Chapter 3: VLAN Trunking Protocol Walk-through Part One (Preview)

Switches acting as VTP Servers allow the creation of VLANs using the vlan vlan-id command and related configurations. VTP Clients do not allow the creation of VLANs, however they synchronize their VLAN configuration with the changes received in VTP advertisements. A switch in the VTP Transparent mode does not participate in VTP synchronization. They simply pass the VTP advertisements they receive out trunk links to other switches.

The following example demonstrates how the VTP messages propagate and the role of VTP configuration revision number. The SW1 is acting as a VTP Server. This enables us to create VLANs on SW1 and to have the SW1 participate in VTP.

The other two switches SW2 and SW3 can be an either a VTP Client or a VTP Server. In addition, at the beginning of the example, all the switches have converged to use the VLAN database that uses revision number 4.

 

  1. The network engineer configures a new VLAN with the vlan 777 command on switch SW1

 

  1. SW1 which is in VTP server mode, changes the VTP configuration revision number for its own VLAN configuration database from 4 to 5.

 

  1. SW1 sends VTP messages over the VLAN trunk to SW2 to begin the process of telling SW2 about the new VTP configuration revision number for the VLAN configuration database.

 

vtp-walkthrough-1

Figure Adding VLAN 777 at Switch SW1, Starting with All at Revision Number 4

 

  1. SW2, upon receiving the VTP messages from SW1, compare it’s VLAN configuration revision numbers to the VLAN configuration revision number found in the VTP messages it received from SW1. Since the VLAN configuration revision number in the VTP messages from SW1 is higher than its own, SW2 increments its VLAN configuration revision number by 1 to 5.

 

  1. SW2 synchronizes its VLAN database with the changes in the VTP messages it received from SW1 by adding VLAN 777 to its VLAN database.

 

  1. SW2 sends VTP messages over the VLAN trunk to SW3 to begin the process of telling SW3 about the new VTP configuration revision number for the VLAN configuration database.

vtp-walkthrough-2

Figure Adding VLAN 777 at Switch SW2, Starting with All at Revision Number 4

  1. Similarly, upon receiving the VTP message from SW2, SW3 compares it’s VLAN configuration revision numbers to the VLAN configuration revision number found in the VTP messages it received from SW2. Since the VLAN configuration revision number in VTP messages from SW2 is higher than its own, SW3 increments its VLAN configuration revision number by 1 to 5.

 

  1. SW3 synchronizes its VLAN database with the changes in the VTP messages it received from SW2 by adding VLAN 777 to its VLAN database.

vtp-walkthrough-3

Figure Adding VLAN 777 at Switch SW3, Starting with All at Revision Number 4

 

Now with VTP operating correctly on all three switches, all the switches have the exact same configuration, with VTP configuration revision number 5, as advertised with VTP messages.

 

 

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