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VLAN(4)                FreeBSD Kernel Interfaces Manual                VLAN(4)

NAME
     vlan - IEEE 802.1Q VLAN network interface

SYNOPSIS
     pseudo-device vlan [count]

DESCRIPTION
     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.

     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.  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
     inform 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
     frames.

SEE ALSO
     ifconfig(8)

BUGS
     No 802.1Q features except VLAN tagging are implemented.

FreeBSD 11.0-PRERELEASE          July 25, 2001         FreeBSD 11.0-PRERELEASE

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | BUGS

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