VLAN(4) BSD Kernel Interfaces Manual VLAN(4)
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 rout-
ing/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.
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 automati-
cally recognized by their capabilities. Depending on the level of sophistication found in a physical interface, it may do full VLAN process-
ing 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.
By now, the list of physical interfaces able of full VLAN processing in the hardware is limited to the following devices: ae(4), age(4),
alc(4), ale(4), bce(4), bge(4), cxgb(4), em(4), ixgb(4), jme(4), msk(4), nge(4), re(4), sge(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, some of them 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.
The following interfaces support long frames for vlan natively: bfe(4), cas(4), dc(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), 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 of theirs. The MTU setting on vlan can be corrected manually if used in conjunction with such a parent interface.
No 802.1Q features except VLAN tagging are implemented.
April 14, 2010 BSD