VLAN Trunking

VLAN Trunks
The VLAN Trunk technology allows you to communicate with various VLAN networks
Cisco has two different VLAN Trunk protocols:
ISL:
ISL is Cisco's VLAN encapsulation method and supported only on Cisco's equipment through Fast and Gigabit Ethernet links. The size of an Ethernet encapsulated ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes.

IEEE 802.1Q:
IEEE 802.1Q, or VLAN Tagging, is a networking standard written by the IEEE 802.1 workgroup allowing multiple bridged networks to transparently share the same physical network link without leakage of information between networks. IEEE 802.1Q — along with its shortened form dot1q — is commonly used to refer to the encapsulation protocol used to implement this mechanism over Ethernet networks.
IEEE 802.1Q defines the meaning of a Virtual LAN (VLAN) with respect to the specific conceptual model underpinning bridging at the MAC layer and to the IEEE 802.1D spanning tree protocol. This protocol allows for individual VLANs to communicate with one another with the use of a switch with layer-3 capabilities, or a router.