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NAME    [Toc]    [Back]

     vlan - IEEE 802.1Q encapsulation/decapsulation pseudo-device

SYNOPSIS    [Toc]    [Back]

     pseudo-device vlan [count]

DESCRIPTION    [Toc]    [Back]

     The  vlan  Ethernet interface allows construction of virtual
LANs when used
     in conjunction with IEEE 802.1Q-compliant Ethernet  devices.

     A  vlan  interface  can  be  created  at  runtime  using the
ifconfig vlanN
     create command or by setting up a hostname.if(5)  configuration file for

     This driver currently supports the following modes of operation:

     802.1Q  encapsulation  over  Ethernet   (Ethernet   protocol

          The 802.1Q header specifies the virtual LAN number, and
thus allows
          an Ethernet switch (or other 802.1Q  compliant  network
devices) to be
          aware  of  which  LAN  the frame is part of, and in the
case of a
          switch, which port(s) the  frame  can  go  to.   Frames
          through  the  vlan  interface  will  be diverted to the
specified physical
 interface with 802.1Q vlan  encapsulation.   Frames
with 802.1Q
          encapsulation received by the parent interface with the
correct vlan
          tag will be diverted to the associated vlan  pseudo-interface.

     Frame  headers  which normally contain the destination host,
source host,
     and protocol, are altered with additional information.   After the source
     host,  a  32-bit 802.1Q header is included, with 16 bits for
the ether type
     (0x8100), 3 bits for the priority field (not  used  in  this
 1 bit for the canonical field (always 0), and 12 bits
for the vlan
     identifier.  Following the vlan header is the  actual  ether
type for the
     frame and length information.

     vlan interfaces support the following unique ioctl(2)s:

          Set the vlan tag and parent for a given vlan interface.

          Get the vlan tag and parent for a given vlan interface.

     vlan interfaces use the following interface capabilities:

          The parent interface can handle full sized frames, plus
the size of
          the vlan tag.

          The parent interface will participate  in  the  tagging
and untagging
          of frames.

DIAGNOSTICS    [Toc]    [Back]

     vlan%d:  initialized  with  non-standard  mtu %d (parent %s)
 capability was not set on the parent interface.
We assume
     in  this  event  that the parent interface is not capable of
handling frames
     larger than its MTU.  This will generally result in  a  noncompliant
     802.1Q implementation.

     Some Ethernet chips will either discard or truncate Ethernet
frames that
     are larger than 1514 bytes.  This causes a problem as 802.1Q
     frames  can  be up to 1518 bytes.  Most controller chips can
be told not to
     discard large frames and/or to increase  the  allowed  frame
size.  Refer to
     the hardware manual for your chip to do this.

     If  the  IFCAP_VLAN_MTU  capability is set on a vlan parent,
vlan assumes
     that the Ethernet chip on the parent  can  handle  oversized
frames.  Either
     the chip allows 1518 byte frames by default (such as rl(4)),
the driver
     has instructed the chip to do so (such as fxp(4) and dc(4)),
or the driver
  also  takes  advantage of a hardware tagging capability,
and thus oversized
 frames are never actually sent or received by  OpenBSD
(such as
     txp(4) and ti(4)).

SEE ALSO    [Toc]    [Back]

     bridge(4),  inet(4), ip(4), netintro(4), hostname.if(5), ifconfig(8),

     IEEE         802.1Q          standard,          http://standards.ieee.org/getieee802/802.1.html.

     The vlan interface is to be configured with ifconfig(8), see
its manual
     page for more information.

AUTHORS    [Toc]    [Back]

     Originally wollman@freebsd.org.

BUGS    [Toc]    [Back]

     The 802.1Q specification allows for operation over FDDI  and
Token Ring as
     well  as Ethernet.  This driver only supports such operation
with Ethernet

     When the IFCAP_VLAN_HWTAGGING capability is set on the  parent interface,
     vlan does not participate in the actual tagging or untagging
of Ethernet
     frames.  It simply passes the vlan ID on to the  parent  interface for tagging
  on transmit, and gets a vlan ID for each packet on receive.  The
     vlan tagged packet is not actually visible to OpenBSD. Thus,
bpf(4) will
     show  untagged  packets  on  the  parent interface, although
frames are actually
 being transmitted and received with tags on the wire.

     This driver could be the basis for support of the Cisco  ISL
VLAN protocol,
      detailed     at     http://www.cisco.com/warp/pub-
lic/473/741_4.html .  Unfortunately,
 public reimplementation  of  this  protocol  is
currently prevented
 by patent (at least in the USA).

OpenBSD      3.6                          January     9,     2000
[ Back ]
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