ISL: Cisco proprietary and encapsulates the original frame in a 26 byte header and a 4 byte trailer. The ISL vlan’s are 1-1001.
802.1Q: Open standard and inserts a field in the 802.1P header, just after the source mac-address. The 802.1Q vlan’s are 0-4094.
If a mixture of ISL and 802.1Q is in use then the matching vlan’s can map across up to the ISL limit.
Points to note include if a non trunking port receives and ISL encapsulated frame then the frame is dropped. This is because the header and trailer cause the frame to exceed the MTU and may be counted as an error.
If an 802.1Q frame is received on a non trunking port then the source and destination mac-addresses are checked and the frame is switched normally at Layer2.