VTP (VLAN Trunking Protocol) is a protocol that runs on the Layer 2 layer of the OSI model used to configure and manage VLANs on Cisco network devices.
What is VTP?
In a physical network, there are usually several switches linking multiple VLANs. Each VLAN must be manually configured on each switch to maintain the connection between VLANs. As the organization grows and additional switches are added to the network, each new switch must be manually configured with information from VLANs, but the process of managing the workload and the workload becomes longer.
Thanks to the VTP feature, all VLANs configured on a central switch are automatically sent to other devices in a domain. Thus, central management is provided over the switches in the network and the created configurations are sent to all devices that are members of the same VLAN and domain.
When devices on both local and remote networks are physically inaccessible, other devices’ configurations can be easily configured by the administrator thanks to the VTP protocol.
VTP performs editing or deletion of created VLANs from a central switch from which all switches in the network can be managed. In this way, it saves time by preventing setting settings separately for each device on the network.
It is much easier to configure VLANs on a single switch in a large or small network environment, but since it will be very troublesome to configure all VLANs on all switches, a VTP domain structure is created and other devices are subscribed to this domain so that all settings are synchronized by all switches.
In short, the Trunking protocol is a Cisco-proprietary Layer 2 protocol that allows the switches on the network to exchange information about VLANs so that they always have a VLAN database synchronized from a central point in the network. It also sends its updates to all devices via multicast.
Switches use VTP version 1 by default, but when you want to change the version, the vtp version command can be used in the general configuration mode of the device.
Its major advantage is that it reduces the need for manual network configuration and allows easily scalable switched network solutions.
What Does It Do?
VTP was developed to be able to propagate VLANs in very large network infrastructures and fundamentally maintains the integrity of VLANs and allows VLAN information to be distributed, deleted, or modified centrally.
In short, it makes it easy for network administrators to automatically deploy VLANs across multiple switches after certain parameters are configured on each switch. However, it is not recommended to enable or configure this feature in networks with a single Switch. In addition, it is recommended that the switch to be added to the network is set in Transparent mode.
NOTE: VTP messages to be sent between network devices are only transmitted over Trunk interfaces.
Trunking protocol has 3 basic operating modes. These; It is Server, Client, and Transparent.
This mode is the default mode on switches and VLANs can be created, modified, or deleted in this mode. A Switch configured in Server mode announces its configuration to all Switches in the domain.
To configure this mode, at least one device in the environment must be configured as Server. Additionally, authentication can be used in this mode.
In this mode, VLANs cannot be deleted, changed, or created on the Switch. In this mode, only VLAN information is retrieved from the Switch configured as VTP Server.
A VTP Client only saves VLAN information for the entire domain with the switch turned on, but this information is cleared and retrieved again upon restarting the device.
In Transparent mode, VLANs that can be synchronized with other devices on the network cannot be created, deleted, or changed. Only locally VLAN information can be changed on a Switch operating in this mode.
This mode does not receive and apply VTP updates, but only sends the received updates to Client devices in the same domain. In VTP version 2, updates are delivered regardless of whether the devices are in the same domain or not.
Thanks to the pruning feature, bandwidth is saved. For example, when a Switch with the same domain controls its ports according to the VLAN information it receives from the server device, if there is no port for the relevant VLAN information, the packets coming to this Switch will be discarded because the ports of the device receiving the updates do not correspond to the relevant VLAN.
So, when there is an incompatible VLAN structure, VTP Pruning is enabled as the device constantly sending packets to an unrelated VLAN will consume the network’s bandwidth.
Such messages are sent to neighboring switches to provide information about the device’s current domain information and VTP revision. The device receiving the message compares its domain information with its local information. If the domain name does not match, the package is canceled, otherwise, the revision number is compared and a request is sent if the revision number of the canceled package is higher than the local value.
Such messages contain an up-to-date VLAN database and the subset message is sent by increasing the revision number as a result of any change.
When the conditions between the Summary Advertisement message and the Switches are completed, VTP information is requested.
Join message type is used to propagate VLANs between the server and client devices after the domain name, password, and version number conditions are met.