Table of Contents

Implement effective VLAN strategies for network segmentation, security, and performance optimization.

VLAN Overview

What are VLANs?

Virtual Local Area Networks (VLANs) allow you to logically segment a physical network into multiple broadcast domains, providing:

  • Security: Isolate sensitive traffic
  • Performance: Reduce broadcast traffic
  • Management: Organize devices by function
  • Flexibility: Easy reconfiguration without physical changes

Common VLAN Assignments

VLAN ID Name Subnet Purpose
1 Default - Native VLAN (avoid using)
10 Management 192.168.10.0/24 Network device management
20 Servers 10.0.20.0/24 Application servers
30 Workstations 10.0.30.0/22 User devices
40 Guest 172.16.40.0/24 Guest network (isolated)
50 IoT 10.0.50.0/24 Internet of Things devices
60 VoIP 10.0.60.0/24 Voice over IP phones
99 Native - Trunk native VLAN

VLAN Implementation

Creating VLANs

To implement VLANs in your network:

  1. Plan VLAN Structure

    • Identify security zones and traffic types
    • Assign VLAN IDs and names
    • Document subnet assignments

  2. Create VLANs on Switches

    • Define VLAN ID and descriptive name
    • Activate VLAN
    • Configure management interfaces (SVIs)

  3. Assign Ports to VLANs

    • Configure access ports for end devices
    • Specify VLAN assignment per port
    • Enable port security features

Trunk Configuration

Trunk links carry traffic for multiple VLANs between switches:

Key Concepts:

  • 802.1Q Encapsulation: Industry-standard VLAN tagging
  • Native VLAN: Untagged traffic on trunk (change from default VLAN 1)
  • Allowed VLANs: Specify which VLANs traverse the trunk
  • DTP (Dynamic Trunking Protocol): Disable for security

Best Practices:

  • Change native VLAN from default (1) to unused VLAN (e.g., 99)
  • Only allow necessary VLANs on trunks
  • Disable automatic trunk negotiation
  • Document trunk links and allowed VLANs

Platform-Specific Implementation

For detailed configuration steps, see platform-specific guides:

Inter-VLAN Routing

VLANs create separate broadcast domains, so routing is required for inter-VLAN communication.

Router-on-a-Stick

Overview:

  • Single physical router interface
  • Multiple virtual subinterfaces (one per VLAN)
  • Each subinterface has VLAN-specific IP address
  • 802.1Q encapsulation tags traffic

Use Cases:

  • Small to medium networks
  • Limited switch ports
  • Budget constraints

Advantages:

  • Cost-effective (single router interface)
  • Simple topology

Disadvantages:

  • Single point of failure
  • Potential bandwidth bottleneck
  • Limited scalability

Layer 3 Switch

Overview:

  • Switching hardware with routing capability
  • Switched Virtual Interfaces (SVIs) for each VLAN
  • Wire-speed routing between VLANs
  • No external router required

Use Cases:

  • Medium to large networks
  • High inter-VLAN traffic
  • Enterprise environments

Advantages:

  • High performance (hardware-accelerated routing)
  • Reduced latency
  • Simplified cabling
  • Scalable solution

Disadvantages:

  • Higher equipment cost
  • More complex configuration

Configuration Examples

For detailed inter-VLAN routing configuration:

Security Best Practices

VLAN Segmentation Strategy

  1. Management VLAN: Isolate with strict ACLs

    • Network device management only
    • Limited access from specific IPs
    • No user devices

  2. Server VLAN: Application-specific rules

    • Group by function or security level
    • Control access with ACLs or firewalls
    • Monitor for anomalies

  3. User VLAN: Standard access

    • Separate by department or function
    • Apply user security policies
    • Internet access with content filtering

  4. Guest VLAN: Internet-only, isolated

    • No access to internal resources
    • Captive portal for authentication
    • Bandwidth limits

  5. IoT VLAN: Minimal required connectivity

    • Isolated from user network
    • Restrict to required services only
    • Monitor for unusual traffic

Access Control Implementation

Implement access controls to enforce VLAN segmentation:

Guest Network Isolation:

  • Deny guest VLAN access to internal networks
  • Allow internet access only
  • Use access control lists or firewall rules

Example ACL Logic:

Deny: Guest VLAN → Internal Networks (10.0.0.0/8, 192.168.0.0/16)
Permit: Guest VLAN → Internet (any other)

For platform-specific ACL configuration, see:

VLAN Management with PowerShell

For automated VLAN management, see the PowerShell Automation guide which includes:

  • NetworkVLAN class for VLAN creation
  • Configuration generation for multiple platforms
  • Network analysis and security validation
  • Documentation generation

Troubleshooting VLANs

Common Issues

Problem Cause Solution
No connectivity between VLANs Missing inter-VLAN routing Configure Layer 3 routing
Devices on same VLAN can't communicate Port not in correct VLAN Verify port VLAN assignment
Trunk not passing VLAN traffic VLAN not allowed on trunk Add VLAN to trunk allowed list
Native VLAN mismatch Different native VLANs on trunk ends Ensure matching native VLAN
VLAN not in database VLAN not created Create VLAN on switch

Verification Steps

  1. Check VLAN Database

    • Verify VLAN exists
    • Confirm VLAN name and status

  2. Verify Port Assignment

    • Check port mode (access or trunk)
    • Confirm VLAN assignment
    • Review switchport configuration

  3. Check Trunk Configuration

    • Verify trunk status
    • Confirm allowed VLANs
    • Check native VLAN setting

  4. Review MAC Address Table

    • Confirm devices learned on correct VLAN
    • Check for MAC address table issues

  5. Test Inter-VLAN Routing

    • Verify routing configuration
    • Check gateway addresses
    • Test connectivity with ping/traceroute

Platform-Specific Troubleshooting

For detailed verification commands and debugging:

VLAN Design Best Practices

Planning Guidelines

  1. Document Everything

    • VLAN IDs, names, and purposes
    • Subnet assignments
    • Port assignments
    • Trunk configurations

  2. Use Consistent Naming

    • Descriptive VLAN names
    • Standardize across devices
    • Follow organizational conventions

  3. Implement Security from Start

    • Change default VLAN
    • Use unique native VLAN on trunks
    • Disable unused ports
    • Assign unused ports to "blackhole" VLAN

  4. Plan for Growth

    • Leave room for new VLANs
    • Design scalable subnetting
    • Document expansion procedures

  5. Test Before Production

    • Verify all VLAN connectivity
    • Test inter-VLAN routing
    • Confirm security policies
    • Validate performance

Common VLAN Assignments Table

VLAN ID Name Purpose Security Level
1 Default Do not use N/A
10 Management Device administration High
20 Servers Application servers High
30 Workstations User devices Medium
40 Guest Guest network Low (isolated)
50 IoT Smart devices Medium (isolated)
60 VoIP IP phones High (QoS)
70 DMZ Public-facing servers High (restricted)
99 Native Trunk native VLAN N/A
999 Unused Disabled ports N/A
Edit this page