What is a VLAN Trunk?

A VLAN (Virtual Local Area Network) trunk is a network link or connection that can carry multiple VLANs. Trunks are commonly used in network environments where there is a need to segregate traffic into different VLANs while efficiently using network resources.

Here are some key points about VLAN trunks:

  1. Carrying Multiple VLANs: VLAN trunks enable a single network link to carry traffic for multiple VLANs simultaneously. This is achieved by tagging each frame with a VLAN identifier.
  2. VLAN Tagging: In a VLAN trunk, each frame is tagged with a VLAN ID. This tag helps the receiving devices understand which VLAN the frame belongs to. The most common tagging protocols are IEEE 802.1Q and ISL (Inter-Switch Link).
  3. IEEE 802.1Q Protocol: This is the industry standard for VLAN tagging. The 802.1Q tag is inserted into the Ethernet frame and includes information about the VLAN to which the frame belongs.
  4. ISL (Inter-Switch Link): ISL is a proprietary VLAN tagging protocol developed by Cisco. It achieves a similar goal as 802.1Q but is specific to Cisco devices.
  5. Trunk Ports: Network devices (such as switches and routers) have specific ports known as trunk ports that are configured to carry traffic for multiple VLANs. These ports are configured to recognize and process VLAN tags.
  6. Avoiding VLAN Hopping: VLAN trunks need to be configured securely to prevent VLAN hopping attacks. VLAN hopping occurs when an attacker gains unauthorized access to traffic on a VLAN other than the one they belong to.
  7. Interconnecting Switches: VLAN trunks are often used to interconnect switches in a network, allowing devices in different VLANs to communicate with each other.

A VLAN trunk is a network connection that carries traffic for multiple VLANs by adding VLAN tags to frames. This enables the efficient segmentation of network traffic while using a single physical link.