What Is VLAN Trunking?

Updated on January 15, 2025

Managing a network isn’t just about hardware and bandwidth. Businesses need to control traffic, make the best use of bandwidth, and improve security. VLAN trunking is a networking tool that can help. In this post, we’ll cover what VLAN trunking is, how to set it up, and important security tips to keep in mind.

Why VLAN Trunking Matters 

VLAN trunking is a networking feature that allows multiple Virtual Local Area Networks (VLANs) to traverse a single physical connection. By tagging network traffic with VLAN information, devices such as switches and routers can manage and isolate traffic without needing a dedicated port for each VLAN.

• Enhanced Network Scalability: VLAN trunking reduces the need for excessive physical connections by consolidating multiple VLANs into a single trunk link. 
• Optimized Traffic Management: It enables better segmentation and prioritization of network traffic. 
• Simplified Infrastructure: VLAN trunking minimizes hardware requirements, reducing costs and complexity.

This technology is widely used in enterprise environments to address the demands of growing network infrastructures and is a critical tool in network segmentation and bandwidth optimization.

Technical Definition and Purpose 

VLAN trunking allows data to move between different VLANs using a shared trunk link, while keeping each VLAN’s traffic separate and organized across the network.

Trunk Ports and Their Role 

A trunk port is a switch port configured to handle data from multiple VLANs. Unlike access ports, which connect to devices in a single VLAN, trunk ports use VLAN tagging to identify and direct traffic to the correct VLAN as it traverses the network.

VLAN Trunking in Context 

VLAN trunking fits into a broader strategy of network segmentation. It enhances traffic isolation, improves security, and ensures efficient communication between departments or organizational units.

How VLAN Trunking Works 

Step 1: VLAN Tagging 

VLAN tagging is the process of adding identification metadata to Ethernet frames. The industry-standard protocol for this is IEEE 802.1Q, which inserts a VLAN tag into the frame header.

Key Components of a Tagged Frame 

• Tag Protocol Identifier (TPID): Indicates the frame is tagged. 
• Priority Code Point (PCP): Used for Quality of Service (QoS) prioritization. 
• VLAN ID (VID): Specifies the VLAN to which the frame belongs.

For example, when a device sends a frame through a trunk port, the switch appends a VLAN tag, enabling other switches and routers to direct the frame to the correct VLAN on the receiving end.

Step 2: Handling Untagged Traffic 

Trunk ports can also handle untagged traffic through a concept called the native VLAN. Frames sent through the trunk port without tags are assigned to the native VLAN, ensuring backward compatibility with devices that don’t support tagging.

Step 3: Trunk Links vs. Access Links 

Trunk links differ significantly from access links:

• Trunk Links: Carry data for multiple VLANs using tagging. 
• Access Links: Limited to a single VLAN, connecting end devices like computers or printers. 

For network administrators, understanding this distinction is crucial when configuring ports for specific roles.

Configuration Basics 

Setting Up VLAN Trunking 

Configuring VLAN trunking requires specifying trunk modes and VLAN assignments in the Command Line Interface (CLI) of networking devices. Below is an example configuration on a Cisco switch:

Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30
Switch(config-if)# switchport trunk native vlan 1
Switch(config-if)# end

Key Commands Explained 

• switchport mode trunk: Enables trunking on the port. 
• switchport trunk allowed vlan: Specifies which VLANs can traverse the link. 
• switchport trunk native vlan: Sets the native VLAN for untagged traffic.

Tips for Successful Configuration 

• Enable Trunking on Both Ends: Both switches on a trunk link must be configured for trunking to ensure proper communication. 
• Match Native VLANs: Native VLAN mismatches can result in dropped packets and network issues.

Configuration mistakes can interrupt communication between VLANs, so testing the setup is always a best practice.

Security Considerations 

Risks of VLAN Trunking 

VLAN trunking improves efficiency, but it can also create risks like VLAN hopping attacks, where attackers exploit weaknesses in VLAN tagging to access unauthorized traffic.

Mitigation Strategies 

• Disable Unused Trunk Ports: To prevent unauthorized access. 
• Configure Allowed VLANs: Specify VLANs allowed on trunk links to limit traffic. 
• VLAN Pruning: Remove unnecessary VLANs from trunk links to reduce the attack surface.

Proactively managing trunk ports and VLAN configurations is essential for maintaining network security.