ISL and 802_1Q Trunking

ISL (Inter-Switch Link) and 802.1Q are two different methods for trunking multiple VLANs between switches in a network.

ISL is a Cisco proprietary protocol that adds a header to the Ethernet frame to identify the VLAN to which the frame belongs. ISL encapsulates the entire Ethernet frame and adds a 26-byte header, increasing the frame size. ISL is no longer widely used, as it has been replaced by the industry-standard 802.1Q.

802.1Q, also known as dot1q, is an industry-standard protocol that tags Ethernet frames with a VLAN ID. Instead of encapsulating the entire Ethernet frame, 802.1Q adds a 4-byte tag to the Ethernet frame, which identifies the VLAN to which the frame belongs. This tag is added between the source MAC address and the EtherType field of the Ethernet frame, which means that the frame size is not increased. Both ISL and 802.1Q can be used to trunk multiple VLANs over a single link between switches. In order to configure trunking, the switch ports connected to each other must be configured with the same trunking protocol and the same VLANs. This allows multiple VLANs to be carried over a single link, which can simplify network management and reduce the number of physical connections required between switches.