Assume we have a switchport with an untagged (native) VLAN 10, plus two tagged VLANs 20 & 30 (see attached switch config). The device connected to that switchport has an internal interface eth0. Use this in /etc/rc.conf
:
# SYNCDHCP makes the boot process wait for address assignment
ifconfig_eth0="SYNCDHCP"
vlans_eth0="20 30"