What is ISL?

Understanding ISL

ISL (Inter-Switch Link) is a Cisco-proprietary VLAN tagging protocol that lets switches carry traffic from multiple VLANs across a single trunk link. It works by fully encapsulating each original Ethernet frame inside a new header and trailer that records the VLAN ID, so the receiving switch knows which VLAN the frame belongs to.

Mechanically, ISL adds a 26-byte header and a 4-byte CRC trailer around the entire original frame, totaling 30 bytes of overhead. The header carries the VLAN identifier, frame type, and source port information. Unlike the IEEE 802.1Q standard, which inserts a small 4-byte tag inside the existing frame, ISL wraps the frame externally without modifying it. Because ISL is Cisco-only, both ends of a trunk must be Cisco devices configured for ISL.

From a security and design standpoint, VLAN trunking is foundational to network segmentation, and the trunking protocol must isolate VLANs reliably. Misconfigured trunks enable VLAN hopping attacks, where an attacker injects double-tagged frames or negotiates an unintended trunk to reach restricted VLANs. ISL is now deprecated; modern Cisco platforms default to or exclusively support 802.1Q, which is the open standard and the only choice for multi-vendor environments. Knowing ISL still matters because legacy gear, exam blueprints, and older topologies reference it.

For example, on an older Cisco Catalyst switch an engineer might configure a trunk between two switches with the command 'switchport trunk encapsulation isl' before setting the port to trunk mode. The link would then carry VLAN 10, 20, and 30 traffic between buildings, with each frame wrapped in ISL headers so the far-side switch forwards it onto the correct VLAN. On any current switch, the engineer would instead use 802.1Q, since ISL is no longer supported. Recognizing this distinction is exactly the kind of detail tested on Cisco CCNA and security certifications.