Virtual LANs (VLANs)

VLANs enable a switch to carry multiple discrete broadcast domains, allowing a single switch to function as if it were multiple switches. VLANs are commonly used for network segmentation in the same way that multiple switches can be used: To place hosts on a specific segment, isolated from other segments. Where trunking is employed between switches, devices on the same segment need not reside on the same switch. Devices that support trunking can also communicate on multiple VLANs through a single physical port.

This chapter covers VLAN concepts, terminology and configuration.

Requirements

There are two requirements, both of which must be met to deploy VLANs.

  1. 802.1Q VLAN capable switch

    Every decent managed switch manufactured in the last 15 years supports 802.1Q VLAN trunking.

    Warning

    VLANs cannot be used with an unmanaged switch.

  2. Network adapter capable of VLAN tagging

    A NIC that supports hardware VLAN tagging or has long frame support is required. Each VLAN frame has a 4 byte 802.1Q tag added in the header, so the frame size can be up to 1522 bytes. A NIC supporting hardware VLAN tagging or long frames is required because other adapters will not function with frames larger than the normal 1518 byte maximum with 1500 MTU Ethernet. This will cause large frames to be dropped, which causes performance problems and connection stalling.

    Note

    If an adapter is listed as having long frame support does not guarantee the specific implementation of that NIC chipset properly supports long frames. Realtek rl(4) NICs are the biggest offenders. Many will work fine, but some do not properly support long frames, and some will not accept 802.1Q tagged frames at all. If problems are encountered using one of the NICs listed under long frame support, we recommend trying an interface with VLAN hardware tagging support instead. We are not aware of any similar problems with NICs listed under VLAN hardware support.

Ethernet interfaces with VLAN hardware support:

ae(4), age(4), alc(4), ale(4), bce(4), bge(4), bxe(4), cxgb(4), cxgbe(4), em(4), igb(4), ixgb(4), ixgbe(4), jme(4), msk(4), mxge(4), nxge(4), nge(4), re(4), sge(4), stge(4), ti(4), txp(4), vge(4).

Ethernet interfaces with long frame support :

axe(4), bfe(4), cas(4), dc(4), et(4), fwe(4), fxp(4), gem(4), hme(4), le(4), nfe(4), nve(4), rl(4), sf(4), sis(4), sk(4), ste(4), tl(4), tx(4), vr(4), vte(4), xl(4).