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.
That's all for this tutorial. In this tutorial, we compared ISL with 802.1Q and discussed the differences.
