bionic (4) if_vlan.4freebsd.gz

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)