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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 nu-
     meric 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 set	to 0 (default).	 However, there	are network
     devices that fail to adjust frame length, should it fall below the	al-
     lowed minimum due to untagging.  Such devices should be able to interop-
     erate 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 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	in-
     cluding 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), 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				 June 14, 2009				   BSD


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