man/man4/vlan.4

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.Dd August 28, 2005
.Dt VLAN 4
.Os
.Sh NAME
.Nm vlan
.Nd "IEEE 802.1Q VLAN network interface"
.Sh SYNOPSIS
To compile support for the
.Nm
driver into your kernel, place the following lines in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device miibus"
.Cd "device vlan"
.Ed
.Pp
Alternatively, to load the
.Nm
driver at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
if_vlan_load="YES"
.Ed
.Sh DESCRIPTION
The
.Nm
driver demultiplexes frames tagged according to
the IEEE 802.1Q standard into logical
.Nm
network interfaces, which allows routing/bridging between
multiple VLANs through a single switch trunk port.
.Pp
Each
.Nm
interface is created at runtime using interface cloning.
This is
most easily done with the
.Xr ifconfig 8
.Cm create
command or using the
.Va cloned_interfaces
variable in
.Xr rc.conf 5 .
.Pp
To function, a
.Nm
interface must be assigned a parent interface and
numeric VLAN tag using
.Xr ifconfig 8 .
A single parent can be assigned to multiple
.Nm
interfaces provided they have different tags.
The parent interface is likely to be an Ethernet card connected
to a properly configured switch port.
The VLAN tag should match one of those set up in the switched
network.
.Sh HARDWARE
The
.Nm
driver supports efficient operation over parent interfaces that can provide
help in processing VLANs.
Such interfaces are automatically recognized by their capabilities.
Depending on the level of sophistication found in a physical
interface, it may do full VLAN processing or just be able to
receive and transmit frames exceeding the maximum Ethernet frame size
by the length of a 802.1Q header.
The capabilities may be user-controlled by the respective parameters to
.Xr ifconfig 8 ,
.Cm vlanhwtag
and
.Cm vlanmtu .
However, a physical interface is not obliged to react to them:
It may have either capability enabled permanently without
a way to turn it off.
The whole issue is very specific to a particular device and its driver.
.Pp
By now, the list of physical interfaces able of full VLAN processing
in the hardware is limited to the following devices:
.Xr bge 4 ,
.Xr em 4 ,
.Xr ixgb 4 ,
.Xr nge 4 ,
.Xr re 4 ,
.Xr ti 4 ,
.Xr txp 4 ,
and
.Xr vge 4 .
.Pp
The rest of the Ethernet interfaces can run
VLANs using software emulation in the
.Nm
driver.
However, most of them lack the capability
of transmitting and receiving oversized frames.
Assigning such an interface as the parent to
.Nm
will result in a reduced MTU on the corresponding
.Nm
interfaces.
In the modern Internet, this is likely to cause
.Xr tcp 4
connectivity problems due to massive, inadequate
.Xr icmp 4
filtering that breaks the Path MTU Discovery mechanism.
.Pp
The interfaces that support oversized frames are as follows:
.Bl -tag -width ".Xr fxp 4 " -offset indent
.It Xr bfe 4
supports long frames for
.Nm
natively.
.It Xr dc 4
supports long frames for
.Nm
natively.
.It Xr de 4
requires defining
.Dv BIG_PACKET
in the
.Pa /usr/src/sys/pci/if_de.c
source file and rebuilding the kernel
or module.
The hack works only for the 21041, 21140, and 21140A chips.
.It Xr fxp 4
supports long frames for
.Nm
natively.
.It Xr gem 4
supports long frames for
.Nm
natively.
.It Xr hme 4
supports long frames for
.Nm
natively.
.It Xr rl 4
supports long frames for
.Nm
natively.
.It Xr sis 4
supports long frames for
.Nm
natively.
.It Xr ste 4
supports long frames for
.Nm
natively.
.It Xr tl 4
has support for long frames.
.It Xr tx 4
supports long frames for
.Nm
natively.
.It Xr xl 4
supports long frames only if the card is built on a newer chip
(Cyclone and above).
.El
.Pp
The
.Nm
driver automatically recognizes devices that natively support oversized frames
for
.Nm
use and calculates the appropriate frame MTU based on the
capabilities of the parent interface.
The other interfaces listed above can handle oversized frames,
but they do not advertise this ability of theirs.
The MTU setting on
.Nm
can be corrected manually if used in conjunction with such parent interface.
.Sh SEE ALSO
.Xr kqueue 2 ,
.Xr miibus 4 ,
.Xr ifconfig 8
.Sh BUGS
No 802.1Q features except VLAN tagging are implemented.
.Pp
.Dv EVFILT_NETDEV
events on a
.Nm
interface will be sent through
.Xr kqueue 2
only if the parent interface uses
.Xr miibus 4
for link state notification.