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Understanding Trunking in Networking – IEEE 802.1Q

 🌍 Understanding Trunking in Networking – IEEE 802.1Q


If you’re working in enterprise networking, VLAN trunking is a must-know concept. Let’s break it down simply 👇

🔌 What is Trunking?
Trunking allows multiple VLANs to travel over a single physical link between switches. Instead of using one cable per VLAN, you use one trunk link that carries traffic for multiple VLANs. This is where IEEE 802.1Q comes in.

📘 What is IEEE 802.1Q?
802.1Q is the industry standard for VLAN tagging.It inserts a VLAN tag into Ethernet frames so switches know which VLAN the traffic belongs to.

👉 This process is called tagging.
🖧 Example Scenario
Switch A Switch B
VLAN 30 VLAN 30
Fa0/24 ⟷ Fa0/24
(802.1Q Trunk Link)
When traffic from VLAN 30 leaves Switch A:
Traffic → Tagged with VLAN ID → Sent over trunk → Received by Switch B → Delivered to VLAN 30
This ensures proper VLAN separation across multiple switches.

🏷 How 802.1Q Tagging Works
• Adds a 4-byte tag inside the Ethernet frame
• Includes VLAN ID (VID)
• Default VLAN = VLAN 1 (native VLAN – untagged by default)
Tagged traffic = VLAN-aware
Untagged traffic = Native VLAN

🎯 Why Trunking is Important
✅ Reduces cable usage
✅ Supports scalable VLAN design
✅ Essential for inter-switch communication
✅ Required for enterprise networks
Without trunking, VLANs cannot extend between switches.
⚙ Common Configuration Concept (Cisco Example)
interface fa0/24
switchport mode trunk
switchport trunk encapsulation dot1q
This enables 802.1Q trunking on the interface.

🚨 Common Troubleshooting Issues
⚠ VLAN not allowed on trunk
⚠ Native VLAN mismatch
⚠ Trunk mode mismatch (access vs trunk)
⚠ VLAN not created on both switches
💡 If you’re preparing for CCNA or working in real-world networking, mastering trunking is non-negotiable.

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💬 Quick Question:
Have you ever faced a native VLAN mismatch issue?
What was the root cause?
#Networking #VLAN #Trunking #8021Q #CCNA #NetworkEngineer #ITCareers #TechGrowth 🌍


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