freebsd-nq/share/man/man4/vlan.4

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.\" Copyright (c) 2001 Yar Tikhiy
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.Dd July 25, 2001
.Dt VLAN 4
.Os
.Sh NAME
.Nm vlan
.Nd IEEE 802.1Q VLAN network interface
.Sh SYNOPSIS
.Cd device vlan Op Ar count
.\"
.Sh DESCRIPTION
The
.Nm
driver demultiplexes frames tagged according to
the IEEE 802.1Q standard into logical
.Nm
network interfaces, which allows routing/bridging between
multiple VLANs through a single switch trunk port.
.Pp
To function, a
.Nm
interface must be assigned a parent interface and
numeric VLAN tag using
.Xr ifconfig 8 .
A single parent can be assigned to multiple
.Nm
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.
.Pp
The
.Nm
driver supports physical devices that do
the VLAN demultiplexing in firmware.
The
.Cm link0
flag should be set on a
.Nm
interface
.Pq Em not on its parent
using
.Xr ifconfig 8
in that case to indicate that the hardware support for
the 802.1Q VLANs is present in its parent.
.\"
.Ss "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
.Fx
are
.Xr nge 4 ,
.Xr ti 4 ,
and
.Xr txp 4 .
.Pp
The rest of the ethernet NICs supported by
.Fx
can run
VLANs using the software emulation in the
.Nm
driver.
However, most of them lack the capability
of transmitting and/or receiving oversized frames.
Using such a NIC as a parent interface for a
.Nm
implies a reduced MTU on the corresponding
.Nm
interfaces.
In the modern Internet, this is likely to cause
.Xr tcp 4
connectivity problems due to massive, inadequate
.Xr icmp 4
filtering that breaks the Path MTU Discovery mechanism.
.Pp
The NICs that support oversized frames are as follows:
.Bl -tag -width ".Xr fxp 4 " -offset indent
.It Xr de 4
requires defining
.Dv BIG_PACKET
in the
.Pa /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.
.It Xr fxp 4
supports long frames for the
.Nm
natively.
.It Xr tl 4
does support long frames.
.It Xr tx 4
may begin supporting long frames soon.
.It Xr xl 4
supports long frames only if the card is built on a newer chip
(Cyclone and above).
.El
.Pp
Note:
Except for
.Xr fxp 4 ,
none of the above drivers inform the
.Nm
driver about their support for long frames.
Just fix the MTU of a
.Nm
interface if it appears to be lower than 1500 bytes after
attaching the
.Nm
to a parent interface that is known to support long frames.
.Sh SEE ALSO
.Xr ifconfig 8
.Sh BUGS
No 802.1Q features except VLAN tagging are implemented.