ISL:
ISL is Cisco Proprietary, not usable in multivendor environments. IT will encapsulate the entire frame and place both header and trailer onto the frame, which will increases the overhead on trunk line. Because ISL does not use the concept of native Vlan (Vlan 1 Default in Cisco switches), every single frame transmitted across the trunk will be encapsulated. ISL adds 26-byte header, which contains Vlan ID and adds 4-byte trailer contains CRC value.
Frames larger then 1518 byte (Max size for Ethernet frame) called GIANTS. ISL encapsulation adds 30 byte to the original frame, which makes them too large for the switch to handle.




Dot 1Q:

Dot 1Q is the IEEE standard suitable for multivendor. Dot1Q does not encapsulate frame, it just adds 4-byte header to the frame and adds nothing to native Vlan frames. Adding only 4-byte to the frame results less overhead then ISL .Maximum frame size of 1522.