/etc/rc.conf:

ifconfig_em0="up promisc mtu 9212"
ifconfig_em1="up promisc mtu 9212"
cloned_interfaces="lagg0"
ifconfig_lagg0="laggproto loadbalance laggport em0 laggport em1 promisc monitor up mtu 9212"


usage: tcpdump -i lagg0 …