ISL is a VLAN protocol that stands for Inter-Switch Link. Cisco's ISL is a proprietary protocol that is only utilized between Cisco switches.
A point-to-point VLAN context can support up to 1000 VLANs and is only compatible with Fast Ethernet and Gigabit Ethernet networks.
ISL encapsulates an Ethernet frame with a header that sends VLAN IDs between switches and routers. The tag in IEEE 802.1Q is internal.
An Ethernet encapsulated ISL frame will typically start at 94 bytes and grow to 1548 bytes in size. Encapsulation is used to develop the protocol.
The frame is given a 26-byte header and a 4-byte CRC trailer by ISL. The data-link layer of the OSI model is where ISL works.
ISL protocol encapsulation contains various fields such as the destination address, type of frame encapsulated, the source address, the length of the packet, its VLAN destination ID, the sub-network protocol (SNAP), high bits of source address, the source of the packet's port index, the actual Ethernet frame, a 4-byte check on the ISL packet to ensure it is not corrupted, and a reserved field for further information.
An Ethernet frame is encapsulated with a 26-byte header and a 4-byte trailer by the ISL. On the ISL packet, the trailer is a Frame Check Sequence (FCS) to guarantee that it is not corrupted. ISL frames range in size from 94 bytes to 1548 bytes.
Throughout, the encased frame remains unchanged. The ISL header's source and destination addresses are not passed down from the encapsulated frame. The target address is one of two ISL-specific multicast MAC addresses.
Unless one of the Cisco switches does not support ISL or is specially set not to utilize it, when two linked Cisco switches automatically negotiate a trunk with DTP, they will pick ISL over 802.1Q.
Inter-Switch Link is used for tagging information. It enables the simultaneous connectivity of several switches while also keeping VLAN information and delivers full wire-speed performance with minimal latency.
ISL eliminates the requirement for a router in order to interact/communicate, allowing users to access servers efficiently and rapidly.
A significant disadvantage of ISL is that only Cisco switches are compatible with ISL, which only supports 1000 VLANs. Due to several underlying reasons and challenges related to switching, particularly in VLAN Trunking, ISL is no longer used today.
Cisco has abandoned ISL and no longer supports it. Instead, 802.1q is widely utilized, and Cisco expressly encourages it. The majority of current switches use this 802.1q VLAN switching protocol.