What is Inter-Switch Link (ISL)?

Inter-Switch Link (ISL) is a proprietary VLAN trunking protocol developed by Cisco Systems for carrying multiple VLAN traffic between Cisco switches. ISL was designed to maintain VLAN information when frames traverse trunk links connecting switches.

Unlike the IEEE 802.1Q standard, ISL uses external tagging by completely encapsulating the original Ethernet frame with an ISL header and trailer, rather than inserting a tag within the frame.

How ISL Works

ISL operates by encapsulating the entire original Ethernet frame with a 26-byte header and a 4-byte CRC trailer. The encapsulated frame remains completely unchanged throughout transmission.

The ISL header contains essential information including VLAN ID, frame type, source address, and port index. The 4-byte trailer provides Frame Check Sequence (FCS) for error detection. Total frame size ranges from 94 to 1548 bytes.

Key Features of ISL

• External encapsulation − Completely wraps the original frame, preserving it intact

• Support for 1000 VLANs − Can handle VLAN IDs from 1 to 1000

• Fast/Gigabit Ethernet compatibility − Works with high-speed Ethernet links

• Proprietary protocol − Only compatible between Cisco switches

ISL vs 802.1Q Comparison

Advantages and Disadvantages

Advantages

• Wire-speed performance − Minimal latency in frame processing

• Multiple VLAN support − Efficiently carries traffic for multiple VLANs simultaneously

• Frame integrity − Original frame remains completely unchanged

Disadvantages

• Vendor lock-in − Only works with Cisco equipment

• Higher overhead − 30-byte overhead compared to 4 bytes for 802.1Q

• Limited VLAN range − Supports fewer VLANs than 802.1Q standard

Current Status

ISL is now considered legacy technology. Cisco has deprecated ISL support in favor of the industry-standard IEEE 802.1Q protocol. Modern Cisco switches primarily use 802.1Q for VLAN trunking due to its broader compatibility, lower overhead, and larger VLAN ID space.

When two Cisco switches negotiate a trunk using Dynamic Trunking Protocol (DTP), they will prefer ISL over 802.1Q only if both switches support ISL and are not configured otherwise.

Conclusion

ISL was Cisco's proprietary solution for VLAN trunking that used external frame encapsulation to preserve VLAN information across switch links. While it provided good performance, ISL has been largely replaced by the industry-standard 802.1Q protocol due to better interoperability and efficiency.