802.1ad interfaces are created using interface cloning ("svlan" prefix).
Eg., the following creates a 802.1Q VLAN (id #42) over a 802.1ad S-VLAN
(id #5) over a physical Ethernet interface (em0).
ifconfig svlan5 create vlandev em0 vlan 5 up
ifconfig vlan42 create vlandev svlan5 vlan 42 inet 10.5.42.1/24
Any combination of vlan and svlan interfaces is supported, as long as
VLAN IDs are unique among the children of the same interface
(ie. can't have both vlan #5 and svlan #5 with em0 as a parent).
VLAN_MTU, VLAN_HWCSUM and VLAN_TSO capabilities should be properly
supported. VLAN_HWTAGGING is only partially supported, as there is
currently no IFCAP_VLAN_* denoting the possibility to set the VLAN
EtherType to anything else than 0x8100 (802.1ad uses 0x88A8).
Sponsored by: RG Nets