What is VLAN Trunking Protocol (VTP) ?

VTP is a Layer 2 messaging protocol that maintains VLAN configuration consistency throughout a common administration domain.VTP manages the additions,deletions,and name changes of VLANs across multiple switches,minimizing misconfigurations and configuration inconsistencies that cna cause problems,such as duplicate VLAN names or incorrect VLAN tpe specifications.
    VTP makes VLAN configuration easier.However,ou have not yet seen how to configure VLANs.To appreciate VTP,consider this example.If a network has ten switches that are interconnected,and parts of VLAN 3 were on all ten switches,you would have to type the same config command on all switched to "create" the VLAN.with VTP,you would do that once and the other nine switches would learn about VLAN 3 dynamically.
                  VTP distributes and synchronizes identifying information abput VLANs configures throughout a switched network.Configurations made to a single switch,which is called the VTP server,are propagated across trunk links to all switches in the same VTP domain.
  VTP allows switched network solutions to scale to large sizes by reducing the manual configuration needs in the network.The VTP domain is created by having administrator chooses which switches are  in the same domain by deciding which switches share common VLANs.Once(or more)switch creates VLANs as the VTP server;then the others are configured as clients for full VTP operation.(VTP transparent mode,a third option,is covered shortly.)

How VTP works

VTP advertisements are flooded throughout the management domain every 5 minutes,or whenever there is a change in VLAN configurations.Included in a VTP advertisement is a configuration revision number,as well a VLAN names and numbers,and information about which switches have ports assigned to each VLAN.By configuring the details on one server and propagating the information through advertisement,all switches know the names and numbers of all VLANs.
          One of the most important components of the VTP advertisements is the configuration revision number.Each time a VTP server modifies its VLAN information,it increments.Each time a VTP server modifies its VLAN information,it increments the configuration revision number by one.The VTP server then sends out a VTP advertisement that includes the new configuration revision number.When a switch receives a VTP advertisement with a larger configuration revision number.it updates its VLAN configuration.

VTP operates in one of three modes:
*Server mode
*Client mode
*Transparent mode

VTP servers can create,modify and delete VLANs and other configuration parameters for the entire VTP domain;this information,in turn,is propagated to the VTP clients in that same domain.VTP servers save VLAN configurations in the Catalyst NVRAM,whereas,in clients,the VLAN configuration is not stored.VTP messages are transmitted by the server out all trunk connections.

                A VTP client cannot create,change or delete VLANs,nor can it save VLAN configurations  in nonvolatile memory.So,why is a VTP  client?Well,if one engineer design and implements the network,it's a lot more convenient to configure the VLANs in one switch (the VTP server)and have the information propagated to VTP clients.
          In some cases,a VLAN exists in multiple switches,but administrative control of those switches is among different departments. VTP transparent mode provides an option so that some switches can use VTP but other switches can ignore VTP,while not stopping the other switches from using it.A switch in transparent mode forwards VTP advertisements received from other switches that are part of the same management domain,while ignoring the information in the VTP messages.A switch configured in VTP transparent mode can create,delete and modify VLANs,but the changes are not transmitted to other switches in the domain;they affect only that local switch.Choosing to use transparent mode is typical when there is a need for distributed administrative control of the switches,in spite of the fact that they each control parts of the same VLANs.

Table below offers a comparative overview of the three VTP modes.

Because ISL trunk lines carry VLAN traffic for all VLANs,some traffic might be needlessly broadcast across links that do not need to carry that traffic. VTP pruning uses VTP advertisements to determine when parts of the network do not have any members in a particular VLAN. By knowing what switches do not have members of a VLAN,VTP can prune some trunks,meaning these trunks do not forward broadcast for that VLAN. By default,a trunk connection carries traffic for all VLANs in the VTP management domain.Commonly,some switches in an enterprise network do not have local ports configured in each VLAN,so VTP pruning provides a way to be ready for future expansion but not waste trunk capacity with needless broadcasts.