what is the advantage of ISL over dot1q on cisco network compatible with ISL

[LuisPantaB]

The entire network has cisco switches that support ISL. Is there any advantage of this protocol over dot1q?

[Mark Snyder · Accepted solution]

The only advantages that I know of are fddi and token ring support, but as those aren't eminent technologies anymore you can kind of see why Cisco deprecated it. The drawbacks - 30-byte header, frame encapsulation instead of tag, needing to recalculate the FCS - outweigh that benefit by far. Also, I can imagine a scenario where a company replaces a switch with a non-Cisco switch and it becomes necessary to migrate everything because ISL is proprietary.

Hope that helps.

[Aryanaresh · Accepted solution]

Cisco developed ISL with lot of plans in there mind but it resulted with 30 byte header tag (Causes processor overhead).

Whereas, 802.1Q is Industry standard and it uses 4 byte VLAN tag (Simple and serves the purpose very well).

ISL was the first trunking protocol, i believe 5 years later IEEE came up with industry standard protocol for Trunking (i.e) 802.1Q

You can find the detailed Frame structure in the below link

http://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html

When it comes to ISL or 802.1q, you can blindly choose 802.1q because Cisco wants to get rid of ISL. Catalyst 2940 and 2950/2955 series switches support only 802.1Q trunking. These switches do not support ISL trunking.

The Catalyst 4500/4000 series, which includes the Catalyst 2948G and Catalyst 2980G, only supports 802.1Q trunking. The series does not support ISL trunking.

Bottom Line is its better to switch over to 802.1q than ISL.