What Is The Purpose Of ISL?

[Ali Hazim]

Hi All
What Made Cisco Work On The ISL Encapsulation Protocol When The IEEE 802.1Q Protocol Was Efficient? What Advantage Does The ISL Has Over The IEEE 802.1Q? 

[Mark Malone · Accepted solution]

Hi
Its legacy and retired now , all dot1q , Cisco removed the ISL trunking feature while back , cant even enable it on the new switches
it was just Cisco's own version proprietary , 802 is the open standard , 802 had less overhead memory
i dont think it really had an advantage thats why they dropped it


some key differences noted on this website
https://ciscohite.wordpress.com/2013/05/14/difference-between-isl-802-1q/
802

It is an IEEE Standard.
802.1q supports 4096 Vlans.
IN 802.1q encapsulation process, a 4 byte tag is inserted into original frame and FCS (Frame Check Sequence) is re-calculated.
802.1q does not tag frames from native Vlans.



ISL is an Cisco proprietary protocol.
Supports up to 1000 Vlans
Original frame is encapsulated and a new header is inserted during encapsulation process.
A 26 byte header and a 4 byte FCS (frame check sequence) are inserted. Hence a total of 30 Bytes of overhead.
ISL tags frames from native Vlans.
ISL is less preferred in networks because of its high overhead value which is added to each Ethernet frame.

[Joseph W. Doherty · Accepted solution]

I recall (?) ISL pre-dates 802.1Q. If correct, Cisco saw the need for VLAN trunking and ISL was their solution. (Keep in mind that Cisco was/is often a pioneer in network technologies. If the technology is good, but if Cisco keeps them proprietary, the industry will often define a somewhat similar technology that all might use. [Some other examples, PVST, with enhancements, vs. RSTP; CDP vs. LLDP, HSRP vs. VRRP, RIP vs. IGRP, OSPF vs. EIGRP, etc.] The industry wide technology might be worse, about the same, or better than Cisco's.)