I found this is new in FRR compared to QUAGGA, what's this used for ? Why I should config interface first ? Is there some DOC about this ?
# neighbor 1.1.1.1/24 ... interface Enable BGP on interface
# neighbor 1.1.1.1/24 remote-as 5001 % Create the peer-group or interface first or specify "interface" keyword % if using an unnumbered interface neighbor