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vlan (IEEE 802.1Q VLAN network interface)


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 create command.

To function, a vlan interface must be assigned a parent interface and numeric VLAN tag using ifconfig. 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.

Initially vlan assumes the same minimum length for tagged and untagged frames. This mode is selected by the sysctl variable set to 0 (default). However, there are network devices that fail to adjust frame length, should it fall below the allowed minimum due to untagging. Such devices should be able to interoperate with vlan after changing the value of to 1. In the latter mode, vlan will pad short frames before tagging them so that their length stays not less than the minimum value after untagging by the non-compliant devices.


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 long frames (up to 1522 bytes including an Ethernet header and FCS). 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: ae(4), age(4), ale(4), bce(4), cxgb(4), ie1g, ixgb(4), jme(4), msk(4), nge(4), rtl1g, stge(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, some of them lack the capability of transmitting and receiving long 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 following interfaces support long frames for vlan natively: bfe(4), dc(4), fwe(4), gem(4), hme(4), le(4), nfe(4), nve(4), sf(4), sis(4), sk(4), ste(4), tl(4), tx(4), vr(4), and xl(4).

The vlan driver automatically recognizes devices that natively support long frames for vlan use and calculates the appropriate frame MTU based on the capabilities of the parent interface. Some other interfaces not listed above may handle long 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 a parent interface.


No 802.1Q features except VLAN tagging are implemented.


Versions Link to
INtime 4.0 netlib.lib
See Also