Differences between ISL and 802.1Q
ISL and 802.1Q are trunking protocols. Trunk ports use them to encapsulate and de-encapsulate Ethernet frames. A trunk port can use only one protocol from these at a time.
Differences between ISL and 802.1Q
The following table compares ISL with 802.1Q and lists the differences between them.
| ISL | 802.1Q |
| ISL stands for Inter-Switch Link. | 802.1Q is also known as dot1Q. |
| Cisco developed this protocol to provide trunking between its devices. | IEEE developed this protocol to provide trunking between all supported devices. |
| It is a Cisco proprietary protocol. | It is an open standard protocol. |
| It works only with Cisco devices. | It works with all vendors. |
| It works only with Fast and Gigabit Ethernet links. | It works with all types of Ethernet links. |
| It does not modify original Ethernet frames. | It modifies original Ethernet frames. |
| A switch cannot forward an ISL frame from an access port. | A switch can forward an 802.1Q frame from an access port. |
| It adds a 26-byte header and a 4-byte CRC trailer to the frame. | It inserts a 4-byte tag field in the frame's header. |
| Cisco no longer uses this protocol. | This protocol is the default trunking protocol on Cisco devices. |
The following image shows how ISL encapsulates Ethernet frames.
The following image shows how 802.1Q inserts a tag field in the frame's header.
Conclusion
ISL and 802.1Q are two trunking protocols. This tutorial compared both and listed their key differences. Understanding the differences enhances your knowledge about trunking mechanism switches used to exchange VLAN information across the network.
Author Laxmi Goswami Updated on 2026-04-01