This command is used under family-address to tell other peers which family-address we support( ipv4/ipv6 unicast, ipv4/ipv6 multicast, ipv4/ipv6 VPN).This is for MP-BGP but now you can use it with BGP as replacement for default ipv4 configuration.
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But why will anybody want to use this command when normal BGP config works just fine? I guess there is no need, just that if IPv6 (or protocol other than IPv4) is used, then this command is a must as IPv4 is default for BGP...Is that right?
This can be used on CE VRFs. CE IP addresses are known only at appropriate VRF level hence it makes no sense in defining the neighbor at process level.
As the IPv4 address families are used to represent routing session for protocols that use IPv4 address prefix. We need to activate in IPv4 routing session.
I guess there is no need, just that if IPv6 (or protocol other than IPv4) is used, then this command is a must as IPv4 is default for BGP...Is that right? - NO that inst correct
By default bgp assumes you want to use ipv4 unicast bgp, however if you disable this prior to defining you bgp peers
no bgp default ipv4-unicast than you need to specify this activate command so that peer will establish.
router bgp xxx
no bgp default ipv4-unicast
neighbor x.x.x.x remote-as xx
neighbor x.x.x.x activate
Hope this have been helpful.