NAME¶
vlan
—
IEEE 802.1Q VLAN network interface
SYNOPSIS¶
To compile this driver into the kernel, place the following line in your kernel
configuration file:
device vlan
Alternatively, to load the driver as a module at boot time, place the following
line in
loader.conf(5):
DESCRIPTION¶
The
vlan
driver demultiplexes frames tagged
according to the IEEE 802.1Q standard into logical
vlan
network interfaces, which allows
routing/bridging between multiple VLANs through a single switch trunk port.
Each
vlan
interface is created at runtime
using interface cloning. This is most easily done with the
ifconfig(8)
create
command or using the
cloned_interfaces variable in
rc.conf(5).
To function, a
vlan
interface must be
assigned a parent interface and numeric VLAN tag using
ifconfig(8). A single parent can be assigned to
multiple
vlan
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.
vlan
initially assumes the same minimum
length for tagged and untagged frames. This mode is selected by setting the
sysctl(8) variable
net.link.vlan.soft_pad to 0 (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
vlan
after changing
the value of
net.link.vlan.soft_pad to 1. In
the latter mode,
vlan
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.
HARDWARE¶
The
vlan
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
ifconfig(8),
vlanhwtag
, and
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.
At present, these devices are capable of full VLAN processing in hardware:
ae(4),
age(4),
alc(4),
ale(4),
bce(4),
bge(4),
bxe(4),
cxgb(4),
cxgbe(4),
em(4),
igb(4),
ixgb(4),
ixgbe(4),
jme(4),
msk(4),
mxge(4),
nxge(4),
nge(4),
re(4),
sge(4),
stge(4),
ti(4),
txp(4), and
vge(4).
Other Ethernet interfaces can run VLANs using software emulation in the
vlan
driver. However, some lack the
capability of transmitting and receiving long frames. Assigning such an
interface as the parent to
vlan
will result
in a reduced MTU on the corresponding
vlan
interfaces. In the modern Internet, this is likely to cause
tcp(4) connectivity problems due to massive,
inadequate
icmp(4) filtering that breaks the Path
MTU Discovery mechanism.
These interfaces natively support long frames for
vlan
:
axe(4),
bfe(4),
cas(4),
dc(4),
et(4),
fwe(4),
fxp(4),
gem(4),
hme(4),
le(4),
nfe(4),
nve(4),
rl(4),
sf(4),
sis(4),
sk(4),
ste(4),
tl(4),
tx(4),
vr(4),
vte(4), and
xl(4).
The
vlan
driver automatically recognizes
devices that natively support long frames for
vlan
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
vlan
can
be corrected manually if used in conjunction with such a parent interface.
SEE ALSO¶
ifconfig(8),
sysctl(8)
BUGS¶
No 802.1Q features except VLAN tagging are implemented.