VLAN(4)

NAME
vlan - IEEE 802.1Q Virtual LAN network device

SYNOPSIS
pseudo-device vlan

DESCRIPTION
The vlan interface provides support for IEEE 802.1Q Virtual Local Area Networks (VLAN). This supports the trunking of more than one network on a single network interface by using 802.1Q tagged and untagged frames.

To use a vlan interface, the administrator must first create the interface and then specify the VID ( VLAN identifier, the first 12 bits from a 16-bit integer which distinguishes each VLAN from any others) and (parent) physical interface associated with the VLAN. This can be done by using the ifconfig(8) create, vlan, and vlanif subcommands from a shell command line or script. From within a C program, use the ioctl(2) system call with the SIOCSIFCREATE and SIOCSIFVLAN arguments.

Packets sent through a vlan interface are tagged with the VID and passed to the parent interface for transmission. Tagged packets received on the parent interface are passed to the vlan interface with the corresponding VID associated with the parent interface. Packets sent directly through the parent interface are transmitted as untagged frames. Untagged frames received on the parent interface are handled by the parent interface. Tagged frames received on the parent interface with a VID of 0 and an EtherType of IP or IPv6 are processed on the parent interface. Tagged frames received on the parent interface for which no vlan interface with a matching VID exists are dropped and counted as “unknown protocol”. ( These are displayed by the ifconfig(8) -v option.)

If the vlan pseudo-device is not configured in the kernel only packets tagged with a VID of 0 are processed.

To be compatible with other IEEE 802.1Q devices, the vlan interface supports a 1500 byte MTU, which means that the parent interface will have to handle packets that are 4 bytes larger than the original Ethernet standard.

vlan can be used with devices not supporting the IEEE 802.1Q MTU, but then the MTU of the vlan interface will be 4 bytes too small and will not interoperate properly with other IEEE 802.1Q devices, unless the MTU of the other hosts on the VLAN are also lowered to match.

EXAMPLES
The following will create interface vlan0 with VID six, on the Ethernet interface tlp0:

    ifconfig vlan0 create
    ifconfig vlan0 vlan 6 vlanif tlp0

After this set up, IP addresses (and/or other protocols) can be assigned to the vlan0 interface. All other hosts on the Ethernet connected to tlp0 which configure a VLAN and use VID six will see all traffic transmitted through vlan0.

The same VLAN can be created at system startup time by placing the following in /etc/ifconfig.vlan0:

    create
    vlan 6 vlanif tlp0

SEE ALSO
bridge(4), ifconfig(8)

HISTORY
The vlan device first appeared in NetBSD 1.5.1, and was derived from a VLAN implementation that appeared in FreeBSD and OpenBSD.

BUGS
The vlan interfaces do not currently inherit changes made to the physical interfaces' MTU.