Virtual Local Area Network and Inter-VLAN Routing

Definition

A Virtual Local Area Network (VLAN) is a logical method of dividing a single physical switched network into multiple smaller broadcast domains. VLANs improve network management, security, and performance by reducing unnecessary broadcast traffic.

Key Points

• VLAN (Virtual Local Area Network) divides one large broadcast domain into multiple logical networks.
• VLANs operate at Layer 2 (Data Link Layer) of the OSI model.
• VLANs reduce:
  • Broadcast traffic
  • Multicast traffic
  • Network congestion
• VLANs provide:
  • Better security
  • Easier network management
  • Logical separation of users

VLAN Range

• VLAN IDs use 12 bits and range from 1 to 4094.
• By default, all switch ports belong to VLAN 1.
• VLAN 1 is also called:
  • Native VLAN
  • Management VLAN

Example / Code

Creating VLANs

Switch(config)# vlan 10
Switch(config-vlan)# name HR

Switch(config)# vlan 20
Switch(config-vlan)# name IT

Explanation:

• vlan 10 Creates VLAN number 10.

• name HR Assigns the name HR to VLAN 10.

• vlan 20 Creates VLAN number 20.

• name IT Assigns the name IT to VLAN 20.

Assigning a Port to a VLAN

Switch(config)# interface ethernet 0/0
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

Explanation:

• interface ethernet 0/0 Selects the switch interface.

• switchport mode access Configures the port as an access port.

• switchport access vlan 10 Places the port into VLAN 10.

Configuring a Trunk Port

Switch(config)# interface ethernet 0/0
Switch(config-if)# switchport trunk encapsulation dot1q
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan all

Explanation:

• switchport trunk encapsulation dot1q Uses IEEE 802.1Q trunking protocol.

• switchport mode trunk Changes the interface into trunk mode.

• switchport trunk allowed vlan all Allows all VLANs through the trunk.

VTP Configuration

Switch(config)# vtp mode server
Switch(config)# vtp domain test.com
Switch(config)# vtp version 2

Explanation:

• vtp mode server Configures the switch as a VTP server.

• vtp domain test.com Places the switch into the VTP domain.

• vtp version 2 Enables VTP version 2.

Explanation

VLAN Operation

A normal switch network has one broadcast domain. When many devices exist in the same broadcast domain, broadcast traffic increases.

VLANs solve this problem by logically separating users:

Example:

Devices in different VLANs cannot communicate directly because each VLAN is a separate broadcast domain.

Access and Trunk Ports

Access Port

An access port connects end devices such as:

• PCs
• Printers
• IP phones

Characteristics:

• Carries traffic from only one VLAN.
• Does not add VLAN tags.

Example:

PC ---- Access Port ---- Switch

Trunk Port

A trunk port connects network devices such as:

• Switch to switch
• Switch to router

Characteristics:

• Carries multiple VLANs.
• Uses VLAN tagging.

Example:

Switch ---- Trunk Link ---- Switch

Frame Tagging

When multiple VLANs travel through a trunk link, switches must identify which VLAN each frame belongs to.

This process is called frame tagging.

Tagging Protocols

802.1Q is the recommended and commonly used standard.

VTP (VLAN Trunking Protocol)

Definition

VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used to distribute VLAN configuration information between switches through trunk links.

VTP Requirements

For switches to exchange VTP information:

• Same VTP domain name
• Same VTP password
• Same VTP version
• Connected through trunk ports

VTP Revision Number

• Identifies the latest VLAN database update.
• Default value is 0.
• Higher revision number means newer configuration.
• Each update increases the revision number.

VTP Modes

VTP Server

A VTP server can:

• Create VLANs
• Delete VLANs
• Modify VLANs
• Advertise VLAN information

Characteristics:

• Default switch mode.
• Stores VLAN information in vlan.dat.

VTP Client

A VTP client:

• Cannot create VLANs.
• Cannot modify VLANs.
• Receives VLAN updates from the server.
• Stores VLAN information in vlan.dat.

VTP Transparent

A VTP transparent switch:

• Can create, delete, and modify VLANs.
• Does not synchronize VLAN information.
• Passes VTP advertisements through.
• Resets the revision number when configured.

Inter-VLAN Routing

Definition

Devices in different VLANs cannot communicate directly because each VLAN is a separate broadcast domain.

Inter-VLAN routing allows communication between different VLANs using a Layer 3 device such as a router or Layer 3 switch.

Methods of Inter-VLAN Routing

1. Router-on-a-Stick

• Uses a single router interface.
• Interface is configured as a trunk.
• Router uses subinterfaces for different VLANs.

Example:

Switch ---- Trunk ---- Router

2. Multiple Links

• Uses separate physical router interfaces.
• Each interface connects to a different VLAN.

Example:

VLAN 10 ---- Router Interface 1

VLAN 20 ---- Router Interface 2

Verification Commands

Output (if any)

Expected Output

VLAN 10 HR
VLAN 20 IT

Trunk ports successfully configured.
VTP information successfully displayed.

Common Mistakes

• Assuming devices in different VLANs can communicate without routing.
• Assigning multiple VLANs to one access port.
• Forgetting that trunk ports are required between switches.
• Using different VTP domains between switches.
• Ignoring VLAN tagging on trunk links.
• Changing VTP revision numbers incorrectly.
• Connecting VLANs without configuring inter-VLAN routing.

Short Exam Notes

• VLAN divides one broadcast domain into multiple logical broadcast domains.

• VLAN operates at Layer 2.

• VLAN range: 1–4094.

• Default VLAN: VLAN 1.

• Access ports carry one VLAN.

• Trunk ports carry multiple VLANs.

• 802.1Q is the standard VLAN tagging protocol.

• VTP distributes VLAN information between Cisco switches.

• VTP modes:

  • Server
  • Client
  • Transparent

• Inter-VLAN routing requires a Layer 3 device.

• Router-on-a-stick is a common method for VLAN communication.