VLAN Trunking Protocol (VTP)

Definition

VLAN Trunking Protocol (VTP) is a Cisco proprietary protocol used to distribute and synchronize VLAN configuration information between switches in the same VTP domain.

VTP allows administrators to create, modify, or delete VLANs on one switch and automatically propagate those changes to other switches.

Key Points

• VTP (VLAN Trunking Protocol) is developed by Cisco.

• It is used to synchronize VLAN databases between switches.

• All switches participating in VTP must:

  • Have the same VTP domain name.
  • Be connected using trunk links.
  • Use the same VTP password.
  • Run compatible VTP versions.

• VTP has three versions:

  • VTP Version 1
  • VTP Version 2
  • VTP Version 3

• VLAN information is stored in:

  • vlan.dat file in Flash memory (Cisco IOS switches).
  • NVRAM configuration file in Cisco CatOS switches.

VTP Requirements

For VTP information exchange to occur:

VTP Modes

VTP operates using different switch modes.

VTP Versions

VTP Version 1

• Original VTP version.
• Supports basic VLAN synchronization.
• Does not support extended-range VLANs.
• Uses VLAN database updates.

VTP Version 2

Improvements over VTP Version 1:

• Supports Token Ring VLANs.
• Provides better consistency checking.
• Still does not support extended-range VLANs.

Automatic Upgrade

• A VTP Version 1 switch that supports Version 2 can automatically upgrade when it detects a VTP Version 2 or Version 3 neighbor.
• A VTP Version 2 switch remains Version 2 even when it detects a Version 3 neighbor.

VTP Version 3

VTP Version 3 provides enhanced security and management features.

Features

• Backward compatible with VTP Version 2 on individual links.
• Supports primary and secondary VTP servers.
• Password is hidden from the running configuration.
• Supports extended-range VLANs.
• Allows VTP to be disabled globally or per interface.

VTP Version 3 Configuration

SW(config)# vtp version 3
SW(config)# vtp domain Kabul
SW# vtp primary
SW(config)# vtp password cisco123

Explanation

VTP Password Options (Version 3)

Normal Password

SW(config)# vtp password cisco123

Hidden Password

SW(config)# vtp password cisco123 hidden

Secret Password

SW(config)# vtp password cisco123 secret

Verification:

SW# show vtp password

VTP Off Mode

VTP Version 3 allows disabling VTP.

Globally

SW(config)# vtp mode off

Per Interface

SW(config-if)# no vtp

VTP Configuration Revision Number

Definition

The VTP revision number is a sequence number that identifies the latest version of the VLAN database.

The switch with the higher revision number has the newest VLAN information.

Key Points

• Higher revision numbers overwrite lower revision number databases.
• VLAN databases synchronize when revision numbers are compared.
• A switch with an incorrect VLAN database and a higher revision number can overwrite the entire VTP domain.

Example:

Switch A Revision Number: 5
Switch B Revision Number: 10

Switch B VLAN information can replace Switch A information.

VTP Authentication

Definition

VTP authentication verifies that received VTP updates are from trusted switches.

Configuration

SW(config)# vtp domain Test
SW(config)# vtp password password123

Verification:

SW# show vtp status
SW# show vtp password

Authentication uses MD5 hash comparison to verify passwords.

VTP Update Exchange Process

Step-by-Step Process

1. A switch sends a summary advertisement containing:

   • VTP domain name
   • VTP password
   • Revision number

2. Receiving switch checks:

   • Domain name
   • Password

3. If the domain name or password does not match:

   • The update is discarded.

4. If authentication succeeds:

   • The switch compares revision numbers.

5. If the received revision number is:

   • Lower or equal: Ignore update.
   • Higher: Request latest VLAN information.

6. The VLAN database is updated.

Example

Received Revision Number: 15
Current Revision Number: 10

Result:

• Switch requests the latest VLAN database.
• VLAN information is updated.

Important VTP Notes

VLAN Database Storage

Cisco IOS switches store VTP and VLAN information in:

Flash Memory:
vlan.dat

Cisco CatOS switches store information in:

NVRAM configuration file

VTP Advertisement Contains

A VTP advertisement includes:

• VLAN information
• VTP domain name
• Configuration revision number

Adding a New Switch

Before adding a switch to an existing VTP domain:

• Reset the revision number.
• Avoid connecting switches with unknown VLAN databases.

Resetting VTP Revision Number

The revision number can be changed by:

Changing VTP Domain

SW(config)# vtp domain NewDomain

Changing VTP Mode to Transparent

SW(config)# vtp mode transparent

Common Mistakes

• Connecting switches without matching VTP domains.
• Forgetting that VTP requires trunk links.
• Adding a switch with a higher revision number without resetting it.
• Assuming VTP client switches can create VLANs.
• Using different VTP passwords between switches.
• Ignoring VTP version compatibility.
• Forgetting that VTP Version 3 provides better security features.

Short Exam Notes

• VTP: Cisco protocol used to synchronize VLAN information between switches.

• Requirements:

  • Same domain.
  • Same password.
  • Same version.
  • Trunk connection.

• VTP Modes:

  • Server
  • Client
  • Transparent
  • Off (VTP v3)

• VTP Versions:

  • V1
  • V2
  • V3

• Revision Number: Determines the latest VLAN database version.

• Higher revision number overwrites lower revision number.

• Authentication uses password verification and MD5 hashing.

• VTP advertisements contain:

  • Domain name
  • Password
  • Revision number

• Reset revision number before joining a new switch to a VTP domain.