VLAN(4) BSD Kernel Interfaces Manual VLAN(4)NAMEvlan — IEEE 802.1Q VLAN network interface
SYNOPSIS
pseudo-device vlan [count]
DESCRIPTION
The vlan driver demultiplexes frames tagged according to the IEEE 802.1Q
standard into logical vlan network interfaces, which allows rout‐
ing/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 con‐
figured 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 sophistica‐
tion found in a physical interface, it may do full VLAN processing or
just be able to receive and transmit frames exceeding the maximum Ether‐
net 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.
Selecting the Right Network Interface Card to Run VLANs Through
By now, the only NICs that have both hardware support and proper driver
hooks for the 802.1Q VLAN technology in DragonFly are bce(4), bge(4),
em(4), jme(4), nfe(4), nge(4), re(4), stge(4), ti(4), txp(4), and vge(4).
The rest of the ethernet NICs supported by DragonFly can run VLANs using
software emulation in the vlan driver. However, most of them lack the
capability of transmitting and/or receiving oversized frames. Using such
a NIC as a parent interface implies 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 NICs that support oversized frames are as follows:
dc(4) supports long frames for vlan natively.
de(4) requires defining BIG_PACKET in the
/sys/dev/netif/de/if_de.c source file and rebuilding the
kernel. The hack works only for the 21041, 21140, and
21140A chips.
et(4) supports long frames for vlan natively.
fxp(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).
Note: Unless marked as having native support for vlan, the above drivers
don't inform the vlan driver about their long frame handling capability.
Just increase the MTU of a vlan interface if it appears to be lower than
1500 bytes after attaching to a parent known to support long frames.
SEE ALSOifconfig(8)BUGS
No 802.1Q features except VLAN tagging are implemented.
BSD July 25, 2001 BSD
