Command Section
VLAN(4)                FreeBSD Kernel Interfaces Manual                VLAN(4)

     vlan - IEEE 802.1Q VLAN network interface

     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):


     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.

     vlan initially assumes the same minimum length for tagged and untagged
     frames.  This mode is selected by setting the sysctl(8) variable 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 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.

     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),
     rl(4), sf(4), sis(4), sk(4), ste(4), tl(4), tx(4), vr(4), vte(4), and

     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.

     ifconfig(8), sysctl(8)

FreeBSD 11.1-RELEASE-p4          June 8, 2016          FreeBSD 11.1-RELEASE-p4
Command Section