What is VTP (Virtual LAN Trunking Protocol)?

VTP (Virtual LAN Trunking Protocol), as it can be understood from the name of the protocol in networks with a large number of switchers, allows us to move VLAN (Virtual Local Area Network) to other switchers thanks to the “trunk” ports. VTP can also be called VLAN Domain in a sense.

Thanks to this, centralized management can be provided in networks with multiple switches. It saves you the hassle of configuring each switcher separately from errors such as adding, deleting, changing names in the managed network and errors that may occur during the configuration. Necessary changes are made on a single switcher and VTP teaches changes to other switches through trunk ports.

The concept of a domain is important. A field is created in a network where VLANs are to be taught with this protocol. Switchers in the same area exchange packages, they do not exchange packages with switchers in different areas. To create a domain, Domain Name is entered in the switcher.

The domain name basically comes empty. Switchers with the same domain are in the same domain. In addition, the Password can be entered optionally for security purposes. Switchers in the same field must be entered with the same password, otherwise, switchers cannot exchange packages.

Trunking Protocol can run in 3 different modes: Server, Client and Transparent.

Server: It is the mode that has the authority to make all the changes on the VLANs, send the change information it has made, receive information and updated features. At least a switcher running in server mode is required for adding and configuring VLANs in each domain. Any changes made in this mode are also announced to that domain and taught to other switchers in the domain. These configurations are stored on NVRAM (Non-Volatile RAM – Nonvolatile memory).

Client: They are switchers that can receive information from their servers, update and send the information they receive, but do not have the authority to make any changes on VLANs. The configuration of this mode is not stored on NVRAM, they are temporary.

Transparent: The switcher working in this mode is actually like a server, VLANs can be created, deleted or changed. These operations only affect the VLAN database of this switcher, the changes made here cannot be transferred to other switchers. In addition, it sends the information received from other switchers to other switchers through trunk ports but does not use the information it receives and does not update its own VLAN database with this information. The configuration for this mode is stored on NVRAM.

VTP Configuration Revision Number

When using VTP to carry VLAN information between switchers, the concept of Revision Number is very important. When a change is made to the VLANs within the network, the switcher in the server mode that has been changed increases one by one.

The switcher whose revision number is changed sends VTP notification to other switchers in the same domain. Switchers receiving the statement compare revision numbers. If the revision number in the notification it receives is larger than its revision number, it updates the VLAN information according to the notification.

There are several ways to reset the revision number of a switcher with a configuration. Some of those;

  • Changing the VTP domain name
  • Deleting the vlan.dat file held in Flash
  • Switching the VTP mode to transparent mode (switching to another mode in transparent mode)

There is also a potentially dangerous aspect of the revision number. If you unwittingly connect a switcher with the same password in the same domain and a larger revision number to the trunk port of a switcher connected to the same network, all VLANs and VLAN information in the network will be destroyed and the VLAN information in the switcher with the high revision number will be valid.


There are 3 types of papers: Summary, Subset and Request Advertisement.

1. Summary Advertisements

Contains information such as domain name, current revision number, and other configuration details. It is sent to switchers with VTP feature running in neighboring server or client mode every 5 minutes.

2. Subset Advertisements

Contains VLAN information. Changes such as creating or deleting a VLAN, turning off or activating a VLAN, changing the VLAN name, and changing the size of the VLAN packet send it immediately after modification to other switchers in the same domain.

3. Request Advertisements

The domain name changes, the receipt of a high summary advertisement from its revision number, the subset advertisement message cannot be received due to any reason, and the switcher is notified to the switchers in the switcher domain if it is closed and opened. The switcher that receives a request advertisement first sends a summary and then subset advertisement.


There are 3 versions: version 1, version 2 and finally version 3. The only difference between versions 1 and 2 is that version 2 supports Token Ring VLANs. Version 3 has brought quite new features compared to previous versions. Some of these features;

Version 3 provides more effective administrative control over the switcher used to update the VLAN information of other switchers. It provides a significant reduction in accidental changes in the network and increases its usability.

With ISL VLANs from 1 to 1001, the Dot1q (802.1Q) VLANs up to 4095 have been transported and functionality has increased significantly in the VLAN environment as well as supporting VLANs as well as Private VLAN (PVLAN) structures.

Version 3 enables the transfer of different databases besides VLAN.


If pruning is enabled in a domain, broadcast traffic related to this VLAN will not flow to a switcher without an active port of a VLAN. VTP pruning; It provides bandwidth by preventing packets of those VLANs from being sent to the device with unused VLANs. In order for VTP pruning to work, it must be implemented in all switchers in the same domain. This feature cannot be activated in client mode. It is activated in Server mode and taught to other devices. VTP pruning is disabled on a device that will be used for the first time and is activated by running the #vtp pruning enable command in global configuration mode on the switcher.

Dot1q (802.1Q)

It is a VLAN encapsulation standard developed by IEEE. It determines what the VLAN ID is, thanks to the bits placed in the Ethernet packets. The 802.1Q standard has been developed to meet the need to divide large networks that occur in the use of VLAN into smaller and manageable parts. Unlike the ISL standard developed by Cisco, its compatibility with different branded devices has made 802.1Q very popular.

In addition, the 802.1Q protocol is 26 bytes smaller than ISL, which makes 30-byte labeling. Although it is not known exactly, it can be said that the size reduction makes the 802.1Q faster than ISL.

ISL (Inter-Switch Link)

ISL is a VLAN encapsulation standard developed by Cisco Systems. It is a protocol especially developed by Cisco that enables the flow of network traffic over the VLAN from the switcher to router or switcher to the switcher. Since this protocol was developed by Cisco, it works only between Cisco devices.

These can only work on Fast or Gigabit Ethernet. This is called “external tagging”, which does not change the original size of the package, but adds a 26 byte ISL header to the package, allowing VLAN recognition between devices. It also adds a 4-byte FCS (frame check sequence) field that controls the pack to the end of the packet. The package can only be recognized by devices that enable ISL after these plugins.

The size of the package can reach up to 1522 bytes so that the maximum length in the ethernet network is 1518 bytes. The ISL protocol does not make any changes to the original frame, so it is very safe and does not cause any changes to the data.

Basic Configuration Commands Related to VTP

To teach VLAN information to other switches, the port to which the switchers are connected must first work as a trunk port. To check this, #show interfaces fastethernet0/1 switch port command must be executed.

If it is not working in trunk mode, you should enter the interface and go to trunk mode with the #switchport mode trunk command.

We can have an idea when we run (vtp ?) Command in global configuration mode.

Whichever version the protocol will run, the version is specified with the command #vtp version “version-number”. After this process, a password must be set for the #vtp domain “domain name” to create a domain and the #vtp password “domain password” domain for security purposes.

In addition, information can be obtained with the protocol executed with the #show vtp status command in privilege mode.

VTP Version: Indicates in which version VTP is working.
Configuration Revision: Specifies the configuration revision number.
Maximum VLANs supported locally: Specifies the maximum number of VLANs that will be stored in the database.
A number of existing VLANs: Specifies the number of VLANs that exist in the database. 5 VLANs are basically created on the device where no process is performed and they cannot be deleted. These VLANs are 1, 1002, 1003, 1004, and 1005 VLANs.
VTP Operating Mode: Indicates in which mode the VTP is operating.
VTP Domain Name: Specifies the VTP domain name. On devices in the same domain, this part should be the same.
VTP Pruning Mode: Indicates whether the VTP Pruning feature is active.

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