VLAN Trunking Explained with Examples

What is a VLAN Trunk?

You can’t describe VLANs without mentioning trunks. It’s a known fact that you can control and segment network broadcasts with VLANs.  VLAN trunking enables the movement of traffic to different parts of the network configured as a VLAN.

A trunk is a point-to-point link between two network devices that carry more than one VLAN. With VLAN trunking, you can extend your configured VLAN across the entire network. Most Cisco switches support the IEEE 802.1Q used to coordinate trunks on FastEthernet and GigabitEthernet.

To enable VLAN configured with trunk link to traffic frames between switches on the network, it made possible by a link protocol called VLAN Trunking Protocol VTP

VLAN Trunking Protocol (VTP) is a  Cisco-proprietary link protocol, it provides a means by which Cisco
switches can exchange VLAN configuration information. In effect, VTP advertises about the existence of each VLAN based on its VLAN ID and the VLAN name.

VTP serves a useful purpose, It enables the distribution of VLAN configuration among switches.

In the figure Below , the links between switches SW1 and SW2, SW1, and SW3 are configured as trunk links to enable traffic between VLAN 10, 20 and 30. This network simply could not function without VLAN trunks.

VTP has three different modes of operation within a domain: Server, Client and transparent.

VTP Server

Every catalyst switch is a server by default. Every network or domain requires a server to propagate VLAN information throughout the network or domain.

 As a server switch, it will be able to create, add and delete VLANs in the domain.

 VTP server controls any change that’s to be made in the entire domain. When a change is made on the server, it will be advertised throughout the entire VTP domain. VTP server configurations are saved in the NVRAM.

VTP Client

Switches in a client mode receive information from VTP servers.

 VTP client switches also send and receive updates, but one difference between clients and server is that; VTP client switches can’t create, change and delete VLANs.  In other words, none of the ports on the client switch can be added to a new VLAN without the authorization or notification from the server switch.

Switches in client mode processes and forwards VLAN information. VLAN information on client switches is not saved in NVRAM.

VTP Transparent

Switches in this mode don’t take part in VTP domain or database. They behave more or less like servers in that they create, modify and delete VLANs because they have and keep their own databases, which is only locally important.

VTP switches in transparent modes only purpose in a VTP domain is to forward VTP advertisements through any configured trunk links.