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 above, 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.