In the realm of network engineering, understanding the distinction between different types of ports is fundamental. Among these, trunk ports play a crucial role in managing network traffic efficiently and securely. In this post, we’ll delve into what a trunk port is, how it differs from an access port, and its importance in network infrastructure. We’ll also cover best practices for configuring trunk ports to ensure optimal security and performance.

What is a Trunk Port?

A trunk port is a type of switch port that is designed to carry traffic for multiple VLANs (Virtual Local Area Networks) across the network. Unlike access ports, which only carry traffic for a single VLAN, trunk ports can handle traffic from multiple VLANs simultaneously. This makes them essential for interconnecting switches and other network devices in a VLAN-segmented network. Trunk Ports vs. Access Ports

Understanding the difference between trunk and access ports is crucial for effective network design:

  • Access Port: An access port connects a switch to end devices like computers, printers, or IP phones. It carries traffic for a single VLAN, which simplifies the network configuration for end devices that do not need to distinguish between VLANs.

  • Trunk Port: A trunk port, on the other hand, connects switches to each other or to other network devices that need to manage traffic from multiple VLANs. It tags the traffic with VLAN identifiers, allowing different VLANs to coexist on the same physical link.

Uses of Trunk Ports

  • Uplinks to Other Switches: Trunk ports are commonly used for connecting switches to one another. This allows VLAN traffic to be carried across multiple switches, maintaining VLAN segmentation throughout the network.

  • Uplinks to Access Points: In wireless networks, trunk ports are used to connect switches to wireless access points (APs). This is crucial for supporting multiple SSIDs (each mapped to different VLANs), ensuring that wireless clients are properly segmented.

Configuring Trunk Ports for Security

Proper configuration of trunk ports is essential for maintaining network security and performance. Here are some best practices:

Restrict VLANs: By default, a trunk port can carry traffic for all VLANs. However, it’s a best practice to restrict the VLANs allowed on a trunk port to only those that are necessary. This minimizes the risk of VLAN hopping attacks and reduces unnecessary traffic on the link.

switchport trunk allowed vlan <VLAN_IDs>

VLAN Pruning: VLAN pruning is a technique used to limit the number of VLANs that can travel over a trunk link. By default, a trunk port can carry traffic for all VLANs, but this can lead to unnecessary traffic and potential security risks. VLAN pruning ensures that only the VLANs required for a particular trunk link are allowed to send traffic across it. How VLAN Pruning Works

Imagine a scenario where you have three VLANs: VLAN 10, VLAN 20, and VLAN 30. Suppose you have two switches, Switch A and Switch B, connected via a trunk port. However, devices on Switch B only need access to VLAN 10 and VLAN 20. Without VLAN pruning, the trunk port would still carry traffic for VLAN 30, even though it’s not needed on Switch B.

With VLAN pruning, you can configure the trunk port to only allow traffic for VLANs 10 and 20, effectively blocking VLAN 30 from using that trunk link. This reduces unnecessary traffic and minimizes the risk of VLAN hopping attacks, where an attacker could potentially access VLANs they’re not supposed to. Configuring VLAN Pruning

To configure VLAN pruning, you would typically specify which VLANs are allowed on a trunk port. Here’s an example command:

switchport trunk allowed vlan 10,20

In this example, only VLANs 10 and 20 are allowed on the trunk link, while VLAN 30 is pruned off.

Native VLAN: Ensure that the native VLAN (the VLAN for untagged traffic) is properly configured. Avoid using VLAN 1 as the native VLAN since it is the default and could be a target for attacks. Assign a different, unused VLAN as the native VLAN. The Importance of Using an Unused VLAN for the Native VLAN

The native VLAN is the VLAN that carries untagged traffic on a trunk port. By default, this is VLAN 1, which can pose a security risk for several reasons:

  • Common Target for Attacks: Since VLAN 1 is the default VLAN on most network devices, it is a common target for attackers. If an attacker gains access to VLAN 1, they could potentially disrupt or intercept untagged traffic.

  • Broadcast Storms: Using VLAN 1 as the native VLAN increases the risk of broadcast storms, which can lead to network congestion and outages.

  • Management Traffic: VLAN 1 often carries management traffic, making it more susceptible to being targeted in attacks.

Best Practice: Use an Unused VLAN

To mitigate these risks, it’s a best practice to assign an unused VLAN as the native VLAN. For example, if you have VLANs 10, 20, and 30 in use, you could assign VLAN 99 as the native VLAN:

switchport trunk native vlan 99

This approach reduces the risk of VLAN hopping and ensures that your critical VLANs are protected.

switchport trunk native vlan <VLAN_ID>

Disable DTP: Dynamic Trunking Protocol (DTP) can automatically negotiate trunking on a port. For security reasons, it’s advisable to disable DTP and manually configure trunking to prevent unauthorized devices from becoming trunks.


switchport mode trunk
switchport nonegotiate

Final Thoughts

Trunk ports are a fundamental component in VLAN-segmented networks, enabling the efficient and secure transport of traffic between switches and other network devices. By understanding the differences between trunk and access ports and following best practices for configuration, you can enhance both the performance and security of your network infrastructure. Remember, the goal is to maintain a streamlined, secure, and efficient network that meets the needs of your organization.