What is Trunking in Networking? The Superhighway for VLAN Traffic

Imagine your office building has multiple departments (VLANs) that need to communicate across different floors. Instead of installing separate elevators for each department, you build one high-capacity express elevator that can carry all groups simultaneously while keeping them separated. That’s exactly what trunking does for your network!

Understanding Trunking: The VLAN Traffic Merger

Trunking is a networking technique that allows:

Multiple VLANs to share a single physical connection
Efficient transport of traffic between switches and routers
Preservation of VLAN identification across network devices
Optimized bandwidth usage by eliminating the need for separate cables per VLAN

Key Benefits of Trunking

Saves Ports – Carries multiple VLANs over one physical link
Reduces Cabling – Fewer connections between devices
Maintains VLAN Integrity – Keeps traffic properly separated
Simplifies Management – Centralized VLAN configuration

How Trunking Works: A Real-World Example

Scenario: A corporate headquarters with:

3 floors (each with its own switch)
4 VLANs (Management, Sales, Engineering, Guest)
Need for inter-floor communication

Without Trunking:

Requires 4 separate cables between each floor’s switches (12 total)
Each cable carries only one VLAN’s traffic
Wastes switch ports and creates cable clutter

With Trunking:

Single high-bandwidth connection between switches
All 4 VLANs share this connection
Special tagging identifies which VLAN each packet belongs to

Trunking Protocols: The Traffic Directors

Two main protocols manage VLAN traffic across trunks:

1. IEEE 802.1Q (Dot1Q)

The industry standard
Adds a 4-byte tag to Ethernet frames
Supports up to 4,094 VLANs
Works with most network equipment

2. ISL (Inter-Switch Link)

Cisco proprietary protocol
Adds 26-byte header and 4-byte trailer
Being phased out in favor of 802.1Q

Protocol Comparison:

Feature	802.1Q	ISL
Compatibility	Multi-vendor	Cisco-only
Overhead	4 bytes	30 bytes
VLAN Support	4,094	1,000
Common Use	Modern networks	Legacy systems

Practical Applications of Trunking

1. Multi-Switch Office Networks

Connects access switches to core switches
Carries all department VLANs between floors

2. Data Center Operations

Links servers to network infrastructure
Supports virtualization environments

3. Campus Networks

Connects buildings while maintaining VLAN separation
Carries voice, data, and security traffic

4. Cloud Connectivity

Extends on-premise VLANs to cloud providers
Maintains network policies in hybrid environments

Configuring Trunking: A Simple Example

Network Setup:

Switch A (Floor 1) needs to connect to Switch B (Floor 2)
VLANs 10 (Management), 20 (Sales), 30 (Engineering)

Configuration Steps:

Designate trunk ports on both switches
Set encapsulation to 802.1Q
Specify which VLANs to allow
Set native VLAN (untagged traffic)

Sample Cisco Commands:

interface GigabitEthernet0/1
 switchport mode trunk
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 10,20,30
 switchport trunk native vlan 99

Common Trunking Mistakes to Avoid

Native VLAN Mismatch – Causes security vulnerabilities
Overlooking Bandwidth Needs – Can create bottlenecks
Improper VLAN Pruning – Allows unnecessary VLAN traffic
Neglecting Security – Failing to restrict unauthorized VLAN access

Trunking vs. Access Ports: Key Differences

Characteristic	Trunk Port	Access Port
VLAN Handling	Multiple VLANs	Single VLAN
Tagging	Uses VLAN tags	No tagging
Typical Use	Switch-to-switch links	End-device connections
Bandwidth	High-capacity needed	Standard capacity