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):

             if_vlan_load="YES"

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), ixgbe(4), jme(4), liquidio(4), msk(4), mxge(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 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)

Debian                                             May 8, 2018                                           VLAN(4)