Default configuration file of bgpd is bgpd.conf.
bgpd searches the current directory first then
/usr/local/etc/bgpd.conf. All of bgpd's command must be
configured in bgpd.conf.
bgpd specific invocation options are described below. Common
options may also be specified (see Common Invocation Options).
-p PORT
--bgp_port=PORT
-r
--retain