What is the name of Cisco's proprietary VLAN trunking protocol?

The correct answer, ISL, refers to Cisco's Inter-Switch Link protocol, which is a proprietary VLAN trunking protocol that encapsulates Ethernet frames with additional header information. This allows for the multiplexing of multiple VLANs on a single physical link between switches, enabling them to communicate while maintaining VLAN separation.

ISL adds a 26-byte header to the original frame and uses a specific encapsulation technique to identify the VLAN to which the frame belongs. This encapsulation is important for ensuring that switches can properly route traffic to the right VLANs when multiple VLANs share the same trunk link.

In contrast, 802.1Q is an industry-standard VLAN tagging protocol that also allows VLAN traffic to be carried on a trunk link but does it by inserting a tag into the original frame's Ethernet header instead of creating a separate header. STP (Spanning Tree Protocol) is a network protocol that ensures a loop-free topology in Ethernet networks, and VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used to manage VLAN configurations across multiple switches but is not a trunking protocol itself.