Supporting Multiple VLANs on a Single Trunk
Comparing ISL and 802.1Q trunking protocols

Depending on the trunking protocol, data frames sent across a trunk link are either encapsulated or tagged. The purpose of encapsulating or tagging frames is to provide the receiving switch with a VLAN ID (VID) which identifies the VLAN from which the frame originated. The trunking protocol options are Inter-Switch Link (ISL), a Cisco proprietary protocol which encapsulates frames, or IEEE 802.1Q, which inserts a tag into the original Layer 2 data frame.

802.1Q is not proprietary and can be deployed in any Ethernet, standards-based Layer 2 device. It is specific to a single Layer 2 protocol (Ethernet) because it modifies the Layer 2 frame by inserting a tag between two specific fields of the frame and therefore must be aware of the frame header details.

ISL is Layer 2 protocol independent. Because the original Layer 2 frame is fully encapsulated and not altered, ISL can transport data frames from various Layer 2 media types.