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VLAN(4)                FreeBSD 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
     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 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.

FreeBSD 4.10                     July 25, 2001                    FreeBSD 4.10


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