VLAN(4) BSD Kernel Interfaces Manual VLAN(4)
vlan, svlan -- IEEE 802.1Q/1AD pseudo-device
The vlan Ethernet interface allows construction of virtual LANs when used
in conjunction with IEEE 802.1Q-compliant Ethernet devices. The svlan
Ethernet interface allows construction of IEEE 802.1AD-compliant provider
bridges. It is normally used for QinQ to stack vlan interfaces on top of
The interfaces can be created at runtime using the ifconfig vlanN create
command or by setting up a hostname.if(5) configuration file for
netstart(8). The interface itself can be configured with ifconfig(8);
see its manual page for more information.
For vlan devices, the 802.1Q header specifies the virtual LAN number, and
thus allows an Ethernet switch (or other 802.1Q compliant network
devices) to be aware of which LAN the frame is part of, and in the case
of a switch, which port(s) the frame can go to. Frames transmitted
through the vlan interface will be diverted to the specified physical
interface with a 802.1Q vlan tag added. 802.1Q frames received by the
parent interface with the correct vlan tag will be diverted to the asso-
ciated vlan pseudo-interface.
Frame headers which normally contain the destination host, source host,
and protocol, are altered with additional information, comprising as fol-
lows: 16 bits for the ether type (0x8100); 3 bits for the priority field;
1 bit for the canonical field (always 0); and 12 bits for the vlan iden-
tifier. The priority field may be altered via pf.conf(5); see the prio
option for more information. Following the vlan header is the actual
ether type for the frame and length information.
For svlan devices, the configuration is identical to the vlan interface,
the only differences being that it uses a different Ethernet type
(0x88a8) and an independent VLAN ID space on the parent interface.
vlan and svlan interfaces support the following unique ioctl(2)s:
SIOCGETVLAN Get the vlan tag and parent for a given vlan interface.
SIOCSETVLAN Set the vlan tag and parent for a given vlan interface.
vlan and svlan interfaces use the following interface capabilities:
IFCAP_VLAN_MTU The parent interface can handle full sized
frames, plus the size of the vlan tag.
IFCAP_VLAN_HWTAGGING The parent interface will participate in the
tagging of frames. (This is not supported by
vlan0: initialized with non-standard mtu N (parent ...) The
IFCAP_VLAN_MTU capability was not set on the parent interface. We assume
in this event that the parent interface is not capable of handling frames
larger than its MTU. This will generally result in a non-compliant
Some Ethernet chips will either discard or truncate Ethernet frames that
are larger than 1514 bytes. This causes a problem as 802.1Q tagged
frames can be up to 1518 bytes. Most controller chips can be told not to
discard large frames and/or to increase the allowed frame size. Refer to
the hardware manual for your chip to do this.
If the IFCAP_VLAN_MTU capability is set on a vlan parent, vlan assumes
that the Ethernet chip on the parent can handle oversized frames. Either
the chip allows 1518 byte frames by default (such as rl(4)), the driver
has instructed the chip to do so (such as fxp(4) and dc(4)), or the
driver also takes advantage of a hardware tagging capability, and thus
oversized frames are never actually sent by OpenBSD (such as txp(4) and
bridge(4), inet(4), ip(4), netintro(4), hostname.if(5), pf.conf(5),
IEEE 802.1Q standard, http://standards.ieee.org/getieee802/802.1.html.
IEEE 802.1AD standard, Provider Bridges, QinQ.
Originally Garrett Wollman <email@example.com>.
BSD April 26, 2017 BSD