freebsd-dev/share/man/man4/vlan.4

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.Dd June 4, 2012
.Dt VLAN 4
.Os
.Sh NAME
.Nm vlan
.Nd "IEEE 802.1Q VLAN network interface"
.Sh SYNOPSIS
To compile this driver into the kernel,
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device vlan"
.Ed
.Pp
Alternatively, to load the driver as a
module 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.
.Pp
.Nm
initially assumes the same minimum length for tagged and untagged frames.
This mode is selected by setting the
.Xr sysctl 8
variable
.Va net.link.vlan.soft_pad
to 0
.Pq default .
However, there are network devices that fail to adjust frame length
when it falls below the allowed minimum due to untagging.
Such devices should be able to interoperate with
.Nm
after changing the value of
.Va net.link.vlan.soft_pad
to 1.
In the latter mode,
.Nm
will pad short frames before tagging them
so that their length is not less than the minimum value
after untagging by the non-compliant devices.
.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 long frames (up to 1522 bytes including an Ethernet
header and FCS).
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
At present, these devices are capable of full VLAN processing
in hardware:
.Xr ae 4 ,
.Xr age 4 ,
.Xr alc 4 ,
.Xr ale 4 ,
.Xr bce 4 ,
.Xr bge 4 ,
.Xr cxgb 4 ,
.Xr cxgbe 4 ,
.Xr em 4 ,
.Xr igb 4 ,
.Xr ixgb 4 ,
.Xr ixgbe 4 ,
.Xr jme 4 ,
.Xr msk 4 ,
.Xr mxge 4 ,
.Xr nxge 4 ,
.Xr nge 4 ,
.Xr re 4 ,
.Xr sge 4 ,
.Xr stge 4 ,
.Xr ti 4 ,
.Xr txp 4 ,
and
.Xr vge 4 .
.Pp
Other Ethernet interfaces can run VLANs using software emulation in the
.Nm
driver.
However, some lack the capability
of transmitting and receiving long 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
These interfaces natively support long frames for
.Nm :
.Xr axe 4 ,
.Xr bfe 4 ,
.Xr cas 4 ,
.Xr dc 4 ,
.Xr et 4 ,
.Xr fwe 4 ,
.Xr fxp 4 ,
.Xr gem 4 ,
.Xr hme 4 ,
.Xr le 4 ,
.Xr nfe 4 ,
.Xr nve 4 ,
.Xr rl 4 ,
.Xr sf 4 ,
.Xr sis 4 ,
.Xr sk 4 ,
.Xr ste 4 ,
.Xr tl 4 ,
.Xr tx 4 ,
.Xr vr 4 ,
.Xr vte 4 ,
and
.Xr xl 4 .
.Pp
The
.Nm
driver automatically recognizes devices that natively support long frames
for
.Nm
use and calculates the appropriate frame MTU based on the
capabilities of the parent interface.
Some other interfaces not listed above may handle long frames,
but they do not advertise this ability.
The MTU setting on
.Nm
can be corrected manually if used in conjunction with such a parent interface.
.Sh SEE ALSO
.Xr ifconfig 8 ,
.Xr sysctl 8
.Sh BUGS
No 802.1Q features except VLAN tagging are implemented.