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Trunking and 802.1Q Encapsulation

CCNA: 0 to Hero - Comprehensive Network Engineering Bootcamp

Lesson 25: Trunking and 802.1Q Encapsulation

A Trunk Port is a link between two switches or between a switch and a router, designed to carry traffic for multiple VLANs simultaneously.

802.1Q Tagging

The IEEE 802.1Q standard (often called 'dot1q') is the industry standard for VLAN tagging. When a switch sends a frame across a trunk link, it inserts a 4-byte tag into the Ethernet frame header.

The 802.1Q Tag includes:

  • TPID: Tag Protocol Identifier (always 0x8100).
  • VLAN ID (VID): A 12-bit field identifying the VLAN (0 to 4095).

The Native VLAN

The Native VLAN is the only VLAN whose traffic is untagged when traversing the 802.1Q trunk. By default, the Native VLAN is VLAN 1. It is best practice to change the Native VLAN to an unused VLAN ID for security reasons.