VLANs and 802.1Q Trunking

Learning objectives

• Explain the purpose and scope of VLANs and 802.1Q Trunking.
• Identify important protocols, roles, fields, and terminology.
• Apply the concept to a practical network scenario.
• Use IOS XE commands to verify operational behavior.

1. Core concepts

A VLAN creates a separate Layer 2 broadcast domain. Devices in different VLANs do not communicate directly at Layer 2 and need Layer 3 routing. VLANs improve segmentation and reduce broadcast scope. VLAN IDs range from 1 to 4094, with reserved values.

2. How it works

An access port carries one data VLAN and normally connects an endpoint. A trunk carries multiple VLANs using IEEE 802.1Q tags. The native VLAN is normally untagged on a traditional trunk. Both ends should agree on native VLAN and allowed VLAN list.

Visual walkthrough

3. Worked example

Create VLAN 20 named USERS, assign Gi1/0/10 as an access port in VLAN 20, and configure Gi1/0/48 as a trunk allowing VLANs 10, 20, and 30. Verify VLAN existence, port membership, and trunk allowance.

IOS XE commands and verification

Syntax and feature availability can vary by device family and software release. Confirm platform support before applying configuration.

vlan 20
 name USERS
interface gi1/0/10
 switchport mode access
 switchport access vlan 20
interface gi1/0/48
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30
show vlan brief
show interfaces trunk

Common mistakes and troubleshooting

• Check physical and interface state before changing higher-layer settings.
• Verify both ends agree on addressing, VLAN, encapsulation, and protocol parameters.
• Use operational show commands; configured text alone does not prove a feature is working.
• Change one variable at a time and verify the result.

Knowledge check

Choose one answer per question. Submit to see why the correct choice is right and why every other option is wrong. These are original practice questions, not Cisco exam questions.