802.1q

IEEE 802.1Q

IEEE 802.1Q is a networking standard that defines Virtual LAN (VLAN) tagging on Ethernet networks. It allows multiple VLANs to be carried over a single physical link, a process known as VLAN trunking.

Overview

802.1Q works by inserting a 4-byte tag into an Ethernet frame. This tag identifies the VLAN to which the frame belongs, enabling network switches to separate traffic logically while sharing the same physical infrastructure.

Frame Format

The 802.1Q tag includes:

• Tag Protocol Identifier (TPID) – Identifies the frame as VLAN-tagged
• Priority Code Point (PCP) – Used for Quality of Service (QoS)
• Drop Eligible Indicator (DEI) – Indicates congestion eligibility
• VLAN ID (VID) – Identifies the VLAN (range 1–4094)

Native VLAN

Frames belonging to the native VLAN are sent untagged on 802.1Q trunk links by default. This behavior can be configured and is often changed for security reasons.

Uses

802.1Q is commonly used in enterprise and data center networks to:

• Segment broadcast domains
• Improve network security
• Support scalable network design

See also

• Virtual LAN
• IEEE 802.1D
• Quality of Service

References

• IEEE Standard 802.1Q