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VTP (VLAN Trunking Protocol): is the VLAN trunk protocol, also known as the virtual LAN trunk protocol. It is a Cisco private protocol. The function is that more than a dozen switches are in the enterprise network, and there is a large amount of work to configure VLAN. You can use VTP protocol to configure one switch as VTP Server and the rest of the switches as VTP Client, so that they can automatically learn the use of VTP:

Usually, we need to keep the VLAN database synchronized in a group of switches in the entire campus network or in the enterprise network to ensure that all switches can read the relevant VLAN information from the data frames for correct data forwarding. However, for large networks, there may be hundreds of switches, and there may be dozens or even hundreds of VLAN on one switch. Manual configuration by a network engineer alone is a lot of work, and it's not good for future maintenance-every time you add, modify or remove VLAN, you need to deploy it on all switches. In this case, we introduced VTP (VLAN Trunking Protocol).

To use VTP, you must first establish a VTP management domain in which switches in the same management domain share vlan information, and a switch can participate in only one management domain. Switches in different domains cannot share vlan information.

It is a communication protocol of the second layer of the OSI reference model, which is mainly used to manage the establishment, deletion and renaming of VLANs within the network scope of the same domain. When a new VLAN is configured on a VTP Server, the configuration information for that VLAN is automatically propagated to all other switches in the domain. These switches automatically receive these configuration information, making their VLAN configuration consistent with VTP Server, thus reducing the workload of configuring the same VLAN information on multiple devices and maintaining the unity of VLAN configuration.

VTP maintains VLAN configuration uniformity over the network (ISL frames or cisco private DTP frames). VTP manages added, deleted, and adjusted VLAN at the system level and automatically broadcasts information to other switches in the network. In addition, VTP reduces configurations that can cause security problems. Easy to manage, as long as the appropriate settings are made in vtp server, vtp client will automatically learn vlan information on vtp server.

The VLAN domain, also known as the VTP management domain, consists of more than one switch that shares the VTP domain name and connects to each other. In other words, a VTP domain is a group of switches that have the same VTP domain name and are connected to each other through trunk links.

The following are the requirements for the VTP domain:

1. Every switch in the domain must use the same domain name, whether it is implemented through configuration or automatically learned by switching.

2. Catalyst switches must be adjacent, that is, adjacent switches need to have the same domain name.

3. Trunk links must be configured between all Catalyst switches.

If any of the above conditions are not met, the VTP domain cannot be connected and the information cannot be transmitted across separate parts.

There are three VTP modes: server mode (Server), client mode (Client), transparent mode (Transparent).

The default configuration of a new switch when it comes out of the factory is preconfigured as a server in VLAN1,VTP mode. In general, there is only one VTP Server for the entire network in a VTP domain. VTP Server maintains a list of all VLAN information in the VTP domain. VTP Server can establish, delete or modify VLAN, send and forward relevant notification information, synchronize vlan configuration, and save the configuration in NVRAM. Although VTP Client also maintains a list of all VLAN information, its VLAN configuration information is learned from VTP Server. VTP Client cannot establish, delete or modify VLAN, but it can forward advertisements, synchronize vlan configurations, and do not save configurations to NVRAM. VTP Transparent is equivalent to an independent switch, it does not participate in VTP work, does not learn the configuration information of VLAN from VTP Server, but only has the VLAN information maintained by itself on this device. VTP Transparent can create, delete, and modify VLAN information on the local machine, while forwarding announcements and saving the configuration to NVRAM.

VTP message type:

 Summary advertisement message (Summary Advertisements)

By default, the Catalyst switch sends a summary advertisement message every 5 minutes. A summary advertisement message notifies the current VTP domain name and configuration revision number of the neighboring Catalyst switch. When the switch receives a summary advertisement message packet, the switch compares the VTP domain name in the packet with its own VTP domain name. If the name is different, the switch ignores the packet. If the name is the same, the switch compares its own configuration revision number with the packet revision number. If your own configuration revision number is greater than or equal to the packet revision number, the packet will also be ignored. If your own configuration revision number is small, the switch sends an advertisement request message.

 subset advertisement message (Subset Advertisements)

When an administrator adds, deletes, or modifies a VLAN on a Catalyst switch, the server switch whose configuration has been modified increases the configuration revision number and sends a summary advertisement message. It then sends one or more subset notification messages. Each subset advertisement message contains a list of VLAN information. If there is more than one VLAN, the switch asks the server switch to send multiple subset advertisement messages to advertise all the VLAN information.

 advertise request message (Request Advertisements)

The switch needs to send an VTP advertisement request message in the following cases.

1. Switch restart

The 2.VTP domain name has been modified

3. The switch receives an VTP summary advertisement message with a higher configuration revision number than its own.

After receiving an advertisement request message, the VTP device sends a summary advertisement message. After that, one or more subset notification messages are sent.

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Teaching assistant: Ma Ji

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