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vlan - IEEE 802.1Q VLAN network interface
To compile this driver into the kernel, place the following lines in your
kernel configuration file:
Alternatively, to load the driver as a module at boot time, place the
following line in loader.conf(5):
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
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.
Initially vlan assumes the same minimum length for tagged and untagged
frames. This mode is selected by the sysctl(8) variable
net.link.vlan.soft_pad 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 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 stays not less than the minimum value after
untagging by the non-compliant devices.
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 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
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.
By now, the list of physical interfaces able of full VLAN processing in
the hardware is limited to the following devices: bge(4), em(4), ixgb(4),
nge(4), re(4), stge(4), ti(4), txp(4), and vge(4).
The rest of the Ethernet interfaces can run VLANs using software
emulation in the vlan driver. However, most of them lack the capability
of transmitting and receiving oversized 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.
The interfaces that support oversized frames are as follows:
bfe(4) supports long frames for vlan natively.
dc(4) supports long frames for vlan natively.
de(4) requires defining BIG_PACKET in the
/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.
fxp(4) supports long frames for vlan natively.
gem(4) supports long frames for vlan natively.
hme(4) supports long frames for vlan natively.
le(4) supports long frames for vlan natively.
rl(4) supports long frames for vlan natively.
sis(4) supports long frames for vlan natively.
sk(4) supports long frames for vlan natively.
ste(4) supports long frames for vlan natively.
tl(4) has support for long frames.
tx(4) supports long frames for vlan natively.
xl(4) supports long frames only if the card is built on a newer
chip (Cyclone and above).
The vlan driver automatically recognizes devices that natively support
oversized frames for vlan 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 vlan can be corrected manually if
used in conjunction with such parent interface.
kqueue(2), miibus(4), ifconfig(8), sysctl(8)
No 802.1Q features except VLAN tagging are implemented.
EVFILT_NETDEV events on a vlan interface will be sent through kqueue(2)
only if the parent interface uses miibus(4) for link state notification.