What are the VTP Modes?

VLAN Trunking Protocol is a Cisco proprietary protocol used for communicating VLAN information by Cisco switches. Through VTP, the user can synchronize VLAN name, VLAN ID, and other VLAN information; with Cisco switches inside the same domain. These VTP domains are a set of trunked switches with a similar VTP domain name, version, password, and some other VTP settings. All Cisco switches inside the same domain share their VLAN data with each other.

There are three VTP versions, i.e., V1, V2, and V3. V1 and V2 versions are alike except that V2 supports token ring VLANs and V3 is quite different as it adds the following features:

1. V3 supports extended VLANs (1006 to 4094). Whereas V1 and V2 can broadcast only VLANs 1 to 1005.
2. V3 supports private VLAN.
3. V3 supports VTP primary server and secondary servers.
4. It also supports enhanced authentication.
5. V3 is backward compatibility with V1 and V2.
6. V3 has the ability to be configured on a per-port basis.

VTP Modes :
The user can configure a switch to work in any one of the following VTP modes:

1. Server –
   In VTP server mode, the user can make VLANs, modify and delete them, and they can also specify additional configuration constraints such as VTP pruning and VTP version for the whole domain. These servers promote their VLAN configuration to additional switches that exist in the same domain, and they also synchronize their configuration with additional switches based upon the announcements acknowledged over trunk links. The default mode in VTP is server.
2. Client –
   VTP clients act in a similar way as the VTP servers, though here the user can’t change, create, or delete VLANs.
3. Transparent –
   VTP transparent switches don’t partake in VTP. A VTP transparent switch doesn’t promote its VLAN configuration and it also doesn’t synchronize its VLAN configuration based on acknowledged announcements, however, these switches do transmit VTP announcements that they receive from trunk ports in Version 2 of VTP.
4. VTP mode Off –
   In the three described modes, VTP announcements are acknowledged and forwarded as soon as the switch goes in the management domain state. In the off mode, switches act in a similar way as they do in VTP transparent mode however there is one difference that is the VTP announcements are not forwarded.

The user can configure VLANs on Catalyst 1900, 2820, and 4500 series switches when the switch is in transparent mode or VTP server. The user can use the MIB (Management information base), CLI, or console menus to modify a VLAN configuration when the switch is in either transparent mode or server.

A switch configured in VTP server mode promotes VLAN configuration to adjoining switches over its trunks and learns new VLAN configurations from those neighboring switches. The user can also use the server mode to add or delete VLANs and to modify VLAN information by using either the CLI, the VTP MIB, or the console. For example, VTP promotes the new VLAN, whenever the user adds a VLAN and both servers and clients prepare to receive traffic on their trunk ports.

Subsequently, the switch automatically changes to VTP client mode, it forwards announcements and learns new data from announcements. Though, the user can’t add, modify, or delete a VLAN over the console, the CLI, or the MIB. The VTP client doesn’t preserve VLAN information in NVM (non-volatile memory); Hence as soon as it starts, it learns the configuration by getting announcements from the trunk ports.

In VTP transparent mode, the switch doesn’t learn or promote VLAN configurations from the network. Whenever a switch is in transparent mode, the user is allowed to add VLANs, modify, or delete them over the CLI, MIB, or the console.

VTP configuration :
For exchanging VTP messages there are some basic conditions that need to be fulfilled.

1. The VTP domain name should be the same on both switches.
2. VTP versions should be the same.
3. VTP domain password should be the same.
4. The switch should be configured as either a VTP client or a VTP server.
5. A trunk link should be used between switches.

VTP 

In the image above three switches are connected through trunk links. On switch1, the VTP domain name will be configured using the “vtp domain” command and VTP password by using the “vtp password” command.

Switch1(config)#vtp domain mlkjr
Changing VTP domain name from NULL to mlkjr
Switch1(config)#vtp password kjtmkcbb
Setting device VLAN database password to kjtmkcbb

Now configuring Switch2 and Switch3 as VTP clients.

Switch2(config)#vtp mode client
Setting device to VTP CLIENT mode.
Switch2(config)#vtp domain mlkjr
Changing VTP domain name from NULL to mlkjr
Switch2(config)#vtp password kjtmkcbb
Setting device VLAN database password kjtmkcbb

Switch3(config)#vtp mode client
Setting device to VTP CLIENT mode.
Switch3(config)#vtp domain mlkjr
Changing VTP domain name from NULL to mlkjr
Switch3(config)#vtp password kjtmkcbb
Setting device VLAN database password kjtmkcbb

Now create a new VLAN on Switch1, the VTP will be sent to Switch2 and Switch3 creating a new VLAN automatically on Switch2 and Switch3.

Switch1(config)#vlan 30

Switch2 and Switch3 will create the VLAN 30 automatically. Now checking if it has been created or not.

Switch2#show vlan
VLAN     Name                                  Status             Ports
----     --------------------------------     ---------         -----------------------------
1         default                             active             Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                               Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                               Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                            Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                               Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                               Fa0/21, Fa0/22, Fa0/23, Fa0/24
2         Accounting                             active             Fa/05
30         VLAN0030                             active
1002     fddi-default                         act/unsup
1003     token-ring-default                     act/unsup
1004     fddinet-default                     act/unsup
1005    trnet-default                         act/unsup

Now checking for Switch3

Switch3#show vlan
VLAN     Name                                 Status             Ports
----     --------------------------------     ---------         -----------------------------
1         default                             active             Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                            Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                            Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                            Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                            Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                            Fa0/21, Fa0/22, Fa0/23, Fa0/24
2         Accounting                             active             Fa/05
30         VLAN0030                             active
1002     fddi-default                         act/unsup
1003     token-ring-default                     act/unsup
1004     fddinet-default                     act/unsup
1005     trnet-default                         act/unsup