Tale of Two Trunking Protocols

Tale of Two Trunking Protocols

It was the best of times, it was the worst of times…whoops, wrong tale! Once upon a time, no VLAN trunking existed. Cisco wanted something like trunking in its switches, but no standards body had created a standard protocol for trunking. So, Cisco created its own standard and named it Inter-Switch Link (ISL). ISL defines the type of header that should be added to the frame, including the field in which the VLAN can be numbered.

Later, the IEEE 802.1Q committee defined a standard for VLAN trunking called 802.1Q trunking, or simply "dot 1 Q." Unsurprisingly, 802.1Q trunking differs from ISL. As a result, when you connect two switches and you want to trunk, you need to pick between the two options. The switches on each end of the trunk must agree to which protocol to use, or trunking will not work.