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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 miibus
	   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.

     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 frames exceeding the maximum Ether-
     net frame size by the length of a 802.1Q header.  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: bge(4), em(4), ixgb(4),
     nge(4), re(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,	most of	them lack the capability of
     transmitting and receiving	oversized 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 interfaces that support oversized frames are as follows:

	   bfe(4)  supports long frames	for vlan natively.

	   dc(4)   supports long frames	for vlan natively.

	   de(4)   requires defining BIG_PACKET	in the
		   /usr/src/sys/pci/if_de.c source file	and rebuilding the
		   kernel or module.  The hack works only for the 21041,
		   21140, and 21140A chips.

	   fxp(4)  supports long frames	for vlan natively.

	   gem(4)  supports long frames	for vlan natively.

	   hme(4)  supports long frames	for vlan natively.

	   le(4)   supports long frames	for vlan natively.

	   rl(4)   supports long frames	for vlan natively.

	   sis(4)  supports long frames	for vlan natively.

	   sk(4)   supports long frames	for vlan natively.

	   ste(4)  supports long frames	for vlan natively.

	   tl(4)   has support for long	frames.

	   tx(4)   supports long frames	for vlan natively.

	   xl(4)   supports long frames	only if	the card is built on a newer
		   chip	(Cyclone and above).

     The vlan driver automatically recognizes devices that natively support
     oversized frames for vlan use and calculates the appropriate frame	MTU
     based on the capabilities of the parent interface.	 The other interfaces
     listed above can handle oversized 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 parent interface.

     kqueue(2),	miibus(4), ifconfig(8)

     No	802.1Q features	except VLAN tagging are	implemented.

     EVFILT_NETDEV events on a vlan interface will be sent through kqueue(2)
     only if the parent	interface uses miibus(4) for link state	notification.

BSD			       January 30, 2006				   BSD


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