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VLAN Trunking Protocol (VTP)–Part I

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Intoduction:

VLAN Trunking Protocol is a Cisco proprietary protocol and it’s purpose is to reduce the administration in a switched network. It helps to maintain consistent VLAN information across all switches in a switched network. It helps to manage the addition, deletion and modification of VLANs across the organisation from a central point of control.

VTP minimizes misconfigurations and configuration inconsistencies that can cause several problems, such as duplicate VLAN names, incorrect VLAN-type specifications, and security violations.

VTP uses Layer 2 trunking to communicate VLAN information with switches in the same VTP management domain(s). VTP packets are sent in either Inter-Switch Link (ISL) frames or in IEEE 802.1Q (dot1q) frames. When you configure VTP, you must configure a trunk port so that the switch can send and receive VTP advertisements to and from other switches in the domain.

VTP Versions:

VTP protocol versions: 1, 2, & 3

VTP version 1 and version 2 support VLANs 1 to 1000 only. There are no major differences between version 1 and version 2 in terms of VTP configuration and operation, except the VTP Transparent mode behavior.

VTP version 3:

  1. In VTP versions 1 & 2, a switch with factory default configuration will automatically adapt the new VTP domain name from a received VTP message. This may lead to serious issues and VTP version 3 addressed this issue by forcing manual configuration.
  2. Only in VTP version 3, the entire VLAN range is supported (VLANs 1 to 4094). (Note: It’s not possible to configure VLANs 1002 to 1005 in VTP client/server mode because these VLANs are reserved for Token Ring.)
  3. Private VLAN support.
  4. Multiple Spanning Tree (MST) Support – VTP version 3 can propagate Multiple Spanning Tree (MST) protocol database information.
  5. VTP primary server and VTP secondary servers. (Note: There can only be one primary server. The primary server is the only server allowed to write the VLAN database.Secondary servers cannot write the VLAN database.)
  6. Support to turn VTP on or off on a per-trunk (per-port) basis.
  7.  VTP can be disabled globally.
  8. In VTP version 3, VLAN configurations are saved in NVRAM in client mode. Earlier version did not save Vlan configurations in NVRAM

VTP Domains:

VTP is organised into management domain which share common VLAN requirements. A VTP domain is made up of one or more interconnected network devices that share the same VTP domain name. A network switch can be configured to be in one and only one VTP domain.

VTP Modes:

A Cisco switch must be configrured in any one of these VTP modes to participate in VTP domain.

  1. Server Mode: This is the default mode of Cisco switches. In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters (such as VTP version) for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on advertisements received over trunk links. Each VTP domain must have atleast one VTP server switch, because VLANs can be created, modified or deleted only in VTP servers.
  2. Client Mode:This mode doesn’t allow the creation, modification and deletion of VLANs from VLAN database. Switches that are configured in VTP client mode listen to VTP advertisements from other switches and modify their VLAN configurations accordingly. They also forward the received VTP advertisments to other switches through the trunk links.
  3. Transparent Mode: VTP transparent switches do not participate in VTP.A In VTP transparent mode, the switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received VTP advertisements. When a VLAN is created or modified in VTP transparent mode, those VLAN changes affect only the local device.  In this role, the switch uses NVAM to store it’s local VLAN database. The local configuration revision number will always be 0 in transparent mode/role.
    • VTP version 1: A transparent mode switch in VTP version 1 does not forward the VTP advertisements to other switches unless it’s VTP domain name and version match with other switches.
    • VTP Version 2: A transparent mode switch in VTP version 1 forwards the VTP advertisement to other switches via trunk links regardless of the VTP domain name setting.
    • VTP Version 3: Allows to relay all VTP protocol packets that it receives on a trunk port to all other trunk ports.
  4. Off:This mode is used to disable all VTP activity on a switch. A switch in VTP off mode functions in the same manner as a VTP transparent switch, except that it does not forward VTP advertisements on trunks.

VTP Requirements:

The following preconditions must be fullfilled by two (or more) switches to participate in the VTP process.

  • The link between the switches must be configured as a VLAN trunk (ISL or 802.1Q).
  • VTP domain name must be the same on all switches participating in that VTP management domain. (Note: VTP domain names are case sensitive).
  • The VTP password (if configured) must be same across all the switches. (VTP password is an optional setting)

Default VTP configuration:

show vtp status command displays the VTP configuration status of a Cisco switch. The below snapshot displays the (factory) default VTP settings of a Cisco switch.

  • Default VTP version is 1.
  • Default VTP mode is Server.
  • The VTP Configuration Revision number always starts from 0.

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