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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.

     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 the hardware
     support 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), nge(4),
     ti(4), and	txp(4).

     The rest of the ethernet NICs supported by	FreeBSD	can run	VLANs using
     the 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 for a vlan 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:

	   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 the	vlan natively.

	   sis(4)  supports long frames	for the	vlan natively.

	   tl(4)   does	support	long frames.

	   tx(4)   does	support	long frames.

	   wx(4)   does	support	long frames.

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

     Note: Except for fxp(4), sis(4) and tx(4),	none of	the above drivers in-
     form the vlan driver about	their support for long frames.	Just fix the
     MTU of a vlan interface if	it appears to be lower than 1500 bytes after
     attaching the vlan	to a parent interface that is known to support long


     No	802.1Q features	except VLAN tagging are	implemented.

BSD				 July 25, 2001				   BSD


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