.\" $OpenBSD: vlan.4,v 1.8 2000/11/10 20:02:17 todd Exp $
.\"
.Dd January 9, 2000
.Dt VLAN 4
.Os
.Sh NAME
.Nm vlan
.Nd "IEEE 802.1Q encapsulation/decapsulation pseudo-device"
.Sh SYNOPSIS
.Cd pseudo-device vlan Op Ar count
.Sh DESCRIPTION
The
.Nm vlan
Ethernet interface allows construction of virtual LANs when used in
conjunction with IEEE 802.1Q-compliant Ethernet devices.
.Pp
This driver currently supports the following modes of operation:
.Bl -tag -width abc
.It 802.1Q encapsulation over Ethernet (Ethernet protocol 0x8100)
.Pp
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.
.El
.Pp
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, with 16 bits for the ether type (0x8100), 3
bits for the priority field (not used in this implementation), 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.
.Pp
The network interfaces are named
.Sy vlan Ns Ar 0 ,
.Sy vlan Ns Ar 1 ,
etc.
The number of interfaces is given by the corresponding
.Sy pseudo-device
line in the system config file.
.Xr vlan 4
interfaces support the following unique
.Xr ioctl 2 Ns s :
.Bl -tag -width aaa
.It SIOCSETVLAN:
Set the vlan tag and parent for a given vlan interface.
.It SIOCGETVLAN:
Get the vlan tag and parent for a given vlan interface.
.Pp
.El
The operation mode can be given as
.Bl -tag -width bbb
.It link0
The parent interface supports VLAN tagging/untagging in hardware,
this requires support from the Ethernet driver as well.
.El
.Pp
to
.Xr ifconfig 8 .
.Pp
.Sh SEE ALSO
.Xr bridge 4 ,
.Xr inet 4 ,
.Xr ip 4 ,
.Xr netintro 4 ,
.Xr ifconfig 8
.Pp
All aspects of the vlan interface can be configured with
.Xr ifconfig 8 ;
see its manual page for more information.
.Pp
A draft of IEEE 802.1Q standard can be obtained from
http://grouper.ieee.org/groups/802/1/pages/802.1Q.html
or the final version can be purchased from the
Institute of Electrical and Electronic Engineers, Inc.
.Sh 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.
.Pp
Some Ethernet chips (notably the Intel 82558 and 82559) automatically
discard frames that are larger then 1514 bytes.
All Ethernet
chips sporting this sort of feature should allow for it to be turned off.
Optimally, upon loading, the chip's driver would do this (The
.Xr fxp 4
driver currently does for the Intel 82558/82559 chips.)
Refer to the hardware manual for your Ethernet chip to determine
if it has this feature, and if so, for information on how to turn it off.
A symptom of this problem is that small frames pass through the VLAN
(such as a frame carrying an ICMP packet), but frames which are at MTU size
are dropped.
When these frames are dropped, the TCP session will hang.
A temporary kludge is to set the MTU for each machine behind each VLAN to
1496 or less.
.Pp
Some Ethernet chips support 802.1Q tagging/untagging in hardware, but
do not have the appropriate hooks in their driver to pass the frame
directly to if_vlan.
The PCI device driver if_ti is able to do this, use it as a reference.
.Pp
This driver could be the basis for support of the Cisco ISL VLAN protocol,
detailed at http://www.cisco.com/warp/public/741/4.html.
Unfortunately, public reimplementation of this protocol is currently prevented
by patent (at least in the USA).
.Pp
.Sh AUTHORS
Originally wollman@freebsd.org.