Understanding how VLAN Trunks work is essential to you passing the CCNA Exam. In this post I’m going to talk about VLAN Encapsulation with 802.1Q, and all the details about the VLAN Trunking Protocol VTP.
Trunks are needed in switching networks with VLANs in order for hosts within different VLANs to still communicate with each other. When you create a VLAN you are supposed to maintain a 1 to 1 ratio with the number of subnets you have. Which means if you have 4 VLANs you should also have 4 subnets. Since each VLAN is in a different subnet they are in different networks and for devices in different networks to communicate with one another a Layer 3 device such as a router is needed. Interfaces on switches by default can only be assigned to one VLAN at a time. So, in order for you to send data about 4 different VLANs configured on a switch to a router you need to configure an interface to be a trunk link. Trunk links are able to send data for many different VLANs at one time. In order to keep them from getting mixed up they tag the switch frame.