Standard: IEEE 802.1Q-2011 (Dot1q).

VLAN is a broadcast domain. VLANs (Virtual LAN) divide a physical network into multiple logistical ones. Communication between VLANs is only possible if a L3 device (a router or a multilayer switch) is connected to them.

Source

Dot1q describes the process of the VLAN encapsulation. An Ethernet frame can contain a 802.1Q tag (32 bit). It consists of a:

1. TPID (16 bit) – Tag protocol identifier – is a marker which shows the frame is dot1q-tagged (valye 0x8100).
2. TCI (16 bit) – Tag control information – contains VLAN details.
   • User Priority (3 bits) or PCP – Priority code point;
   • (formerly) CFI (1 bit) – Canonical format indicator. MAC address format: 0 – canonical [Ethernet, Token Bus; least significant bit in each byte first], 1 – noncanonical (the bits within each byte are transposed) [token ring, fddi; most significant bit in each byte first];
   • DEI (1 bit) – drop elegible indicator – identifies the drop priority of packets. If set to 1, the frame will be first to be discarded during congestion.
   • VID (12 bits) – VLAN identifier – specifies to which VLAN the fram belongs (from 0 to 4095). 0x000 is reserved, it indicates the tag is just a priority tag. On bridges 0x001 (default VID) is often reserved for a network management VLAN (depends on the vendor). 0xFFF is reserved for implementation use.