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VLAN(4) Device Drivers Manual VLAN(4)

vlan, svlanIEEE 802.1Q/1AD pseudo-device

pseudo-device vlan

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 it.

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 associated vlan pseudo-interface.

Frame headers which normally contain the destination host, source host, and protocol, are altered with additional information, comprising as follows: 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 identifier. 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:

Get the vlan tag and parent for a given vlan interface.
Set the vlan tag and parent for a given vlan interface.

vlan and svlan interfaces use the following interface capabilities:

The parent interface can handle full sized frames, plus the size of the vlan tag.
The parent interface will participate in the tagging of frames. (This is not supported by svlan interfaces.)

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 802.1Q implementation.

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 ti(4)).

bridge(4), inet(4), ip(4), netintro(4), hostname.if(5), pf.conf(5), ifconfig(8), netstart(8)

IEEE 802.1Q standard,

IEEE 802.1AD standard, Provider Bridges, QinQ.

Originally Garrett Wollman <>.

January 15, 2015 OpenBSD-5.8