What value is the TPID field always set to in the 802.1Q tag?

The TPID (Tag Protocol Identifier) field in an 802.1Q VLAN tag is always set to 0x8100. This is a standardized value indicating that the frame is a VLAN-tagged frame. The 0x8100 TPID is important because it distinguishes network traffic that is using VLAN tagging from regular Ethernet frames. When a switch or other networking device receives a frame, it checks the TPID to determine how to handle the frame appropriately.

Other values listed in the choices represent different protocols or identification methods. For instance, 0x8021 is not a recognized TPID for VLAN tagging, while 0x0800 is commonly used to indicate IPv4 traffic, and 0x9000 is sometimes associated with jumbo frames. Therefore, 0x8100 is the only correct answer as it is defined by the IEEE specification for VLAN tagging and is used universally across networking devices that support 802.1Q.