What is the primary purpose of the Tag Protocol Identifier (TPI)?

The primary purpose of the Tag Protocol Identifier (TPI), commonly used in VLAN tagging protocols such as IEEE 802.1Q, is to specify VLAN membership. The TPI serves as a unique identifier that designates the VLAN to which a particular data frame belongs. By including this identifier in the frame header, network devices can properly route and segregate data traffic between different VLANs, ensuring that network traffic remains organized and secure.

VLANs allow network administrators to create separate broadcast domains within a single physical network, improving performance and security by isolating traffic. The TPI makes it possible to distinguish between these segments, allowing switches to understand where to forward the traffic while maintaining the integrity of the VLAN configuration.

The other choices refer to separate functionalities. User authentication, data encryption, and establishing user permissions do not relate to VLAN tagging or the role of the TPI in network traffic management. Therefore, specifying VLAN membership is the correct function of the Tag Protocol Identifier.