Manual Reference Pages - VLAN (4)
- IEEE 802.1Q VLAN network interface
To compile this driver into the kernel,
place the following lines in your
kernel configuration file:
.Cd device miibus
.Cd device vlan
Alternatively, to load the driver as a
module at boot time, place the following line in
driver demultiplexes frames tagged according to
the IEEE 802.1Q standard into logical
network interfaces, which allows routing/bridging between
multiple VLANs through a single switch trunk port.
interface is created at runtime using interface cloning.
most easily done with the
command or using the
To function, a
interface must be assigned a parent interface and
numeric VLAN tag using
A single parent can be assigned to multiple
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
assumes the same minimum length for tagged and untagged frames.
This mode is selected by the
set to 0 (default).
However, there are network devices that fail to adjust frame length,
should it fall below the allowed minimum due to untagging.
Such devices should be able to interoperate with
after changing the value of
In the latter mode,
will pad short frames before tagging them
so that their length stays not less than the minimum value
after untagging by the non-compliant devices.
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
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.
By now, the list of physical interfaces able of full VLAN processing
in the hardware is limited to the following devices:
The rest of the Ethernet interfaces can run
VLANs using software emulation in the
However, some of them lack the capability
of transmitting and receiving long frames.
Assigning such an interface as the parent to
will result in a reduced MTU on the corresponding
In the modern Internet, this is likely to cause
connectivity problems due to massive, inadequate
filtering that breaks the Path MTU Discovery mechanism.
The following interfaces support long frames for
driver automatically recognizes devices that natively support long frames
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 of theirs.
The MTU setting on
can be corrected manually if used in conjunction with such a parent interface.
No 802.1Q features except VLAN tagging are implemented.
events on a
interface will be sent through
only if the parent interface uses
for link state notification.
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