/etc/rc.conf:
ifconfig_bge1="up"
ipv4_addrs_bge1="10.200.0.1/24" # a.k.a. Cisco 'switchport trunk native vlan xxx'
cloned_interfaces="disc0 vlan100 vlan101 vlan102"
ifconfig_vlan100="vlan 100 vlandev bge1 up" # a.k.a. Cisco 'switchport trunk allowed vlan add 100'
ipv4_addrs_vlan100="10.1.100.1/24 10.1.1.1/21 100.100.100.1/24 172.24.0.1/16"
ifconfig_vlan101="vlan 101 vlandev bge1 up"
ipv4_addrs_vlan101="10.101.0.1/21"
ifconfig_vlan102="vlan 102 vlandev bge1 up"
ipv4_addrs_vlan102="10.102.0.1/21"