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NAME    [Toc]    [Back]

     vlan -- IEEE 802.1Q VLAN network interface

SYNOPSIS    [Toc]    [Back]

     device vlan

DESCRIPTION    [Toc]    [Back]

     The vlan driver demultiplexes frames tagged according to the IEEE 802.1Q
     standard into logical vlan network interfaces, which allows routing/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 configured
 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.  Devices that have hardware support for 802.1Q VLANs are
     automatically recognized by their interface capabilities.

   Selecting the Right Network Interface Card to Run VLANs Through    [Toc]    [Back]
     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 or module.  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).

     The vlan driver automatically recognizes devices that support oversized
     frames for vlan use and calculates the appropriate frame MTU based on the
     capabilities of the parent interface.

SEE ALSO    [Toc]    [Back]


BUGS    [Toc]    [Back]

     No 802.1Q features except VLAN tagging are implemented.

FreeBSD 5.2.1		       November 14, 2002		 FreeBSD 5.2.1
[ Back ]
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