Master Cisco VLAN Configuration: Step-by-Step Guide

Introduction

In this lesson, I will explain how to configure, manage, and troubleshoot VLANs (Virtual Local Area Networks) on Cisco switches. VLANs are fundamental to network segmentation and security, allowing you to create logical network boundaries within a physical switch infrastructure.

Lesson Objectives

After completing this lesson, you will be able to:

• Create and manage VLANs on Cisco switches
• Configure access and trunk ports
• Verify VLAN configurations
• Troubleshoot common VLAN issues
• Implement VLAN best practices

Prerequisites

• Basic understanding of switching concepts
• Access to a Cisco switch (physical or virtual)
• Knowledge of basic Cisco IOS commands
• Familiarity with Ethernet networking concepts

Core Lesson Content

Part 1: VLAN Basics and Creation

Explanation

VLANs allow you to segment your network logically without requiring separate physical switches. Each VLAN represents a broadcast domain, and devices in different VLANs cannot communicate directly without routing.

Creating VLANs

Switch> enable
Switch# configure terminal
Switch(config)# vlan 10
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit
Switch(config)# vlan 20
Switch(config-vlan)# name ENGINEERING
Switch(config-vlan)# exit

Understanding the Output

• The vlan command creates a new VLAN with the specified ID
• Valid VLAN IDs range from 1-4094 (1, 1002-1005 are reserved)
• The name command is optional but recommended for management

Key Points to Check

• VLAN status (active/suspended)
• VLAN name configuration
• Available VLAN IDs
• VLAN database consistency

Part 2: Port Configuration

Access Ports

Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# no shutdown

Trunk Ports

Switch(config)# interface gigabitethernet 0/2
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# no shutdown

Understanding Port Configuration

• Access ports belong to a single VLAN
• Trunk ports carry traffic for multiple VLANs
• Native VLAN is untagged on trunk ports
• Use allowed vlan to restrict VLAN traffic on trunks

Part 3: VLAN Verification

Show Commands

Switch# show vlan brief
Switch# show interfaces trunk
Switch# show interfaces status
Switch# show running-config interface gigabitethernet 0/1

Example Output

VLAN Name                             Status    Ports
---- -------------------------------- --------- ------------------------
1    default                          active    Gi0/3, Gi0/4
10   SALES                            active    Gi0/1
20   ENGINEERING                      active    

What to Look For

• VLAN status should be “active.”
• Ports should appear under the correct VLANs
• Trunk ports should show the correct encapsulation
• Allowed VLANs on trunks should match the design

Common Issues and Solutions

Problem 1: Port in Wrong VLAN

Symptoms:

• Devices cannot communicate
• The Port shows up in the wrong VLAN

Solution:

Switch# show interfaces gigabitethernet 0/1 switchport
Switch(config)# interface gigabitethernet 0/1
Switch(config-if)# switchport access vlan 10

Problem 2: Trunk Negotiation Issues

Symptoms:

• VLANs not passing through the trunk
• Encapsulation mismatch

Solution:

Switch(config)# interface gigabitethernet 0/2
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk

Practical Exercise

Configure a small network with the following requirements:

1. Create VLANs 10 (Sales) and 20 (Engineering)
2. Configure ports 1-5 as access ports in VLAN 10
3. Configure ports 6-10 as access ports in VLAN 20
4. Configure port 24 as a trunk port, allowing all VLANs
5. Verify your configuration

Verification Checklist

• [ ] VLANs created and named
• [ ] Access ports assigned to correct VLANs
• [ ] Trunk port configured properly
• [ ] Inter-VLAN routing working (if configured)
• [ ] All ports in correct operational state

Quick Reference Commands

show vlan brief                    # Display VLAN summary
show interfaces trunk              # Show trunk port information
show interfaces status             # Display port status
show running-config interface      # Show interface configuration
show mac address-table vlan 10     # Show MAC addresses in VLAN 10