TB - GNS3 Lab: VLANs and Trunks (Part 2) with GNS3Vault

by C. Casquatch

In this blog, I walk through the lab step-by-step and include screenshots to document my process and learning.

Shoutout to Rene Molenaar for giving people access to these practices through labs like these: https://gns3vault.com/switching/vlans-and-trunks Which is what I am basing this post off of. I have tweaked only one thing and that is which interfaces I use, as mine are gigabit ethernet and not fast ethernet.

🧪 VLAN & Trunking Lab – Step-by-Step Configuration Guide (Gigabit Interfaces)

This lab builds on VLAN and trunking configuration using Gigabit interfaces between three Layer 2 switches (SW1, SW2, SW3) in a custom topology. I will configure VLANs, trunking protocols, DTP behavior, allowed VLANs, and native VLANs.

🖥️ Lab Topology Overview

Connections:

• SW2 Gi0/2 ↔ SW1 Gi0/2
• SW2 Gi0/0 ↔ SW1 Gi0/0
• SW2 Gi0/1 ↔ SW3 Gi0/1
• SW2 Gi0/3 ↔ SW3 Gi0/3
• SW1 Gi0/1 ↔ SW3 Gi0/2
• SW1 Gi0/3 ↔ SW3 Gi0/0

🎯 Lab Goals

• Create and name VLANs: 10, 20, 30, 40, 50
• Configure SW1 Gi0/1 as access port in VLAN 10
• Configure SW2 Gi0/1 as access port in VLAN 20
• Use ISL encapsulation between SW2 Gi0/2 and SW1 Gi0/2
• Prevent trunk negotiation on link SW2 Gi0/1 ↔ SW3 Gi0/1
• Block DTP on SW1 Gi0/3 ↔ SW3 Gi0/0
• Allow only VLANs 1,10,20 on SW2 Gi0/2 ↔ SW1 Gi0/2
• Allow only VLANs 1,10,20,40,50 on SW2 Gi0/1 ↔ SW3 Gi0/1
• Set VLAN 50 as native VLAN on SW1 Gi0/3 ↔ SW3 Gi0/0

🔧 Step 1: Create VLANs on All Switches

configure terminal
vlan 10
 name Engineering
vlan 20
 name Marketing
vlan 30
 name Research
vlan 40
 name Sales
vlan 50
 name Management
exit

🔌 Step 2: Configure Access Ports

On SW1 (Gi0/1):

interface gi0/1
 switchport mode access
 switchport access vlan 10

On SW2 (Gi0/1):

interface gi0/1
 switchport mode access
 switchport access vlan 20

Checking the access port vlan assignments on sw1 and sw2 with

show int 'interface' switchport

🔗 Step 3: Trunk with ISL – SW2 Gi0/2 ↔ SW1 Gi0/2

On SW2 and SW1:

interface gi0/2
 switchport trunk encapsulation isl
 switchport mode trunk
 switchport trunk allowed vlan 1,10,20

Checking the trunk interfaces for sw1 and sw2 with:

show interfaces trunk

🚫 Step 4: No Trunk Negotiation – SW2 Gi0/1 ↔ SW3 Gi0/1

On both switches:

interface gi0/1
 switchport mode trunk
 switchport nonegotiate
 switchport trunk allowed vlan 1,10,20,40,50

Checking the dtp interfaces with:

show dtp trunk

🔕 Step 5: Block DTP – SW1 Gi0/3 ↔ SW3 Gi0/0

On both switches:

interface gi0/3  ! (SW1) or gi0/0 (SW3)
 switchport mode trunk
 switchport nonegotiate
 switchport trunk native vlan 50

Checking the interfaces on sw1 and sw3:

show interface gi0/3 switchport (sw1)
OR
show interface gi0/0 switchport (sw3)

🔍 Step 6: Verify Trunks & VLAN Filtering

Check trunk status and allowed VLANs:

show interfaces trunk
show running-config interface gi0/x

SW1:

💾 Step 7: Save Configuration

If write memory fails, use:

copy running-config startup-config

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