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.