VLAN(4) OpenBSD Programmer's Manual VLAN(4)NAME
vlan, svlan - IEEE 802.1Q/1AD encapsulation/decapsulation pseudo-device
SYNOPSIS
pseudo-device vlanDESCRIPTION
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 802.1Q vlan encapsulation. Frames with 802.1Q
encapsulation 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. After the source
host, a 32-bit 802.1Q header is included, 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 ifconfig(8); see the vlanprio 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.
SIOCGETVLANPRIO Get the vlan priority for a given vlan interface.
SIOCSETVLAN Set the vlan tag and parent for a given vlan
interface.
SIOCSETVLANPRIO Set the vlan priority 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
svlan interfaces.)
DIAGNOSTICS
vlan%d: initialized with non-standard mtu %d (parent %s) 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)).
SEE ALSObridge(4), inet(4), ip(4), netintro(4), hostname.if(5), ifconfig(8),
netstart(8)
IEEE 802.1Q standard, http://standards.ieee.org/getieee802/802.1.html.
IEEE 802.1AD standard, Provider Bridges, QinQ.
AUTHORS
Originally wollman@freebsd.org.
BUGS
The 802.1Q specification allows for operation over FDDI and Token Ring as
well as Ethernet. This driver only supports such operation with Ethernet
devices.
When the IFCAP_VLAN_HWTAGGING capability is set on the parent interface,
vlan does not participate in the actual tagging of Ethernet frames. It
simply passes the vlan ID on to the parent interface for tagging on
transmit. The vlan tagged packet is not actually visible to OpenBSD.
Thus, bpf(4) will show untagged packets on the parent interface, although
frames are actually being transmitted with tags on the wire.
OpenBSD 4.9 June 8, 2010 OpenBSD 4.9
