====== VLAN ======

Standard: IEEE 802.1Q-2011 (Dot1q).

==== What is a VLAN? ====

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

==== How does it work? ====

[[https://www.alliedtelesis.com/sites/default/files/8021q_encapsulation_feature_config_guide_rev_b.pdf|Source]]

Dot1q describes the process of the VLAN encapsulation. An Ethernet frame can contain a 802.1Q tag (32 bit). It consists of a:
{{ :vlangraph.png?400|}}
  - TPID (16 bit) -- Tag protocol identifier -- is a marker which shows the frame is dot1q-tagged (valye 0x8100).
  - 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.