vlan - IEEE 802.1Q encapsulation/decapsulation pseudo-device
pseudo-device vlan [count]
The vlan Ethernet interface allows construction of virtual
LANs when used
in conjunction with IEEE 802.1Q-compliant Ethernet devices.
A vlan interface can be created at runtime using the
create command or by setting up a hostname.if(5) configuration file for
This driver currently supports the following modes of operation:
802.1Q encapsulation over Ethernet (Ethernet protocol
The 802.1Q header specifies the virtual LAN number, and
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
through the vlan interface will be diverted to the
interface with 802.1Q vlan encapsulation. Frames
encapsulation received by the parent interface with the
tag will be diverted to the associated vlan pseudo-interface.
Frame headers which normally contain the destination host,
and protocol, are altered with additional information. After the source
host, a 32-bit 802.1Q header is included, with 16 bits for
the ether type
(0x8100), 3 bits for the priority field (not used in this
1 bit for the canonical field (always 0), and 12 bits
for the vlan
identifier. Following the vlan header is the actual ether
type for the
frame and length information.
vlan interfaces support the following unique ioctl(2)s:
Set the vlan tag and parent for a given vlan interface.
Get the vlan tag and parent for a given vlan interface.
vlan 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
vlan%d: initialized with non-standard mtu %d (parent %s)
capability was not set on the parent interface.
in this event that the parent interface is not capable of
larger than its MTU. This will generally result in a noncompliant
Some Ethernet chips will either discard or truncate Ethernet
are larger than 1514 bytes. This causes a problem as 802.1Q
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,
that the Ethernet chip on the parent can handle oversized
the chip allows 1518 byte frames by default (such as rl(4)),
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 or received by OpenBSD
txp(4) and ti(4)).
bridge(4), inet(4), ip(4), netintro(4), hostname.if(5), ifconfig(8),
IEEE 802.1Q standard, http://standards.ieee.org/getieee802/802.1.html.
The vlan interface is to be configured with ifconfig(8), see
page for more information.
The 802.1Q specification allows for operation over FDDI and
Token Ring as
well as Ethernet. This driver only supports such operation
When the IFCAP_VLAN_HWTAGGING capability is set on the parent interface,
vlan does not participate in the actual tagging or untagging
frames. It simply passes the vlan ID on to the parent interface for tagging
on transmit, and gets a vlan ID for each packet on receive. The
vlan tagged packet is not actually visible to OpenBSD. Thus,
show untagged packets on the parent interface, although
frames are actually
being transmitted and received with tags on the wire.
This driver could be the basis for support of the Cisco ISL
detailed at http://www.cisco.com/warp/pub-
lic/473/741_4.html . Unfortunately,
public reimplementation of this protocol is
by patent (at least in the USA).
OpenBSD 3.6 January 9, 2000
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