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

     vlan -- IEEE 802.1Q VLAN network interface

     pseudo-device vlan	[count]

     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 physical devices that do the VLAN	demultiplexing
     in	firmware.  The link0 flag should be set	on a vlan interface (not on
     its parent) using ifconfig(8) in that case	to indicate that hardware sup-
     port for the 802.1Q VLANs is present in its parent.

   Selecting the Right Network Interface Card to Run VLANs Through
     By	now, the only NICs that	have both hardware support and proper driver
     hooks for the 802.1Q VLAN technology in FreeBSD are bge(4), em(4),	gx(4),
     nge(4), ti(4), and	txp(4).

     The rest of the ethernet NICs supported by	FreeBSD	can run	VLANs using
     software emulation	in the vlan driver.  However, most of them lack	the
     capability	of transmitting	and/or receiving oversized frames.  Using such
     a NIC as a	parent interface implies 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 NICs that support oversized frames are	as follows:

	   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.  The	hack works only	for the	21041, 21140, and
		   21140A chips.

	   fxp(4)  supports long frames	for vlan natively.

	   sis(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).

     Note: Unless marked as having native support for vlan, the	above drivers
     don't inform the vlan driver about	their long frame handling capability.
     Just increase the MTU of a	vlan interface if it appears to	be lower than
     1500 bytes	after attaching	to a parent known to support long frames.


     No	802.1Q features	except VLAN tagging are	implemented.

BSD				 July 25, 2001				   BSD


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