vlan man page on FreeBSD

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VLAN(4)			 BSD Kernel Interfaces Manual		       VLAN(4)

     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 rout‐
     ing/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 con‐
     figured 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
     net.link.vlan.soft_pad 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 inter‐
     operate 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 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 sophistica‐
     tion 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-con‐
     trolled 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

     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 emula‐
     tion 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) con‐
     nectivity 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 abil‐
     ity 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.

BSD				April 14, 2010				   BSD

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