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), 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 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)