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vlan - "IEEE 802.1Q VLAN network interface"


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To compile support for the vlan driver into your kernel, place the following lines in your kernel configuration file:

.Cd "device miibus"

.Cd "device vlan"

Alternatively, to load the vlan driver at boot time, place the following line in loader.conf(5):


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.

By now, the list of physical interfaces able of full VLAN processing in the hardware is limited to the following devices: bge(4), em(4), ixgb(4), nge(4), re(4), ti(4), txp(4), and vge(4).

The rest of the Ethernet interfaces can run VLANs using software emulation in the vlan driver. However, most of them lack the capability of transmitting and receiving oversized frames. Assigning such an interface as the parent to vlan will result in 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 interfaces that support oversized frames are as follows:

bfe(4) supports long frames for vlan natively.
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 or module. The hack works only for the 21041, 21140, and 21140A chips.
fxp(4) supports long frames for vlan natively.
gem(4) supports long frames for vlan natively.
hme(4) supports long frames for vlan natively.
rl(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).

The vlan driver automatically recognizes devices that natively support oversized frames for vlan use and calculates the appropriate frame MTU based on the capabilities of the parent interface. The other interfaces listed above can handle oversized frames, but they do not advertise this ability of theirs. The MTU setting on vlan can be corrected manually if used in conjunction with such parent interface.


kqueue(2), miibus(4), ifconfig(8)


kqueue(2) miibus(4)

Created by Blin Media, 2008-2013