The Basic Definition

The acronym VLAN expands to Virtual Local Area Network. A VLAN is a logical local area network (or LAN) that extends beyond a single traditional LAN to a group of LAN segments, given specific configurations. Because a VLAN is a logical entity, its creation and configuration is done completely in software.


How Is a VLAN Identified

Since a VLAN is a software concept, identifiers and configurations for a VLAN must be properly prepared for it to function as expected. Frame coloring is the process used to ensure that VLAN members or groups are properly identified and handled. With frame coloring, packets are given the proper VLAN ID at their origin so that they may be properly processed as they pass through the network. The VLAN ID is then used to enable switching and routing engines to make the appropriate decisions as defined in the VLAN configuration.

Configuring VLANs
A virtual LAN (VLAN) permits multiplexing multiple IP and/or PPPoE and/or MPLS interfaces over a single physical Ethernet port. This multiplexing is accomplished through VLAN subinterfaces. Ethernet interfaces support the 802.1q-1998 IEEE Standards for Local and Metropolitan Area Networks: Virtual Bridged Local Area Networks, which the router uses as its standardized format for frame tagging.

The Ethernet V2 frame format allows multiplexing of different protocols over a single physical link. IEEE 802.1q compatibility extends the frame format by adding a tag that contains a VLAN ID. This feature allows multiplexing of different channels (VLANs) over the physical link; each channel is able to multiplex different protocols.

This capability works very much like ATM encapsulation as described in RFC 2684 - Multiprotocol Encapsulation over ATM Adaptation Layer 5 (September 1999). This encapsulation type allows multiplexing of multiple protocols over a single ATM virtual circuit (VC).

As shown in Figure 1, VLANs are similar to ATM VCs, with the VLAN ID serving the same function as the virtual path identifier (VPI) and virtual channel identifier (VCI) to multiplex the different channels over the physical link. The Ethernet protocol type serves the same function within a VLAN as the logical link control (LLC) subnetwork attachment point (SNAP) within a VC, to multiplex the different protocols over the channel.