv4, IPv4 multicast, IPv6 unicast, as well as IPv6 multicast. As with most routing protocols, if the address family is not configured, the default address family being used is IPv4 unicast.
The configuration of address families is the basis for the operation of Multi Protocol BGP (MP-BGP).
Links:
https://forum.networklessons.com/t/multiprotocol-bgp-mp-bgp-configuration/1177/66?u=lagapides
https://networklessons.com/bgp/multiprotocol-bgp-mp-bgp-configuration