To enter address family configuration mode to configure a routing session using standard IP Version 4 address prefixes, use the address-family ipv4 command in router configuration or router scope configuration mode. To exit address family configuration mode and remove the IPv4 address family configuration from the running configuration, use the no form of this command.
Syntax Available Under Router Configuration Mode
address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name]
mdt : (Optional) Specifies an IPv4 multicast distribution (MDT) tree address-family session.
multicast : (Optional) Specifies IP Version 4 multicast address prefixes.
For more information please click following URL:
http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp1.html#wp1010944
