Load drivers on boot

echo "if_lagg_load=\"YES\"" >> /boot/loader.conf

Add to rc.conf

ifconfig_em0="UP" ifconfig_em1="UP" cloned_interfaces="lagg0" ifconfig_lagg0="laggproto failover laggport em0 laggport em1 ip.ad.dr.ess netmask net.ma.sk"

probably good idea to turn on spanning tree on your port .