Cisco 802.1Q Trunk Configuration on Switch-to-Switch Links

Introduction

When connecting multiple Cisco switches, you need a way for multiple VLANs to pass through a single physical link. That’s precisely what 802.1Q trunking does: it tags Ethernet frames so that traffic from different VLANs can traverse a single connection.

In this tutorial, you’ll configure and verify trunk ports using GigabitEthernet0/0 (g0/0) interfaces on Cisco switches.
You’ll also learn how to check trunk status, disable DTP, and troubleshoot common issues.

You’ll Learn:

• How to configure trunk ports using 802.1Q encapsulation
• How to set native and allowed VLANs
• How to disable DTP for security
• How to verify and troubleshoot trunk links

Network Topology

This lab focuses on trunking between two Cisco switches using g0/0 interfaces.

Lab Requirements

• Two Cisco switches (e.g., Catalyst 2960, 3850, or virtual in CML / EVE-NG / Packet Tracer)
• Cisco IOS 15.x or later
• Interfaces: GigabitEthernet0/0 On each switch
• VLANs are already created or preconfigured.

Step 1: Configure Trunk on Switch-1

Configure the g0/0 interface as a trunk port using 802.1Q encapsulation.

Switch1# configure terminal
Switch1(config)# interface g0/0
Switch1(config-if)# switchport trunk encapsulation dot1q
Switch1(config-if)# switchport mode trunk
Switch1(config-if)# switchport trunk allowed vlan all
Switch1(config-if)# switchport trunk native vlan 99
Switch1(config-if)# description *** Trunk link to Switch2 ***
Switch1(config-if)# end
Switch1# write memory

Step 2: Configure Trunk on Switch-2

Repeat the process for the g0/0 port on the second switch.

Switch2# configure terminal
Switch2(config)# interface g0/0
Switch2(config-if)# switchport trunk encapsulation dot1q
Switch2(config-if)# switchport mode trunk
Switch2(config-if)# switchport trunk allowed vlan all
Switch2(config-if)# switchport trunk native vlan 99
Switch2(config-if)# description *** Trunk link to Switch1 ***
Switch2(config-if)# end
Switch2# write memory

Note: The native VLAN and allowed VLANs must match on both switches to avoid warnings or traffic drops.

Step 3: Disable DTP (Optional but Recommended)

Dynamic Trunking Protocol (DTP) automatically negotiates trunking. In most production environments, it’s best to disable it for security.

Switch1(config)# interface g0/0
Switch1(config-if)# switchport nonegotiate

Switch2(config)# interface g0/0
Switch2(config-if)# switchport nonegotiate

Security Note: Disabling DTP helps prevent VLAN hopping attacks and ensures predictable link behavior.

Step 4: Verify the Trunk Configuration

Check Trunk Status

Switch# show interfaces trunk

Expected Output:

Port    Mode         Encapsulation  Status        Native vlan
Gi0/0   on           802.1q         trunking      99

Port    Vlans allowed on trunk
Gi0/0   1-4094

Port    Vlans allowed and active in management domain
Gi0/0   10,20,30

Check Switchport Details

Switch# show interfaces g0/0 switchport

Key Lines to Look For:

Administrative Mode: trunk
Operational Mode: trunk
Trunking Encapsulation: dot1q
Trunking Native Mode VLAN: 99
Administrative Native VLAN tagging: enabled

Step 5: Troubleshooting Common Trunk Issues

“Command rejected” error

If you see:

Command rejected: An interface whose trunk encapsulation is "Auto" cannot be configured to trunk mode.

Fix:
Set encapsulation before enabling trunking.

switchport trunk encapsulation dot1q

VLAN mismatch warning

If you see a “Native VLAN mismatch” in show interfaces trunk:

Fix:
Make sure both switches use the same native VLAN (e.g., VLAN 99).

switchport trunk native vlan 99

Trunk not active

If trunk status shows “not-trunking”:

Fix:
Check that both sides are configured as trunk.

switchport mode trunk

Pro Tip: You can verify DTP negotiation with:

show dtp interface g0/0

Summary

You’ve successfully configured 802.1Q trunking on Cisco switches using the g0/0 interfaces.
This allows multiple VLANs to traverse a single link while maintaining VLAN separation.

You learned how to:

• Configure trunk ports using 802.1Q encapsulation
• Modify native VLANs
• Disable DTP for security
• Verify and troubleshoot trunk connections