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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 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.

     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.