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.

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 efficient operation over parent interfaces that can provide help in processing VLANs. Such interfaces are automatically recognized by their capabilities. Depending on the level of sophistication found in a physical interface, it may do full VLAN processing or just be able to receive and transmit frames exceeding the maximum Ethernet frame size by the length of a 802.1Q header. The capabilities may be user-controlled by the respective parameters to ifconfig(8), vlanhwtag and vlanmtu. However, a physical interface is not obliged to react to them: It may have either capability enabled permanently without a way to turn it off. The whole issue is very specific to a particular device and its driver.

  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 DragonFly are bce(4), bge(4), em(4), jme(4), nfe(4), nge(4), re(4), stge(4), ti(4), txp(4), and vge(4).

The rest of the ethernet NICs supported by DragonFly 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 /sys/dev/netif/de/if_de.c source file and rebuilding the kernel. The hack works only for the 21041, 21140, and 21140A chips.

et(4)
    supports long frames for vlan natively.

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.

SEE ALSO
ifconfig(8)

BUGS
No 802.1Q features except VLAN tagging are implemented.