Frame tagging places a unique identifier in the header of each frame as it is forwarded throughout the network backbone. The VTP header varies based on the type of VTP message, but generally, the same four items are found in all VTP messages: * VTP protocol version - Either version 1 or 2 * VTP message type - Indicates one of four types of messages * Management domain name length - Indicates the size of the name that follows * Management domain name - Name configured for the management domain VTP switches operate in one of three modes: * Server * Client * Transparent VTP servers can create, modify, and delete VLAN and VLAN configuration parameters for the entire domain. VTP servers save VLAN configuration information in the switch NVRAM. VTP servers send VTP messages out to all trunk ports. VTP clients cannot create, modify, or delete VLAN information. This mode is useful for switches that lack the memory to store large tables of VLAN information. The only role of VTP clients is to process VLAN changes and send VTP messages out all trunk ports. Switches in VTP transparent mode forward VTP advertisements but ignore information contained in the message. A transparent switch will not modify its database when updates are received, or send out an update that indicates a change in its VLAN status. Except for forwarding VTP advertisements, VTP is disabled on a transparent switch. There are two types of VTP advertisements: * Requests from clients that want information at bootup * Response from servers There are three types of VTP messages: * Advertisement requests * Summary advertisements * Subset advertisements Each advertisement starts as configuration revision number 0. As changes are made, the configuration revision number is increased incrementally by one, or n + 1. The revision number continues to increment until it reaches 2,147,483,648. When it reaches that point, the counter will reset back to zero. With advertisement requests, clients request VLAN information and the server responds with summary and subset advertisements. Subset advertisements contain detailed information about VLANs such as VTP version type, domain name and related fields, and the configuration revision number. Certain actions can trigger subset advertisements: * VLAN creation or deletion * VLAN suspension or activation * VLAN name change * VLAN maximum transmission unit (MTU) change Advertisements can contain some or all of the following information: * Management domain name - Advertisements with different names are ignored. * Configuration revision number - The higher number indicates a more recent configuration. * Message Digest 5 (MD5) - MD5 is the key that is sent with the VTP when a password has been assigned. If the key does not match, the update is ignored. * Updater identity - The updater identity is the identity of the switch that sends the VTP summary advertisement. Specific steps must be considered before VTP and VLANs are configured on the network: 1. Determine the version number of VTP that will be utilized. 2. Decide if the switch will be a member of a management domain that already exists, or if a new domain should be created. If a management domain exists, determine the name and password of the domain. 3. Choose a VTP mode for the switch. Two different versions of VTP are available, Version 1 and Version 2. The two versions are not interoperable. The following command can be used to enter VLAN database mode and configure the VTP version number. Switch#vlan database Switch(vlan)#vtp v2-mode f the switch is the first switch in the network, the management domain should be created. If the management domain has been secured, configure a password for the domain. The following command can be used to create the management domain. Switch(vlan)#vtp domain cisco The domain name can be between 1 and 32 characters in length. The password must be between 8 and 64 characters long. To add a VTP client to a VTP domain that already exists, verify that its VTP configuration revision number is lower than the configuration revision number of the other switches in the VTP domain. Use the show vtp status command. Switches in a VTP domain always use the VLAN configuration of the switch with the highest VTP configuration revision number. If a switch is added with a higher revision number than what is currently in the VTP domain, it can erase all VLAN information from the VTP server and VTP domain. The following command can be used to set the correct mode of the switch: Switch(vlan)#vtp {client | server | transparent} Figure shows the output of the show vtp status command. This command is used to verify VTP configuration settings on a Cisco IOS command-based switch. Figure shows an example of the show vtp counters command. This command is used to display statistics about advertisements sent and received on the switch. The following are some VLAN configuration issues: * A switch creates a broadcast domain * VLANs help manage broadcast domains * VLANs can be defined on port groups, users or protocols * LAN switches and network management software provide a mechanism to create VLANs VLANs help control the size of broadcast domains and localize traffic. VLANs are associated with individual networks. Therefore, network devices in different VLANs cannot directly communicate without the intervention of a Layer 3 routing device. Because routers prevent broadcast propagation and use more intelligent forwarding algorithms than bridges and switches, routers provide more efficient use of bandwidth. In order for multiple devices on the same VLAN to communicate, the IP addresses of all meshed subinterfaces must be on the same network or subnetwork. It is important to understand that a trunk link does not belong to a specific VLAN. special NIC installed that supports trunking By default, server and client Catalyst switches issue summary advertisements every five minutes. What should be done before moving a Catalyst switch to a new VTP management domain? Verify that the new switch has a lower configuration revision number than the other switches in the domain. Select the correct VTP mode and version. Configure the switch with the name of the new management domain.