Free Cisco CCNA Training: VLAN Trunking Protocol (VTP)

In any network setup, the switches must know about all the VLANs running and not just the ones they are immediately connected to through their ports. The VLAN Trunking protocol, or the VTP, allows switches to send out advertisements to other members of the same VTP domain, containing information about all the VLANs on the network. For example, if a VLAN is added on a switch, all the switches on the network get to know about it and not just the ones directly connected to it. Servers running VTP know about all the VLANs, even those which do not have any members attached to that particular switch.

The Modes

The information travels between the VTP devices in the form of summary advertisements. A VTP server sends out these “advertisements” over five-minute intervals, or immediately upon a change in the VTP database. From the point of view of CCNA examination and application in the real world, it is necessary to know about the modes in which VTP may run.

1. Server Mode

A VTP server is not a separate device but simply a mode within the protocol. In this mode it is possible to create, delete, modify, and rename VLANs. All such changes are advertised to all the VTP devices in the domain. Server mode devices can originate, transmit, or process summary advertisements. The information is stored on NRAM, or non-volatile RAMs, which allow it to be retained even on reboots.

2. Client Mode

Client mode switches can’t add, remove, or modify VLANs. The information is not retained upon a reboot either; it is simply stored in its running configuration. Upon a reboot, the information is again obtained from the closest VTP server. Clients can only accept and process summary ads.

3. Transparent Mode

This is a specialty VTP mode. Not used very often, it still finds a lot of application in the real world. However such a scenario is mostly out of the scope of the CCNA. Switches in transparent mode simply forward the ads they receive; they do not process this information. VLANs may be created, deleted or modified, but this information is not advertised. It is kept to the device and is significant only locally. Data in transparent mode is also saved on NRAM and is available after reboots.

VTP Domain Command

For VLAN information to be correctly exchanged the VTP domain name must match. The names are case sensitive and can be set using the vtp domain command. The default is NULL which indicates that there was no name existent before the assignment of the current name for that domain.

Ignoring the Ad

Every summary ad has its own configuration revision number which is increased by 1 every time it updates its own VTP database. This number is sent alongside the advertisements. If the number of the receiving device is lower than the ad, it removes all of its own data and replaces it with the data in the incoming advertisement which is more recent. In the other case if the revision number of the receiving device is higher, the ad is ignored.

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