When using VLANs in networks that have multiple interconnected switches, you need to use VLAN
trunking between the switches. With VLAN trunking, the switches tag each frame sent between switches so
that the receiving switch knows to what VLAN the frame belongs. End user devices connect to switch ports
that provide simple connectivity to a single VLAN each. The attached devices are unaware of any VLAN
structure.
A trunk link can transport more than one VLAN through a single switch port. A trunk link is not assigned to
a specific VLAN. Instead, one or more active VLANs can be transported between switches using a single
physical trunk link. Connecting two switches with separate physical links for each VLAN is also possible. In
addition, trunking can support multiple VLANs that have members on more than one switch.
Cisco switches support two trunking protocols, namely, Inter-Switch Link (ISL) and IEEE 802.1Q.