A trunk port is a switch port that carries traffic belonging to multiple VLANs over a single physical Ethernet link. Trunk ports are mainly used to connect network devices such as switches, routers, firewalls, and Layer 3 switches when several VLANs need to travel between them.
In an industrial network, a trunk link can allow CCTV, SCADA/PLC, access control, corporate IT, and management VLANs to share the same fiber or Ethernet uplink while remaining logically separated.
For example, a industrial managed switch may have the following VLANs:
- VLAN 10: CCTV Surveillance
- VLAN 20: SCADA / PLC
- VLAN 30: Access Control
- VLAN 40: Corporate IT
- VLAN 99: Network Management
A trunk port can carry these VLANs between the access switch and the network core.
How Does a Trunk Port Work?
A trunk port uses VLAN tagging to identify which VLAN a particular Ethernet frame belongs to. The most widely used VLAN tagging standard is IEEE 802.1Q.
Consider two switches connected using a single fiber link:
Industrial Switch A
│
│ 802.1Q Trunk
│
▼
Industrial Switch B
The same physical link can carry:
VLAN 10 → CCTV
VLAN 20 → SCADA
VLAN 30 → Access Control
VLAN 40 → Corporate IT
The VLAN information allows the receiving switch to understand which logical network each frame belongs to.
Without trunking, separate physical links might be required for each VLAN.
What Is 802.1Q VLAN Tagging?
IEEE 802.1Q adds VLAN information to an Ethernet frame. A simplified representation looks like this:
Destination MAC
Source MAC
802.1Q VLAN Tag
EtherType
Data
FCS
The VLAN tag contains the VLAN ID, allowing network equipment to identify the frame’s VLAN. When these VLANs travel over a trunk, the switch can distinguish them using their VLAN information.
Why Are Trunk Ports Important in Industrial Networks?
Industrial facilities often have several systems operating on a shared physical network infrastructure.
Core Switch
│
Trunk
│
Comxus Industrial
Switch
┌─────────┼─────────┐
CCTV SCADA Access
VLAN 10 VLAN 20 VLAN 30
A trunk allows the required VLANs to travel between switches without requiring a separate physical uplink for every system.
This is especially useful when industrial switches are connected through fiber uplinks across production areas, control rooms, substations, warehouses, or other buildings.
Where Are Trunk Ports Used?
Trunk ports are commonly used for:
- Switch-to-Switch Connections: Multiple VLANs can be transported between access and distribution switches.
- Switch-to-Core Connections: An access switch can carry several VLANs to a Layer 3 core switch.
- Switch-to-Firewall Connections: Where supported by the network architecture, a trunk can transport multiple VLANs to a firewall for routing and security policies.
- Switch-to-Router Connections: A router can use VLAN sub interfaces to provide routing between multiple VLANs over one physical interface.
Frequently Asked Questions
What is a trunk port in VLAN?
A trunk port is a switch port that carries traffic from multiple VLANs over one physical Ethernet link, typically using IEEE 802.1Q VLAN tagging.
Is a trunk port tagged or untagged?
Trunk traffic is commonly VLAN-tagged using IEEE 802.1Q. However, specific behavior can vary depending on the native VLAN and vendor configuration.
Can a trunk port connect to a PC?
It can, but a normal PC connection is usually configured as an access port. A trunk connection is generally used when the endpoint or network device needs to handle multiple VLANs.
Can a trunk port carry CCTV and SCADA traffic?
Yes. A properly configured trunk can carry both CCTV and SCADA VLANs over the same physical uplink while keeping them logically separated.
Does a trunk port increase bandwidth?
No. Trunking does not increase the physical bandwidth of a link. If a trunk uses a 1 Gbps uplink, all VLAN traffic sharing that trunk collectively uses that 1 Gbps capacity.
A trunk port is a key component of VLAN-based network architecture. It allows multiple VLANs to travel across a single physical connection while preserving their logical separation.
