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. This is particularly useful on routers or on hosts which must be connected to many different networks through a single physical interface.
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 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.
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. Drivers supporting this increased MTU are:
–
drivers using the DP8390 core (such as
ec(4),
ne(4),
we(4), and possibly others)
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.