Where is the 802.1Q tag inserted in the Ethernet frame?

The 802.1Q tag is inserted after the Source MAC Address in the Ethernet frame. This tagging process is part of VLAN (Virtual Local Area Network) implementation, which allows multiple VLANs to share the same physical network infrastructure while maintaining separation between the traffic of different VLANs.

In an Ethernet frame, the structure typically includes fields such as the Destination MAC Address, Source MAC Address, Ethertype, and Frame Check Sequence (FCS). When a frame is tagged with 802.1Q, the Tag Protocol Identifier (TPID) and the Tag Control Information (TCI) are inserted directly after the Source MAC Address. This added tag includes the VLAN ID and priority information, which is crucial for proper handling and identification of the frame as it traverses a switched network.

Understanding this insertion point is vital as it allows network devices to process and forward frames according to the VLAN configuration, ensuring that frames are appropriately managed within their respective VLANs. This is essential for maintaining network segmentation and performance.