MirBSD manpage: vlan(4)

VLAN(4)                    BSD Programmer's Manual                     VLAN(4)


     vlan - IEEE 802.1Q encapsulation/decapsulation pseudo-device


     pseudo-device vlan [count]


     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 0x8100)

          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 transmitted
          through the vlan interface will be diverted to the specified physi-
          cal interface with 802.1Q vlan encapsulation. Frames with 802.1Q en-
          capsulation 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 implementa-
     tion), 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.

     The network interfaces are named vlan0, vlan1, etc. The number of inter-
     faces is given by the corresponding pseudo-device line in the system con-
     fig file. 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.


     vlan%d: initialized with non-standard mtu %d (parent %s)  The
     IFCAP_VLAN_MTU 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 non-compliant 802.1Q

     Some Ethernet chips will either discard or truncate Ethernet frames that
     are larger than 1514 bytes. This causes a problem as 802.1Q tagged 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)).


     bridge(4), inet(4), ip(4), netintro(4), 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.


     Originally wollman@freebsd.org.


     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 tag-
     ging 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 proto-
     col, detailed at http://www.cisco.com/warp/public/473/741_4.html . Unfor-
     tunately, public reimplementation of this protocol is currently prevented
     by patent (at least in the USA).

MirBSD #10-current             January 9, 2000                               1

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