A Virtual LAN (VLAN) is a logical network group that limits the broadcast domain, and allows the manager to isolate network traffic. Data communications are limited in the VLAN group, which means that only the members of the same VLAN group will receive traffic from the ones of the same VLAN group. Basically, creating a VLAN from a switch is logically equivalent of reconnecting a group of network devices to another Layer 2 switch. However, all the network devices are still plugged into the same switch physically.

The diagram consists of three groups which connect to one switch. The members of yellow group could exchange data with each other, but couldn’t exchange data with the member of blue group or green group. This is the theorem of VLAN.

Port based VLAN

Port based VLAN allows manager to assign VLAN group based on the destination MAC address and its associated port, and the packet forwarding decision is based on the assignment. Managers are able to assign several ports among different switches as a VLAN group. Since it is based on physical layer, port based VLAN is best for small organizations.

IEEE 802.1Q Tag VLAN

Defined in IEEE 802.1Q standard, Tag VLAN allows VLAN information to be written into the Ethernet packet itself. It adds a 32-bit field between the source MAC address and the EtherType/Length fields of the original frame which means that each packet carries a VLAN ID as it travels across the network via trunk link.

GVRP

GVRP (GARP VLAN Registration Protocol) is a standard-based protocol to automatically register VLAN members on ports across the network and defines the way for different vender’s switches to exchange VLAN information.

The GVRP Bridge Protocol Data Units (GVRP BPDUs) are able to advertise static VLANs to other switches in the network. Any device that is GVRP-enabled can dynamically join the advertised VLAN by receiving the advertisements.