| VLAN(4) | Device Drivers Manual | VLAN(4) |
vlan — IEEE 802.1Q
VLAN network interface
To compile this driver into the kernel, place the following line in your kernel configuration file:
device vlanAlternatively, to load the driver as a module at boot time, place the following line in loader.conf(5):
if_vlan_load="YES"
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.
vlan initially assumes the same minimum
length for tagged and untagged frames. This mode is selected by setting the
sysctl(8) variable
net.link.vlan.soft_pad to 0 (default). However, there
are network devices that fail to adjust frame length when it falls below the
allowed minimum due to untagging. Such devices should be able to
interoperate with vlan after changing the value of
net.link.vlan.soft_pad to 1. In the latter mode,
vlan will pad short frames before tagging them so
that their length is 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.
At present, these devices are capable of full VLAN processing in hardware: ae(4), age(4), alc(4), ale(4), bce(4), bge(4), bxe(4), cxgb(4), cxgbe(4), em(4), igb(4), ixgbe(4), jme(4), liquidio(4), msk(4), mxge(4), nge(4), re(4), sge(4), stge(4), ti(4), txp(4), and vge(4).
Other Ethernet interfaces can run VLANs using software emulation
in the vlan driver. However, some 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.
These interfaces natively support long frames for
vlan: axe(4),
bfe(4), cas(4), dc(4),
et(4), fwe(4), fxp(4),
gem(4), hme(4), le(4),
nfe(4), rl(4), sf(4),
sis(4), sk(4), ste(4),
tl(4), tx(4), vr(4),
vte(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. The MTU setting on
vlan can be corrected manually if used in
conjunction with such a parent interface.
| May 8, 2018 | Debian |