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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:

           device vlan

     Alternatively, to load the driver as a module at boot time, place the following line in



     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.

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


     ifconfig(8), sysctl(8)


     No 802.1Q features except VLAN tagging are implemented.