Trunking is the process of creating logical connection between two physically connected switches allowing frames to flow between them.
Members of same VLAN communicate when physically attached to different switches.
ISL: cisco proprietary. encapsulates the entire frame before sending it across the trunk. results in greater overhead than ieee 802.1q. does not recognize native vlan
dot1q: industry standard. does not encapsulate frame. inserts 4byte value with vlan id in ethernet header. recognize native vlan.
native vlan: default vlan. usually vlan 1.
Dot1Q will not put 4byte value into ethernet header if frame is destined for native vlan. When remote switch receives an untagged frame and it knows it is destined to native vlan frame is forwarded to ports belonging to that vlan.